Chapter 25: Alkylation of Enolates

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Have you ever found yourself maybe looking at a complex molecule on paper and just thinking, how on earth do chemists actually build that specific carbon -carbon bond?

It's almost like building with Lego, but you know, the really crucial connection pieces are missing and you have to somehow invent them.

Yeah, it's a great analogy.

And that's exactly what we're diving into today.

Welcome to the deep dive, where we're unpacking an alkylation of enolets, pulling directly from chapter 25 of Clayton, Greaves, and Warren's organic chemistry.

And our mission here isn't just like listing a bunch of reactions.

No, definitely not.

We want to get into the core strategies, the why behind it.

Understand the mechanistic reasoning that lets chemists design these powerful synthetic pathways.

Absolutely.

Because, you know, carbonyl compounds, they have this fascinating dual personality.

Sometimes they're electrophiles, right?

They're on electrons.

But then, with a bit of strategic thinking, they become nucleophiles, ready to donate electrons.

And this deep dive is all about harnessing that nucleophilic side specifically, their enol or enolate forms, to build those essential carbon -carbon bonds.

I mean, it really is a cornerstone of organic synthesis.

It absolutely is.

But here's the big challenge, the main hurdle you face.

If your carbonyl compound is playing the role of a nucleophile, you have to stop it from reacting with itself acting as an electrophile.

Think back to those Lego bricks.

You don't want them just randomly sticking together in ways you didn't intend.

Right.

That self -condensation, like the aldol reaction you might remember.

Exactly.

Designing these reactions needs, well, careful thought, as the book says, to dodge those problematic side reactions.

Thankfully, chemists have put, what, decades of really brilliant thinking into solving this exact problem.

And the good news for us learning this is that they've developed a pretty amazing toolkit of solutions.

A whole toolkit.

We'll look at everything from, let's say, simple systems that are just inherently less reactive, all the way up to sophisticated strategies involving different kinds of enol equivalents and really precise control over which part of the molecule reacts.

It sounds like chemical elegance in action.

It really is.

Okay, so let's unpack this toolkit then.

Before we dive head first into the really complex carbonyl stuff,

where's a good starting point for making CC bonds using these stabilized anions, especially when we want to avoid that self -reaction issue?

Well, a good place to start is with functional groups that are, let's say, less electrophilic than carbonyls, but they still do a good job of stabilizing an adjacent anion.

This just makes the whole alkylation process easier, kind of sidesteps the self -reaction problems.

Okay, like what?

Good examples are nitriles and nitroalkanes.

Nitriles, the C -triple bond end group.

Right.

How do they simplify things?

So that nitrile group, it's much less likely to get attacked by a nucleophile compared to a carbonyl CO group.

So if you deprotonate it with a strong base, say sodium hydride, you form what's called a nitrile enolate engine.

This anion is a pretty potent nucleophile, and it reacts really efficiently with alkyl halides through an SN2 mechanism.

That linear shape of the nitrile actually helps here, too.

And there are practical ways to manage this.

Oh, yeah.

A really clever application is making something like two -phenylpentanetrile.

You don't even necessarily need a super strong base.

You can use sodium hydroxide, which is weaker, but combine it with phase transfer catalysis.

Ah, the phase transfer trick.

Exactly.

You use a catalyst to kind of shuttle the base into the organic layer where the nitrile is.

It allows the deprotonation to happen smoothly right where you need it and keeps the reagents nicely separated, preventing unwanted stuff.

It's like a chemical bouncer at a club.

Huh.

I like that.

A chemical bouncer.

That's a neat way to control reactivity.

And I read these nitrile enions can even form quite crowded centers.

They can.

They're nucleophilic enough to create highly substituted quaternary centers.

You know, carbons bonded to four other carbons.

Even double alkylation is possible if you use enough base and alkyl halide.

And if you have a dinitrile, the anion is so stable that you might even get away with using a mild neutral base like triethylamine for deprotonation and still get fantastic yields.

That's versatile.

Does it work for making rings, too?

It does.

Intermolecular reactions are definitely possible.

A nitrile anion within the same molecule can attack an alkyl halide part elsewhere in the molecule, closing up to form rings, even cyclopropanes.

OK.

Nitriles make sense.

What about the nitrile alkynes?

How do they fit in?

Nitrile alkynes are pretty similar in utility.

That nitro group, the NO2, is incredibly electron withdrawing.

More so than nitrile.

Oh, yes.

It makes the adjacent protons surprisingly acidic.

Nitromethane, for example, has a pKi around 10, which is similar to phenol.

Quite acidic for a CH bond.

So these nitrinate anions,

form readily and react well with carbon electrophiles.

You can make, say, 3 -nitroheptane from nitropropane.

Now, if you use a really strong base like butylythium, you generally need two steps.

Make the anion first, then add the electrophile, because Booley reacts with alkyl halides.

Right.

You can't have them both in the pot.

Exactly.

But you can often use milder conditions like hydroxide under phase transfer again, or even something like potassium carbonate, especially for intramolecular cyclizations where you want to avoid the base reacting with the halide.

Just need to watch out for multiple alkylations if you still have acidic protons left after the first step.

So nitriles and nitrole canes seem a bit more forgiving, maybe, for forging these C -C bonds.

In terms of self -reaction, yes.

But let's get back to the classic carbonyl compounds.

That's where this problem of self -condensation really becomes a major issue.

How do we actually stop them from attacking themselves when we want them to be nucleophiles?

Yeah, you've nailed the central problem.

And the really elegant solution chemists develop is this.

Ensure complete conversion to the enolate before you introduce any electrophile.

Okay, complete conversion.

How?

You need a sufficiently strong base, typically one with a pKa that's at least, say, three or four units higher than the pKa of the carbonyl compounds alpha proton.

And you need conditions where that enolate is stable once formed.

This is where lithium enolates really become the stars of the show.

And the champion base for making these is usually LDA, right?

Lithium disipropylamide.

What's so special about LDA?

LDA is, well, it's kind of the workhorse for a reason.

You make it easily from butyl lithium and disipropylamine.

It's very strong, but it's also very hindered, very bulky.

But bulkiness is key.

It's crucial.

It means LDA deprotonates ketones and esters rapidly, completely, and essentially irreversibly, usually at very low temperatures, like minus 78 degrees Celsius in THF.

That low temperature freezes outside reactions, like self -condensation, letting the enolate form cleanly.

And the bulkiness helps direct which proton it takes, which we'll get to later.

Okay, so low temp, strong, bulky base.

We've got our clean lithium enolate.

Now, how do we use it to build that C -C bond?

Well, the alkylation of these lithium enolates with alkyl halides is, I'd say, one of the most fundamentally important C -C bond forming reactions in synthesis.

Sounds critical.

It is.

The typical lab procedure is form the enolate cleanly at naga 78 degrees C, then you add your alkyl halide, and then you let the whole mixture slowly warm up to room temperature.

This allows the SN2 reaction to happen smoothly.

You can get really high yields doing this, like methylating a ketone quite effectively.

Okay, that sounds straightforward for ketones.

But esters, I remember they have that tricky reaction, the Claisen condensation.

How do you alkylate an ester without just attacking itself?

Yeah, the Claisen is definitely a major concern with esters.

That's where the enolate of one ester molecule attacks the carbonyl group of another.

Very.

So the trick is technique.

You must add the ester, slowly, drop -wise, into the LDA solution.

This ensures you never have a significant concentration of unenolized ester floating around waiting to be attacked.

Always keep the LDA in excess until all the ester is added.

Ah, stewy geometry and addition order are critical.

Absolutely.

Another smart tactic is to use bulky ester groups.

Think tert -butyl esters.

Their sheer size discourages unwanted attack on the carbonyl carbon, but you can still easily remove the t -butyl group later via hydrolysis when you need to.

Clever.

What about carboxylic acids themselves?

Can you make an enolate from the acid directly?

Seems like that acidic OH group would interfere.

It does.

You can't just use one equivalent of base.

For a carboxylic acid, you actually need two equivalents of a strong base, like LDA.

Two?

Why two?

Well, the first equivalent just deprotonates the very acidic carboxylic acid proton itself, forming the carboxylate anion.

Okay, makes sense.

Then the second equivalent of LDA comes in and deprotonates the alpha carbon, the one next to the carbonyl, to form the actual enolate, so you end up with a dianion.

A dianion.

Wow.

And where does the alkylation happen, then?

Typically it happens at the carbon site, the alpha carbon, the one that lost its proton That tends to be the most reactive nucleophilic site in the dianion.

You see this, for example, when alkylating derivatives of amino acids like glycine.

Even though there are multiple acidic protons, using enough LDA ensures alkylation happens specifically at that alpha carbon.

It shows remarkable control.

Okay, so we're making CC bonds,

but enolates, they're often called ambidant nucleophiles, right?

They have two potential reaction sites,

the carbon and the oxygen.

Why do we usually get C alkylation with alkyl halides, not O alkylation?

That's a great question, and it comes down to the hard -soft acid -base principle, or HSAB.

Think of the enolate.

The carbon atom is considered the softer nucleophilic site.

It's where the highest occupied molecular orbital, the HOMO, has its largest coefficient.

The oxygen, while having more negative charge density, is the harder site.

Okay, soft carbon, hard oxygen.

And alkyl halides, especially iodides and bromides, are considered soft electrophiles.

The principle is that soft nucleophiles prefer to react with soft electrophiles.

Soft interaction, got it.

Exactly.

So the soft carbon of the enolate reacts with the soft alkyl halide, leading to the desired CC bond.

Hard electrophiles, on the other hand, tend to react at the hard oxygen site, giving O alkylation, but that's less common with simple alkyl halides.

That clarifies things nicely.

But I remember reading there's one major class of carbonyls where this whole lithium enolate strategy just falls apart.

Aldehydes.

You remembered correctly.

Aldehydes are, unfortunately, just too electrophilic.

Even trying this LDA approach at netting of 78 degrees C, they react with themselves via aldol condensation, or the LDA itself just adds directly to the carbonyl group way too fast.

So no useful lithium enolates from aldehydes for direct alkylation.

Generally, no.

It's a significant limitation.

For aldehydes, we need to get more creative.

We need ways to sort of mask their reactivity temporarily.

Right.

So we've kind of hit a wall with aldehydes using the direct lithium enolate method.

If that doesn't work, what are these clever workarounds, these masked forms or enol equivalents you mentioned?

This is where the toolkit gets really sophisticated.

Chemists have devised several ways to temporarily disguise the aldehyde or ketone, control its reactivity, and then reveal the desired product.

The main players here are enamines, ESA enolates, which come from enamines, and CILI and all ethers.

Okay.

Three main strategies.

Let's take enamines first.

How do they solve the aldehyde problem or help with ketones?

Enamines are formed by reacting an aldehyde or ketone with a secondary amine, like pyrrolidine or morpholine.

You might remember that from earlier chapters.

Vaguely, yeah.

Nitrogen replacing the oxygen, double bond shifts.

Exactly.

Now, the genius part for alkylation is this.

When you alkylate an enamine, there's no strong base involved and no actual enolate anion floating around.

So no self -condensation danger.

Precisely.

The anamine itself acts as a nucleophile.

It attacks the alkylating agent, forms an intermediate called an iminium ion, and then you just hydrolyze that intermediate with some mild acid to get back your alkylated aldehyde or ketone.

So it's a way to achieve the outcome, the alkylation, but using much milder, less reactive intermediates.

What's the catch?

Are there limitations?

There are.

Enamines are significantly less nucleophilic than lithium enolates.

So they generally only work well with reactive electrophiles, think allelic halides, benzylic halides, or alpha halo ketones and esters, things that are really good at SN2 reactions.

Okay, so not just any alkyl halolide will do.

Oh, right.

And another thing is regioselectivity anamines tend to form from the less substituted alpha carbon because that anamine is thermodynamically more stable.

That can be useful sometimes.

But the biggest problem is N -alkylation.

If you use a less reactive electrophile, like metal iodide, it might just react with a nitrogen atom instead of the carbon.

Giving you an unwanted side product.

Yeah, leading to low yields of the C -alkylated product you actually want.

But still for aldehydes, especially if you have one of those highly reactive electrophiles, enamines are a very useful tool.

Okay, that's a clever workaround.

What's the next tool in the masked aldehyde docks?

Next up are azanenolates.

These come from enamines.

Now, enamines are the nitrogen analogues of aldehydes and ketones, but they're formed using primary amines.

Primary amines this time, okay.

How does that help?

The critical advantage here is that the enamine group itself is less electrophilic than the original aldehyde or ketone carbonyl group, especially under basic or neutral conditions.

Less prone to attack.

Exactly.

So when you treat an enamine with a strong base, like LDA or even a Grignard region, to rip off an alpha proton and form the azanenolate… The remaining enamine doesn't attack the newly formed azanenolate.

Precisely.

There's no significant self -condensation problem like you have with aldehydes themselves.

The azanenolate forms cleanly.

That sounds like a major improvement.

A built -in safety feature.

So how does the alkylation sequence work?

It's pretty straightforward.

You take your aldehyde or ketone, convert it to an amine often using a bulky primary amine and like butylamine to help direct reactivity later.

Then you deprotonate that amamine with your strong base, LDA usually, to get the azanenolate.

That azanenolate then reacts nicely with SN2 reactive alkylating agents like benzyl chloride for instance.

And finally, you just hydrolyze the alkylated amamine product, usually with

And voila, you get your desired alkylated aldehyde or ketone back.

That sounds much more general than enemines.

It is.

Many consider azanenolates the best general solution for alkylating aldehydes with a wide range of standard SN2 -type electrophiles.

Okay, that handles aldehydes and SN2 electrophiles really well.

But you mentioned SN1 reactive electrophiles earlier.

Things like tertiary alkyl halides.

They often cause elimination problems with strong bases like LDA used for enolites or manzoyl enolites.

How do we alkylate with those?

Ah, yes.

You've pinpointed the exact situation where both lithium enolates and azanenolates tend to fail.

Being strong bases, they just rip a proton off the tertiary halion, giving you elimination instead of substitution.

So we need something less basic.

Exactly.

And the solution is the third tool.

Silly enol ethers.

These are formed, for example, by reacting a ketone with trimethylsilyl chloride and a base like triethylamine.

They essentially trap the enol form with a silly group.

And these are less reactive?

Much less reactive, yes.

They aren't strong bases.

So they don't react well with typical SN2 electrophiles on their own.

They actually require a potent electrophile, typically one that can form a stable carbocation, precisely the kind that SN1 reactive halides generate.

So they react via a different mechanism.

Right.

You usually need a Lewis acid catalyst like titanium tetrachloride, TICL4, or tin tetrachloride, SNCl4.

The Lewis acid activates the electrophile, helping it form a carbocation in situ,

or it might activate the silly enol ether itself.

This carbocation then reacts with the relatively mild silly enol ether nucleophile.

Ah, so they have this kind of complementary reactivity.

Exactly.

Silly enol ethers work beautifully in situations where lithium enolates is ather enolates, and even enamines struggle or fail, specifically with things like tertiary alcohol halides or other SN1 -prone electrophiles.

So just to recap these equivalents quickly.

Lithium enolates are the go -to for ketones and esters with standard SN2 electrophiles.

Aza enolates are the best general way to alkylate aldehydes with SN2 electrophiles.

Enamines work for aldehydes as ketones too, but mainly with very reactive electrophiles.

And silly enol ethers fill the gap for SN1 -type electrophiles.

That's a perfect summary.

Each strategy has its specific niche and advantages.

This is fantastic.

But you know, all these methods involve fairly strong bases or reactive intermediates.

What if we had starting materials where the alpha protons were so acidic that we didn't need LDA or anything like it?

That sounds like it would simplify things immensely.

It absolutely does.

And that brings us to a really important class of compounds, then dicarbonyl compounds, also called 1 -phil -3 -dicarbonyl.

Beta -dicarbonyls.

Think about structures where you have two carbonyl groups separated by a single carbon atom, or maybe even three electron -withdrawing groups attached to that central carbon.

Like diethylmalinate or ethylacetoacetate.

Exactly.

Those are the classic examples you absolutely need to know.

Because you have two electron -withdrawing groups pulling electron density away from that central carbon, the protons attached to it are exceptionally acidic.

Their pK values are often down in the range of 10 -15.

Wow.

That's almost like phenols or water territory.

Pretty much.

And the huge advantage is that you don't need super strong bases like LDA.

Even mild bases like sodium ethoxide or potassium carbonate are strong enough to achieve complete deprotonation, forming the enolate quantitatively.

So much easier conditions.

No need for digative 78 degrees, presumably.

Often, yes.

And crucially, because the resulting enolate anion is highly stabilized by resonance across both carbonyl groups, it's much less reactive and basic than a simple ketone enolate.

So self -condensation isn't really an issue here either.

That's a double win.

Easier formation and less reactivity.

And how well do these stabilized anions alkylate?

They alkylate very efficiently.

You can, for example, take a simple diketone, treat it with potassium carbonate, a really mild base in the presence of methyl iodide, and the alkylation proceeds smoothly in high yield.

And you mentioned diethylmalinate and ethyl acetoacetate.

Any specific tips for working with those esters?

Yes.

One important detail.

When you're deprotonating these esters, you should use an alkoxide base that matches the alkoxy group of the ester.

So for diethylmalinate, you'd use sodium methoxide.

For dimethylmalinate, sodium methoxide.

Why is that important?

It's to avoid a side reaction called transesterification, where the alkoxide base might swap with the esters alkoxy group, leading to a mixture of products you probably don't want.

Sticking with the matched alkoxide prevents this.

Good tip.

Now you mentioned these beta -dicarbonyls are foundational.

A big part of that is this process called decarboxylation, right?

It seems almost like magic.

You alkylate it and then you can just pop off one of the carbonyl groups.

How on earth does that work?

It's one of the most powerful tricks in the synthetic chemist's playbook.

It lets you use the activating effect of the two carbonols to easily make a C -C bond and then get rid of the activating group you don't need anymore.

Okay, how?

The key is that carboxylic acids that have a carbonyl group in the beta position, so a beta -keto acid or malonic acid derivative, readily lose carbon dioxide when you heat them.

Just heat?

Often just heating, sometimes with a bit of acid or base catalyst, is enough.

The mechanism typically involves a cyclic six -membered transition state where the carboxylic acid proton transfers to the keto oxygen as CO2 leaves.

This initially forms an enol.

Which then tautomerizes to the more stable keto form.

Exactly.

The net result is that if you start with an alkylated B -keto ester, like from ethyl acetoacetate, after hydrolysis and decarboxylation, you end up with a simple ketone.

If you start with an alkylated malonate ester, you end up with a carboxylic acid.

That's incredibly useful.

So you can use ethyl acetoacetate to make substituted ketones and diethyl malonate to make substituted carboxylic acids.

Precisely.

For instance, alkylate ethyl acetoacetate with, say, betal bromide, then hydrolyze the ester and heat it, you'll get hexan -2 -1.

It's a classic synthesis.

And there are even milder ways to do this, like the crap -show decarboxylation, which specifically removes an ester group, often a methyl or ethyl ester, using salts like NaCl or allelial in wet DMSO at high temperatures.

It works via an SN2 attack by chloride on the ester -alkyl group.

Wow.

Multiple ways to achieve this transformation.

And since these beta -dicarbinals have two acidic protons on that central carbon,

I guess you can alkylate twice.

Absolutely.

Double alkylation is very common.

You can add two identical alkyl groups, or you can do it sequentially and add two different groups.

Can you use this for making rings, too?

Yes, definitely.

If you use a dihaloalkane as your electrophile, like 1 -phiral -3 -di -bromo -propane, you can perform an intramolecular alkylation after the first intermolecular one, leading to cyclization.

This is a great way to make cycloalkane rings, even strained ones like four -membered cyclobutanins or cyclobutane carboxylic acids.

OK, this beta -dicarbonyl chemistry is clearly super powerful.

But let's circle back to simpler ketones for a moment.

This is where things get, I think, really interesting.

What happens if your ketone isn't symmetrical?

Say, 2 -methylcyclohexanone.

It has alpha protons on both sides of the carbonyl.

Which side gets deprotonated?

Do you get a mixture?

That's the million -dollar question in enolate chemistry.

Regioselectivity.

You're absolutely right.

An unsymmetrical ketone, like 2 -methylcyclohexanone, can potentially form two different enolates, one by removing a proton from the methyl group, carbon 2, and another by removing a proton from the methylene group, carbon 6.

And if you alkylate at that mixture, you get a mixture of products, right?

Which is usually bad news in synthesis.

Usually, yeah.

You want control.

So the key here is understanding how to selectively form one of those regiosameric enolates over the other.

And chemists control this using either thermodynamic or kinetic control conditions.

OK, let's break those down.

Thermodynamic control, what does that favor?

Thermodynamic control favors the formation of the more stable enolate, which enolate is more stable.

Generally, it's the one with the more substituted double bond, think Zaitsev's rule, but for enolates.

More substituted double bond is more stable.

How do you achieve thermodynamic control in the lab?

You need conditions that allow the system to reach equilibrium.

That means the initially formed enolates need to be able to interconvert, usually via proton transfer, mediated by a small amount of unreacted ketone or a weaker base.

So you typically use conditions like Maybe a slightly weaker base or not quite a full equivalent of base, so there's excess ketone around, higher reaction temperatures, and longer reaction times.

Let it cook and settle into the lowest energy state.

Allowing equilibration to the most stable product.

Exactly.

Sometimes this happens automatically.

Like with ethyl acetoacetate, the enolate involving the proton between the two carbonals is vastly more stable.

Or, with something like 2 -phenylcyclohexanone, the enolate that puts the double bond in conjugation with the phenyl ring is strongly favored thermodynamically.

Can solenol ethers play a role here, too?

They can.

Often, when you form solenol ethers under conditions that allow equilibration, you preferentially get the more substituted thermodynamically favored solenol ether.

You can potentially purify this regioasomer and then convert it cleanly back into a specific lithium enolate using something like methyl lithium, mellolot.

Ah, so you can isolate the thermodynamic intermediate.

Clever.

Now what about the opposite?

Kinetic control.

That sounds like it's all about speed, not stability.

Precisely.

Kinetic control aims to form the enolate that is generated fastest.

Which proton is removed fastest?

Usually it's the one that is less sterically hindered and sometimes slightly more acidic due to fewer electron -donating alkyl groups.

This leads to the less substituted enolate.

The less stable one, but the one that forms quicker.

How do you force the reaction down that path?

You need conditions that prevent equilibration and favor the fastest reaction.

This means 1.

A very strong hindered base, like our friend LDA.

Its bulkiness makes it attack the less hindered proton, preferentially.

2.

Low temperatures, typically negative 78 degrees C, to slow down all reactions, especially equilibration.

3.

Short reaction times, just enough to form the enolate.

4.

And crucially, adding the ketone to the LDA solution.

This ensures LDA is always in excess, immediately grabbing the most accessible proton before any equilibration can happen.

No leftover ketone to mediate proton transfer.

So LDA's bulkiness is key again, steering it away from the more crowded side.

Exactly.

It physically can't reach the more substituted alpha proton as easily.

Plus statistically, there might be more protons on the less substituted side, like the three protons of methyl group versus two on a methylene.

So for methyl ketones or two substituted cyclohexanones, LDA at low temp gives excellent selectivity for the less substituted kinetic enolate.

Okay, let me try summarizing that.

Thermodynamic control.

More stable, more substituted enolate.

Uses conditions allowing equilibrium, excess ketone, higher temp, longer time.

Kinetic control.

Faster formed, less substituted, less stable enolate.

Requires LDA, low temp, short time, adding ketone to base.

Perfect.

That's the core difference.

And just as a fascinating aside, remember we talked about dianion.

Yeah, with carboxylic acids.

You can use that idea for unusual regioselectivity, too.

Take methylacetoacetate.

If you deprotonate it twice with a very strong base, you form a dianion.

The second proton removed, the one from the terminal methyl group, forms the less stable but more reactive anion site.

So alkylation actually occurs preferentially at that terminal methyl group, which is completely opposite to the normal beta -dicarbonyl reactivity.

Wow, that's using multi -step deprotonation to override the inherent acidity rules.

Very clever.

It shows the level of control possible.

This control over regioselectivity is amazing.

But thinking about controlling where the enole forms,

what if we kind of predetermine it, build the regioselectivity right into the starting material, using something like an enon, maybe?

That's another very powerful strategy.

Enons, those omicara unsaturated carbonyl compounds, provide fantastic platforms for generating regiospecific enolates, meaning enolates formed at a very specific, predictable location.

How does that work?

What are the methods?

One really elegant method is the dissolving metal reduction of enons.

Think back to the Birch reduction we might have discussed.

If you take an enon and treat it with lithium metal in liquid ammonia, usually with a proton source like an alcohol present, it selectively reduces the carbon -carbon double bond.

Okay, the CAC gets reduced.

And in the process, it generates an enolate intermediate, where the negative charge ends up specifically on the alpha carbon, the carbon that used to be part of the double bond next to the carbonyl.

Ah, so the reduction itself dictates exactly where the enolate forms.

Precisely.

So if you then add an alkylating agent in situ while that enolate is present, you achieve highly regioselective alkylation at that specific alpha position.

For example, reducing methylcyclohexanone this way and trapping with methyl iodide gives almost exclusively the 2 -3 -dimethylcyclohexanone product.

It's very clean.

That's incredibly neat.

And often stereoselective too.

Yes.

The alkylating agent usually approaches from the less sterically hindered face of the intermediate enolate, leading to predictable stereochemistry as well.

So dissolving metal reduction is one way to get a specific enolate from an enone.

What about the other major reaction of enolanes conjugate addition, the Michael reaction?

That generates an enolate too, doesn't it?

It certainly does.

When a nucleophile adds to the beta carbon of an enone, the carbon from the carbonyl and the CCO system, that's conjugate addition or 1 -alpha -4 addition.

The electrons shuffle and you end up with an enolate where the negative charge is again on the alpha carbon.

Okay.

Another way to get a specific enolate.

But isn't there always a competition between that conjugate 1 -alpha -4 addition and direct 1 -2 addition to the carbonyl carbon itself?

There absolutely is.

That's the main challenge.

You need conditions that favor the desired conjugate addition.

Generally, conjugate addition is the thermodynamically favored pathway.

It leads to a more stable overall product because you're breaking a C -C pi bond and ultimately forming a C -O pi bond, which is stronger.

Direct addition to the carbonyl is often faster, kinetically favored, but needs to be reversible for conjugate addition to win out.

So how do chemists push the reaction towards conjugate addition when they want to trap that specific enolate intermediate for alkylation?

There are several excellent strategies.

One is using bulky hydride reagents like L -selectride or K -selectride.

Their steric bulk prevents them from attacking the carbonyl directly, so they selectively deliver hydride to the beta carbon, generating the enolate for subsequent alkylation.

Okay.

Bulky hydrides work.

Yes.

Organocopper reagents like lithium dialkyl cuprates, Gilman reagents, are masters of conjugate addition.

They selectively add an alkyl or aryl group to the beta position of anonase.

And here's a really neat trick.

You can trap the resulting enolate intermediate not with an alkyl halide, but with trimethylsilychloride, mathri -siloh.

Ah, to make a silyanol ether.

Exactly.

And it's a regioselective silyanol ether formed precisely where the conjugate addition occurred.

And then you can use that silyanol ether.

You bet.

This opens the door to brilliant pandem reactions.

You form the regiosecific silyanol ether via conjugate addition trapping.

Then you can convert that silyanol ether back into a lithium enolate, using Mellie again for example, and alkylate that with a different electrophile.

So in one sequence, you've added two new different groups and formed two new C -C bonds with precise control.

That sounds incredibly powerful for building complex molecules.

Is there a famous example?

Absolutely.

A landmark example is Ryōji Noyori's Nobel Prize -winning synthesis of prostaglandin E2.

It features exactly this type of sequence.

A highly stereoselective conjugate addition of a complex organocopper regent to an enun trapping the enolate and then alkylating it with a specific allylic iodide to install the second side chain with the correct trans stereochemistry.

Just beautiful chemistry.

Wow.

Okay, so far we've mostly seen enuns as the electrophiles.

Let's flip the script again.

What happens when the enolate is the nucleophile and the enun or a similar unsaturated system is the electrophile, the Michael acceptor.

Excellent perspective shift.

Yes, enolates themselves can act as nucleophiles in Michael additions.

Again, the key is usually setting up conditions that favor the conjugate one before addition over direct one or two attack on the carbonyl.

And how do you achieve that when the enolate is the attacker?

Well, stabilized enolates like the ones from our friends of the decarbonyl compounds, malonates, acetoacetates, etc.

are ideal Michael donors.

Why them specifically?

Because these enuns are relatively weak bases and soft nucleophiles, direct addition to carbonyl is more easily reversible with these, allowing the thermodynamically favored conjugate addition to dominate.

Plus the direct addition product would be very sterically hindered.

So diethyl malonate adding to something like diethyl fumarate works very cleanly.

Makes sense.

What about less stabilized enolates like from simple ketones?

Those can work too, but conditions matter more.

Often using alkali metal enolates like sodium, net plus, or potassium, K plus, kanparians, helps promote conjugate addition compared to lithium li plus enolates, possibly because they are more dissociated or softer.

Using bases like potassium tert -vitoxide is common.

And what about our other enol equivalents?

Can they be Michael donors?

Oh, absolutely.

Enamines are fantastic Michael donors.

Being neutral soft nucleophiles, they readily undergo conjugate addition, often needing just mild heating or acid catalysis.

Silo -enol ethers are also very effective, frequently used with Lewis acid catalysts like TiCl4, and they can even create new quaternary centers via Michael addition.

There are also silly ketene acetyls, which are like silly enol ethers of esters, and they're even more nucleophilic.

So lots of options for the nucleophile.

What about the Michael acceptor side?

Does it have to be an enano?

Not at all.

The scope is much broader.

Enodo and saturated esters, amines, especially tertiary ones where direct attack is less favorable.

And very importantly, AMO unsaturated nitriles like acrylonitrile are excellent Michael acceptors.

Why are nitriles good acceptors?

They provide the necessary electron withdrawal to activate the double bond for conjugate addition.

But the nitrile group itself isn't easily attacked directly by many nucleophiles, which simplifies things compared to, say, an aldehyde acceptor.

Any other interesting nucleophiles or acceptors?

Well, you can use cyanide as the anion -stabilizing group in the nucleophile.

And even simple amino acids like glycine can be functionalized, convert it to an imamine first, then use the resulting stabilized anion for conjugate addition.

And let's not forget nitrile alkanes.

You said earlier they were super acidic.

Are they also super -nupiophiles for Michael additions?

They're absolutely superb nucleophiles for conjugate addition.

That nitrile group is so powerfully electron withdrawing, it's considered equivalent to two carbonyl groups in its ability to stabilize an adjacent anion.

So nitrile alkanes undergo conjugate addition exceptionally well, often requiring only very mild bases like imions or even just basic alumina as a catalyst.

They are fantastic for making new quaternary centers and can participate in tandem reactions like a Michael addition followed by an intramolecular SN2 alkylation.

That sounds incredibly versatile.

And you mentioned before the nitrile group could be transformed afterwards.

Critically important, yes.

After you've used the nitrile cane in a Michael addition, the nitrile group isn't necessarily stuck there.

You can reduce it to a primary amine using various methods.

Or you can hydrolyze it to a ketone via the Neff reaction, although that can be harsh.

A milder alternative for converting the nitrile cane product to a ketone or aldehyde is often ozonolysis of the intermediate nitrinate anion, which is particularly useful for making one very four dicarbonyl compounds.

So the nitrile group acts as a powerful activator and directing group, and then it can be swamped for other useful functional groups.

Exactly.

It's a real workhorse in synthesis.

This whole deep dive has been just an incredible journey through the art and science of making carbon carbon bonds to really bring it all home.

Can you show us how these reactions we've talked about actually come together in the real world, maybe in the synthesis of a drug?

Absolutely.

That's where you really see the power and elegance of understanding these principles.

A fantastic example is the commercial synthesis of a drug called Vivalon, which is described as a dopaminergic antagonist.

Okay, Vivalon, how does its synthesis use the chemistry we've discussed?

It beautifully integrates several of the key reactions.

The synthesis reportedly involves a sequence that starts with a conjugate addition, likely using a stabilized enolate, adding to acrylonitrile, that unsaturated nitrile we just mentioned.

Yeah, Michael addition first, then what?

Then the resulting nitrile group is reduced down to a primary amine.

Nitrile to amine, okay.

Following that, there's an alkylation step, presumably adding another piece to the molecule.

This cleverly sets up a spontaneous cyclization where the amine likely attacks an ester or similar group elsewhere in the molecule to form a cyclic amide, a lactam.

Wow, intramolecular reaction forming a ring.

Exactly.

Then as a final step, that cyclic amide is reduced to give the final amine structure of Vivalon.

You see conjugate addition reduction, alkylation, cyclization, another reduction, all fundamental transformations we've touched upon string together logically.

That's amazing.

It really drives home how understanding these individual reactions,

enolate, formation, regioselectivity, conjugate addition, functional group conversions, allows chemists to piece together incredibly complex and important molecules like medicines.

It really is like building with those custom molecular legos.

It truly is.

Each step, each choice of reagent, each reaction condition, it's all guided by these fundamental mechanistic principles we've been unpacking from Clayton.

So as you've heard today, crafting these vital new carbon -carbon bonds isn't just a matter of mixing chemicals together.

It's this intricate dance, this deep understanding of electron flow, steric effects, stability, reaction rates.

It's strategic design.

Yeah, the thought that Clayton emphasizes really is key.

So the next time you see a complex structure, maybe pause and consider the elegant thought process, the chemical logic that must have gone into planning its synthesis step by step.

Think about how these fundamental principles of enolate chemistry unlock transformations that might otherwise seem impossible.

And maybe think about other complex molecules, you know, perhaps other drugs or natural products or even materials.

How might they have been constructed using these powerful C -C bond forming reactions?

You start to see this chemistry everywhere once you know what to look for.

It really opens up a new way of looking at the molecular world.

Thank you for joining us on this deep dive.

Thank you for being part of the Last Minute Lecture family.