Allylic sources acids

Catalytic asymmetric allylations of aldehydes or ketones are roughly classified into two methods, namely, those using chiral Lewis acid catalysts and those using chiral Lewis base catalysts. The former method uses less reactive allylsilanes or allylstannanes as the allyl source. The latter method requires allyltrichlorosi-lane or more reactive allylmetals. Both processes are applicable to the reactions with substituted allylmetal compounds or propargylation. 

R-M Organo- metallics CHjLi CHaMgl (CH3)2Cu-Li+ The more ionic RM bond is more reactive Substitutions Additions. Deprotonates acidic H s See enolates (allylic sources)... 

Under equilibrium conditions (thermodynamic control), the allylic source adds to the polarized multiple bond (path AdN). However, the allylic source can also serve as a base and may deprotonate the sink, creating a mixture of sources and sinks and thus a messy statistical mixture of products. Clean products result if the source is just the deprotonated sink or if the sink has no acidic protons. With ketones, the equilibrium of the attack step favors the starting materials, and therefore the reaction goes to completion only if driven by a following elimination. In the next Adisj2 example, the source is the deprotonated sink. The product is an aldehyde-alcohol, or aldol, a name now used for the general process of an enol (acidic media) or enolate (basic) reacting with an aldehyde or ketone. 

Simple pi bonds are not good enough nucleophiles to react with conjugate acceptors. It is basically a case of little push and little pull, so no electron flow occurs. If the pi bond is made more nucleophilic by adding a pi donor (making it an allylic source), then the reaction is similar to the one above. Alternatively, a Brpnsted acid or a Lewis acid could be added to improve the electron sink few examples of this reaction are known. 

The carbon-carbon bond-forming step of the acid-catalyzed aldol reaction has an enol (allylic source) attacking a protonated carbonyl (which is just a lone-pair-stabilized carbocation). With those hints, give a mechanism for the acid-catalyzed aldol reaction. 

In acidic media, the electron sink is most often the carbocation produced from protonating the acylating agent, and therefore the sink is very hard. Attack by the Z end (harder end) of the allylic source is very fast. For enols, the Z-acylated kinetic product can be isolated. Since the Z-acylated enol is itself an allylic source (but weaker), it can be forced by more vigorous conditions to equilibrate to the more stable C-acylated product. For enamines, the Z-acylated enamine is a good acylating agent any excess of enamine will attack it and equilibrate it to the more stable C-acylated product. 

The ester enolate is an allylic source that can serve as a base or a nucleophile. The original ester has acidic hydrogens within range of the enolate. The ester carbonyl is an electrophilic sink (a polarized multiple bond with attached leaving group). 

EXPLOSION and FIRE CONCERNS flammable liquid NFPA rating Health 4, Flammability 3, Reactivity 3 vapors are heavier than air and may travel to an ignition source and flashback dangerous fire and explosion hazard when exposed to heat, flame or oxidizers violent reaction with acids, aluminum chloride and substituted anilines, acetic acids, acetic anhydride, acrolein, acrylic acid, allyl chloride, chlorosulfonic acid, epichlorohydrin, hydrogen... 

The enzyme catalyzed reactions that lead to geraniol and farnesol (as their pyrophosphate esters) are mechanistically related to the acid catalyzed dimerization of alkenes discussed m Section 6 21 The reaction of an allylic pyrophosphate or a carbo cation with a source of rr electrons is a recurring theme m terpene biosynthesis and is invoked to explain the origin of more complicated structural types Consider for exam pie the formation of cyclic monoterpenes Neryl pyrophosphate formed by an enzyme catalyzed isomerization of the E double bond m geranyl pyrophosphate has the proper geometry to form a six membered ring via intramolecular attack of the double bond on the allylic pyrophosphate unit... 

Scheme 10.17 illustrates allylation by reaction of radical intermediates with allyl stannanes. The first entry uses a carbohydrate-derived xanthate as the radical source. The addition in this case is highly stereoselective because the shape of the bicyclic ring system provides a steric bias. In Entry 2, a primary phenylthiocar-bonate ester is used as the radical source. In Entry 3, the allyl group is introduced at a rather congested carbon. The reaction is completely stereoselective, presumably because of steric features of the tricyclic system. In Entry 4, a primary selenide serves as the radical source. Entry 5 involves a tandem alkylation-allylation with triethylboron generating the ethyl radical that initiates the reaction. This reaction was done in the presence of a Lewis acid, but lanthanide salts also give good results. 

The valuable and versatile study was conducted by the group of Bosnich [143]. The defined nature and amounts of by-products in the reaction mixture allowed to judge about the possible mechanism of the catalysis. It was proved that the aldohzation and allylation reactions proceed with the evolution of TMS salt that is itself a strong Lewis acid and can catalyze the reaction in high rate (Scheme 52). The possible sources of the TMS salt production in the reaction medium were investigated. The utilization of a hindered base suppressed the influence of the silyl salt, and the rate of the reaction was dramatically diminished. This consequence was considered to confirm that the TMS salt can catalyze the reaction even in the undetectable quantities of 10 mol. 

Various allylhalostannanes can transfer allyl groups to carbonyl compounds. In this case, the reagent acts both as a Lewis acid and as the source of the nucleophilic allyl group. Reactions with halostannanes are believed to proceed through cyclic transition states. 

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