Enediolate, from acyloin reaction

Bis(tributyltin) enediolates can be prepared from the reaction of acyloins with bis(tributyltin) oxide or tributyltin methoxide (equation 14-61).112... 

The formation of acyloins (a-hydroxyketones of the general formula RCH(OH)COR, where R is an aliphatic residue) proceeds best by reaction between finely-divided sodium (2 atoms) and esters of aliphatic acids (1 mol) in anhydrous ether or in anhydrous benzene with exclusion of oxygen salts of enediols are produced, which are converted by hydrolysis into acyloins. The yield of acetoin from ethyl acetate is low (ca. 23 per cent, in ether) owing to the accompanying acetoacetic ester condensation the latter reaction is favoured when the ester is used as the solvent. Ethyl propionate and ethyl ji-butyrate give yields of 52 per cent, of propionoin and 72 per cent, of butyroin respectively in ether. 

This procedure is representative of a new general method for the preparation of noncyclic acyloins by thiazol ium-catalyzed dimerization of aldehydes in the presence of weak bases (Table I). The advantages of this method over the classical reductive coupling of esters or the modern variation in which the intermediate enediolate is trapped by silylation, are the simplicity of the procedure, the inexpensive materials used, and the purity of the products obtained. For volatile aldehydes such as acetaldehyde and propionaldehyde the reaction Is conducted without solvent in a small, heated autoclave. With the exception of furoin the preparation of benzoins from aromatic aldehydes is best carried out with a different thiazolium catalyst bearing an N-methyl or N-ethyl substituent, instead of the N-benzyl group. Benzoins have usually been prepared by cyanide-catalyzed condensation of aromatic and heterocyclic aldehydes.Unsymnetrical acyloins may be obtained by thiazol1um-catalyzed cross-condensation of two different aldehydes. -1 The thiazolium ion-catalyzed cyclization of 1,5-dialdehydes to cyclic acyloins has been reported. 

Whereas condensation of a-hydroxy ketones such as benzoin and acetoin on heating with formamide [240] or ureas in acetic acid [239, 242] to form imidazoles such as 769 or 770 is a well known reaction, only two publications have yet discussed the amination of silylated enediols, prepared by Riihlmann-acyloin condensation of diesters [241], by sodium, in toluene, in the presence of TCS 14 [241, 242]. Thus the silylated acyloins 771 and higher homologues, derived from Riihl-... 

Cyclobutanediones, once exotic compounds represented by a few perhalo derivatives, have become readily available as a result of new synthetic developments in recent years. These include the modified acyloin condensation 52) in which the intermediate enediolate is trapped as bis-trimethylsilyl ether (28) which can be converted to cyclobutanedione by reaction with bromine or hydrolyzed to acyloin and oxidized in a separate step. In addition to this efficient and general method, bi- or polycyclic unsaturated cyclobutanediones (30) have become available from photolysis of bridged cyclohexenediones (29) to be discussed in the following section. Photocycloaddition of dichlorovinylene carbonate (DCVC) to olefins53) promises to provide a third route if the problems associated with hydrolysis of the photoadducts (31) can be overcome. 

There are currently two proposed mechanisms for the acyloin ester condensation reaction. In mechanism A the sodium reacts with the ester in a single electron transfer (SET) process to give a radical anion species, which can dimerize to a dialkoxy dianion. Elimination of two alkoxide anions gives a diketone. Further reduction (electron transfer from the sodium metal to the diketone) leads to a new dianion, which upon acidic work-up yields an enediol that tautomerizes to an acyloin. In mechanism B an epoxide intermediate is proposed. ... 

In the presence of TMS-Cl the enediolate dianion and, importantly, the alkoxide ions, are trapped as their neutral silyl ethers (Scheme 5). This leads to much improved yields of the coupled product the acyloin is isolated in the form of its silyl enediol ether (3). Work-up is much easier. It is only necessary to filter the solution, evaporate the solvent, and isolate the product by distillation or chromatography. The TMS-Cl should be purified by distillation from calcium hydride, under a nitrogen or argon atmosphere, before use. A convenient procedure when using an organic solvent is to add the ester and the TMS-Cl together, dropwise, to the alkali metal finely dispersed in the solvent, at a rate sufficient to maintain the reaction. An explosion has been reported where this procedure was not followed. For a reaction conducted in liquid ammonia the TMS-Cl is added at the end of the reaction and after all the ammonia has been allowed to evaporate. Particularly in cases where sodium-potassium alloy has been used, a pyrophoric residue may have formed, so that the filtration must be carried out under an inert atmosphere. 

The reductive coupling of carbonyl compounds with active metals (Na, Mg, Al) yields pinacols. An electron transfer from the metal surface to the carbonyl oxygen (ketyl formation), a soft-soft interaction, is undoubtedly involved. The conversion of esters to acyloins (22, 23) on the surface of metallic sodium is well known. Here the enediolate products can be trapped in situ by Me3SiCl (24). The chlorosilane does not interfere with the coupling, yet it effectively removes the alkoxide ions and neutralizes the enediolate ions immediately on formation. The elimination of RO is imperative, for otherwise Claisen or Dieckmann condensations would compete with the normal course of reaction. These complicating processes require a hard base (e.g. RO ) to abstract a proton from the starting esters, whereas the desired coupling is accomplished by a soft base which is the electrons on the metal surface. 

Acyloins (la) [29,30] and bis-silylated derivatives of their enediols (lb) [31] have been demonstrated to be useful acyl anion equivalents (RCO ) and have been employed in an indirect but facile way for the preparation of ketones (3) (Equation 7.1). The hydroxy ketones (2), which are the key intermediates in this method, have been prepared by the reaction of acyloins (la) with organic halides in the presence of sodium hydroxide in dimethyl sulfoxide [29, 30] according to the Hein s procedure [32] or using sodium hydride in 1,2-dimethoxyethane (DME) (glyme) [33]. They also have been prepared by the alkylation of lithium enedi-olates of acyloins prepared from the corresponding lb and methyllithium [31]. These reactions proceed with C-alkylation however, the reaction conditions employed are quite basic which would limit substrate choice. 

Russell has reported the preparation of bicyclo[3,2,0]hept-2-en-6,7-semidione (582). Reaction of the bicyclo[2,2,l]heptene acyloin (583) with potassium t-butoxide in DMSO gives (582). The rearrangement may occur via the enediol dianion, as the bicyclo[2,2,l]heptene semidione itself, generated by an alternative route, is stable. The methylated bicyclo[2,2,l]heptene acyloins (584a) and (584b) both yielded mixtures of the possible bicydo[3,2,0]heptene semidiones (585) and (586) on treatment with butoxide in DMSO. The rearrangement was shown to be reversible, since the same mixture of semidiones was formed from l-methylbicyclo[3,2,0]hept-2-en-6,7-acyloin (587). 

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