Acyl silanes, «,/5-unsaturated

The addition of carbonyl compounds towards lithiated 1-siloxy-substituted allenes does not proceed in the manner described above for alkoxyallenes. Tius and co-work-ers found that treatment of 1-siloxy-substituted allene 67 with tert-butyllithium and subsequent addition of aldehydes or ketones led to the formation of ,/i-unsaturated acyl silanes 70 (Scheme 8.19) [66]. This simple and convenient method starts with the usual lithiation of allene 67 at C-l but is followed by a migration of the silyl group from oxygen to C-l, thus forming the lithium enolate 69, which finally adds to the carbonyl species. Transmetalation of the lithiated intermediate 69 to the corresponding zinc enolate provided better access to acylsilanes derived from enolizable aldehydes. For reactions of 69 with ketones, transmetalation to a magnesium species seems to afford optimal results. 

When substituted silanes are used instead of hydrogen, the process is referred to as silylformylation or silylcarbonylation. Only rhodium complexes catalyze the transformation of unsaturated compounds to silylaldehydes via the silylformylation reaction. Iridium complexes also are able to catalyze the simultaneous incorporation of substituted silanes and CO into unsaturated compounds, although during the reaction other types of product are formed. In the presence of [ IrCl(C03) ] and [Ir4(CO)i2]) the alkenes react with trisubstituted silanes and CO to give enol silyl ethers of acyl silanes [58] according to Scheme 14.10. 

In a related process, Johnson and co-workers have developed an asymmetric metallophosphite-catalyzed intermolecular Stetter-hke reaction employing acyl silanes [81, 82], Acyl silanes are effective aldehyde surrogates which are capable of forming an acyl anion equivalent after a [l,2]-Brook rearrangement. The authors have taken advantage of this concept to induce the catalytic enantioselective synthesis of 1,4-dicarbonyls 118 in 89-97% ee and good chemical yields for a,p-unsaturated amides (Table 11). Enantioselectivities may be enhanced by recrystallization. 

Other magnesium allenyl enolates, such as 22, obtained by transmetallation of the lithium species have been used successfully in the preparation of a,-unsaturated acyl silanes (equation 28). ... 

The very different reactivity toward allylation of a,P-unsaturated esters and acyl-silanes [132] is due to stabilization of the allyl cation intermediates by the alkoxy group in the former class of substances, in contrast to those derived from acylsilanes which are expected to be less stable than 3-monooxyallyl cations. In the 1,1-dioxy substituted cations two donor atoms are bonded to the acceptor site, but in the 1-oxy-3 -silyl congeners a rf,a-antagonism (captodative) exists. 

Generally, protons attached to the a-carbon atoms of acyl silanes are somewhat deshielded relative to their carbon analogues (Table 2). Differences in magnetic anisotropy and electronegativity are no doubt responsible for this effect. It is interesting that a,/i-unsaturated acyl silanes appear to be an exception (entry 12). 

In another simple procedure, deprotonation of methoxy bis(trimethylsilyl)methane with butyl lithium and addition of the resulting anion to aldehydes induces Peterson elimination (Scheme 27). The product methyl enol ethers could be hydrolysed to the parent acyl silanes with hydrochloric acid-THF or could be treated with electrophiles such as M-halosuccinimides to give a-haloacyl silanes105. Alternatively, treatment with phenyl selenenyl chloride, oxidation at selenium and selenoxide elimination afforded a,/3-unsaturated acyl silanes. 

Enantioselective Claisen rearrangement of allyl (a-trimethylsilyl)vinyl ethers in the presence of aluminium binaphthol derivatives gives ft-chiral y, <5-unsaturated acyl silanes with good ee (Scheme 33)114. 

An excellent synthesis of a,/J-unsaturated acyl silanes from allyl silyl ethers is shown in Scheme 4020. This simple two-step procedure hinges on the Wittig rearrangement127 128,... 

One of the simplest methods for preparation of an a,/J-unsaturated acyl silane is by hydrolysis of a 1-alkoxy-l-trimethylsilylbutadiene, the conjugated dienol ether of a,fi-unsaturated acyl silane, prepared by deprotonation and alkylation of the 1-alkoxydiene (Scheme 42)11. This method is generally limited in application to simple substrates, presumably due to the complexity of preparation of more highly functionalized 1-alkoxy dienes. 

The most versatile synthesis of w./i-unsaUi rated acyl silanes involves the use of allene methodology, developed by a number of groups14,22. Deprotonation and silylation of allenyl ethers followed by hydrolysis gives rise directly to ,/3-unsaturated acyl silanes via their enol ethers, 1-alkoxy-l-trimethylsilylallenes (Scheme 43). Indeed, the first example of an a,/l-unsaturated acyl silane was prepared by such a route223, as was the first example of an allenic acyl silane (from a l-trimethylsilyl-l-trimethylsilyloxy-l,2,3-alkatriene)22b. 

Alkoxy-l-trimethylsilylallenes also undergo a Lewis acid-induced rearrangement to give 2-substituted cr,/J-unsaturated acyl silanes in reasonable yields (Scheme 46)130. The related 1-methylthio-l-trialkylsilylallenes undergo Lewis acid-induced aldol and Mukaiyama reactions to produce 2-alkoxyalkylated a,/i-unsaturated acyl silanes (Scheme 47)131. 

Photochemically-induced addition of bromotrichloromethane to 1-ethoxy-1-trimethyl-silylethene, the ethyl enol ether of acetyl trimethylsilane, generates a 1 1 adduct which provides 3,3-dichloropropenoyl trimethylsilane (20) on solvolysis. Treatment of this material with lithium alkyl cyanocuprates resulted in addition-elimination to give the -isomers of the 3-substituted a,/J-unsaturated acyl silane products (Scheme 50)135. 

Nowick and Danheiser have employed the Horner-Emmons reaction of a-phosphon-oacyl silanes to prepare a,/l-unsaturated acyl silanes in 54-97% yields116. The a-phosphonoacetyl silane intermediate (21), prepared from a-iodoacetyl t-butyldimethylsilane through the Arbuzov reaction, undergoes enolate alkylation, for example using potassium t-butoxide and methyl iodide the alkylated products also underwent Horner-Emmons reaction (Scheme 53). 

In an interesting and unusual reaction, Paquette and Maynard have shown that peracid oxidation of l,2-disiIyl-3,3-dimethylcyclopropene gives rise to an o -silyl-aq/i-unsaturated acyl silane, through rearrangement of the intermediate epoxycyclopropane. Further treatment with peracid gave the a-epoxy acyl silane (Scheme 54)145. 

The first a-cyclopropyl acyl silanes to be isolated were generated by treatment of a,f-unsaturated acyl silanes with diazomethane, followed by vapour-phase pyrolysis of the intermediate pyrazoline derivatives (vide infra, Section IV.D)141. They suffer acid-induced cleavage or rearrangement under more mild conditions than do their carbon analogues146. 

Nowick and Danheiser148 have explored a-cyclopropyl acyl silane generation from a-haloacyl silanes through McCoy reactions (Scheme 57) and via sulphur ylids (Scheme 58). Ylid species such as 24 were found to be stable in aprotic solvents in the presence of lithium salts, and were used for the cyclopropanation of a,/3-unsaturated aldehydes. 

In an interesting transformation, reaction of benzoyl trimethylsilane with lithium enolates derived from various methyl ketones gives rise to 1,2-cyclopropanediols, predominantly with the cis configuration, in good yields (Scheme 77). The reaction, which proceeds through addition, Brook rearrangement and cyclization, is also successful with a,/l-unsaturated acyl silanes vide infra, Section IV.D)187. 

Silyl enol ethers of acyl silanes have been used in Lewis acid-mediated Mukaiyama reactions with acetals. Treatment of the resulting /1-alkoxy acyl silanes with tetrabutylammonium hydroxide or tetrabutylammonium fluoride gave the corresponding a,/J-unsaturated aldehydes (Scheme 99)210. 

The related Brook rearrangement of a-halo-a,jS-unsaturated acyl silanes produces silyloxy allenes (63), from which several sesquiterpenes have been synthesized (Scheme 106)21. Silyloxy allenes may also be prepared by the alkylation of silyloxy allenyl lithium reagents the acyl silane route is, however, less sensitive to solvent effects and other experimental parameters. An outline of the synthesis of dehydrofukinone (64), which elegantly exemplifies this methodology, appears in Scheme 107. 

The Degl Innocenti group has published a number of reactions of a, -acetylenic acyl silanes, mostly involving initial conjugate addition to give a,/i-unsaturated acyl silanes... 

Some /J-heteroatom substituted a,/J-unsaturated acyl silanes react with methyl ketone enolates in a stepwise stereoselective cyclopentannelation process, formally a [3 + 2] annelation, which may proceed through aldol reaction followed by Brook rearrangement and cyclization (Scheme 111)223. 

The first recorded cyclopropyl acyl silane (69) was generated by vapour phase pyrolysis of a pyrazoline derived from a,/)-unsaturated acyl silane by 1,3-dipolar cycloaddition of diazomethane (vide supra, Section DTE)141. Exposure of 69 to titanium tetrachloride induced ring expansion to give the cyclobutanone in 75% yield (Scheme 113). 

The [4 + 2] cycloaddition of a-phenylselenopropenoyl trimethylsilane (75) with 2,3-dimethylbuta-1,3-diene is unusual in that a significant portion of product mixture consists of the hetero-Diels-Alder dihydropyran adduct 76. The phenylselenenyl substituent appears to be responsible for this unusual pattern of reactivity, since propenoyl trimethylsilane gives only the expected regioisomer (77, X = H) (Scheme 116)14. a-Selenenyl substituted a,/l-unsaturated acyl silanes such as 75 were used to prepare a series of substituted dienes in excellent yields through the addition of a-sulphinyl carbanions, Brook rearrangement and expulsion of sulphinate, in a reaction pathway recognisably more typical of acyl silanes (Scheme 117). 

Phenylthiotrimethylsilane adds to propenoyl trimethylsilane under the influence of Lewis acid to give l,3-bis(phenylthio)-l-trimethylsilylprop-l-ene (18). This enol thioether may be deprotonated with f-butyl lithium and alkylated with any of a large range of electrophiles. Subsequent hydrolysis-elimination with mercuric chloride in aqueous acetonitrile provides -substituted a,/J-unsaturated acyl silanes (vide supra, Section III.D.3)132. It should be noted that, in this transformation, the /1-substituent has... 

Allenyl trialkylsilyl ethers 771 can be a-deprotonated with f-BuLi in THF at — 78 °C to give the allenyllithiums 77210901091. They underwent reverse Brook rearrangement to afford the silaacrolein enolates 773, which react with aldehydes and ketones to yield the a,/9-unsaturated acyl silanes 774 (Scheme 200). For enolizable aldehydes transmetallation with ZnCl2-TMEDA, and MgBr2 for ketones, provided better yields. 

Thomas, S.E., Tustin, G.J., and Ibbotson, A., Synthesis and reactivity of iron carbonyl complexes of a,P-unsaturated acyl silanes. Tetrahedron, 48, 7629, 1992. 

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