Acylsilanes carbenes

Silylfiirans are available from acylsilane dicarbonyl confounds <96JOC1140>. Fhotocycloaddition of 35 with aUcenes leads cleanly to tetrasubstituted furans 38 in yields of 85%. A mechanism is proposed involving an alkyl propargyl biradical (as 36) that closes first to a vinyl carbene (as 37) and than to 38 <96JOC3388>. 

Addition of acyl anions generated from acylsilanes to a,(3-unsaturated ketones using N-heterocyclic carbenes (NHCs) derived from thiazolium salts as catalyst produced 1,4-diketones, which cyclized to form the corresponding furans in good yields under an acidic condition <06JOC5715>. 

Aryl(trimethylsiloxy)carbenes. Acylsilanes (153) undergo a photoinduced C —> O silyl shift leading to aryl(trimethylsiloxy)carbenes (154).73,74 The carbenes 154 can be captured by alcohols to form acetals (157) 73 or by pyridine to give transient ylides (Scheme 29).75 LFP of 153 in TFE produced transient absorptions of the carbocations 155 which were characterized by their reactions with nucleophiles.76 The cations 155 are more reactive than ArPhCH+, but only by factors < 10. Comparison of 154 and 155 with Ar(RO)C and Ar(RO)CH+, respectively, would be of interest. Although LFP was applied to generate methoxy(phenyl)carbene and to monitor its reaction with alcohols,77 no attempt was made to detect the analogous carbocation. 

In the rationale of Scheidt et al., a carbene catalyst (48) adds to the acylsilane 49 to produce - after 1,2-silyl migration - the intermediate 50 (Scheme 9.13). The alcohol is believed to induce a desilylation, leading to the Breslow-intermediate 26. Due to the reduced electrophilicity of the acylsilane (in comparison with an aldehyde), the conjugate addition predominates. 

Photolysis of acylsilanes produces siloxysubstituted carbenes, which could be trapped by an excess of electrondeficient olefins. This route to the resulting alkyl 2-siloxycyclopropanecarboxylates seems not to be of preparative value, however 67). 

Lithium phosphites also can catalyze the silyl benzoin reaction of acylsilanes. Its asymmetric version is successfully achieved by a lithium phosphite derived from a homochiral diol.236 Thiazolium salt 32 effectively promotes conjugate acylation of a, 3-unsaturated carbonyls with acylsilanes in the presence of DBU (Equation (61)).237,237a The active catalyst of this sila-Stetter reaction would be a carbene species generated from 32 by deprotonation. 

A proposed reaction mechanism is shown in Scheme 130. It involves the addition of a neutral carbene/zwitterionic species 514 (generated in situ from the exposure of thiazolium salt to DBU) to an acylsilane <2001JOC5124, 1996TL8241>. This nucleophilic addition initiates a 1,2-silyl group migration (Brook rearrangement)... 

Carbene 13 is extremely photolabile, and 570 nm irradiation results in the rearrangement to acylsilanes 14. The calculated activation barrier for this highly exothermic rearrangement is only 4 7 kcal/mol. If we assume that the analogous reaction of silanone and formaldehyde also leads to a labile siloxycarbene, the formation of formylsilanol 11 is easily explained. 

Vice versa, siloxycarbenes have been generated as short lived intermediates by irradiation of acylsilanes [27-29]. Irradiation of matrix isolated acylsilanes 14c, e and f resulted both in the a-cleavage to give radical pairs and in the rearrangement to siloxycarbenes 13 [29]. Since the formation of these intermediates is reversible, the radical pairs and carbenes 13 could only be identified by oxygen trapping. 

The nucleophilic and electron-accepting properties of heterocyclic nucleophilic carbenes 36 were also used in combination with the electrophilic/nucleophilic character of acylsilanes via Brook rearrangement, leading to the invention of a sila-Stetter reaction by Scheidt and coworkers fScheme 6.24). The iminium structure in 37, generated by addition of the carbene catalyst 36 to the acylsilanes, promotes a Brook rearrangement to afford enol silyl ether 38. The alcohol additive present in the reaction causes desilylation to produce nucleophilic enaminol 39, which adds to a,p-unsaturated ketones to give 40. The formation of aryl ketone expels the carbene catalyst and produces 1,4-diketone 41. 

Scheme 6.24 Sila-Stetter reactions between acylsilanes and a,p-unsaturated ketones catalyzed by heterocyclic nucleophilic carbenes. 

N-Heterocychc carbenes (NHCs) derived from thiazolium salts (e.g., X) have been utihzed as catalysts in benzoin condensations, in additions of aldehydes to enones (Stetter reaction [17]) and recently in the reaction of acylsilanes with a, p-unsaturated carbonyl compounds (sila-SrEiTER reaction [18]), as exemplified in the following sequence of reactions ... 

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