Acylsilanes

Acylsilanes react with alkylmetals to give a-silylated alcohols, which can be readily desilylated by fluoride ion-induced hydrolysis with stereochemical retention. For this reason, they have been used as aldehyde equivalents for highly stereo- and regioselective synthesis of alcohols.227-228b When acylsilanes or alkylmetals have a leaving group at 

Nucleophilic addition of anionic species to acylsilanes followed by the Brook rearrangement provides an efficient route to siloxy-substituted carbon nucleophiles valuable for further carbon-carbon bond formation. The reaction of 

Cyanide addition to acylsilanes leads to cyano-stabilized carbanions. Cyanide-catalyzed processes via such an 

It has been proposed that metal cyanides generated from metal alkoxides and cyanoformates act as the 

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. 

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Substituted aroyl- and heteroaroyltrimethylsilanes (acylsilanes) are prepared by the coupling of an aroyl chloride with (Me3Si)2 without decarbonylation, and this chemistry is treated in Section 1.2[629], Under certain conditions, aroyl chlorides react with disilanes after decarbonylation. Thus the reaction of aroyl chlorides with disilane via decarbonylation is a good preparative method for aromatic silicon compounds. As an interesting application, trimel-litic anhydride chloride (764) reacts with dichlorotetramethyidisilane to afford 4-chlorodimethylsilylphthalic anhydride (765), which is converted into 766 and used for polymerization[630]. When the reaction is carried out in a non-polar solvent, biphthalic anhydride (767) is formed[631]. Benzylchlorodimethylsilane (768) is obtained by the coupling of benzyl chloride with dichlorotetramethyl-disilane[632,633]. 

The ( )-vinylsilane 151 was prepared by treatment of the silylstannation product 150 with hydrogen iodide[75] and the silylzincation product with water[70]. The silylstannylation of 1-ethoxyacetylene proceeds at room temperature using Pd(OAc)i and 1,1,3,3-tetramethylbutyl isocyanide regioselec-tively and an Si group is introduced at the ethoxy-bearing carbon. Subsequent Cul- and Pd-catalyzed displacement of the stannyl group in the product 152 with allyl halide, followed by hvdrolvsis, affords the acylsilane 153[79],... 

Portella reported the Paal-Knorr condensation of l,4-bis(acylsilanes) 38 in the presenc( of p-toluenesulfonic acid to yield a variety of 2,5-disilylfurans 1)9. Presumably due to steri( constraints, bis(acylsilanes) substituted in the 2-position failed to undergo the Paal-Knor reaction to provide any of the expected trisubstituted furan products. 

The reactions of a-methyl-branched acylsilanes with 2-propenylmagnesium bromide exhibit surprisingly high diastereoselectivilies, although further improvement is accomplished by application of allyltrimethylsilane/ titanium(IV) chloride (> 100 I)27. The fluoride-induced desilylation proceeds with retention of configuration. 

A convenient route to both saturated and unsaturated acylsilanes lies in the hydroboration-oxidation of alkynylsilanes (Chapter 7). Recent improvements (6) to this method involve the use of the borane-dimethyl sulphide complex for hydroboration, and of anhydrous trimethylamine A-oxide for the oxidation of the intermediate vinyl boranes. 

Acylsilanes of a variety of substitution patterns have been employed (9) in routes to allenyl silyl enol ethers. 

BF,.OF.t2-induced reaction of acylsilane silyl enol ethers with acetals... 

A solution of the trimethylsilyl enol ether of propionyl trimethylsilane (5 mmol) (Chapter 12) and benzaldehyde diethyl acetal (5 mmol) in dichloromethane (10ml) was added to a solution of BF3.OEt2 (5 mmol) in dichloromethane (5ml), cooled to —78 C. After being stirred for lh at -78°C and 2h at -30°C, the mixture was quenched with excess saturated sodium hydrogen carbonate solution, and extracted with ether. Concentration and distillation gave the product -ethoxy acylsilane, (4.6mmol, 95%). b.p. 105-106 C/2mmHg. Treatment of this alkoxy... 

Acylsilanes are most useful synthetic intermediates (1), providing, inter alia, controlled routes to silyl enol ethers. They are relatively unreactive towards nucleophilic reagents for both steric and electronic reasons. 

Enolate generation, 106-7 Enolate trapping, 99-101 Enones, 34-5 Epoxidation, 21-3 a/3-Epoxysilanes, 21-4, 78 -Ethoxy acylsilane, 110 1-Ethoxy-l-trimethylsilyloxycyclo-propane,133 Ethyl bromoacetate, 123 Ethyl 2-chloropropanoate, 133 Ethyl glycinate, 87,88-9 Ethyl m-nitrobenzene, 137 Ethyl irimethylsilylacetate. 71, 123-4, 134 Ethyllithium, 66... 

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>. 

Triphenylsilanol is one of several products obtained in the thermally induced decarbonylation of Ph3SiC02H (129). Acylsilanes react readily with aqueous base for example, Ph3SiC(0)Ph affords Ph3SiOH and PhC(0)H when treated with aqueous sodium hydroxide solution (130). Similarly, hydrolysis of the allenyl silane 22 leads to the formation of 23 via the acylsilane 24 [Eq. (24)] (131). (For further examples of acylsilane chemistry, see Refs. 132-134.)... 

The unusual l-sila-l,2-diol (CH2=CH)CH(OH)SiPh2OH, prepared by hydrolysis of the acylsilane (CH2=CH)C(0)SiHPh2, has a structure comprising hydrogen-bonded double chains as shown in Fig. 9. Only COH - HOSi hydrogen bonds are present, with the bonds between the chains being shorter than those that build up the chain, 2.666 and 2.739 A, respectively (131, 210). 

Scheme 4.22 Coupling reaction using aldehyde, acylsilane and oxo-acid.

If cinnamic acid imidazolide is introduced into this reaction, the double bond of the corresponding acylsilane is reduced as well.[105]... 

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