Silanes allenyl

Allenyl(trimethyl)silanes also react with aldehydes in the presence of titanium(IV) chloride to give the. vy -products with moderate syn stereoselectivity57. 

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.)... 

A new type of asymmetric hydrosilylation which produces axially chiral allenylsilanes has been reported by use of a palladium catalyst coordinated with the bisPPFOMe ligand 51b.64 The hydrosilylation of l-buten-3-ynes substituted with bulky groups such as tert-butyl at the acetylene terminus took place in a 1,4-fashion to give allenyl(trichloro)-silanes with high selectivity. The highest enantioselectivity (90% ee) was observed in the reaction of 5,5-dimethyl-T hexen-3-yne with trichlorosilane catalyzed by the bisPPFOMe-palladium complex (Scheme 13). 

Allenyltrichlorosilanes can also be prepared by Sn2 displacement of propargylic chlorides with a Cu or Ni complex of HSiCl3 [56]. The reaction requires an amine base and a donor solvent such as THF or propionitrile (Table 9.32). Conditions can be adjusted to favor the propargylic or allenic silane, which is not isolated, but treated directly with various aldehydes to afford allenylcarbinols (A) or homopropargylic alcohols (B). These reactions presumably proceed by an SE2 pathway, such that the allenyl products arise from the propargylic silane and vice versa. 

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 acid portion of 1 was assembled by enantio-and diastereocontrolled addition of Z-crotyl borane to the aldehyde 8, following the Brown protocol. Hydroboration and oxidation led to 9, which was condensed with the allenyl silane 10 to give 11 with high diastereocontrol. Conversion of the alcohol to the iodide followed by three-carbon homologation by the Myers procedure then led to 1, which was cyclized with > 10 1 regio- and diasterocontrol to give 12. Ozonolysis and methylenation of the less hindered ketone then delivered 3. 

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. 

In a further development of this approach, the synthesis of cr,/J-acetylenic acyl silanes has been achieved as shown in Scheme 4514. Oxidation of the 3-selenenyl allenyl ethers (16) with m-chloroperbenzoic acid at —78 °C gave the corresponding unstable selenoxides, which underwent in situ [2,3] sigmatropic shift producing acetals (17). Loss of selenenyl ester on work-up gave the cr,/J-acetylenic acyl silanes in ca 50% yields. 

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 /3-lactone was formed by the cyclization of a 3-hydroxycarboxylic acid with sulfonyl chloride. An alternative synthesis attempted to control all stereochemical relationships in the molecule using the properties of silyl moieties attached to substrates and reagents <20040BC1051>. Stereoselective reactions of this type included the use of silyl groups in enolate alkylations, hydroboration of allylsilanes, and an anti Se2 reaction of an allenyl silane with an aldehyde and ry -silylcupration of an acetylene. The /3-lactone was again formed by the standard sulfonyl chloride cyclization method. 

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. 

Analogous to the allylation with allylsilanes and -stannanes, the transformations, vinylallylation, propargylation, allenylation, alkenylation, alkynylation, and arylation, are viable by the use of an appropriate reagent in the presence of a titanium Lewis acid these are surveyed in the review articles cited both in the Introduction and in this section. The stereochemistry of the reaction of a (vinylallyl)silane in the presence of TiCU has been reported [234]. Equation (113) shows that the major reaction of this silane and isobutyraldehyde occurred mainly in the anti sense with a ratio of anti to syn attack of 90 10 at the terminus remote from the silyl group. Essentially the same stereochemical outcome was observed for the same reaction with the corresponding trimethylsilyl derivative. The intramolecular reaction with an acetal, however, proceeded less selectively the anti syn ratio was 60 40 (Eq. 114) [234]. 

Organo-silanes and -stannanes possessing sp caibon-metal bonds also add to carbonyls in the presence of Lewis acids. Alkynylation with silyl- and stannyl-alkynes is promoted by AlCb or Z11CI2 (equations 9 and lO). " Notably, the reaction of l,3-bis(trimethylsilyl)-l-prcq>yne with chloral affords ynic alcohol instead of allenylic alcohol, showing the preferential cleavage of sp C—Si bonds (equation 11). ... 

Homopropargyl and allenyl alcohols. Chiral silanes are formed from propargylic mesylates. Subsequent reaction with aldehydes proceeds in a manner depending on the substituent on the sp-carbon of the mesylates. Starting from nonracemic mesylates the products are obtained with excellent ee. 

Allenyl-, Propargyl-, Vinyl- andEthynyl-silanes and -stannanes... 

Propargyltrimethylsilanes give allenyl derivatives on reaction with -acyliminium precursors. - On the other hand, application of allenyl-silanes or -stannanes provides access to propargyl-substituted lactams. All reactions proceed with high regio- and stereo-selectivity (equations 54 and 55). 4-Allenyl-azetidinones can be cyclized to A -carbapenem systems, mediated by Ag or Pd salts. - The reactions of ethoxylactams with allenylsilanes can be modified in such a way that bicyclic systems are obtained in one step (equation 56). ... 

---