Coupling reactions of allylsilanes

The coupling reaction of allylsilane with the w-thiomethoxyacetal is catalyzed by TMSOTf51. TiCLj-mediated reaction of a-bromoallylsilane 7 with 1,1-diethoxyethane leads to homoallylic ether 12 stereoselectively in excellent yield (equation 9)40. Under similar reaction conditions, double substitution of allylsilane to diketals 13 affords 14 in high diastereoselectivity (equation 10)52. 

The kind of a fluoride ion activator and the leaving group in electrophiles affects the stereochemistry in the cross-coupling reaction of allylsilanes as exemplified with 2-cyclohexenyl(difluoro)phenylsilane (Eq. 31) 135]. 

Scheme 10-9 Mechanism of regiochemical control for coupling reactions of allylsilanes.

Difficulties have been fonnd in the intermolecular coupling reactions of allylsilanes with epoxides other than the simplest ethylene oxide . For example, reactions of allylsilane 99 with epichlorohydrins in the presence of TiCLj or EtAlCl2 give chlorohydrins in moderate to good yields. However, treatment of epichlorohydrin with 99 fnrnishes the expected allylated chlorohydrin product 100 (equation 78). In the intramolecnlar addition of the allylsilane moiety to 2,3-epoxyether moiety of 101, the ratio of 6- and 7-membered ring products is affected by the nature of the Lewis acid. When BF3 OEta is nsed, a mixture of 102 and 103 is obtained (equation 79). Interestingly, 102 is the exclusive product in the TiCU-catalyzed reaction . 

The TiCU-induced three-component coupling reaction of an a-haloacylsilane, allylsilane and another carbonyl compound gives 48 in good yield. A silyl enol ether intermediate is suggested (equation 31)82. The reaction of a cyclopropyl ketone with allylsilane yields a mixture of skeletal rearranged products83. 

More recently, allylic carbonates 115 have been found to couple with alkenylfluo-rosilanes in the presence of a palladium catalyst to yield the corresponding 1,4-dienes (equation 93)174. The corresponding palladium-catalyzed reaction of allylsilane with allyl acetate 116 has been explored (equation 94)175. 

These have been much less popular, but some examples have been reported. These involve only alkenyltins containing one additional moiety on the G-carbon this substituent can be trimethylsilylmethyl (leading to functionalized allylsilanes [37] or divinyl ketones (Scheme 4-10) [38]) or trifluoromethyl [39]. The substituent can also be in the /3-position (E-geometry) examples have been reported by Parrain et al. [40] and Castano et al., the latter having used the coupling reaction of a jS-stannyl enone as the key step in the preparation of the indolizidine alkaloid ( )-monomorine here the coupling step is followed by an immediate in-situ reduction of the intermediate enone, the mechanism of which is unclear (Scheme 4-11) [41J. 

Coupling reactions of alkyl Grignard reagents lacking -H substitution are exemplified by the stereoselective synthesis of a trideuteriomethylated alkene (equation 10) and of allylsilanes (equations 11 and 12). ° Cyclic bromoalkenes also undergo coupling smoothly (equation 13). ... 

The total synthesis of 5(5)-hydroxy-12-oxo-20 3(6Z,8.E,14Z) involving Wittig coupling reactions of tailored intermediates was described (38). 10(5)-Hydroxy-20 4(5Z,8Z,11Z,14Z) was synthesized in eight steps starting from enantiomerically pure (7 )-glyceraldehyde acetonide (39). Chiral adducts from Grignard or allylsilane... 

Several unusual silametacyclophanes, macrocyclic cage compounds, have been prepared by multiple Pd(0)-catalyzed Suzuki coupling reactions of 9-BBN adducts of allylsilane and bromobenzene. For example, reaction of 9-BBN adduct of methyltriallylsilane and 1,3,5-tribromobenzene leads to 4-methyl-4-sila[3 - ][7] metacyclophane, 4-(2-propenyl)-4-methyl-4-sila[7]metacyclophane, which results from an intramolecular coupling of two legs of the silane and -elimination of the third leg and 4-(3-phenylpropyl)-4-methyl-4-sila[7]metacyclophane (Eq. 

A zinc-silver couple has been reported to be an improved reagent for the reductive addition of allylic bromides to nitriles. l-Trimethylsilyl-2-methyl-cyclopropane is a convenient regio- and stereospecific precursor of unsaturated ketones [equation (37)]. The reaction of allylsilanes with a-nitro-olefins has been found to proceed smoothly in the presence of aluminium trichloride [equation (38)]. The unstable nitronic acid product may be converted readily into the unsaturated ketone by the Nef reaction. 

The alkoxycarbenium ions generated by the cation pool method react with various carbon nucleophiles such as substituted allylsilanes and enol silyl ethers to give the corresponding coupling products in good yields. It should be noted that the reactions of alkoxycarbenium ion pools with such nucleophiles are much faster than the Lewis acid promoted reactions of acetals with similar nucleophiles. A higher concentration of the cationic species in the cation pool method seems to be responsible. 

The intramolecular coupling of enolethers with enolethers, styrenes, alkyl-substituted olefins, allylsilanes, and vinylsilanes was systematically studied by Moeller [69]. Many of these coupling reactions turned out to be compatible with the smooth formation of quaternary carbon atoms (Eq. 11) [70], which were formed diastere-oselectively and led to fused bicyclic ring skeletons having a ds-stereochemistry [71]. The cyclization is compatible with acid-sensitive functional groups as the allylic alkoxy group. Moeller has demonstrated in some cases that these reactions can be run without loss of selectivity and yield in a simple beaker with either a carbon rod or reticulated carbon as anode without potential control and a 6-V lantern battery as power supply [71]. 

Allyl- and vinylsilane chemistry was one of the first areas of reagent synthesis impacted by CM methodology. Allylsilanes are commonly employed in nucleophilic additions to carbonyl compounds, epoxides, and Michael acceptors (the Sakurai reaction) vinylsilanes are useful reagents for palladium-coupling reactions. As the ubiquitous application of CM to this substrate class has recently been described in several excellent reviews, this topic will not be discussed in detail, with the exception of the use of silane moieties to direct CM stereoselectivity (previously discussed in Section 11.06.3.2). 

The asymmetric cross-coupling was successfully applied to the synthesis of optically active allylsilanes [27,28] (Scheme 8F.9). The reaction of a-(trimethylsilyl)benzylmagnesium bro-... 

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