Allylsilanes addition reactions

Chiral oxazolidinones have also been used to induce chirality in TiCU-mediated allylsilane addition reactions to a-keto imides (eq 14). ... 

Allyltrialkoxy- or -tris(dialkylamino)titanium reagents are not capable of chelation-controlled addition reactions with oxy- or amino-substituted carbonyl compounds due to their low Lewis acidity87. To attain chelation control, the application of allylsilanes (Section 1.3.3.3.5.2.2.) and allylstannanes (Section I.3.3.3.6.I.3.2.) in the presence of bidentate Lewis acids like titanium(IV) chloride, tin(lV) chloride or magnesium bromide are the better options. 

The addition reaction of allylsilane to acetaldehyde with BF3 as the Lewis acid has been modeled computationally.95 The lowest-energy TSs found, which are shown in Figure 9.2, were of the synclinal type, with dihedral angles near 60°. Although the structures are acyclic, there is an apparent electrostatic attraction between the fluorine and the silicon that imparts some cyclic character to the TS. Both anti and syn structures were of comparable energy for the model. However, steric effects that arise by replacement of hydrogen on silicon with methyl are likely to favor the anti TS. 

The addition of an allyl metal to a-amino aldehydes has been used by Vara Prasad and Rich (Scheme 12)J23l After the addition step, the allyl group is oxidized to a carboxylic acid and lactonized. Then, the a-carbon of the lactone is alkylated stereoselectively. These investigators also systematically examined the addition reaction to determine its diastereo-selectivity. The highest diastereoselectivity was obtained when allyltrimethylsilane was used in the presence of tin(IV) chloride. An increase in the steric bulk of the protecting group and of the side chain also resulted in a better diastereoselection. Alternatively, Taddei and co-workers 24-26 used a 2-(halomethyl)allylsilane and the side chains were introduced by nucleophilic substitution of the halogen (Scheme 13). 

Significantly better results in addition of non-stabilized nucleophiles have come from hydrogenolysis reactions using formate as a hydride donor as shown in Scheme 8E.46. The racemic cyclic acetate and prochiral linear carbonates were reduced in good enantioselectivities by monophosphine ligands (/ )-MOP (16) and (Zf)-MOP-phen (17), respectively [195]. The chirality of the allylsilane can be efficiently transferred to the carbinol center of the homoallylic alcohol by the subsequent Lewis acid catalyzed carbonyl addition reaction 1196], The analogous... 

Carbamate and amide groups have been found to be stable under these coupling conditions73. In the presence of TiCLt or SnCLt, chiral a-keto amides 36 react with allyl-silane to produce, after hydrolysis, optically active tertiary alcohols 37 with extremely high optical selectivity (equation 23)74. The addition reaction appears to occur from the Si face of the carbonyl group. In a similar manner, a high degree of stereoselectivity is obtained from the reactions of A-Boc-a-amino aldehydes 38 with 2-substituted allylsilanes (equation 24)75. 

In photochemical reactions, some examples have been known of fragmentation of cation radicals of silyl compounds into carbon radicals. Mizuno and colleagues reported a photochemical addition reaction of allylsilane to dicyanophenylethene. Photochemical electron transfer between excited phenanthrene E50 and dicyano-... 

High 1,3- and 1,4-asymmetric induction is possible in addition reactions to chiral P-alkoxy aldehydes. The reaction with allylsilanes catalyzed by TiC shows high diastcr-eofacial selectivity (equation II). Essentially only one. vv -adduct of two possible dias-tereomers is obtained in reactions with silyl enol ethers (equation III). 

Stereochemistry of addition reactions involving allylsilanes 23.4.12 Intermolecular additions to aldehydes, ketones and acetals... 

C(3)-substituted allylsilanes (crotylsilanes) participate in chelation-controlled addition reactions with aldehydes to give syn addition products as the major stereoisomers. Generally, the ( )-crotylsilanes are highly selective in the syn sense (>9S 5). In contrast, the (Z)-crotylsilanes are much less selective (60-70 40-30 syn anti Scheme 42). Hayashi and Kumada have report a successful approach to optically active homoallyl alcohols using this strategy. They have reported that useful levels of asymmetric... 

Table 11 summarizes the important results of asymmetric induction in addition reactions of optically active allylsilanes (90) and (92). The origin of the ir-facial selectivity in these reactions can be traced to the anti selectivity that is commonly observed for 5e2 reactions of lylsilanes (Figure 14). The syn stereochemistry with respect to the double bond configuration (E or Z) is also very high even with aldehydes bearing sterically bulky substituent gro q)s, as illustrated by entry (/ )-(Z)-(92b). 

Usefiil levels of stereoselectivity were obtained in intermolecular addition reactions of C(3)-sub-stituted allylsilanes to chiral aldehydes. Lewis acids that are citable of chelating to heteroatoms have been used to direct the stereochemical course of allylsilane additions to a-alkoxy and a,p-dialkoxy carbonyl compounds. The allylation of a-benzyloxy iddehyde (94) in the presence of TiG4 and SnOt furnished products with high levels of syn stereoselection (syn-9. In contrast, under nonchelation-controlled reaction conditions (BF3-OEt2) allyltrimethylsilane reacted to form predonunantly the anti-1,2-diol product (anti-95), as shown in Scheme 45. 

Propargylsilanes undergo electrophilic addition reactions to generate -silyl carbocations (109) that can, in p ciple, react further to give eidier addition (110) or substimtion (111) products, as illustrated in Scheme S1. As in the case of allylsilanes, however, substimtion predominates. ... 

---