Aldehydes, reaction with allylsilanes

In reactions of chiral aldehydes, TiIV compounds work well as both activators and chelation control agents, a- or A-oxygcnated chiral aldehydes react with allylsilanes to afford chiral homoallylic alcohols with high selectivity (Scheme 22).85 These chiral alcohols are useful synthetic units for the synthesis of highly functionalized chiral compounds. Cyclic chiral 0,0- and A/O-acetals react with allylsilanes in the same way.86,87 Allenylsilanes have also been reported as allylation agents. 

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). 

The fact that ketones, aldehydes and geminal diacetates are readily available from these reactions illustrates their complementarity to reactions with allylsilanes. Specifically, the equivalency of allylic ethers to homoenolates allows for the formation of compounds extended... 

Addition to C=0. Hydrates of a-ketoaldehydes react selectively at the aldehyde group with allylsilanes under the influence of YbfOTfjj at room temperature. The allylation of aldehydes with allylstannanes is accelerated by benzoic acid." Another method for the synthesis of homoallylic alcohols is by the ene reaction, thus y,6-unsaturated a-hydroxy esters are obtained from glyoxylic esters at room temperature in a catalyzed process. ... 

The CAB-catalyzed reactions of aldehydes with allyl- and crotylsilanes are summarized in Table 11-28 [288, 309], These data show that the CAB-catalyzed crotylation reactions with crotylsilanes 123b and 460 are highly diastereo- and en-antioselective, while the corresponding allylation reactions with allylsilanes 131 occur with somewhat lower enantioselectivity. 

Allylation. Various aldehydes react with allylsilanes on catalysis by YbCl, in MeCN. o-Anisaldehyde is converted to 2-(l,6-heptadien-4-yl)anisole under these reaction conditions. 

The cyclohexyloxy(dimethyl)silyl unit in 8 serves as a hydroxy surrogate and is converted into an alcohol via the Tamao oxidation after the allylboration reaction. The allylsilane products of asymmetric allylboration reactions of the dimethylphenylsilyl reagent 7 are readily converted into optically active 2-butene-l, 4-diols via epoxidation with dimethyl dioxirane followed by acid-catalyzed Peterson elimination of the intermediate epoxysilane. Although several chiral (Z)-y-alkoxyallylboron reagents were described in Section 1.3.3.3.3.1.4., relatively few applications in double asymmetric reactions with chiral aldehydes have been reported. One notable example involves the matched double asymmetric reaction of the diisopinocampheyl [(Z)-methoxy-2-propenyl]boron reagent with a chiral x/ -dialkoxyaldehyde87. 

Optically active (Z)-l-substituted-2-alkenylsilanes are also available by asymmetric cross coupling, and similarly react with aldehydes in the presence of titanium(IV) chloride by an SE process in which the electrophile attacks the allylsilane double bond unit with respect to the leaving silyl group to form ( )-s)vr-products. However the enantiomeric excesses of these (Z)-allylsilanes tend to be lower than those of their ( )-isomers, and their reactions with aldehydes tend to be less stereoselective with more of the (E)-anti products being obtained74. 

Si. rra(pentafluorophenyl)boron was found to be an efficient, air-stable, and water-tolerant Lewis-acid catalyst for the allylation reaction of allylsilanes with aldehydes.167 Sc(OTf)3-catalyzed allylations of hydrates of a-keto aldehydes, glyoxylates and activated aromatic aldehydes with allyltrimethylsilane in H2O-CH3CN were examined. a-Keto and a-ester homoallylic alcohols and aromatic homoallylic alcohols were obtained in good to excellent yields.168 Allylation reactions of carbonyl compounds such as aldehydes and reactive ketones using allyltrimethoxysilane in aqueous media proceeded smoothly in the presence of 5 mol% of a CdF2-terpyridine complex (Eq. 8.71).169... 

Recently, a new multicomponent condensation strategy for the stereocontrolled synthesis of polysubstituted tetrahydropyran derivatives was re-published by the Marko group, employing an ene reaction combined with an intramolecular Sakurai cyclization (IMSC) (Scheme 1.14) [14]. The initial step is an Et2AlCl-promoted ene reaction between allylsilane 1-50 and an aldehyde to afford the (Z)-homoallylic alcohol 1-51, with good control of the geometry of the double bond. Subsequent Lewis acid-media ted condensation of 1-51 with another equivalent of an aldehyde provided the polysubstituted exo-methylene tetrahydropyran 1-53 stereoselectively and... 

Polystyrene-bound allylsilanes react with /V-(alkoxycarbonyl)imincs under Lewis acid catalysis to yield /V-homoallylcarbamates (Entry 4, Table 14.9). Similarly, Wang resin bound carbamates have been successfully N-alkylated with allylsilanes and aldehydes in a Mannich-type reaction (Entry 5, Table 14.9). Resin-bound /V-(alkoxycarbo-nyl)imines can be generated either from unsubstituted carbamates ROCONH2 by... 

Reaction with Further Electrophiles of Group IVA (Sl,Ge,Sn). IV-Silylated aziridines can be prepared from ethyleneimine by amination of chlorosilanes in the presence of an HC1 acceptor, by dehydrocondensation with an organosilicon hydride or by cleavage of a silicon—carbon bond in 2-furyl-, 2-thienyl-, benzyl-, or allylsilanes in the presence of an alkali metal catalyst (262—266). N-Silylated aziridines can react with carboxylic anhydrides to give acylated aziridines, eg, A/-acetylaziridine [460-07-1] in high yields (267). At high temperatures, A/-silylaziridines can be dimerized to piperazines (268). Aldehydes can be inserted... 

A similar transition state 22 is calculated for the reaction of allylsilanes with aldehydes (equation 10)43. In this case, however, the calculations show that the oxygen attacks at an apical site of the silicon centre, while the allyl group departs directly from an equatorial position without causing a pseudorotation, in contrast to the mechanisms previously discussed. 

Silyl-l,3-dienes undergo anodic methoxylation in methanol to give 1,4-addition products with an allylsilane structure as intermediates. Therefore, they are further oxidized to give l,l,4-trimethoxy-2-butene derivatives as the final products. The products are easily hydrolyzed to provide the corresponding y-methoxy-a, /t-unsaUirated aldehydes. Since 1-trimethylsilyl-l,3-dienes are readily prepared by the reaction of the anion of l,3-bis(trimethylsilyl)propene with aldehydes or ketones, l,3-bis(trhnethylsilyl)propene offers a, /i-formylvinyl anion equivalent for the reaction with carbonyl compounds (equation 15)16. 

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. 

Parallel to an earlier work on the highly diastereoselective reactions of aliphatic aldehydes with allylsilane in the presence of 3976, treatment of methyl ketones under the same conditions yields the corresponding tertiary homoallylic ether with a diastereomeric excess of up to 90% (equation 25)77. 

Another approach leading to pyrrolidine 210 via [3 + 2] cycloaddition is the reaction of A-protected a-amino aldehydes with allylsilanes in the presence of a catalytic amount of BF3 OEt2 (equation 172). No 1,2-silyl group migration occurs in these annulation processes313. 

The (F,)- and (Z)-(R)-allylsilanes (3b) react with aldehydes to give optically active (F)-homoallylic alcohols 5 or 6. In reactions with (E)-3b, essentially only the erythro-diustcreoisomer 5 is formed, regardless of the aldehyde used. In reactions with (Z)-3b, both vrythro and fAmi-diastereoisomers (6) are obtained, the ratio depending on the itructiircol the aldehyde but the configuration at C, is identical in both products and... 

---