Homoallylic alcohol, from allyl silane

The radical anions of carbonyl groups can also be generated via PET from activated alkenes, e.g. allylic silanes or stannanes. Triplet excited aromatic ketones, a-dicarbonyls and Michael systems are suitable substrates for oxidizing allylic Group 14 organometallic compounds with subsequent formation of homoallylic alcohols or S-allylated ketones (Scheme 32) [120-122]. 

As shown in racemic series [220], the stereoselectivity of the allylation of chiral 1,3-dioxanes 6.20 depends upon the reaction conditions and upon the nature of the reagent (silane or stannane). The most selective reagent is CH2=CHCH2SnBu3. Most of the reactions have been carried out with trimethyl-allylsilane in the presence of TiC1.4/Ti(0/-Pr)4 under the conditions recommended by Johnson and coworkers [213], After oxidation of the products and treatment with a base, nonracemic homoallylic alcohols are obtained with excellent enantiomeric excesses (Figure 6.58). From CH2=C=CHCH2SiMe3, dienyl alcohols are similarly obtained [1228, 1229] (Figure 6.58). The TiC -catalyzed reactions of dioxanes 6.20 with alkynylsilanes also lead to nonracemic a-alkynyl alcohols [223],... 

Non-racemic epoxide 307, prepared from (+)-epichlorohydrin, was alkeny-lated to give allyl silane 308, which was converted into allyl stannane 309. Corey s chiral boron reagent-mediated asymmetric allylation [120] to aldehyde 311 provided homoallylic alcohol 306 with a 11 1 diastereomer ratio. Intramolecular Sn2 cyclization of the corresponding 7-hydroxy-3-TsO unit derived from 306, followed by hydrolysis of the dithiane, afforded aldehyde 312 (Scheme 66). 

Homoallyl bromide 314, prepared from readily available non-racemic ester 313, was converted to the Grignard reagent, which reacted with non-racemic epoxide, derived from D-maUc acid, to afford the alcohol 305. Ozonolysis of the alkene gave a ketone, which was converted into enol tri-flate 316. Ni-catalyzed cross coupling with trimethylsilylmethyl magnesium chloride afforded the allyl silane, which was converted into the allyl stan-nane 317. The asymmetric allylation of 313 with 317 provided 304 with a ration of 8.5 1. Methyl etherification and oxidative cleavage of exo-methylene... 

While the chiral aldehydes or allyl nucleophiles are based on stoichiometric amounts for the control of diastereoselectivity [74, 77], it has been found that catalytic amounts of titanium complexes derived from BINOL can mediate the enantioselective addition of allyl stannanes to aldehydes, giving the homoallyl alcohols high enantioselectivity. Mikami reported that the BINOL-Ti complexes prepared in situ in the presence of 4A molecular sieves (MS) catalyze the carbonyl addition reaction of allyl silanes or stannanes to afford the syn product in high enantiomeric excess [78] (Scheme 14.21). 

Studies on the stereochemical outcome of the reaction of 2-alkenyl organometallic reagents with aldehydes have increased recently, largely because stereoselectively produced homoallylic alcohols are synthetically equivalent, by cleavage at the carbon-carbon double bond, to the type of aldol adduct stereoisomers required for macrolide antibiotic total synthesis. A new stereoselective synthesis of (Z)-2-alkenyltins (46a) or the corresponding silanes (46b) from allyl or vinyl 9-BBN derivatives has appeared (Scheme 20). The... 

On the other hand, homoallylic alcohols containing a (Z)-vinylic silane moiety transfer the allylic group to afford ( )-l-aryl-2-alkenylsilanes (Scheme 5,18). The -selectivity would originate from the absence of severe 1,3-diaxial repulsion between the Si group and the R group in the transition state toward ( )-crotylsilane. 

The synthesis of homoallylic alcohols was achieved via a Pd-catalyzed cross coupling of allyl(isopropoxy)dimethyl-silane. Similarly to the previous example (eq 84), the dimer (1) was in some cases more efficient than Pd(dba)2. The reaction of 2-methyliodobenzene was improved by using 1, and the coupling of 2-methoxyiodobenzene was only observed in the presence of 1 (eq 85). A one-pot process was optimized starting from allyl(isopropoxy)dimethylsilane and benzaldehyde. Another strategy for the preparation of homoallylic alcohols was described via a Pd-catalyzed three-component coupling of an arylboronic acid with allenes and aldehydes. 

Non-allylic Unsaturated Alcohols. Two reports have described the formation of yS-unsaturated alcohols (homoallylic) from the reaction of allyl silanes and aldehydes or ketones under the influence of a Lewis acid (TiCI or AICI3 ). 

In 1976, Akira Hosomi and Hideki Sakurai of Tohoku University in Sendai, Japan, published a letter entitled Syntheses of y,S-Unsaturated Alcohols From Allylsilanes And Carbonyl Compounds In The Presence of Titanium Tetrachloride"The letter describes the reaction depicted in equation 1, in which allyl silanes react with aldehydes or ketones to provide homoallylic alcohols. The following year, Hosomi and Sakurai extended their finding to ketals, which provide homoallylic ethers (eq. 2)," and to a,P-unsaturated ketones (eq. 3), in which case the addition occurs in a 1,4-fashion and becomes a valuable method to generate quaternary centers. 

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