Allylic silanes carbonyl addition reactions

Allylic silanes react with aldehydes, in the presence of Lewis acids, to give an allyl-substituted alcohol. In the case of benzylic silanes, this addition reaction has been induced with Mg(C104)2 under photochemical conditions. The addition of chiral additives leads to the alcohol with good asymmetric induction. In a related reaction, allylic silanes react with acyl halides to produce the corresponding carbonyl derivative. The reaction of phenyl chloroformate, trimethylallylsilane, and AICI3, for example, gave phenyl but-3-enoate. ... 

BINOL-Ti complexes (1) has been shown to serve as efficient asymmetric catalysts for the carbonyl addition reaction of allylic stannanes and silanes 152,53]. The addition reactions to glyoxylates of ( )-2-butenylsilane and -stannane proceed smoothly to afford the corresponding syn-product with high enantiomeric excess (Scheme 8C.21) [52]. 

The chiral titanium complex BINOL-T1CI2 also catalyzes the asymmetric carbonyl addition reaction of allylic silanes and stannanes. Thus the addition reaction of glyoxylate with (E)-2-butenylsilane and -stannane proceeds smoothly to give the syn product in high enantiomeric excess (eq 12). The syn product thus obtained can be readily converted to the lactone portion of verrucaline A. The reaction of aliphatic and aromatic aldehydes with allylstannane is also catalyzed by BINOL-TiC to give... 

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). 

A variety of electrophilic catalysts promote the addition of allylic silanes to carbonyl compounds.86 The original catalysts included typical Lewis acids such as TiCl4 or BFj.87 This reaction is often referred to as the Sakurai reaction. 

Although the allylation reaction is formally analogous to the addition of allylic boranes to carbonyl derivatives, it does not normally occur through a cyclic TS. This is because, in contrast to the boranes, the silicon in allylic silanes has little Lewis acid character and does not coordinate at the carbonyl oxygen. The stereochemistry of addition of allylic silanes to carbonyl compounds is consistent with an acyclic TS. The -stereoisomer of 2-butenyl(trimethyl)silane gives nearly exclusively the product in... 

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. 

Ketones were also reacted under these conditions, leading to tertiary ethers. Thus, by mixing equimolar quantities of a carbonyl (aldehyde or ketone), allyl-silane and a silylated alcohol, followed by the addition of a catalytic amount of TMSOTf, homoallylic ethers can be obtain in good yields via a three-component coupling reaction (Scheme 13.22). 

Dipolar cycloaddition of alkenes with carbonyl ylides generated in situ is a versatile method for tetrahydrofuran synthesis. The synthetic potential of such transformations has been reviewed <2005JOM(690)5533, 2003BMI6-253>. In addition, the stereoselective [3 + 2] annulation of allyl silanes has become a reliable protocol for the synthesis of tetrahydrofurans as demonstrated in several total syntheses . Such a [3 + 2] annulation, for example, affords the tetrahydrofuran product 11 as a single stereoisomer (Scheme 15) <2002OL2945>. Lanthanide salts serve as efficient Lewis acid catalysts in similar [3 + 2] cycloaddition reactions . 

The Lewis acid-mediated addition of allyl silanes and allyl stannanes to carbonyl compounds has been a topic of interest for many years [103,104]. Stereochemical studies of reactions of allylsilanes and allylstannanes with aldehydes have included FeCl3-mediated intramolecular reactions (Sch. 20) [105],... 

Crotyl silanes offer the possibility of diastereoselectivity in reactions with aldehydes in the same way as the corresponding boranes. The mechanism is completely different because crotyl trialkylsilanes react via an open transition state as the silicon is not Lewis acidic enough to bind the carbonyl oxygen of the electrophile. Instead, the aldehyde has to be activated by an additional Lewis acid or by conversion into a reactive oxonium ion by one of the methods described above. The stereoelectronic demands of the allylic silane system contribute to the success of this transformation. Addition takes place in an Se2 sense so that the electrophile is attached to the remote carbon on the opposite side of the n system to that originally occupied by silicon and the newly formed double bond is trans to minimize allylic strain. 

The (dienyl)iron cations of type (248) and (265) are susceptible to reaction with nucleophiles. For the (cyclohexadienyl)iron cations, nucleophilic attack always occurs at a terminal carbon, on the face of the ligand opposite to the metal, to afford -cyclohexadiene products. Typical nucleophiles used are malonate anions, amines, electron-rich aromatics, silyl ketene acetals, enamines, hydrides, and allyl silanes intramolecular nucleophilic addition is also possible. The addition of highly basic organometallic nucleophiles (Grignard reagents, organolithiums) is often problematic this may be overcome by replacing one of the iron carbonyl... 

C—C Bond Formation using Organosilanes. Under catalysis by TiCU, allylsilanes transfer the allyl group, with rearrangement, to ketones, a/3-unsaturated ketones (conjugate addition), and quinones (initially giving the carbonyl addition product which then further rearranges to the 2-allyl-l,4-hydroquinone). But-3-enyl-silanes react with acid chlorides to yield cyclopropylmethyl ketones [reaction (3)) under similar conditions. 

Titanium tetrachloride is a moisture-sensitive, highly flammable liquid reacting violently with water (34). It is a strong Lewis acid capable of promoting Diels-Alder reactions (35) and induces the addition of silyl enol ethers and allyl silanes to carbonyl compounds and derivatives (34r-36). It is a less commonly used catalyst in Friedel-Crafts reactions but very useful for the acylation of activated alkenes and in the Fries rearrangement. 

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