Aldehydes reaction with silanes

The conversion of acid chlorides to aldehydes has been achieved by platinum metal-catalyzed reaction with silanes. The general reaction. 

Morken and coworkers [39b] developed the first asymmetric reductive aldol reaction with silanes as reductants in combination with a chiral rhodium catalyst. a,P-Unsaturated esters were reacted with several aldehydes to provide the corresponding aldol products 79 in good yields and enantio- and diastereoselectivities (Scheme 8.23). Both aliphatic and aromatic aldehydes could be converted into aldol products 79 under these conditions. Furthermore, the group reported an iridium-catalyzed asymmetric version that tolerated various protected hydroxyaldehydes [39aj. On the basis of this precedence, a highly enantio- and diastereoselective... 

The Houk group reported a theoretieal study of the Ni-catalyzed alkyne-aldehyde reductive coupling reactions with silane as reducting agent. They showed that the selectivity of the [5+2] cycloaddition is controlled by the anisotropic steric environment of the NHC ligands. 

In contrast, ( )- and (Z)-2-butcnyl(trifluoro)silanes undergo stereoselective fluoride-induced reactions with aldehydes to give anti- and. sy -products, respectively37. 

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. ... 

It is postulated that the mechanism of the silane-mediated reaction involves silane oxidative addition to nickel(O) followed by diene hydrometallation to afford the nickel -jr-allyl complex A-16. Insertion of the appendant aldehyde provides the nickel alkoxide B-12, which upon oxygen-silicon reductive elimination affords the silyl protected product 71c along with nickel(O). Silane oxidative addition to nickel(O) closes the catalytic cycle. In contrast, the Bu 2Al(acac)-mediated reaction is believed to involve a pathway initiated by oxidative coupling of the diene and... 

Homoallylic alcohols.1 In the presence of fluoride ion these silanes react with aldehydes to form homoallylic alcohols in high yield (equation I). Crotyltrifluo-rosilanes undergo a similar reaction with aldehydes, but in this case the stereose-... 

Allenylsilanes react with acetals to afford homopropargylic ethers (Table 9.37) [61]. These reactions are promoted by silyl and carbocation species. A variation of this conversion involves in situ formation of the acetal from an aldehyde and Me3SiOMe (Eq. 9.55). The success of this method indicates that conversion of the aldehyde to the acetal and its subsequent reaction with the silane must be faster than direct reaction of the aldehyde with the silane. 

An alternative approach to difluoroenoxy silanes developed by the Reims group was described in Sect. 2.3.1 (Eq. 16). Lewis acid-mediated reactions with aldehydes and with alkyl halides were also described, constituting a valuable complementary approach to the Ishihara chemistry. The third approach to the... 

Reaction with dimethylphenylsilane is catalyzed at room temperature under 250 psi of carbon monoxide. Other silanes tested, triethyl- and triphenylsi-lane, are not effective reagents in this system. A variety of aldehydes are good substrates for the reaction, including benzaldehyde, substituted benzaldehydes, and heterocyclic aldehydes. Aliphatic aldehydes also yield a-siloxy aldehyde products, but the reaction must be run at higher CO pressure (1000 psi) to avoid hydrosilylation. The reaction does not tolerate substrates bearing strong electron-withdrawing substituents, such as p-nitrobenzaldehyde. 

PEG-grafted polystyrene is also well suited for reactions with highly reactive orga-nometallic reagents, provided that the support has been dried. PEG-containing polymers are generally more difficult to dry than pure polystyrene. Cross-linked PEG is stable towards Lewis acids, and can be used for SnCl4-mediated allylations of aldehydes with allyl silanes [21],... 

Silyl enol ethers of acyl silanes have been used in Lewis acid-mediated Mukaiyama reactions with acetals. Treatment of the resulting /1-alkoxy acyl silanes with tetrabutylammonium hydroxide or tetrabutylammonium fluoride gave the corresponding a,/J-unsaturated aldehydes (Scheme 99)210. 

Bis(trimethylsilyl) sulphide reacts with acyl silanes in the presence of a cobalt chloride catalyst to afford the corresponding thiocarbonyl derivatives, thioacyl silanes (Scheme 100)214. The reaction is mild and proceeds in good yields. It is also applicable to aldehydes. Thioacyl silanes have been prepared from the corresponding haloacyl silanes by reaction with hydrogen sulphide (vide supra, Section IV.A.3)196. 

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. 

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