Alkoxysilanes aldehydes

Vinylsilane to copper transmetallation has entered the literature,93 93a,93b and a system suitable for catalytic asymmetric addition of vinylsilanes to aldehydes was developed (Scheme 24).94 A copper(l) fluoride or alkoxide is necessary to initiate transmetallation, and the work employs a copper(ll) fluoride salt as a pre-catalyst, presumably reduced in situ by excess phosphine ligand. The use of a bis-phosphine was found crucial for reactivity of the vinylcopper species, which ordinarily would not be regarded as good nucleophiles for addition to aldehydes. The highly tailored 5,5 -bis(di(3,5-di-tert-butyl-4-methoxyphenyl)phosphino-4,4 -bis(benzodioxolyl) (DTBM-SEGPHOS) (see Scheme 24) was found to provide the best results, and the use of alkoxysilanes is required. Functional group tolerance has not been adequately addressed, but the method does appear encouraging as a way to activate vinylsilanes for use as nucleophiles. 

When phenyl(trimethylsilyl)diazomethane (20) is pyrolyzed in the gas phase, typical reactions of carbene 21 can be observed (see Section III.E.4). However, copyrolysis with alcohols or carbonyl compounds generates again products which are derived from silene 2239,40 (equation 6). Thus, alkoxysilanes 23 are obtained in the presence of alcohols and alkenes 24 in the presence of an aldehyde or a ketone. 2,3-Dimethylbuta-l,3-diene traps both the carbene (see Section ni.E.4) and the silene. 

Even if the SMS reaction typically involves allylsilanes, carbonyls and alcohols (or silyl ethers), some transformations can be considered as belonging to the same family. For example, in 2001, Yokozawa et al. described [43] a three-component reaction between aldehydes 6, alkoxysilanes 38 and propargylsilane 88 (instead of allylsilane). Tritylperchlorate was used as the catalyst and a-allenyl ethers 89 were... 

Yokozawa [44] also inverted the methodology and generated a-propargyl ethers 91 from carbonyls 6, alkoxysilanes 38 and allenylsilanes 90. Aromatic aldehydes remained the best substrates but aliphatic aldehydes or ketones could be induced to react, though the yields remained modest (Scheme 13.36). 

Over the past ten years, absolute rate data have been reported on the kinetics of several bimolecular silene reactions in solution, including both head-to-tail and head-to-head dimerization the [l,2]-addition reactions of nucleophilic reagents such as water, aliphatic alcohols, alkoxysilanes, carboxylic acids and amines and the ene-addition, [2 + 2]-cycloaddition and/or [4 + 2]-cycloaddition of ketones, aldehydes, esters, alkenes, dienes and oxygen. The normal outcomes of these reactions are summarized in Scheme 1. 

High-yield anodic oxidation of a-alkoxysilanes (XCV) to )5-hydroxy aldehyde acetals was used as the key step in an iterative preparation of optically active polyols [Eq. (59)]. The reaction apparently involves formation of the ether radical cation, followed by rapid C-Si bond scission and nucleophilic trapping by solvent [142]. 

Rhodium-phosphine complexes are usually active and effective in the asymmetric hydrosilylation of olefins, ketones, and aldehydes, allowing for the virtual synthesis of optically active alkoxysilanes and organic compounds of high purity. Chiral rhodium-phosphine catalysts predominate in the hydrosilylation of pro-chiral ketones. This subject has been comprehensively reviewed by several authors who have made major contributions to this field [52-54]. A mechanism for the hydrosilylation of carbonyl groups involving the introduction of asymmetry is shown in Scheme 3 [55]. 

When prochiral silane and ketone are used, hydrosilylation, in the presence of a chiral catalyst, results in asymmetric induction at both the silicon and carbon centers. Treatment of the diastereomeric alkoxysilane by a Grignard reagent leads to recovery of an organosilane and an alcohol of different optical purity. Results obtained in the asymmetric hydrosilylation of ketones and aldehydes by prochiral silanes in the presence of an asymmetric catalyst are summarized in Tables 3 and 4. 

Diene (14) reacted with a series of aldehydes under BFs-OEtz catalysis in CH2CI2 to give predominantly trans products (Table lO). " Aldol-type products, such as p-hydroxy enones, are isolated (along with dihydropyrones) from the reaction mixtures. Using TFA as a catalyst, the p-hydroxy enones are, as previously described, converted into dihydropyrones. The stereoselectivity of these reactions is consistent with a Mukaiyama-aldol reaction rather than a Diels-Aider cycloaddition. The stereochemistry of the P-hydroxy enones is also consistent with the observation that the (Z)-alkoxysilane reacts with the aldehyde in an extended transition state to give anti (threo) aldol products (Scheme 16). In the cases using ZnCh or lanthanide ions as catalysts aldol products have not been detected. 

Petasis and Teets reported the study shown in Scheme 7-30 [158]. The silylallene ester 130 was metalated with LDA to give an alkynyllithium enolate intermediate which was reacted with the propaigyl aldehyde 129, and the resulting alkoxysilane eliminated under acidic condi-... 

C-to-O silyl-group transfer without alkene formation is relatively uncommon. However, Oshima, Utimoto, and co-workers reported that metalation of dichloromethylsilane 96 and addition to aldehydes provided fi-alkoxysilanes (e.g., 97) that imderwent 1,3-rearrangement and alkylation to 98. Notably, the addition of HMPA promoted the 1,3-Brook rearrangement, presumably by increasing the reactivity of the lithium alkoxide intermediate. As the authors pointed out, this sequence demonstrated 96 as a methylene chloride dianion equivalent. ... 

Piers examined the equilibria between B and B-carbonyl complexes in a study of C=0 hydrosilylation (Fig. 3c, X = O, Y = H, Me, OEt). Equilibrium constants were found to favor the carbonyl complex 3 by about 10 depending on the nature of Y [19]. However, the rates of the hydrosilylation reaction were inversely proportional to the carbonyl concentration, suggesting that complexes such as 3 inhibit the reaction. These observations led to a mechanistic interpretation involving the reversible formation of a borohydride complex 2, which is the active species in the subsequent reductive silylation process via 4 or 5, shown for carbonyl reduction (Fig. 4). Analogous processes have been invoked for the conversion of aldehydes/ketones to alkanes [20], alcohols to alkoxysilanes [21], and then to alkanes [22], and the hydrosilylation of C=C bonds [23]. 

The hydrosilylation of unsaturated carbon-rhodium-catalyzed silylcarbocyclizations. In the presence of Rli4(CO)i2 and triethoxysilane, a rigid triyne backbone can undergo a silylcarbotricyclization cascade reaction to yield [5,6,5]-tricycles (eq 16). Similar to the results observed by Sieburth for the hydrosilylation of enamines, the alkoxysilane functionality provides significant rate enhancement in comparison to silylcarbocyclizations using alkyl- and arylsilane reagents. The incorporation of carbonyl functionality as terminal electrophiles into these cyclizations has also been successful. Rhodium-catalyzed carbonylative silylcarbocyclizations proceed in the presence of carbon monoxide (10 atm) to incorporate a carbonyl unit, usually as the aldehyde. Both of these tandem ad-dition/cyclization strategies produce functionalized carbocycles with simultaneous incorporation of sUyl functionality as aryl- and vinylsilanes. These alkenylsilanes can then be exploited for further synthetic manipulations as discussed above. "" ... 

Aldehydes and ketones are readily acetalized by alkoxysilanes under aprotic conditions in the presence of trimethylsilyl trifluoromethanesulphonate as catalyst,and anhydrous aluminium chloride has been found to be an efficient reagent for promoting thioacetalization of carbonyl compounds. Monothiols reacted with highly enolizable carbonyl compounds to give mainly the vinyl sulphide under these conditions. 1,3-Dicarbonyl compounds react with propane-... 

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