Alkenes reduction with silanes

When double bonds are reduced by lithium in ammonia or amines, the mechanism is similar to that of the Birch reduction (15-14). ° The reduction with trifluoro-acetic acid and EtsSiH has an ionic mechanism, with H coming in from the acid and H from the silane. In accord with this mechanism, the reaction can be applied only to those alkenes that when protonated can form a tertiary carbocation or one stabilized in some other way (e.g., by a OR substitution). It has been shown, by the detection of CIDNP, that reduction of a-methylstyrene by hydridopenta-carbonylmanganese(I) HMn(CO)5 involves free-radical addition. ... 

Isomerization does not take place on the products. No isomerization of alkenes occurs with the action of chloroplatinic acid alone, but recovered alkenes from a silane-alkene mixture often contain isomers of the alkene. D-H exchange is observed for the recovered hydrosilane from the hydrosilation mixture with DSiCb. These facts suggest the mechanism outlined in Scheme 2, which is known as the Chalk-Harrod mechanism. " The mechanism involves oxidative addition of hydrosilane to the active species of the catalyst, coordination of alkene, followed by <7 — tt conversion, and reductive elimination of the product. 

C-Glycosidation of enol silane 279 to lactol acetate 278, prepared from 277 in two steps, furiushed ynone 280 as a single isomer. Reduction of the ketone with L-selectride furnished alcohol 270 with poor selectivity, but the minor isomer can be converted into the desired isomer via the Mitsunobu protocol Dihydroxylation of the terminal alkene, reduction of alkyne, and oxidative cleavage of the resulting triol gave the intermediate hydroxy aldehyde, which was spontaneously transformed into macrolactol 281 as a single diastereomer. 

Terminal alkenes are prepared by the Peterson elimination of yS-hydroxyalkyl-silanes having no further substituents on the carbon atom bearing the hydroxy group. The 8-hydroxyalkylsilane 188 is prepared from a yS-silyl carboxylic acid 187 by reduction with LAH, and further mesylation initiates the Peterson elimination affording the terminal alkene 189 (Scheme 2.118) [322]. 

Titanocene derivatives catalyze reductive cyclization of an alkene with a hetero-atom-containing functional group and the cleavage of the titanium-oxygen bond in these metallacycles was promoted by reaction with silanes, with concomitant formation of Ti-H and Si-O bonds via a <7-bond metathesis process (Scheme 12.45) [65],... 

In an effort to identify a more stereoselective route to dihydroagarofuran (15), trimethylsilylated alkyne 17 was utilized as a substrate for radical cyclization (Scheme 2). Treatment of 17 with a catalytic amount of AIBN and tri-n-butyltin hydride (1.25 equiv) furnishes a mixture of stereoisomeric vinyl silanes 18 (72% combined yield) along with an uncyclized reduction product (13% yield). The production of stereoisomeric vinyl silanes in this cyclization is inconsequential because both are converted to the same alkene 19 upon protodesiiyiation. Finally, a diastereoselective di-imide reduction of the double bond in 19 furnishes dihydroagaro-... 

Enones and enoates undergo 1,2-reduction [115, 191]. Lipshutz et al. reported the effective protection of carbonyl functions by the triisopropylsilyl acyl silane group (TIPS), which allowed the selective conversion of alkenes or alkynes to the corresponding zirconocene complexes [24]. The aldehyde could subsequently be regenerated by desilylation with TBAF [186]. 

Abstract Significant advances have been made in the study of catalytic reductive coupling of alkenes and alkynes over the past 10 years. This work will discuss the progress made in early transition metal and lanthanide series catalytic processes using alkyl metals or silanes as the stoichiometric reductants and the progress made in the use of late transition metals for the same reactions using silanes, stannanes and borohydrides as the reductant. The mechanisms for the reactions are discussed along with stereoselective variants of the reactions. 

The acid or base elimination of a diastereoisomerically pure p-hydroxysilane, 1, (the Peterson olefination reaction4) provides one of the very best methods for the stereoselective formation of alkenes. Either the E- or Z-isomer may be prepared with excellent geometric selectivity from a single precursor (Scheme 1). The widespread use of the Peterson olefination reaction in synthesis has been limited, however, by the fact that there are few experimentally simple methods available for the formation of diastereoisomerically pure p-hydroxysilanes.56 One reliable route is the Cram controlled addition of nucleophiles to a-silyl ketones,6 but such an approach is complicated by difficulties in the preparation of (a-silylalkyl)lithium species or the corresponding Grignard reagents. These difficulties have been resolved by the development of a simple method for the preparation and reductive acylation of (a-chloroalkyl)silanes.7... 

Alkenes.1 Esters can be converted into 1-alkenes by a-silylation with this silane followed by a reductive Peterson elimination. 

The low regioselectivity of the enc reaction of singlet oxygen with mono-alkenes can also be improved by incorporation of trialkylsilyl groups at a vinylic position. Thus, simple vinyl-silanes afforded, after photooxygenation and reduction, regioselectively, /1-silyl allylic alcohols35. The diastereoseleclivity of the titanium-catalyzed reaction of the /1-silyl allylic hydroperoxides is described in Section 4.9.4.2. 

Bromine azide, prepared by different methods, undergoes addition to alkenes stereoselectively by an ionic mechanism 33- 37 (Table 2). Improved yields are obtained by using azidotrimethyl-silane and A-bromosuccinimide (NBS) in the presence of Nafion-FI as catalyst33. Alternatively, A-bromosuccinimide may be added to the mixture of the alkene and sodium azide in dimeth-oxyethane/water34. Aziridines were prepared stereoselectively by reduction of (i-bromo azides with lithium aluminum hydride34. 

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