Alkenes reductive silylation

Reductive silylation of a-diketones or quinones.1 a-Diketones are conveniently converted into l,2-bis(silyloxy)alkenes by reaction with ClSi(CH3)3 and zinc. Yields are increased by use of THF rather than ether as solvent and by ultrasonic irradiation. 

Reductive coupling of carbonyls to alkenes Titanium(IV) chloride-Zinc, 310 of carbonyls to pinacols Titanium(III) chloride, 302 Titanium(IV) chloride-Zinc, 310 of other substrates Samarium(II) iodide, 270 Reductive cyclization 2-(Phenylseleno)acrylonitrile, 244 Tributylgermane, 313 Tributyltin hydride, 316 Triphenyltin hydride, 335 Trityl perchlorate, 339 Reductive hydrolysis (see Hydrolysis) Reductive silylation Chlorotrimethylsilane-Zinc, 82... 

Alkenes can be converted via reductive silylation to allylsilanes by the system chloroalkylsilane/Mg/HMPA in the presence of FeCl3 and/or TiCl4 as catalysts, involving an allylic rearrangement by-products are some hydrogenated compounds... 

Alkenes. Alk-3-en-l-ynes are obtain in the presence of SnC and (PhiPl.PdCl.-carboxylic acids are degraded by one cartx Reductive silylation. a-Diketoncs i while methyl benzoylformate undergoei 2,3-diphenyl-2,3-bis(( -trimethylsilyl)tartn (Me,P)2PdCl2]. 

An Q-arylalkanoate is prepared by the reaction of aryl halide or triflate with the ketene silyl acetal 74 as an alkene component. However, the reaction is explained by transmetallation of Ph - Pd—Br with 74 to generate the Pd eno-late 75, which gives the a-arylalkanoate by reductive elimination[76]. 

The carbon-centered radical R, resulting from the initial atom (or group) removal by a silyl radical or by addition of a silyl radical to an unsaturated bond, can be designed to undergo a number of consecutive reactions prior to H-atom transfer. The key step in these consecutive reactions generally involves the intra-or inter-molecular addition of R to a multiple-bonded carbon acceptor. As an example, the propagation steps for the reductive alkylation of alkenes by (TMSfsSiH are shown in Scheme 6. 

High levels of diastereocontrol in an ISOC reaction were induced by a stereogenic carbon center that bears a Si substituent (Scheme 23) [55]. For instance, conversion of nitro alkenes (e.g., 199) to j3-siloxyketones (e.g., 203) has been accomplished via a key ISOC reaction-reduction sequence with complete control of 1,5-relative stereochemistry. The generality of the ISOC reaction of a silyl nitronate with a vinylsilane was demonstrated with seven other examples. Corresponding INOC reaction proceeded with lower stereoselectivity. 

Silyl(pinacol)borane (88) also adds to terminal alkenes in the presence of a coordinate unsaturated platinum complex (Scheme 1-31) [132]. The reaction selectively provides 1,2-adducts (97) for vinylarenes, but aliphatic alkenes are accompanied by some 1,1-adducts (98). The formation of two products can be rationalized by the mechanism proceeding through the insertion of alkene into the B-Pt bond giving 99 or 100. The reductive elimination of 97 occurs very smoothly, but a fast P-hydride elimination from the secondary alkyl-platinum species (100) leads to isomerization to the terminal carbon. 

The reductive coupling/silylation reaction was extended to more complicated polyenes, such as the triene-substituted cyclopentanol 73, which cyclizes to provide 74 with a 72% yield and 6 1 dr after oxidation (Eq. 10) [44], The reaction is chemoselective the initial insertion occurs into the allyl substituent, which then inserts into the less hindered of the two remaining olefins, leaving the most hindered alkene unreacted. 

The mechanism for the reaction catalyzed by cationic palladium complexes (Scheme 24) differs from that proposed for early transition metal complexes, as well as from that suggested for the reaction shown in Eq. 17. For this catalyst system, the alkene substrate inserts into a Pd - Si bond a rather than a Pd-H bond [63]. Hydrosilylation of methylpalladium complex 100 then provides methane and palladium silyl species 112 (Scheme 24). Complex 112 coordinates to and inserts into the least substituted olefin regioselectively and irreversibly to provide 113 after coordination of the second alkene. Insertion into the second alkene through a boat-like transition state leads to trans cyclopentane 114, and o-bond metathesis (or oxidative addition/reductive elimination) leads to the observed trans stereochemistry of product 101a with regeneration of 112 [69]. 

Perlmutter used an oxymercuration/demercuration of a y-hydroxy alkene as the key transformation in an enantioselective synthesis of the C(8 ) epimeric smaller fragment of lb (and many more pamamycin homologs cf. Fig. 1) [36]. Preparation of substrate 164 for the crucial cyclization event commenced with silylation and reduction of hydroxy ester 158 (85-89% ee) [37] to give aldehyde 159, which was converted to alkenal 162 by (Z)-selective olefination with ylide 160 (dr=89 l 1) and another diisobutylaluminum hydride reduction (Scheme 22). An Oppolzer aldol reaction with boron enolate 163 then provided 164 as the major product. Upon successive treatment of 164 with mercury(II) acetate and sodium chloride, organomercurial compound 165 and a second minor diastereomer (dr=6 l) were formed, which could be easily separated. Reductive demercuration, hydrolytic cleavage of the chiral auxiliary, methyl ester formation, and desilylation eventually led to 166, the C(8 ) epimer of the... 

Despite the presence of two, relatively weak, N—O bonds ( 53 kcal/mol) (255) nitroso acetals are stable under neutral conditions. Therefore, a number of transformations can be performed on the periphery of the cycloadducts without effecting the nitroso acetal. These include both oxidation of alcohols, dihydroxyla-tion of alkenes (247,248,256), Tamao-Heming type oxidation (248-250), reductions of ketones and esters (244,257-261), silylation and desilylation (247,257, 260), and activation with a variety of sulfonylating agents (247,248,257-260). 

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