Trans-addition reduction with metals

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]. 

The method has been apphed to the reductive cross-coupling of ethyl -arylacrylates with aldehydes in the presence ofTMSCl [160]. Addition of Mg metal to a solution of ethyl -arylacrylate, aldehyde, and TMSCI in DMF provided the corresponding y-lac-tones as a cis/trans stereoisomeric mixture in good to excellent yields (Scheme 3.142). 

There is great interest in understanding the preference of different transition metals for cis or trans isomers or the way in which these complexes undergo geometrical isomerization. Indeed, the course of many reactions of these species, such as nucleophilic substitution, electron transfer, oxidative addition, reductive elimination, thermal decomposition, interaction with molecules of biological interest, and so on, is dictated by the geometry of these compounds. 

The hydrosilylation of terminal alkynes disclosed by Trost can be applied to internal alkynes as well. i Remarkably, the (Z)-isomer is generated in this process, resulting from trans addition during hydrosilylation. The protodesilylation of these sily-lated products in the presence of copper(I) iodide and tetrabuty-lammonium fluoride (TBAF) or silver(I) fluoride (eq 15) leads to internal fraws-olefins. This two-step method is a useful synthetic transformation to access ( j-alkenes from internal alkynes. In contrast, the chemoselective reduction of alkynes to the corresponding ( -alkenes is conventionally accomplished readily with Lindlar s catalyst. The complementary process to afford ( )-olefins has proven much more difficult. Methods involving metal hydrides, dissolving metal reductions, low-valent chromium salts provide the desired chemical conversion, albeit with certain limitations. For example, functional substitution at the propargylic position (alcohols, amines, and carbonyl units) is often necessary to achieve selectivity in these transformations. Conversely, the hydrosilylation/protodesilyla-tion protocol is a mild method for the reduction of alkynes to ( )-alkenes. 

Alkenes are reduced by addition of H2 in the presence of a catalyst such as platinum or palladium to yield alkanes, a process called catalytic hydrogenation. Alkenes are also oxidized by reaction with a peroxyacid to give epoxides, which can be converted into trans-l,2-diols by acid-catalyzed hydrolysis. The corresponding cis-l,2-diols can be made directly from alkenes by hydroxylation with OSO4. Alkenes can also be cleaved to produce carbonyl compounds by reaction with ozone, followed by reduction with zinc metal. In addition, alkenes react with divalent substances called carbenes, R2C , to give cyclopropanes. Nonhalo-genated cyclopropanes are best prepared by treatment of the alkene with CH2I2 and zinc-copper, a process called the Simmons-Smith reaction. 

Very similar results were obtained from the CV studies of ( )-38 and ( )-39, but the observed anodic shifts of the first redox couples upon complexation with K+ were smaller (50 mV for ( )-38 and 40 mV for ( )-39). The reduction of the anodic shift from 90 mV (in ( )-37) to 40 mV (in ( )-38) can be explained by an increasing average distance between the cation bound to the crown ether and the fullerene surface, as the addition pattern changes from trans-1, to trans-2, and to trans-3 [55], Additionally, the effects of different alkali- and alkaline-earth-metal ion salts on the redox properties of ( )-37 were investigated. As expected, all electrochemical data clearly demonstrate a much larger interaction between crown-ether-bound cations with the negatively charged than with the neutral fullerene core [55],... 

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