Apparent trans Addition

Apparent trans addition of hydrogen to a carbon-carbon double bond has been the subject of intense investigations and several mechanism have been proposed the stereochemistry of hydrogenation of alkenes has been comprehensively reviewed by Siegel in 1966,66 by Molnar in 1983,67 and again by Bartok and Molnar in 1997,68 so only a brief history is related here. 

This last mechanism (Fig. 2.9) consists of known common reactions and appears to be the most widely accepted 84 however, it appears to lack full 

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Nevertheless, during hydrogenation of the dimethylcyclohexenes, the fact that the quantities of trans isomer increase with the double bond migration ability of the various catalysts90 suggests that double bond migration sites (or the sites nearby) are involved in apparent trans addition. 

Since edges (and presumably ledges) are now associated with double bond migration,98 and since apparent trans addition is a function of the double bond migration ability of various catalysts, perhaps such locations can produce both processes. The fact that tetrasubstituted alkenes hydrogenate much more slowly than tri-, di-, or monosubsituted alkenes would allow greater... 

Although it seems clear that much of the apparent trans addition comes about through double bond migration, double bond migration seems unlikely... 

The stereospecific cis-addition of diboron tetrachloride to alkynes and alkenes (37) may be interpreted as an interaction of the empty 7r-orbitals of the boron atoms with the 7r-orbital of the organic species. According to this picture, boron-boron bond breaking would lag behind boron-carbon bond formation. The transition state is a 4 + 2 Hiickel aromatic ( .=0), and thermal addition is allowed. If bond making and breaking were synchronous, this four-center reaction would be more like the <7-77 exchange reactions, which we shall discuss later. With regard to (37), there is a discrepant case in which an apparent trans addition of diboron tetrachloride to cyclopentadiene has been found (Saha et al., 1967). 

Accrombessi et al. studied the hydrogenolysis of various epoxycyclohexanes over 10% Pd-C in different solvents at 20° and 1 atm H2.25 The hydrogenolysis of both trans- and cA-l,2-epoxy-4-f-butylcyclohexanes (13 and 14) occurs to give preferentially the axial alcohols by apparent trans addition of hydrogen (eq. 13.8), as evidenced by deuterolysis. 

A mechanistic rationale for the apparent trans -addition in some of these processes is shown in Scheme 6. Initial insertion of the alkyne into the metal-silyl bond yields an intermediate with an unfavorable interaction between the metal and the silyl group. Coordination of the vinyl part to the metal gives formally a metaUacyclopropene. 

It has been shown that the stereochemistry of the hydrosilylation of 1-aUcynes giving 1-silyl-l-alkenes depends on the catalysts or promoters used. For example, the reactions under radical conditions give the cis-product predominantly via trans-addition , while the platinum-catalyzed reactions afford the trans-product via exclusive cts-addition. In the reactions catalyzed by rhodium complexes, thermodynamically unfavorable c/s-1-silyl-l-alkenes are formed via apparent trans-addition as the major or almost exclusive product. Since the trans-addition of HSiEts to 1-alkynes catalyz by RhCl(PPh3)3 was first reported in 1974 , there have been controversy and dispute on the mechanism of this mysterious trans-addition that is vray rare in transition-metal-catalyzed addition reactions to aUtynes. Recently, iridium 4i6 mthenium complexes were also found to give the ds-product with extremely high selectivity (vide supra). 

It should be mentioned that nmr shows the final result of monomer enchainment and does not necessarily provide information about the stereochemistry of the transition state for enchainment [34,37]. For example, the net result of 1.2-cis addition, followed by inversion of the configuration of the 2-carbon atom is apparent trans-addition. 

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