HYDROGEN-TRANSFER REACTIONS OF THE TRIHYDRIDE [(MEC(CH2PPH2)3)RHH3] - A SIMPLE ROUTE TO RHODIUM CIS HYDRIDE ETA-2-ALKENE COMPLEXES

The number, the disposition, and the nature of the substituents play an important role in determining the product composition of the reactions between the trihydride [(triphos)RhH3] (1) and mono- or 1,2- disubstituted alkenes (triphos = MeC(CH2PPh2)3). In particular, alkenes bearing electron-withdrawing substituents such as CO2R or Ph stabilize cis hydride η2-alkene complexes of the formula [(triphos)Rh- (H)(η2-alkene)], whereas electron-rich alkenes do not form 2 adducts but are hydrogenated to alkanes by 1. Alkynes are more easily hydrogenated by 1 than comparable alkenes. The crystal structure of [(triphos)Rh(H)(η2-DMFU)·C4H9OH has been determined by X-ray methods (DMFU = dimethyl fumarate). The coordination geometry is a distorted octahedron with rhodium bound to the three phosphorus atoms of triphos, to a hydride ligand, and to the two carbon atoms of a DMFU moiety. The C=C bond distance in the alkene ligand is significantly lengthened by interaction with the metal at a distance of 1.45 (2) Å, thus indicating a remarkable π-back-bonding contribution. Crystallographic details: space group P212121(orthorhombic); a = 14.277 (2), b = 17.065 (2), c = 19.429 (3) Å; V = 4734 Å3; Z = 4; R = 0.051, Rw= 0.063 for 3847 observed reflections. The reactions of dimethyl maleate, cis-stilbene, and cis-4-methyl-2-pentene with a catalytic amount of 1 in tetrahydrofuran have been studied in detail. The former two alkenes are catalytically isomerized to their trans isomers, whereas both double-bond and cis → trans isomerizations occur for cis-4-methyl-2-pentene. © 1990, American Chemical Society. All rights reserved.