Cyclometalation reactions on rhodium(I). Evidence for a chelate effect and competing C-H and C-O oxidative additions

The ligand Ph2PC6H4(CH2N(Me)CO(Et)) reacts with (bis(diphenylphosphino)butane) (2,5-norbornadiene)rhodium tetrafluoroborate (2a) and (bis(diphenylphosphino)ethane)(2,5-norbornadiene) rhodium tetrafluoroborate (2b) at room temperature in the presence of hydrogen to displace the norbornadiene and give the chelate complexes 3a,b, in which the phosphorus and the oxygen atoms are coordinated to give an eight-membered ring. At elevated temperatures these complexes are converted into cyclometalated Rh(III) benzyl hydride complexes. Rate law, activation parameters, and reactivity trends of this latter transformation indicate that displacement of one of the phosphine functionalities of the bis-chelating phosphine takes place before the C-H activation. Complex 3b was characterized by X-ray crystallography. On the other hand, under ambient conditions the ligand (±)-Ph2PC6H4(CH(Me)O(CO)Et) undergoes activation of either the benzylic C-O or C-H bond, depending on the nature of the Rh precursor used. Thus, 2b gives overall elimination of propionic acid to result in a styrene complex, which was characterized by X-ray crystallography, whereas bis(2,5-norbornadiene)rhodium tetrafluoroborate gives a mixture of Rh(III) benzyl hydride stereoisomers. The difference in reactivity is discussed in terms of different mechanisms for the two processes