DIRECT SYNTHESIS AND REACTIVITY OF UNSUPPORTED (ETA-3-OXAALLYL)RHODIUM(I) COMPLEXES

Addition of 2 equiv of K+PhC(O)CH2- to [(Ph3P)2RhCl]2 (1) gave the mon omeric (eta3-oxaallyl)rhodium complex (Ph3P)2Rh(eta3-CH2C(O)Ph) (3). R eaction of 1 with K+t-BuC(O)CH2-produced a similar oxaallyl complex (6 ), which was characterized by spectroscopic methods and by X-ray cryst allography. Both 3 and 6 showed dynamic NMR spectra which equilibrated the methylene protons at 25-degrees-C. A general methodology for the preparation of (eta3-oxaallyl)rhodium complexes was developed by start ing from [(COD)RhCl]2(7) and 4 equiv of phosphine. Complexes (Et3P)2Rh (eta3-CH2C(O)Ph) (8) and (Et3P)2Rh((Z)-eta3-CH3CHC(O)-t-Bu) (10) were prepared by this methodology. No fluxional behavior was observed with either 8 or 10. Oxaallyl 8 reacted rapidly with CO and t-BuNC to produ ce the eta1-oxygen-bound rhodium enolates trans-(Et3P)2-(CO)Rh(OC(Ph)C H2) (12) and trans-(Et3P)2(t-BuNC)Rh(OC(Ph)CH2) (13). However, unlike complex 8, oxaallyl 6 added 2 equiv of t-BuNC, giving a trigonal-bipyr amidal carbon-bound rhodium enolate complex (16). Notable differences in reactivity between rhodium oxaallyl and rhodium allyl complexes are explained in terms of enhanced stability of the eta1-oxygen-bound rho dium complex relative to the eta1-allyl complex.