FISCHER-TYPE RHENACYCLOBUTADIENE COMPLEXES - SYNTHESIS, STRUCTURE, AND NUCLEOPHILIC-ADDITION SUBSTITUTION AND OXIDATION REACTIONS

Reactions of Na[Re(CO)5] with the activated acetylenes RC=CCO2Me (R = CO2Me (a), Me (b), H (c)) afford the rhenacyclobutenone complexes Na[(CO)4ReC(R)C(CO2Me)C(O)] (1), which undergo alkylation with Et3OPF6 to the novel Fischer-type rhenacyclobutadiene complexes (CO)4ReC(R)C(CO2Me)C(OEt) (2). A single-crystal X-ray diffraction analysis of 2b revealed a planar unsymmetrical ring structure (C=C bond distances 1.45(2) and 1.36(2) angstrom) and Re-C(ring) bond lengths (2.13(1) and 2.18(1) angstrom) Intermediate between those of Re-C and Re=C, as for the simple Fischer metal carbene complexes. The Fischer-type carbenoid character of 2 is further evidenced by the downfield position of the C-13 NMR signals of both Re=C(ring) atoms of 2. 2a reacts with PR3 (R = Et or p-Tol) at ambient temperatures to give phosphine-addition products (CO)4ReC(CO2Me)(PR3)C(CO2Me)C-(OEt) (3a and 4a, respectively). The reaction of 2b and PEt3 affords the analogous addition product 3b; however, that of 2b and P(p-Tol)3 yields the CO-substitution derivative fac-(CO)3(P(p-Tol)3)ReC(Me)C(CO2Me)C(OEt) (5b). Upon heating in hexane at reflux, 3b undergoes conversion to the CO-substitution product fac-(CO)3-(PEt3)-ReC(Me)C(CO2Me)C(OEt) (6b), which reacts with additional PEt3 to afford fac-(CO)3-(PEt3)-ReC(Me)(PEt3)-C(CO2Me)C(OEt) (7b). Oxidation of 2 with (NH4)2[Ce(NO3)6] results in the insertion of an oxygen atom into the Re=C(ring) bonds to yield the five-membered oxametallacyclic products (CO)4ReOC(R)C(CO2-Me)C(OEt) (8) and, for 2a,b, also (CO)4ReC(R)C(CO2Me)C(OEt)O (9). The foregoing nucleophilic addition/substitution and oxygen atom insertion reactions are analogous to those of simple Fischer metal carbene complexes. The structures of (1a)2.THF, 4a, and 8b were also elucidated by X-ray diffraction techniques.