Cobalt(ii) acyl intermediates in carbon-carbon bond formation and oxygenation

摘要

The organocobalt scorpionate compounds To(M)CoR (To(M) = tris(4,4-dimethyl-2-oxazolinyl)phenylborate; R = Bn, 1; CH2SiMe3, 2; Ph, 3; Et, 4; Bu-n, 5; Me, 6) react in carbonylation, oxidation, and carboxylation reactions via pathways that are distinctly influenced by the nature of the organometallic moiety. The compounds are prepared by reaction of To(M)CoCl with the corresponding organolithium or organopotassium reagents. Compounds 1-6 were characterized by 8-line hyperfine coupling to cobalt in EPR spectra and solution phase magnetic measurements ((eff) = 4-5(B)) as containing a high-spin cobalt(ii) center. The UV-Vis spectra revealed an intense diagnostic band at ca. 700 nm (epsilon 1000 M-1 cm(-1)) associated with the tetrahedral organocobalt(ii) center that was assigned to a d d transition on the basis of configuration interaction (CI) calculations. Complexes 1-6 react rapidly with CO to form equilibrating mixtures of the low spin organocobalt carbonyl To(M)Co(R)CO, acyl To(M)CoC(?O)R, and acyl carbonyl To(M)Co{C(?O)R}CO. The H-1 and B-11 NMR spectra contained only one set of signals for the CO-treated solutions, whereas the solution-phase IR spectra contained up to two (CO) and three (C(?O)R) signals with intensities varying depending on the R group (R = Bn, 7; CH2SiMe3, 8; Ph, 9; Et, 10; Bu-n, 11; Me, 12). Single crystal X-ray diffraction of To(M)Co{C(?O)Et}CO (10) supports its assignment as a square pyramidal cobalt(ii) acyl carbonyl complex. Upon evaporation of volatiles, solutions of 8-12 revert to the CO-free organocobalt starting materials 2-6, whereas attempts to isolate benzyl-derived 7 provide an unusual -alkoxyketone species, characterized by single crystal X-ray diffraction. Despite the differences observed in the carbonylation of 1-6 as a result of varying the R group, compounds 7-12 all react rapidly with O-2 through an oxygenation pathway to afford the corresponding carboxylate compounds To(M)CoO(2)CR (R = Bn, 13; CH2SiMe3, 14; Ph, 15; Et, 16;