Osmium porphyrin silylene and carbene complexes: synthesis, characterization, and catalytic reactions

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Smith, Daniel
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L. Keith Woo
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The Department of Chemistry seeks to provide students with a foundation in the fundamentals and application of chemical theories and processes of the lab. Thus prepared they me pursue careers as teachers, industry supervisors, or research chemists in a variety of domains (governmental, academic, etc).

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The preparation and characterization of carbene complexes and the first donor-stabilized silylene complexes of osmium meso-tetra-p-tolylprophyrin, Os(TTP), are described. The silylene complex (TTP)Os = SiMe[subscript]2·THF (1·THF) is prepared by the reaction of (Os(TTP)) [subscript]2 with hexamethylsilacyclopropane. Treating K[subscript]2 (Os(TTP)) with Cl[subscript]2SiR[subscript]2 also generates the silylene complexes (TTP)Os = SiR[subscript]2·THF (R = Me (1·THF), Et (2·THF), or [superscript] iPr (3·THF)). [superscript]1H NMR spectroscopy indicates that, in solution, one molecule of THF is coordinated to the silicon in these complexes. This has been verified by 2D-NOESY experiments. Addition of one equivalent of pyridine to 1·THF or 2·THF replaces the THF on silicon with pyridine, as verified by a 2D-NOESY experiment. The structure of the diethyl complex 2·THF was determined by a single-crystal X-ray diffraction experiment. This complex crystallizes with two additional THF molecules as solvates in the monoclinic space group P2[subscript]1/c with a = 21.649(5) A, b = 13.829(3) A, c = 19.526(3) A, [beta] = 98.08(2)[superscript]·, V = 5788(4) A[superscript]3, Z = 4, R = 5.0%, and R[subscript] w = 5.7%. The Os = Si distance (2.325(8) A) is the shortest observed to date;The carbene complexes, (TTP)Os = CRR[superscript]' (R = R[superscript]' = p- C[subscript]6H[subscript]4- CH[subscript]3 (5); R = H, R[superscript]' = TMS (6), Co[subscript]2ET (7), 2,4,6-( CH[subscript]3)[subscript]3C[subscript]6H[subscript]2 (8), p- C[subscript]6H[subscript]4- CH[subscript]3 (9); or R = CH[subscript]3, R[superscript]' = C[subscript]6H[subscript]5 (10)), are prepared by treating (Os(TTP)) [subscript]2 with N[subscript]2CRR[superscript]'. Complex 5 crystallizes in the space group C2/c with a = 28.003(6) A, b = 14.385(3) A, c = 33.748(8) A, [beta] = 114.51(2)[superscript]·, V = 12370(5) A[superscript]3, Z = 8, R = 4.4%, and R[subscript] w = 5.4%. The Os = C distance is 1.847(5) A. (TTP)Os(CO)(Py), (TTP)Os(Py)[subscript]2 and the carbene complexes 5-7, were found to catalyze the highly stereoselective formation of alkenes from N[subscript]2CHCO[subscript]2Et, z/e = 26, in high yield. (TTP)Os(CO)(Py) is also a stereoselective cyclopropanation catalyst for the reaction of ethyl diazoacetate with styrene, producing disubstituted cyclopropanes with a/s = 10(1). The formation of bicyclobutanes from acetylenes, and the generality of the cyclopropanation reaction to other diazoreagents and substrate alkenes is also discussed. Evidence is presented for the involvement of a trans-biscarbene in the catalytic cycle.

Wed Jan 01 00:00:00 UTC 1992