STEREOSELECTIVE ALPHA-ALKYLATION OF METALLACYCLIC ZIRCONOXYCARBENE COMPLEXES - A CASE OF ASYMMETRIC 1,5-INDUCTION

Coupling of W(CO)6, butadiene, and pinacolone or acetone at the Cp2Zr template yields the chiral nine-membered metallacyclic zirconoxycarbene complexes Cp2Z activated rOC[ = W(CO)5]CH2CH = CHCH2CR1R2O (3a) (R1 = CH3,R2 = C(CH3)3) and (3b) (R1 = R2 = CH3), respectively, exhibiting a trans C = C double bond in the ring. Complex 3b is deprotonated by the ylide Ph3P = CH2 at the alpha-position to the carbene carbon center to yield the chiral unconjugated metallacyclic carbene complex anion 5b. Ylide deprotonation of 3a gives the carbanion 5a which is stereoselectively alkylated at C6 to yield predominately the (2R*,6S*)(4,5,6-pS*) configurated carbene complexes Cp2Z activated rOC[ = W(CO)5]CR3R4CH = CHCH2CR1R2O (e.g., 6a, R3 = H,R4 = CH3, 70% de). Repetition of the deprotonation/alkylation reaction sequence stereoselectively yields doubly alpha-alkylated carbene complexes (e.g., 10, R3 = CH3, R4 = CD3, 86% de or 15, R3 = CH2CH = CH2, > 96% de). The stereo-and regiochemical assignments are based on X-ray crystal structure analyses of the representative complexes 6a and 9. Complex 6a crystallizes in the space group P1BAR with cell parameters a = 11.036 (2) angstrom, b = 12.998 (3) angstrom, c = 13.259 (3) angstrom, alpha = 97.59 (1)-degrees, beta = 103.88 (1)-degrees, gamma = 107.59 (1), Z = 2, R = 0.058, R(w) = 0.058. Complex 9 crystallizes in the space group Pna2(1) with cell parameters a = 15.779 (2) angstrom, b = 13.736 (3) angstrom, and c = 13.311 (3) angstrom, Z = 4, R = 0.050, R(w) = 0.027. Hydrolysis of the alpha-methylated zirconoxycarbene complex 6a in the presence of diazomethane gives the enol ether HOC(CH3)(CMe3)-CH2CH = CHCH(CH3)C(OCH3) = CH2 with conservation of the stereochemistry introduced at the metallacyclic starting material. Similarly, treatment of 6a with water/pyridine N-oxide produces (2R*,6S*)-trans-6-hydroxy-2,6,7-tetramethyl-3-octenoic acid (19).