(eta6-Acetophenone)Cr(CO)3 Enolates Complexed to Bis(cyclopentadienyl) titanium(IV) and -zirconium(IV)

This is a synthetic and structural report on substituted acetophenone-Cr(CO)(3) enolate complexes of titanium(IV) and zirconium(IV). The deprotonation of[(eta(6)-2,4,6-R'R''R'''C6H2-COMe)Cr(CO)(3)] [R' = R'' = R''' = H, 1; R' = Me, R'' = R''' = H, 2; R' = R'' = Me, R''' = H, 3, R' = R'' = R''' = Me, 4] with KH in the presence of 18-crown-6 in THF gave the ion-pair enolates [{eta(6)-2,4,6-R'R''R'''C6H2-C(CH2)O-K(18-crown-6)} Cr(CO)(3)] [R' = R'' = Me, R''' = H, 5; R' = R'' = R''' = Me, 6]. When the deprotonation of 1-4, carried out using LiNPr2i in THF, was followed by reaction with (cp)(2)MCl(2) [M = Ti, Zr], dimetallic enolates were isolated and characterized [eta(6)-2,4,6-R'R''R'''C6H2-C(CH2)O-M(Cl)(cp)(2)][R' = R'' = R''' = H, M = Ti, 7; R' Me, R'' = R''' = H, M = Zr, 8; R' = R'' = Me, R''' = H, M = Zr, 9;R' = R'' = R''' Me, M = Ti, 10; R' = R'' = R''' = Me, M = Zr, 11]. In all of these complexes, the enolato functionality is O-bonded to the oxophilic [(cp)(2)M(Cl)] fragment. Complexes 10 and 11 have been isolated as an 85:15 diastereoisomeric mixture with a significant diastereoselection for the most hindered form. The two diastereoisomers cannot be interconverted thermally because of the very high barrier to free rotation around the phenyl-C(enolate) bond, as from our calculations. An X-ray crystallographic analysis has been carried out on the most hindered diastereoisomer of 11 and compared with that of the starting complex 4, where the oxygen is directed to the opposite face of the arene ring. In order to clarify the mechanism of the diastereoisomerization, the protolysis of the potassium and lithium enolates, including that of the pyridine adduct 12[{eta(6)-2,4,6-Me(3)C(6)H(2)-C(CH2)OLiPy(2)}Cr(CO)(3)], has been carried out. The protolysis, which produces exclusively the starting diastereoisomer of 4 having the oxygen syn to the [Cr(CO)(3)] group, supports strongly the fact that there is no change in the conformation at the level of the enolato formation. Then the comparison between results from the reaction of the lithium enolate of 4 and 12 with Me(3)SiCl, MeBu(2)(t)SiCl, Me(3)SiOSO(2)-CF3, and (cp)(2)ZrCl2 shows that the diastereoselection depends mostly on steric,rather than on electronic; factors. Further support for the steric argument comes when some steric hindrance has been introduced in the starting ketone, i.e. using [(eta(6)-2,4,6-R'R''R'''C6H2COCH2-CH3)Cr(CO)(3)]. The corresponding lithium enolate reacted with (cp)(2)ZrCl2 gave a diastereoselection with a 67:33 ratio. The only plausible hypothesis for explaining the diastereoisomerization occurring at the stage of the reaction of the enolate with the electrophile is an intermolecular migration of the [Cr(CO)(3)] fragment from the more to the less hindered face of the arene ring. Crystallographic details: 4 is monoclinic, space group P2(1)/c, a 6.858(1) Angstrom, b = 13.977(2) Angstrom, c 14.557(2) Angstrom, alpha = gamma = 90 degrees, beta = 94.46(1)degrees, Z = 4, and R = 0.034; 11 is monoclinic, space group P2(1)/c, a = 11.238(1) Angstrom, b = 14.144(1) Angstrom, c = 14.808(1) Angstrom, alpha = gamma = 90 degrees, beta = 95.12(1)degrees, Z = 4, and R = 0.033.