CHROMIUM AND MOLYBDENUM CARBONYL-COMPLEXES OF C(7)PH(7)H AND C(7)PH(5)ME(2)H AND OF C(7)PH(7)H(CO), THE DIELS-ALDER ADDUCT OF TETRACYCLONE AND TRIPHENYLCYCLOPROPENE - VARIABLE-TEMPERATURE NMR AND X-RAY CRYSTALLOGRAPHIC STUDY

The Diels-Alder reaction of tetraphenylcyclopentadienone (tetracyclone) with triphenylcyclopropene yields the adduct 2 with an endo hydrogen at C(7); thermal elimination of CO produces heptaphenylcycloheptatriene (la). This initially formed isomer with a pseudoequatorial phenyl substituent at the sp(3) position is sterically hindered and undergoes a conformational flip, forming Ib, which allows the phenyl group to occupy the favored pseudoaxial site. However, the remaining phenyls are sterically encumbered and slowed rotation of these rings can be monitored by H-1 and C-13 NMR spectroscopy. The intermediacy of conformer la is shown by the formation of isomers of C-7(p-tolyl)(2)Ph(5)H, arising via a series of [1,5] hydrogen shifts that are possible only for 1a and not for 1b. Similarly, use of 2,5-dimethyl-3,4-diphenylcyclopentadienone yields C(7)Me(2)Ph(5)H, 3, in which [1,5]-hydrogen shifts occur. 1 crystallizes in the triclinic space group P $($) over bar$$ 1 with a = 9.8320(10) Angstrom, b = 10.0260(10) Angstrom, c = 19.166(2) Angstrom, alpha = 92.194(1)degrees, beta = 90.57(1)degrees, gamma = 108.94(1)degrees, and V = 1785.2(3) Angstrom(3) for Z = 2. 2 crystallizes in the monoclinic space group P2(1)/n with a 12.829(4) Angstrom, b = 16.456(2) Angstrom, c = 18.226(3) Angstrom, beta = 110.06(1)degrees, and V = 3614.3(13) Angstrom(3) for Z = 4. The reaction of Cr(CO)(6) with C(7)Ph(7)H leads to incorporation of a Cr(CO)3 group on a peripheral phenyl ring rather than on the seven-membered ring. However, the tricyclic ketone 2, which is the precursor to 1, reacts with Mo(CO)(6) to give the pi-allyl complex (eta(5)-C(5)Ph(4)OH)Mo(CO)(2)(eta(3)C(3)Ph(3)H(2)) (10), in which the triphenylcyclopropene ring has been opened. 10 crystallizes in the triclinic space group P $($) over bar$$ 1 with a = 13.265(3) Angstrom, b = 13.561(3) Angstrom, c = 13.635(4) Angstrom, or 71.84(2)degrees, beta = 65.04(2)degrees, gamma = 61.20(1)degrees, and V = 1930.3(12) Angstrom(3) for Z = 2.