REGIOCONTROLLED AND STEREOCONTROLLED DIMERIZATION OF TERT-BUTYLACETYLENE TO (Z)-1,4-DI-TERT-BUTYLBUTATRIENE BY RUTHENIUM CATALYSIS - REACTION-MECHANISM INVOLVING ALKYNYL VINYLIDENE COUPLING AND REARRANGEMENT OF THE METAL-BOUND C4 UNIT

Ruthenium complexes, Ru(cod)(cot) (cod = 1,5-cyclooctadiene, cot = cyclooctatriene) and RU(CO)(PPh3)3(H)2 (1), catalyze the dimerization of tert-butylacetylene to (Z)-1,4-di-tert-butylbutatriene ((Z)-dbb). In the case of the Ru(cod)(cot) catalyst, the addition of bulky tertiary phosphines is an essential factor to effect good selectivity to dbb, the byproducts being isomers of 1,3-enynes. From the reaction of 1 with tert-butylacetylene, Ru(CO)(PPh3)3(H)(C = C(t)Bu) was isolated, which reacted further with HC = C(t)Bu to give Ru(CO)(PPh3)3(C = C(t)Bu)2 (3). The halogen analogue of this complex, RuX2(PPh3)3, reacted with HC = C(t)Bu to give vinylidene complexes RuX2(PPh3)2(C = CH(t)Bu) (11a, X = Cl; 11b, X = Br). Reaction of 11a with a calculated amount of LiC = C(t)Bu followed by bubbling of CO yielded RuCl(CO)(PPh3)2{C(C = C(t)Bu) = CH(t)Bu} (12), where the C = C(t)Bu group and (t)Bu are mutually cis to each other with respect to the double bond of the C4 unit. Complex 12 in benzene decomposed at 50-degrees-C releasing the C4 chain as (Z)-dbb. From these reactions, the mechanism for the catalytic production of (Z)-dbb has been derived. The X-ray structure of 12 is reported.