[2+2+2] and [2+2+1] cyclization reactions catalyzed by titanium aryloxide compounds

A variety of titanium aryloxide reagents catalyze the cross coupling of two alkyne units with one equivalent of olefin to produce the 1,3-cyclohexadiene nucleus. Catalysts include isolated titanacyclopentadiene or titanacyclopentane complexes. The reaction proceeds via attack of the olefin upon a titanacyclopentadiene compound initially formed by coupling of two alkyne units. Mechanistic studies show that a isomerization processes occur via sequential, metal mediated 1,5-hydrogen shifts upon a single face of the six-membered ring, exclusively leading to a cis-stereochemistry within the final products. In the reactions of diynes coupled with ethylene and $\alpha$-olefins, a variety of substituted hexalins are produced. A mixture of the dichlorides ((ArO)$\rm\sb2TiCl\sb2$) 55 (ArO = 2,6-diphenylphenoxide, a; 2,6-di-isopropylphenoxide, b; 2,6-dimethylphenoxide, c; activated with $\u3e$2 equiv of $\rm LiC{\equiv}CBu\sp{t}$ catalyze the trimerization of ($\rm Bu\sp{t}C{\equiv}CH\rbrack$ to 1,3,6-tri-tert-butylfulvene 51. Reaction of dichloride 55a-b tvith 2 equiv of (Bu$\rm\sp{t}C{\equiv}$CLi) leads to the bis(alkynyl) compounds 38 and 39. 38 is converted into a new organometallic product 43 by addition of a further equiv of (Bu$\rm\sp{t}C{\equiv}$CLi). The solid state structure of 43 shows it to be a Li/Ti ate species (Ti(OC$\rm\sb6H\sb3Ph$-2-C$\rm\sb6H\sb3\{\eta\sp2$- C(CMe$\sb3)$=C-C=CH(CMe$\sb3)\})$(OC$\rm\sb6H\sb3Ph\sb2$- 2,6)(C$\equiv$CBu$\rm\sp{t})$(Li)) (43), which catalyzes fulvene 51 formation. These results lead to the hypothesis that catalytic fulvene formation in these systems has as a key step the coupling of an alkynyl group with a titanacyclopentadiene ring. The alkylation of fulvene 51 with LAH, MeLi, LiCH$\rm\sb2SiMe\sb3$ and LiC$\rm\sb6H\sb4$-OMe-2 produces lithium cyclopentadienides. The bulky LiCp compounds are reacted with Ti, Zr, and Ta chlorides to yield new and interesting bulky cyclopentadienyl metal derivatives