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

Eric Scott Johnson, Purdue University

Abstract

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 $>$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.

Degree

Ph.D.

Advisors

Rothwell, Purdue University.

Subject Area

Organic chemistry

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