Dimerization of Alkynes Promoted by a Pincer-Ligated Iridium Complex. C-C Reductive Elimination Inhibited by Steric Crowding

详细信息    查看全文

• 作者：Rajshekhar Ghosh ; Xiawei Zhang ; Patrick Achord ; Thomas J. Emge ; Karsten Krogh-Jespersen ; Alan S. Goldman
• 刊名：Journal of the American Chemical Society
• 出版年：2007
• 出版时间：January 31, 2007
• 年：2007
• 卷：129
• 期：4
• 页码：853 - 866
• 全文大小：517K
• 年卷期：v.129,no.4(January 31, 2007)
• ISSN：1520-5126

文摘

The pincer-ligated species (PCP)Ir (PCP = ³-C₆H₃-2,6-(CH₂P^tBu₂)₂) is found to promotedimerization of phenylacetylene to give the enyne complex (PCP)Ir(trans-1,4-phenyl-but-3-ene-1-yne). Themechanism of this reaction is found to proceed through three steps: (i) addition of the alkynyl C-H bondto iridium, (ii) insertion of a second phenylacetylene molecule into the resulting Ir-H bond, and (iii) vinyl-acetylide reductive elimination. Each of these steps has been investigated, by both experimental andcomputational (DFT) methods, to yield unexpected conclusions of general interest. (i) The product of alkynylC-H addition, (PCP)Ir(CCPh)(H) (3), has been isolated and, in accord with experimental observations, iscalculated to be 29 kcal/mol more stable than the analogous product of benzene C-H addition. (ii) Insertionof a second PhCCH molecule into the Ir-H bond of 3 proceeds rapidly, but with a 1,2-orientation. Thisorientation gives (PCP)Ir(CCPh)(CPh=CH₂) (4) which would yield the 1,3-diphenyl-enyne if it were toundergo C-C elimination; however, the insertion is reversible, which represents the first example, to ourknowledge, of simple -H elimination from a vinyl group to give a terminal hydride. The 2,1-insertion product(PCP)Ir(CCPh)(CH=CHPh) (6) forms more slowly, but unlike the 1,2 insertion product it undergoes C-Celimination to give the observed enyne. (iii) The failure of 4 to undergo C-C elimination is found to begeneral for (PCP)Ir(CCPh)(vinyl) complexes in which the vinyl group has an -substituent. Thus, althoughC-C elimination relieves crowding, the reaction is inhibited by increased crowding. Density-functional theory(DFT) calculations support this surprising conclusion and offer a clear explanation. Alkynyl-vinyl bondformation in the C-C elimination transition state involves the vinyl group -system; this requires that thevinyl group must rotate (around the Ir-C bond) by ca. 90 to achieve an appropriate orientation. Thisrotation is severely inhibited by steric crowding, particularly when the vinyl group bears an -substituent.