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

The pincer-ligated species (PCP)Ir (PCP = κ3-C6H3-2,6-(CH2PtBu2)2) is found to promote
dimerization of phenylacetylene to give the enyne complex (PCP)Ir(trans-1,4-phenyl-but-3-ene-1-yne). The
mechanism of this reaction is found to proceed through three steps:  (i) addition of the alkynyl C−H bond
to 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 and
computational (DFT) methods, to yield unexpected conclusions of general interest. (i) The product of alkynyl
C−H addition, (PCP)Ir(CCPh)(H) (3), has been isolated and, in accord with experimental observations, is
calculated to be 29 kcal/mol more stable than the analogous product of benzene C−H addition. (ii) Insertion
of a second PhCCH molecule into the Ir−H bond of 3 proceeds rapidly, but with a 1,2-orientation. This
orientation gives (PCP)Ir(CCPh)(CPhCH2) (4) which would yield the 1,3-diphenyl-enyne if it were to
undergo C−C elimination; however, the insertion is reversible, which represents the first example, to our
knowledge, of simple β-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−C
elimination to give the observed enyne. (iii) The failure of 4 to undergo C−C elimination is found to be
general for (PCP)Ir(CCPh)(vinyl) complexes in which the vinyl group has an α-substituent. Thus, although
C−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 bond
formation in the C−C elimination transition state involves the vinyl group π-system; this requires that the
vinyl group must rotate (around the Ir−C bond) by ca. 90° to achieve an appropriate orientation. This
rotation is severely inhibited by steric crowding, particularly when the vinyl group bears an α-substituent.