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

Abstract

The pincer-ligated species (PCP)Ir (PCP = kappa3-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 beta-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 alpha-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 pi-system; this requires that the vinyl group must rotate (around the Ir-C bond) by ca. 90 degrees to achieve an appropriate orientation. This rotation is severely inhibited by steric crowding, particularly when the vinyl group bears an alpha-substituent.

Transition Metal Catalyzed CarbonCarbon Bond Formation: The Key of Homogeneous Catalysis

“…IIIa ! IV), which also proceeds through the sequence of RaH bond oxidative addition, alkyne insertion into the MaH bond of VI and reductive elimination (M ¼ Ru, Rh, Ir) [38][39][40][41][42][43].…”

Activation of C–O and C–F Bonds by Pincer–Iridium Complexes

“…We have also found that the Ir-H bond resulting from alkynyl C-H addition could add across an acetylene triple bond to give a vinyl-alkynyl iridium complex that undergoes C-C elimination to give the corresponding enyne [10]. This led us to study C-C elimination from a range of complexes (PCP)IrRR [30].…”

“…Our group has largely focused on pincer-iridium complexes, which have shown a strong tendency toward the activation of C-H bonds. These complexes have been found to effect the oxidative addition of a variety of C-H bonds including those with sp 2 -and sp-hybridized carbon [8][9][10]. Most notable, however, has been the activation of C(sp 3 )-H bonds, leading to alkane dehydrogenation [6,7].…”

Simultaneous Orthogonal Methods for the Real-Time Analysis of Catalytic Reactions

“…Reversible hydride insertion into alkynes is rare. 21 These observations combine to suggest that 7 is the resting state. Under this assumption that hydride insertion into the alkyne is irreversible, the observation of a species consistent with acyl-hydride 5 as an intermediate, rather than 6b, suggests that it precedes the turnover-limiting step.…”