PMID- 17243822
OWN - NLM
STAT- PubMed-not-MEDLINE
DCOM- 20070412
LR  - 20220311
IS  - 0002-7863 (Print)
IS  - 0002-7863 (Linking)
VI  - 129
IP  - 4
DP  - 2007 Jan 31
TI  - Dimerization of alkynes promoted by a pincer-ligated iridium complex. C-C 
      reductive elimination inhibited by steric crowding.
PG  - 853-66
AB  - 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.
FAU - Ghosh, Rajshekhar
AU  - Ghosh R
AD  - Department of Chemistry and Chemical Biology, Rutgers, the State University of 
      New Jersey, Piscataway, New Jersey 08854, USA.
FAU - Zhang, Xiawei
AU  - Zhang X
FAU - Achord, Patrick
AU  - Achord P
FAU - Emge, Thomas J
AU  - Emge TJ
FAU - Krogh-Jespersen, Karsten
AU  - Krogh-Jespersen K
FAU - Goldman, Alan S
AU  - Goldman AS
LA  - eng
PT  - Journal Article
PL  - United States
TA  - J Am Chem Soc
JT  - Journal of the American Chemical Society
JID - 7503056
EDAT- 2007/01/25 09:00
MHDA- 2007/01/25 09:01
CRDT- 2007/01/25 09:00
PHST- 2007/01/25 09:00 [pubmed]
PHST- 2007/01/25 09:01 [medline]
PHST- 2007/01/25 09:00 [entrez]
AID - 10.1021/ja0647194 [doi]
PST - ppublish
SO  - J Am Chem Soc. 2007 Jan 31;129(4):853-66. doi: 10.1021/ja0647194.