Abstract
The pincer-ligated species (PCP)Ir (PCP = κ3-C 6H3-2,6-(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) vinylacetylide 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.
| Original language | English |
|---|---|
| Pages (from-to) | 853-866 |
| Number of pages | 14 |
| Journal | Journal of the American Chemical Society |
| Volume | 129 |
| Issue number | 4 |
| DOIs | |
| Publication status | Published - Jan 31 2007 |
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Dimerization of alkynes promoted by a pincer-ligated iridium complex. C-C reductive elimination inhibited by steric crowding. / Ghosh, Rajshekhar; Zhang, Xiawei; Achord, Patrick; Emge, Thomas J.; Krogh-Jespersen, Karsten; Goldman, Alan S.
In: Journal of the American Chemical Society, Vol. 129, No. 4, 31.01.2007, p. 853-866.Research output: Contribution to journal › Article
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TY - JOUR
T1 - Dimerization of alkynes promoted by a pincer-ligated iridium complex. C-C reductive elimination inhibited by steric crowding
AU - Ghosh, Rajshekhar
AU - Zhang, Xiawei
AU - Achord, Patrick
AU - Emge, Thomas J.
AU - Krogh-Jespersen, Karsten
AU - Goldman, Alan S
PY - 2007/1/31
Y1 - 2007/1/31
N2 - The pincer-ligated species (PCP)Ir (PCP = κ3-C 6H3-2,6-(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) vinylacetylide 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.
AB - The pincer-ligated species (PCP)Ir (PCP = κ3-C 6H3-2,6-(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) vinylacetylide 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.
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U2 - 10.1021/ja0647194
DO - 10.1021/ja0647194
M3 - Article
VL - 129
SP - 853
EP - 866
JO - Journal of the American Chemical Society
JF - Journal of the American Chemical Society
SN - 0002-7863
IS - 4
ER -