Dimerization of alkynes promoted by a pincer-ligated iridium complex. C-C reductive elimination inhibited by steric crowding

Rajshekhar Ghosh, Xiawei Zhang, Patrick Achord, Thomas J. Emge, Karsten Krogh-Jespersen, Alan S Goldman

Research output: Contribution to journalArticle

105 Citations (Scopus)

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 languageEnglish
Pages (from-to)853-866
Number of pages14
JournalJournal of the American Chemical Society
Volume129
Issue number4
DOIs
Publication statusPublished - Jan 31 2007

Fingerprint

Iridium
Alkynes
Dimerization
Density functional theory
Molecules
Benzene
Hydrides
phenylacetylene
diphenyl
1-butene

ASJC Scopus subject areas

  • Chemistry(all)

Cite this

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 journalArticle

Ghosh, Rajshekhar ; Zhang, Xiawei ; Achord, Patrick ; Emge, Thomas J. ; Krogh-Jespersen, Karsten ; Goldman, Alan S. / Dimerization of alkynes promoted by a pincer-ligated iridium complex. C-C reductive elimination inhibited by steric crowding. In: Journal of the American Chemical Society. 2007 ; Vol. 129, No. 4. pp. 853-866.
@article{3b42ecadd60a464dbc022e21585aac7b,
title = "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 = 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.",
author = "Rajshekhar Ghosh and Xiawei Zhang and Patrick Achord and Emge, {Thomas J.} and Karsten Krogh-Jespersen and Goldman, {Alan S}",
year = "2007",
month = "1",
day = "31",
doi = "10.1021/ja0647194",
language = "English",
volume = "129",
pages = "853--866",
journal = "Journal of the American Chemical Society",
issn = "0002-7863",
publisher = "American Chemical Society",
number = "4",

}

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.

UR - http://www.scopus.com/inward/record.url?scp=33846611597&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=33846611597&partnerID=8YFLogxK

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 -