Abstract
The mechanism of (PCP)Ir-catalyzed transfer-dehydrogenation has been elucidated for the prototypical substrate/acceptor couple, COA/TBE, at 55 °C (COA = cyclooctane; TBE = tert-butylethylene). The catalytic cycle may be viewed as the sum of two reactions: (i) hydrogenation of TBE by (PCP)IrH2 and C-H addition of a second mole of TBE to give (PCP)IrH(tert-butylvinyl), and (ii) dehydrogenation of COA by (PCP)IrH(tert-butylvinyl) to give (PCP)IrH2, COE, and TBE. These two stoichiometric reactions have been observed independently and their kinetics determined. The overall catalysis has also been monitored in situ, and (PCP)IrH2 and (PCP)IrH(tert-butylvinyl) have been observed as the resting states; the ratio of these two complexes is found to be proportional to [TBE]2. Based upon the proportionality constant thus obtained and the catalytic rate as a function of [TBE] (which reaches a maximum at ca. 0.3 M), the respective rate constants for the hydrogenation and dehydrogenation segments can be obtained. Good agreement is found between the rates independently obtained from stoichiometric and catalytic runs. Within the overall TBE-hydrogenation reaction, labeling experiments indicate that the rate-determining step is the reductive elimination of TBA (2,2-dimethylbutane) from (PCP)IrH(tert-butylethyl) (which is formed via insertion of TBE into an Ir-H bond of (PCP)IrH2). Based upon considerations of microscopic reversibility, it can be further inferred that the rate-determining step for the alkane dehydrogenations is C-H addition (and not β-H elimination).
| Original language | English |
|---|---|
| Pages (from-to) | 7770-7771 |
| Number of pages | 2 |
| Journal | Journal of the American Chemical Society |
| Volume | 125 |
| Issue number | 26 |
| DOIs | |
| Publication status | Published - Jul 2 2003 |
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Mechanism of alkane transfer-dehydrogenation catalyzed by a pincer-ligated iridium complex. / Renkema, Kenton B.; Kissin, Yury V.; Goldman, Alan S.
In: Journal of the American Chemical Society, Vol. 125, No. 26, 02.07.2003, p. 7770-7771.Research output: Contribution to journal › Article
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TY - JOUR
T1 - Mechanism of alkane transfer-dehydrogenation catalyzed by a pincer-ligated iridium complex
AU - Renkema, Kenton B.
AU - Kissin, Yury V.
AU - Goldman, Alan S
PY - 2003/7/2
Y1 - 2003/7/2
N2 - The mechanism of (PCP)Ir-catalyzed transfer-dehydrogenation has been elucidated for the prototypical substrate/acceptor couple, COA/TBE, at 55 °C (COA = cyclooctane; TBE = tert-butylethylene). The catalytic cycle may be viewed as the sum of two reactions: (i) hydrogenation of TBE by (PCP)IrH2 and C-H addition of a second mole of TBE to give (PCP)IrH(tert-butylvinyl), and (ii) dehydrogenation of COA by (PCP)IrH(tert-butylvinyl) to give (PCP)IrH2, COE, and TBE. These two stoichiometric reactions have been observed independently and their kinetics determined. The overall catalysis has also been monitored in situ, and (PCP)IrH2 and (PCP)IrH(tert-butylvinyl) have been observed as the resting states; the ratio of these two complexes is found to be proportional to [TBE]2. Based upon the proportionality constant thus obtained and the catalytic rate as a function of [TBE] (which reaches a maximum at ca. 0.3 M), the respective rate constants for the hydrogenation and dehydrogenation segments can be obtained. Good agreement is found between the rates independently obtained from stoichiometric and catalytic runs. Within the overall TBE-hydrogenation reaction, labeling experiments indicate that the rate-determining step is the reductive elimination of TBA (2,2-dimethylbutane) from (PCP)IrH(tert-butylethyl) (which is formed via insertion of TBE into an Ir-H bond of (PCP)IrH2). Based upon considerations of microscopic reversibility, it can be further inferred that the rate-determining step for the alkane dehydrogenations is C-H addition (and not β-H elimination).
AB - The mechanism of (PCP)Ir-catalyzed transfer-dehydrogenation has been elucidated for the prototypical substrate/acceptor couple, COA/TBE, at 55 °C (COA = cyclooctane; TBE = tert-butylethylene). The catalytic cycle may be viewed as the sum of two reactions: (i) hydrogenation of TBE by (PCP)IrH2 and C-H addition of a second mole of TBE to give (PCP)IrH(tert-butylvinyl), and (ii) dehydrogenation of COA by (PCP)IrH(tert-butylvinyl) to give (PCP)IrH2, COE, and TBE. These two stoichiometric reactions have been observed independently and their kinetics determined. The overall catalysis has also been monitored in situ, and (PCP)IrH2 and (PCP)IrH(tert-butylvinyl) have been observed as the resting states; the ratio of these two complexes is found to be proportional to [TBE]2. Based upon the proportionality constant thus obtained and the catalytic rate as a function of [TBE] (which reaches a maximum at ca. 0.3 M), the respective rate constants for the hydrogenation and dehydrogenation segments can be obtained. Good agreement is found between the rates independently obtained from stoichiometric and catalytic runs. Within the overall TBE-hydrogenation reaction, labeling experiments indicate that the rate-determining step is the reductive elimination of TBA (2,2-dimethylbutane) from (PCP)IrH(tert-butylethyl) (which is formed via insertion of TBE into an Ir-H bond of (PCP)IrH2). Based upon considerations of microscopic reversibility, it can be further inferred that the rate-determining step for the alkane dehydrogenations is C-H addition (and not β-H elimination).
UR - http://www.scopus.com/inward/record.url?scp=0038203411&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=0038203411&partnerID=8YFLogxK
U2 - 10.1021/ja0289200
DO - 10.1021/ja0289200
M3 - Article
C2 - 12822973
AN - SCOPUS:0038203411
VL - 125
SP - 7770
EP - 7771
JO - Journal of the American Chemical Society
JF - Journal of the American Chemical Society
SN - 0002-7863
IS - 26
ER -