Abstract
The octahedral alkoxo complexes mer-ci.s-HIr(OR)Cl(PR′3)3 (R = Me, Et, i-Pr; R′ = Me, Et; H trans to Cl) decompose at room temperature in an alcohol/benzene solution, forming the dihydrido products mer-cis-H2IrCl(PR′3)3 and the corresponding aldehyde or ketone. The reaction rate is of first order in the iridium complex and of 1.33 order in the alcohol, which serves as a catalyst. The rate depends on the nature of the phosphine (PEt3 > PMe3), on the alkyl substituent of the alkoxide (Me > Et ≫ i-Pr), and on the medium (benzene > N-methylpyrrolidone) but is not effected by excess phosphine. The activation parameters obtained for the decomposition of mer-cis-HIr(OCH3)Cl(PMe3)3 are ΔH‡ obs = 24.1 ± 1.8 kcal mol-1, ΔS‡ 0bs = 0.6 ± 5.9 eu, and ΔG‡ 0bS (298 K) = 23.9 ± 3.6 kcal mol-1. The kinetic isotope effect (combined primary and secondary effects) for the decomposition of mercis-DIr(OCD3)Cl(PMe3)3 at 22 °C is kH/kD = 2.45 ± 0.10, and the secondary kinetic isotope effect for the decomposition of DIr(OCH3)Cl(PMe3)3 at 22 °C is 1.10 ± 0.06. Both DIr(OCH3)Cl(PMe3)3 and HIr(OCD3)Cl(PMe3)3 produce only the two mer-cis isomers of HDIrCl(PMe3)3, but in different ratios. The following steps are involved in the β-hydride elimination process: (a) pre-equilibrium generation of a free coordination site by chloride dissociation, which is induced by hydrogen bonding of a methanol molecule to the chloride; (b) irreversible ratedetermining β-C-H cleavage through the sterically favored transition state; (c) facile, irreversible dissociation of the aldehyde; (d) ligand rearrangement; and (e) irreversible reassociation of the chloride. Selective deuterium labeling enables the elucidation of a competing minor mechanism through the electronically favored transition state, operative for the trimethylphosphine complex only.
| Original language | English |
|---|---|
| Pages (from-to) | 4582-4594 |
| Number of pages | 13 |
| Journal | Journal of the American Chemical Society |
| Volume | 117 |
| Issue number | 16 |
| Publication status | Published - Apr 26 1995 |
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Mechanism of a directly observed β-hydride elimination process of indium alkoxo complexes. / Blum, Ofer; Milstein, David.
In: Journal of the American Chemical Society, Vol. 117, No. 16, 26.04.1995, p. 4582-4594.Research output: Contribution to journal › Article
}
TY - JOUR
T1 - Mechanism of a directly observed β-hydride elimination process of indium alkoxo complexes
AU - Blum, Ofer
AU - Milstein, David
PY - 1995/4/26
Y1 - 1995/4/26
N2 - The octahedral alkoxo complexes mer-ci.s-HIr(OR)Cl(PR′3)3 (R = Me, Et, i-Pr; R′ = Me, Et; H trans to Cl) decompose at room temperature in an alcohol/benzene solution, forming the dihydrido products mer-cis-H2IrCl(PR′3)3 and the corresponding aldehyde or ketone. The reaction rate is of first order in the iridium complex and of 1.33 order in the alcohol, which serves as a catalyst. The rate depends on the nature of the phosphine (PEt3 > PMe3), on the alkyl substituent of the alkoxide (Me > Et ≫ i-Pr), and on the medium (benzene > N-methylpyrrolidone) but is not effected by excess phosphine. The activation parameters obtained for the decomposition of mer-cis-HIr(OCH3)Cl(PMe3)3 are ΔH‡ obs = 24.1 ± 1.8 kcal mol-1, ΔS‡ 0bs = 0.6 ± 5.9 eu, and ΔG‡ 0bS (298 K) = 23.9 ± 3.6 kcal mol-1. The kinetic isotope effect (combined primary and secondary effects) for the decomposition of mercis-DIr(OCD3)Cl(PMe3)3 at 22 °C is kH/kD = 2.45 ± 0.10, and the secondary kinetic isotope effect for the decomposition of DIr(OCH3)Cl(PMe3)3 at 22 °C is 1.10 ± 0.06. Both DIr(OCH3)Cl(PMe3)3 and HIr(OCD3)Cl(PMe3)3 produce only the two mer-cis isomers of HDIrCl(PMe3)3, but in different ratios. The following steps are involved in the β-hydride elimination process: (a) pre-equilibrium generation of a free coordination site by chloride dissociation, which is induced by hydrogen bonding of a methanol molecule to the chloride; (b) irreversible ratedetermining β-C-H cleavage through the sterically favored transition state; (c) facile, irreversible dissociation of the aldehyde; (d) ligand rearrangement; and (e) irreversible reassociation of the chloride. Selective deuterium labeling enables the elucidation of a competing minor mechanism through the electronically favored transition state, operative for the trimethylphosphine complex only.
AB - The octahedral alkoxo complexes mer-ci.s-HIr(OR)Cl(PR′3)3 (R = Me, Et, i-Pr; R′ = Me, Et; H trans to Cl) decompose at room temperature in an alcohol/benzene solution, forming the dihydrido products mer-cis-H2IrCl(PR′3)3 and the corresponding aldehyde or ketone. The reaction rate is of first order in the iridium complex and of 1.33 order in the alcohol, which serves as a catalyst. The rate depends on the nature of the phosphine (PEt3 > PMe3), on the alkyl substituent of the alkoxide (Me > Et ≫ i-Pr), and on the medium (benzene > N-methylpyrrolidone) but is not effected by excess phosphine. The activation parameters obtained for the decomposition of mer-cis-HIr(OCH3)Cl(PMe3)3 are ΔH‡ obs = 24.1 ± 1.8 kcal mol-1, ΔS‡ 0bs = 0.6 ± 5.9 eu, and ΔG‡ 0bS (298 K) = 23.9 ± 3.6 kcal mol-1. The kinetic isotope effect (combined primary and secondary effects) for the decomposition of mercis-DIr(OCD3)Cl(PMe3)3 at 22 °C is kH/kD = 2.45 ± 0.10, and the secondary kinetic isotope effect for the decomposition of DIr(OCH3)Cl(PMe3)3 at 22 °C is 1.10 ± 0.06. Both DIr(OCH3)Cl(PMe3)3 and HIr(OCD3)Cl(PMe3)3 produce only the two mer-cis isomers of HDIrCl(PMe3)3, but in different ratios. The following steps are involved in the β-hydride elimination process: (a) pre-equilibrium generation of a free coordination site by chloride dissociation, which is induced by hydrogen bonding of a methanol molecule to the chloride; (b) irreversible ratedetermining β-C-H cleavage through the sterically favored transition state; (c) facile, irreversible dissociation of the aldehyde; (d) ligand rearrangement; and (e) irreversible reassociation of the chloride. Selective deuterium labeling enables the elucidation of a competing minor mechanism through the electronically favored transition state, operative for the trimethylphosphine complex only.
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M3 - Article
VL - 117
SP - 4582
EP - 4594
JO - Journal of the American Chemical Society
JF - Journal of the American Chemical Society
SN - 0002-7863
IS - 16
ER -