López-Serrano, J., Duckett, S.B., Aiken, S. et al. (5 more authors) (2007) A para-hydrogen investigation of palladium-catalyzed alkyne hydrogenation. Journal of the American Chemical Society, 129 (20). pp. 6513-6527. ISSN 0002-7863
Abstract
The complexes [Pd(bcope)(OTf)2] (1a), where bcope is (C8H14)PCH2−CH2P(C8H14), and [Pd(tbucope)(OTf)2] (1b), where tbucope is (C8H14)PC6H4CH2P(tBu)2, catalyze the conversion of diphenylacetylene to cis- and trans-stilbene and 1,2-diphenylethane. When this reaction was studied with para-hydrogen, the characterization of [Pd(bcope)(CHPhCH2Ph)](OTf) (2a) and [Pd(tbucope)(CHPhCH2Ph)](OTf) (2b) was achieved. Magnetization transfer from the α-H of the CHPhCH2Ph ligands in these species proceeds into trans-stilbene. This process has a rate constant of 0.53 s-1 at 300 K in methanol-d4 for 2a, where ΔH = 42 ± 9 kJ mol-1 and ΔS = −107 ± 31 J mol-1 K-1, but in CD2Cl2 the corresponding rate constant is 0.18 s-1, with ΔH = 79 ± 7 kJ mol-1 and ΔS = 5 ± 24 J mol-1 K-1. The analogous process for 2b was too fast to monitor in methanol, but in CD2Cl2 the rate constant for trans-stilbene formation is 1.04 s-1 at 300 K, with ΔH = 94 ± 6 kJ mol-1 and ΔS = 69 ± 22 J mol-1 K-1. Magnetization transfer from one of the two inequivalent β-H sites of the CHPhCH2Ph moiety proceeds into trans-stilbene, while the other site shows transfer into H2 or, to a lesser extent, cis-stilbene in CD2Cl2, but in methanol it proceeds into the vinyl cations [Pd(bcope)(CPhCHPh)(MeOD)](OTf) (3a) and [Pd(tbucope)(CPhCHPh)(MeOD)](OTf) (3b). When the same magnetization transfer processes are monitored for 1a in methanol-d4 containing 5 μL of pyridine, transfer into trans-stilbene is observed for two sites of the alkyl, but the third proton now becomes a hydride ligand in [Pd(bcope)(H)(pyridine)](OTf) (5a) or a vinyl proton in [Pd(bcope)(CPhCHPh)(pyridine)](OTf) (4a). For 1b, under the same conditions, two isomers of [Pd(tbucope)(H)(pyridine)](OTf) (5b and 5b‘) and the neutral dihydride [Pd(tbucope)(H)2] (7) are detected. The single vinylic CH proton in 3 and the hydride ligands in 4 and 5 appear as strong emission signals in the corresponding 1H NMR spectra.
Metadata
Item Type: | Article |
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Authors/Creators: |
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Dates: |
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Institution: | The University of York |
Academic Units: | The University of York > Faculty of Sciences (York) > Chemistry (York) |
Depositing User: | York RAE Import |
Date Deposited: | 03 Apr 2009 10:45 |
Last Modified: | 03 Apr 2009 10:45 |
Published Version: | http://dx.doi.org/10.1021/ja070331c |
Status: | Published |
Publisher: | ACS American Chemical Society |
Identification Number: | 10.1021/ja070331c |
Open Archives Initiative ID (OAI ID): | oai:eprints.whiterose.ac.uk:6906 |