A para-hydrogen investigation of palladium-catalyzed alkyne hydrogenation

Joaquín López-Serrano, Simon B. Duckett, Stuart Aiken, Karina Q. Almeida Leñero, Eite Drent, John P. Dunne, Denis Konya, Adrian C. Whitwood

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Abstract

The complexes [Pd(bcope)(OTf)2] (1a), where bcope is (C 8H14)PCH2-CH2P(C8H 14), and [Pd(tbucope)(OTf)2] (1b), where tbucope is (C8H14)PC6H 4CH2P(tBu)2, catalyzed the conversion of diphenyl-acetylene 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 a-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)(CPh=CHPh)(MeOD)](OTf) (3a) and [Pd(tbucope)(CPh=CHPh) (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)(CPh=CHPh)-(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.

Original languageEnglish
Pages (from-to)6513-6527
Number of pages15
JournalJournal of the American Chemical Society
Volume129
Issue number20
DOIs
Publication statusPublished - 1 May 2007
Externally publishedYes

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López-Serrano, J., Duckett, S. B., Aiken, S., Almeida Leñero, K. Q., Drent, E., Dunne, J. P., ... Whitwood, A. C. (2007). A para-hydrogen investigation of palladium-catalyzed alkyne hydrogenation. Journal of the American Chemical Society, 129(20), 6513-6527. https://doi.org/10.1021/ja070331c