A para-Hydrogen Investigation of Palladium-Catalyzed Alkyne Hydrogenation

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 alpha-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 DeltaH = 42 +/- 9 kJ mol-1 and DeltaS = -107 +/- 31 J mol-1 K-1, but in CD2Cl2 the corresponding rate constant is 0.18 s-1, with DeltaH = 79 +/- 7 kJ mol-1 and DeltaS = 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 DeltaH = 94 +/- 6 kJ mol-1 and DeltaS = 69 +/- 22 J mol-1 K-1. Magnetization transfer from one of the two inequivalent beta-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 microL 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.

Parahydrogen-based NMR methods as a mechanistic probe in inorganic chemistry

“…Duckett and co-workers initially demonstrated that, in the catalytic hydrogenation of diphenylacetylene by Pd(bcope)(OTf) 2 , enhanced resonances could be observed for intermediate species containing alkyl ligands derived from the hydrogenation of the alkyne substrate, but not containing discrete hydride ligands [81]. While the key intermediate was initially formulated as a palladium alkyl hydride species, subsequent experiments demonstrated it to be a cationic palladium alkyl species, stabilised by an additional interaction between the metal centre and the aromatic moiety of the coordinated substrate [82]. In this later report, Duckett and co-workers used isotopically labelled alkyne substrate to delineate the species involved in the catalytic hydrogenation.…”

Following palladium catalyzed methoxycarbonylation by hyperpolarized NMR spectroscopy: a parahydrogen based investigation

“…20 The addition of CO to the H 2 feed (1:2 ratio) proved to change this product mix, with the methyl-ester C(Ph)(H)=C(Ph)CO 2 CD 3 (6) of Scheme 2 also being formed according to gas chromatographic analysis. Furthermore, when a series of 31 Scheme 2: Summary of the organic products detected during the reaction of diphenylacetylene with CO and H2 in the presence of 1 in methanol-d4; the blue labels reflect atoms that show the PHIP effect.…”

“…68 Interesting, the corresponding 31 P coupling increases to around 70 Hz in related hexadiene based complexes. 69 In the case of related Pd(II) bis phosphine complexes containing H and MeOH ligands couplings of [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30] Hz are typical and so a virtual triplet would be expected for the alkenic protons in both of these types of product if such large couplings are retained. We therefore suggest in 10 that CO binding to the fourth site occurs, and that the palladium centre is charged, which means that an out-of-plane cis-stilbene ligand is indicated ( Figure 5).…”

“…11,14,15 In addition, as PHIP enables the analogous detection of scalar coupled heteronuclei through polarisation transfer, the reliable characterisation of such species is possible 16 and when this approach is coupled with Exchange Spectroscopy (EXSY) experiments their kinetic significance can be determined. [17][18][19][20] Workers have also used this method to examine not only a range of heterogeneous reactions [21][22][23][24][25] but some that don't involve a metal centre. [26][27][28][29] The PHIP approach has therefore developed substantially from the early starting point of Weitekamp, 30, 31 Eisenberg 32,33 and Bargon, 34,35 as illustrated in several reviews.…”

Catalytic hydrogenation of Nb2O on palladium cis-dihydride (Meb2PCHb2CHb2PMeb2)PdHb2 by para-hydrogen as a promising route to para-water

“…33 The PHIP effects were observed in the hydroge nation of diphenylacetylene on such dihydrides. 34 As a palladium dihydride precursor, we used the 14 e palladium(0) complex Рd(Me 2 PCH 2 CH 2 PMe 2 ) (1) (Fig. 1).…”