SHORT COMMUNICATION DOI: 10.1002/ejic.200800356 The Detection of PHIP Effects Allows New Insights into the Mechanism of Olefin Isomerisation during Catalytic Hydrogenation Alessandra Viale, [a] Daniela Santelia, [a] Roberta Napolitano, [a] Roberto Gobetto, [a] Walter Dastrù, [a] and Silvio Aime* [a] Keywords: Isomerization / Parahydrogen / Olefins / Hydrogenation / Catalysis PHIP (parahydrogen-induced polarisation) effects in the 1 H NMR spectra of the products of Rh-complex-catalysed al- kyne hydrogenation brings to light the fact that the cis–trans isomerisation of the formed olefin occurs through the forma- Introduction It has been shown that under ordinary conditions hydro- gen gas is a mixture of two kinds of molecules, known as ortho- and parahydrogen, that correspond to the triplet and the singlet state of the hydrogen molecule, respectively. The concentrations of the two species in equilibrium are deter- mined by Boltzmann averaging and are accordingly tem- perature-dependent. At room temperature they are in a 3:1 ratio (due to the spin triplet/singlet ratio), but enrichment of the para isomer can be easily obtained. [1] Upon carrying out hydrogenation reactions with parahydrogen-enriched mixtures, the alteration of the spin-level populations yields dramatic changes in the 1 H NMR spectra of the hydroge- nated products (parahydrogen-induced polarisation ef- fect). [1–2] The use of the parahydrogen-induced polarisation (PHIP) effect in 1 H NMR spectra has proved to be ex- tremely valuable to assess the occurrence of low-concentra- tion reaction products, which are undetectable under ordi- nary conditions. [1–12] Moreover, information on the occur- ring reaction mechanism can be assessed by the simple ap- pearance of a PHIP effect on a hydrogenation product even when the molecule contains chemically and magnetically equivalent protons. [13–15] In this article, we aim to solve the basic problem of olefin isomerisation that takes place at Rh centres during hydro- genation of alkyne substrates. Our goal is to evaluate whether the use of parahydrogen can give new insights into the mechanism of the cis–trans isomerisation process occur- ring during catalytic hydrogenation with [Rh(diene)- (diphos)] + complexes. [16,17] [a] Dipartimento di Chimica IFM, University of Torino, V. P. Giuria 7, 10125 Torino, Italy Fax: +39 011 6707855 E-mail: silvio.aime@unito.it © 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Eur. J. Inorg. Chem. 2008, 4348–4351 4348 tion of a σ-bonded intermediate stabilised by the reversible addition of a hydrogen molecule at the metal centre. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) Results and Discussion The substrate chosen for the hydrogenation catalysed by [Rh(cyclooctadiene)(diphenylphosphanylbutane)][BF 4 ] was methyl 2-butynoate (1) (Scheme 1). Scheme 1. Hydrogenation of methyl 2-butynoate (1) leads to a mix- ture of methyl cis- and trans-2-butenoate. The reaction, which has been carried out in the NMR tube, quickly leads to methyl trans-2-butenoate. The hydro- genation reaction mechanism with [Rh(diene)(diphos)] + catalysts usually leads to the cis isomer, but due to fast- occurring cis–trans isomerisation, the predominant product is the trans isomer. However, when parahydrogen (50% para) is used, methyl cis-2-butenoate is clearly detected in the 1 H NMR spectrum by its enhanced signals (Figure 1a). The trans isomer is also polarised, but the intensities of the absorption/emission signals resulting from the PHIP effect are significantly lower than those observed for the cis iso- mer. It is noteworthy that in the completely relaxed spec- trum the trans isomer is present in higher concentration (Figure 1b): thus the PHIP experiment highlights the fact that the cis isomer is formed in the first step of the catalytic hydrogenation, and then isomerisation to the trans form readily occurs.