MAGNETIC RESONANCE IN CHEMISTRY Magn. Reson. Chem. 2002; 40: 153–156 Complexation of selenium to (R)-Rh 2 (MTPA) 4 : thermodynamics and stoichiometry Helmut Duddeck, 1∗ Shahid Malik, 1 Tam ´ as G ´ ati, 2 G´ abor T ´ oth 2 and Muhammad Iqbal Choudhary 3 1 Universit ¨ at Hannover, Institut f ¨ ur Organische Chemie, Schneiderberg 1B, D-30167 Hannover, Germany 2 Institute for General and Analytical Chemistry of the Budapest University of Technology and Economics, Szent Gell ´ ert t´ er 4, H-1111 Budapest, Hungary 3 H. E. J. Research Institute of Chemistry, International Center for Chemical Sciences, University of Karachi, Karachi-75270, Pakistan Received 10 September 2001; Accepted 23 October 2001 Variable-temperature 1 H and 77 Se NMR data for 3-phenylselenenyl-1-phenyl-1-propene (1) in the presence of Rh 2 (MTPA) 4 (Rh*) prove that the equilibria are strongly shifted towards the adduct Rh*···1; free selenide molecules cannot be detected as long as uncomplexed rhodium atoms are available. In the case of excess Rh*, both 1 : 2 and 1 : 1 adducts (Rh* vs 1) are formed, and the latter is slightly favoured. With excess selenide, the system strongly favours the complexation of two selenide molecules (1 : 2 adduct), i.e. one at each rhodium atom. In this situation, intermolecular selenide exchange can be monitored by variable-temperature 1 H NMR spectroscopy and the energy barrier is estimated to be 54–55 kJ mol −1 . Copyright  2001 John Wiley & Sons, Ltd. KEYWORDS: NMR; 1 H NMR; 13 C NMR; 77 Se NMR; phenyl alkyl selenide; dirhodium complexes; temperature-dependent spectra INTRODUCTION In a series of papers, 1 we have reported studies of the potential of the dirhodium complex (R)-Rh 2 (MTPA) 4 [Rh*, MTPA-H D methoxytrifluoromethylphenylacetic acid D Mosher’s acid; Scheme 1(a)] as a solvating agent for the determination of enantiomeric ratios of various chiral mono- functional ligands L. It was shown that our ‘dirhodium method’ is particularly suitable for soft-base funtionalities where the classical method with chiral lanthanide shift reagents (CLSR) 2 usually fails. Typically, Rh* and L form kinetically labile adducts so that for the L molecules in equilibria, as depicted in Scheme 1(a), only averaged NMR signals can be observed (in analogy with the CLSR method). During the course of our earlier studies, we demonstrated that even selenides can easily be discriminated by 1 H NMR spectroscopy in the presence of Rh* if they are chiral. 3 To our surprise, a recent re-investigation by 77 Se NMR spectroscopy showed that here the exchange rate of the selenide ligands at the rhodium atoms seemed to be much lower than in the case of previously studied ligands (olefins, Ł Correspondence to: H. Duddeck, Universit¨ at Hannover, Institut f¨ ur Organische Chemie, Schneiderberg 1B, D-30167 Hannover, Germany. E-mail: duddeck@mbox.oci.uni-hannover.de Contract/grant sponsor: Deutsche Forschungsgemeinschaft. Contract/grant sponsor: German Academic Exchange Service (DAAD). Contract/grant sponsor: Hungarian Academy of Sciences; Contract/grant number: 127 MTA/DFG. Contract/grant sponsor: Hungarian National Research Foundation (OTKA); Contract/grant number: T032180. Contract/grant sponsor: Fonds der Chemischen Industrie. epoxides, nitriles, sulfoxides, phosphine sulfides, etc.). This prompted us to record low-temperature 1 H and 77 Se NMR spectra, hoping to freeze exchange processes and obtain a closer insight into the equilibria themselves. The model com- pound of choice was 3-phenylselenenyl-1-phenyl-1-propene [1, Scheme 1(b)] because it features several well-separated 1 H signals. Moreover, it is achiral, avoiding dispersion effects due to the existence of diastereomeric Rh*1 adducts which may obscure the NMR resonances. EXPERIMENTAL Syntheses The synthesis of Rh* has been described previously. 1a 3- Phenylselenenyl-1-phenyl-1-propene (1) was prepared from the corresponding chloride (commercially available) by substitution as reported earlier. 4 Yield after chromatographic purification: 65% as a yellow oil. IR (neat): 3057, 3025, 1576, 1476, 1435, 725, and 687 cm 1 ; 1 H NMR (CDCl 3 ), υ D 3.69 (d, 2H, 2 JSe,H D 12.5 Hz, H-1), 6.32 (dt, 1H, H-2), 6.25 (d, 1H, H-3), 7.55 (m, 2H, ortho-H of Se-Ph), 7.20–7.32 (m, 8H, aromatic H); 13 C NMR (CDCl 3 ), υ D 30.7 (C-1), 125.8 (C-2), 132.0 (C-3), 136.7 (ipso Se-Ph), 133.9 (ortho Se-Ph), 129.7 (ipso C-Ph), 128.9, 128.5, 127.4, 127.3, 126.2 (aromatic CH); 77 Se NMR (CDCl 3 ), υ D 342, 2 JSe,H D 12.5 Hz. EI-MS m/z [ 80 Se](relative intensity, % D 274 19, M C ), 189 (9), 157 (14), 131 (18), 117 (100), 91 (26). NMR measurements Room-temperature 1 H (400.1 MHz), 13 C (100.6 MHz) and 77 Se NMR measurements (76.3 MHz) of the free ligand 1 were DOI: 10.1002/mrc.990 Copyright  2001 John Wiley & Sons, Ltd.