Published: March 09, 2011 r2011 American Chemical Society 1812 dx.doi.org/10.1021/om1008555 | Organometallics 2011, 30, 1812–1817 ARTICLE pubs.acs.org/Organometallics Synthesis and Structure of Gold and Platinum Menthyl Complexes Ashley A. Zuzek, † Samantha C. Reynolds, † David S. Glueck,* ,† James A. Golen, ‡ and Arnold L. Rheingold ‡ † 6128 Burke Laboratory, Department of Chemistry, Dartmouth College, Hanover, New Hampshire 03755, United States ‡ Department of Chemistry, University of California, San Diego, 9500 Gilman Drive, La Jolla, California 92093, United States b S Supporting Information ’ INTRODUCTION The properties of metal-carbon bonds have been studied in detail during the continuing development of organometallic che- mistry. 1 However, surprisingly little is known about chiral alkyl ligands, especially those with a stereogenic sp 3 carbon bound to a transition metal. 2 Such complexes may be useful in studies of the stereochemistry of fundamental reactions of the metal-carbon bond 3 and as chiral reagents or catalysts for asymmetric synthesis. 4 The simplest syntheses of chiral nonracemic metal alkyls start with enantiomerically pure organic precursors, which are transferred to the metal with control of stereochemistry. Such processes have been reported recently in main group chemistry using (-)-menthyl magnesium chloride ((-)-menthyl = (1R,5S,2R-menthyl)), espe- cially for the preparation of menthyl-tin complexes. 5 Related menthyl-germanium 6 and -aluminum 7 complexes have also been prepared. This chemistry is particularly attractive since menthyl chloride is commercially available; it may also be prepared on a large scale from menthol, 8 one of the cheapest available chiral materials. 9 However, it is complicated by the formation of a 1:1 mixture of menthyl magnesium chloride and the C1-epimer neomenthyl magnesium chloride on treatment of (-)-menthyl chloride with magnesium (Scheme 1). 10,11 Beckmann and co-workers studied the reactivity of this mixture in detail, including quenching experiments with D 2 O and a variety of other electrophiles, and concluded that the menthyl and neomenthyl Grignard reagents did not interconvert. They proposed that selective menthyl transfer to an electrophile, observed when an excess of the Grignard mixture was used, occurred because MenMgCl was more reactive than NeomMgCl (Scheme 1). 10,12 We planned to use this chemistry to prepare enantiomerically pure transition metal menthyl complexes. Previously, the group 12 complexes dimenthylmercury 13,14 and dimenthylzinc, 15 as well as MenHgX (X = carboxylate), 13 were reported, but very little or no characterizing data were included, and the stereo- chemistry of these compounds was not elucidated (Chart 1). More recently, the catalytic cross-coupling of MenZnCl and bromo- indenes, which was used to prepare chiral indenyl ligands, presum- ably proceeded via menthyl-palladium and -nickel intermediates. 16a Similar intermediates in stereoselective catalytic cross-couplings, formed by Zn-to-Pd transmetalation, were recently proposed, and NMR evidence for a Pd-cyclohexyl species was reported. 17 Finally, menthyl-nickel, -palladium, and -copper intermediates were likely involved in the synthesis of menthylpyridine via cross-coupling. 16b,c Here we describe the synthesis and structural characterization of gold and platinum menthyl complexes. ’ RESULTS AND DISCUSSION Synthesis and Structure of Gold and Platinum Menthyl Complexes. The thermally stable Au(I) complexes Au(PPh 3 )(R) have been prepared for a range of primary, secondary, and tertiary alkyl groups R. 18 We extended this series to the chiral (-)-menthyl group. Treatment of Au(PPh 3 )(Cl) with an excess (10 equiv) of Grignard reagent prepared from MenCl gave Au(PPh 3 )(Men) (1) as fine white crystals in 67% yield (Scheme 2). The menthyl group gave rise to characteristic 1 H and 13 C NMR resonances, 19 including that due to the Au-bound carbon (δ 55.2, J PC = 97 Hz, THF-d 8 ; see the Experimental Section for complete NMR spectral assignments). Complex 1 decomposed quickly in CDCl 3 or acetone-d 6 solution (see below) but was more robust in C 6 D 6 or THF-d 8 . Received: September 3, 2010 ABSTRACT: A Grignard reagent derived from (-)-menthyl chloride has been reported to be a 1:1 mixture of menthyl magnesium chloride and neomenthyl magnesium chloride, which do not interconvert. Addi- tion of an excess of this reagent to Au(PPh 3 )(Cl) or Pt(dppe)Cl 2 gave Au(PPh 3 )(Men) (1) and Pt(dppe)(Men)(Br) (2), respectively. Crystal- lographic studies of these first well-characterized transition metal menthyl complexes showed that the menthyl group adopts a conforma- tion with all three substituents in equatorial positions. NMR spectro- scopic data for 2 showed that menthyl has a large trans influence, comparable to other alkyl groups. Decomposition of 1 in CDCl 3 gave Au(PPh 3 )(Cl) and a mixture of menthyl chloride and neomenthyl chloride, while 2 formed the halide complexes Pt(dppe)Cl 2 , Pt(dppe)Br 2 , and Pt(dppe)(Br)(Cl) and a mixture of 2-menthene and 3-menthene.