Gold Cations DOI: 10.1002/ange.201102750 The Interaction of Gold(I) Cations with 1,3-Dienes** Rajashekharayya A. Sanguramath, Thomas N. Hooper, Craig P. Butts, Michael Green,* John E. McGrady, and Christopher A. Russell* In memory of F. Gordon A. Stone The activation of C ÀC multiple bonds by substituted gold cations [R 3 PAu] + is an important theme in contemporary homogeneous catalysis. [1] However, it is interesting to note that studies of the reactivity of conjugated 1,3-dienes using gold catalysts are relatively rare, although Au/ZrO 2 catalysts were found to be effective in the heterogeneous selective hydrogenation of 1,3-butadiene. [2] Furthermore, homoge- neous gold catalysts have been used to synthesize function- alized cyclopentadienes [3] and have been employed, for example, in both hydrothiolation [4] and hydroamination [5] of conjugated dienes. In the last of these reactions, a mechanism was suggested whereby the gold center binds h 4 to the diene, which is thus activated towards nucleophilic attack by the amine. Subsequent theoretical studies [6] have refined the mechanism suggesting that the gold actually binds h 2 to the diene prior to attack of the N-nucleophile on the coordinated double bond; protodeauration by proton transfer from the NH 2 group to the unsaturated carbon atom regenerates the catalyst. For further advances to be made, a detailed under- standing of the interaction of gold centers with 1,3-dienes would of course be beneficial. [7] Since the submission of this manuscript, a related study of gold cation interactions with 1,3-dienes has been published. [8] Initially we sought confirmation that the ligand and diene substrates which we employed were active in the hydro- amination of 1,3-dienes. Pleasingly, 2,3-dimethylbuta-1,3- diene undergoes near-quantitative hydroamination (by NMR spectroscopy) with both benzylcarbamate and p- toluenesulfonamide in 1,2-dichloroethane in the presence of 10 mol % [tBu 3 PAuCl]/AgSbF 6 catalyst under mild conditions to form an allylic amine; similar results were observed under the same conditions using the combination of 2,5-dimethyl- hexa-2,4-diene with [(tBu 2 (o-biphenyl)PAuCl]/AgSbF 6 . We also noted that under the same conditions, the normally highly reactive 2,3-dimethoxybuta-1,3-diene did not show any evidence of reaction ( 1 H NMR spectroscopy), which sug- gested to us that the interaction of this substrate and gold may be rather different to its counterparts. The nature of the interaction between gold and the 1,3- diene substrate was then probed using the room-temperature reaction of a slurry of [LAuCl] (L = tBu 3 P; tBu 2 (o-biphe- nyl)P) and AgSbF 6 in CH 2 Cl 2 with a series of symmetric conjugated dienes R 2 C =CR’ ÀCR’ =CR 2 (Scheme 1). Filtration of the resulting white precipitate of AgCl and recrystallisation of the product yielded the cationic gold butadiene products [(h 2 -1,3-diene)Au(L)] + . The mixing time for the initial reaction was found to be critical in determining the outcome of the reaction : Whereas the reaction using 2,3-dimethylbuta- 1,3-diene was invariant to long reaction times (typically 16 h), side reactions were seen (by 31 P NMR spectroscopy) for 2,5- dimethylhexa-2,4-diene after only 15 minutes. Most strikingly, immediate filtration of the mixture using 2,3-dimethoxylbuta- 1,3-diene (typically within 30 seconds) was necessary to avoid intense colorations typically associated with the formation of colloidal gold solutions. A series of cationic gold–h 2 -alkene complexes has been recently reported which were prepared from the interaction of an alkene with either “free” [Au] + or [AuL] + centers and employing a suitable weakly-coordinating anion; [9] similar methods have also been used to generate the corresponding silver salts. [10] In common with these limited examples of cationic gold(I) alkene complexes, both 1 and 2 are remark- ably stable, being ostensibly air- and moisture-stable and showing no sign of decomposition ( 1 H and 31 P NMR spec- troscopy) after 2 h reflux in CDCl 3 . The onset of decom- position upon heating in the solid state does not occur until 158 8C for 1 and 218 8C for 2. Futhermore, both compounds are indefinitely stable to light (daylight) and show no sign of decomposition when the solid is subjected to a vacuum (ca. 10 À2 torr, 30 min). In contrast, 3 is sensitive to air and moisture, decomposing upon brief exposure to air to form unidentified product(s). Scheme 1. Synthesis of compounds 1–3. [*] R. A. Sanguramath, Dr. T. N. Hooper, Dr. C. P. Butts, Prof. M. Green, Dr. C. A. Russell School of Chemistry, University of Bristol Cantock’s Close, Bristol, BS8 1TS (U.K.) E-mail: chris.russell@bristol.ac.uk Homepage: http://www.inchm.bris.ac.uk/people/russell.shtm Prof. J. E. McGrady Department of Chemistry, Inorganic Chemistry Laboratory University of Oxford, South Parks Road, Oxford OX1 3QR (U.K.) [**] We thank the University of Bristol (M.G., C.A.R., T.N.H., R.A.S.) for financial support. We thank Umicore AG & Co. KG for the generous donation of HAuCl 4 . Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.201102750. Zuschriften 7734  2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. 2011, 123, 7734 –7737