Stereospecific and Regioselective Isocyanide Insertions into Siliranes and Reactions of the Resulting Iminosilacyclobutanes Phuc T. Nguyen, Wylie S. Palmer, and K. A. Woerpel* Department of Chemistry, University of California, Irvine, California 92697-2025 Received August 11, 1998 The insertions of p-tolyl and tert-butyl isocyanide into siliranes yielded iminosilacyclobutanes with stereospecific retention of configuration. Monosubstituted siliranes underwent insertion into the more substituted Si-C bond of the ring, although this regioselectivity was eroded as substitution increased on the silirane ring. The iminosilacyclobutane products tautomerized thermally or in the presence of a palladium catalyst to yield the thermodynamically more stable aminosilacy- clobutenes. Ring-expansion reactions of iminosilacyclobutanes were promoted by acids: treatment with aqueous copper sulfate produced an oxasilacyclopentane in high yield, whereas with trifluoroacetic acid, oxasilacyclohexanes were formed. Siliranes undergo a variety of ring-expansion reactions such as the two-atom insertions originally studied by Seyferth 1,2 and Ando. 3,4 A few single-atom insertions, including reactions with isocyanides, 5,6 oxygen, 7 sulfur, 8 and selenium, 9 have been reported, although the question of stereochemical control was not addressed systemati- cally. In our own studies of the reactions of siliranes, 10-13 we found that these strained ring systems undergo a variety of stereospecific two-atom insertion reactions. In this paper, we detail our investigations of the stereospe- cific and regioselective one-atom insertions of isocyanides into siliranes. The stereospecificity of this reaction is not only important from a mechanistic perspective, but it also permits the stereocontrolled preparation of synthetically useful oxasilacyclopentane hemiacetals. 14,15 Isocyanide Insertion. As with other carbon-carbon bond-forming reactions of siliranes, 11 the insertions of isocyanides, first reported by Weidenbruch, 5 proceed stereospecifically. Siliranes cis-1 and trans-1 react with a slight excess of p-tolyl isocyanide at 23 °C or tert-butyl isocyanide at 80 °C. After removal of volatile materials, analytically pure iminosilacyclobutanes 2 and 3, respec- tively, were isolated in high yield (eqs 1 and 2). Analysis of 1 H NMR spectra indicated that the products of each reaction were formed as single stereoisomers. NOE difference measurements indicated that the products were formed with stereospecific retention of configuration about the C-Si bond. 16 The stereospecificity of these reactions indicates that homolysis or heterolysis of a C-Si bond prior to C-C bond formation is unlikely. 17 As with two-atom insertions, 11,12,15 the insertions of an isocyanide into monosubstituted siliranes proceed regio- selectively into the more substituted C-Si bond (eq 3, Table 1). Only one regioisomer of the iminosilacyclobu- tane 5a was detected using NMR spectroscopy upon reaction of silirane 4a (R ) n-Bu) with tert-butyl isocya- nide (Table 1, entry 1). The regioselectivity decreased as the substituent on the silirane ring became sterically more demanding and the temperature required to effect insertion increased (entries 2 and 3). The erosion in regioselectivity reaches an extreme for geminally disub- stituted siliranes, which undergo insertion of phenyl isocyanide into the less-substituted C-Si bond. 5 Although we do not have any detailed information about the reaction mechanism, these reactions likely (1) Seyferth, D.; Duncan, D. P.; Shannon, M. L.; Goldman, E. W. Organometallics 1984, 3, 574-578. (2) Seyferth, D.; Duncan, D. P.; Shannon, M. L. Organometallics 1984, 3, 579-583 and references therein. (3) Saso, H.; Ando, W. Chem. Lett. 1988, 1567-1570. (4) Saso, H.; Ando, W.; Ueno, K. Tetrahedron 1989, 45, 1929-1940. (5) Kroke, E.; Willms, S.; Weidenbruch, M.; Saak, W.; Pohl, S.; Marsmann, H. Tetrahedron Lett. 1996, 37, 3675-3678. (6) Brook, A. G.; Saxena, A. K.; Sawyer, J. F. Organometallics 1989, 8, 850-852. (7) Saso, H.; Yoshida, H.; Ando, W. Tetrahedron Lett. 1988, 29, 4747-4750. (8) Boudjouk, P.; Samaraweera, U. Organometallics 1990, 9, 2205- 2206. (9) Boudjouk, P.; Black, E.; Kumarathasan, R.; Samaraweera, U.; Castellino, S.; Oliver, J. P.; Kampf, J. W. Organometallics 1994, 13, 3715-3727. (10) Bodnar, P. M.; Palmer, W. S.; Shaw, J. T.; Smitrovich, J. H.; Sonnenberg, J. D.; Presley, A. L.; Woerpel, K. A. J. Am. Chem. Soc. 1995, 117, 10575-10576. (11) Bodnar, P. M.; Palmer, W. S.; Ridgway, B. H.; Shaw, J. T.; Smitrovich, J. H.; Woerpel, K. A. J. Org. Chem. 1997, 62, 4737-4745. (12) Shaw, J. T.; Woerpel, K. A. J. Org. Chem. 1997, 62, 442-443. (13) (a) Palmer, W. S.; Woerpel, K. A. Organometallics 1997, 16, 1097-1099. (b) Palmer, W. S.; Woerpel, K. A. Organometallics 1997, 16, 4824-4827. (14) Shaw, J. T.; Woerpel, K. A. J. Org. Chem. 1997, 62, 6706-6707. (15) Shaw, J. T.; Woerpel, K. A. Tetrahedron 1997, 53, 16597-16606. (16) Empirically, the 29 Si NMR chemical shifts of 2 and 3 correlate to stereochemistry: the two cis products cis-2 and cis-3 have chemical shifts of 31.4 and 31.5 ppm, respectively, whereas the two trans products trans-2 and trans-3 have chemical shifts of 30.2 and 30.6 ppm, respectively. (17) These results are in contrast to the one- or two-atom insertions of sulfur into silacyclopropanes 1, which are not stereospecific. Radical intermediates were suggested for those transformations (ref 9). 1843 J. Org. Chem. 1999, 64, 1843-1848 10.1021/jo9816257 CCC: $18.00 © 1999 American Chemical Society Published on Web 02/27/1999