β-Lactam-Forming Photochemical Reactions of N-Trimethylsilylmethyl- and N-Tributylstannylmethyl-Substituted r-Ketoamides Runtang Wang, Chuanfeng Chen,1 Eileen Duesler, and Patrick S. Mariano* Department of Chemistry, University of New Mexico, Albuquerque, New Mexico 87131
Ung Chan Yoon Department of Chemistry, Chemistry Institute for Functional Materials and College of Natural Sciences, Pusan National University, Pusan 609-735, Korea
[email protected] Received November 10, 2003
Two mechanisms have been proposed for the β-lactam-forming photochemical reactions of R-ketoamides. One, suggested by Aoyama, involves excited-state H-atom abstraction while the other, put forth by Whitten, follows a sequential SET-proton-transfer route. The photochemical properties of N-trimethylsilylmethyl- and N-tributylstannylmethyl-substituted R-ketoamides were explored in order to gain information about the mechanism of this process and to develop a regioselective method for β-lactam formation. The results of this effort show that (1) photoreactions of N-trimethylsilylmethyl-substituted R-ketoamides proceed by competitive H-atom abstraction and sequential SET-desilylation pathways and (2) a sequential SET-destannylation pathway is preferentially followed in photochemical reactions of the tributylstannylmethyl-substituted R-ketoamides. Introduction
SCHEME 1
Far too seldom has the long-range goal of mechanistic photochemical studies been the development of synthetically useful reactions. As a result, the synthetic potential of exited-state reactions has often gone unnoticed. An example of this is found in studies of photoinduced reactions of R-ketoamides 1 that produce β-lactams 5 (Scheme 1). Following a number of earlier studies,2-6 Aoyama and co-workers7-10 conducted a series of exploratory investigations that focused on the scope and mechanism of this process. This group proposed that this reaction is initiated by hydrogen atom abstraction by the carbonyl oxygen of the excited ketone from an amide R-carbon. The initially formed intermediate in this pathway is either a triplet 1,4-biradical 3 or a singlet 1,4dipole 4, depending upon the multiplicity of the reacting excited state of 1. Electrocyclic ring closure of the dipole then produces the β-lactam 5 along with other products, (1) Current address: Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China. (2) Akermark, B.; Johansson, N.-G. Tetrahedron Lett. 1969, 371. (3) Henery-Logan, K. R.; Chen, C. G. Tetrahedron. Lett. 1973, 1103. (4) Zhehavi, U. J. Org. Chem. 1977, 42, 2821. (5) Shozaki, M.; Hiraoka, T. Synth. Commun. 1979, 9, 179. (6) Shima, K.; Tanabe, K.; Furukawa, S.; Saito, J.; Shirahashi, K. Bull. Hem. Soc. Jpn. 1984, 57, 1515. (7) Hasegawa, T.; Watabe, M.; Aoyama, H.; Omote, Y. Tetrahedron 1977, 33, 485. (8) Aoyama, H.; Hasegawa, T.; Watabe, M.; Shirashi, H.; Omote, Y. J. Org. Chem. 1978, 43, 419. (9) Aoyama, H.; Sakamoto, M.; Omote, Y. J. Chem. Soc., Perkin Trans. 1 1981, 1357. (10) Aoyama, H.; Sakamoto, M.; Kuwabara, K.; Yoshida, K.; Omote, Y. J. Am. Chem. Soc. 1983, 105, 1958.
including oxazolidinone 6 that derives from intramolecular addition of the hydroxyl group to the iminium cation. Additional evidence for the intermediacy of dipole 4 in this process comes from the observation that R-hydroxamides 7 are produced by photoreactions conducted in MeOH. As with other photochemical reactions promoted by carbonyl H-atom abstraction, this process proceeds in high yields when the amide nitrogen contains identical alkyl substituents. Although an insufficient number of examples exist8 to conclusively prove the point, one expects that R-ketoamides which possess two different
10.1021/jo030343q CCC: $27.50 © 2004 American Chemical Society
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J. Org. Chem. 2004, 69, 1215-1220
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Wang et al. SCHEME 2
alkyl groups on nitrogen would be transformed to mixtures of β-lactams by this pathway. In a later discussion of this novel photochemical process, Chesta and Whitten11 suggested that a single electron transfer (SET) mechanism is operable. In the SET pathway, intramolecular proton transfer from the amide cation radical in zwitterionic biradical 2 to the ketone anion radical generates the same intermediates, 3 and 4, that are produced by the H-atom abstraction route. However, Chesta and Whitten’s proposal11 was not supported by any conclusive experimental observations, and consequently, it cannot be used reliably to predict the behavior of unsymmetric and/or structurally complex systems. The interrelated issues of mechanism and synthetic application of the β-lactam forming photoreactions of R-ketoamides were of interest to us as result of earlier investigations carried out in our laboratories in the area of SET photochemistry. In particular, our earlier studies focusing on the kinetics of aminium radical reactions12,13 and the excited-state reactions of N-silylmethylimides14,15 seemed to be related to these issues. By using a combination of product distribution and laser flash photolysis methods, we showed that the rates of silophile induced desilylation of R-silylamide cation radicals far exceed those of base-induced R-deprotonation.12,13 An example of this kinetic preference is found in the photochemical generation of azomethine 10 from the N-silymethylmaleimide 8, formed by a pathway involving the intermediacy of zwitterionic biradical 9 (Scheme 2). In this process, transfer of trimethylsilyl group to the oxyanion center occurs more rapidly than does proton transfer. Other efforts in our laboratories demonstrated that the rates of R-destannylation reactions of cation radicals exceeds those of R-desilylation.16,17 In another pertinent investigation, Yoshida and co-workers18,19 observed that R-trialkylsilyl and R-tributylstannyl substitution dramatically increases the thermodynamic stability of amine and amide cation radicals. These effects, seen in the oxidation potentials of the corresponding nitrogen containing electron donors, makes SET from R-silyl- and R-stannylamines and R-silyl- and R-stannylamides thermodynamically/kinetically more favorable. (11) Chesta, C. A.; Whitten, D. G. J. Am. Chem. Soc. 1992, 114, 2188. (12) Zhang, X.; Yeh, S.-R.; Hong, S.; Freccero, M.; Albini, A.; Falvey, D. E.; Mariano, P. S. J. Am. Chem. Soc. 1994, 116, 4211. (13) Su, Z.; Mariano, P. S.; Falvey, D. E.; Yoon, U. C.; Oh, S. W. J. Am. Chem. Soc. 1998, 120, 10676. (14) Yoon, U. C.; Kim, D. U.; Lee, C. W.; Choi, Y. S.; Lee, Y. J.; Ammon, H. L.; Mariano, P. S. J. Am. Chem. Soc. 1995, 117, 2698. (15) Yoon, U. C.; Cho, S. J.; Lee, Y. J.; Mancheno, M. J.; Mariano, P. S. J. Org. Chem. 1995, 60, 2353. (16) Borg, R. M.; Mariano, P. S. Tetrahedron Lett. 1986, 2821. (17) Yoon, U. C.; Jin, Y. X.; Oh, S. W.; Cho, D. W.; Park, K. H.; Mariano, P. S. J. Photochem. Photobiol. Chem. A 2002, 150, 77. (18) Yoshida, J.; Maekawa, T.; Murata, T.; Matsunaga, S.; Isoe, S. J. Am. Chem. Soc. 1990, 112, 1962. (19) Yoshida, J.; Itoh, M.; Isoe, S. J. Chem. Soc., Chem. Commun. 1993, 547.
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Based on these observations, we anticipated that zwitterionic biradicals 12, formed by intermolecular SET in the excited states of N-trimethysilylmethyl- or Ntributylstannylmethyl-substituted R-ketoamides 11 would preferentially participate in either intermolecular or intermolecular silyl or stannyl group transfer rather than proton-transfer processes (Scheme 3). In addition, we believed that this selectivity might serve as the foundation for regioselective and, thus synthetically useful, β-lactam-forming photochemical reactions. The proposal presented above guided the current studies, in which product distributions, arising from photoreactions of R-silyl- and R-stannyl-substituted Rketoamides, were determined and translated into conclusions about the mechanism(s) for β-lactam formation. Results emanating from this effort demonstrate that (1) β-lactam forming photoreactions of R-silyl-substituted R-ketoamides are not regioselective because they occur by competitive H-atom abstraction and sequential SETdesilylation pathways and (2) N-tributylstannylmethylsubstituted R-ketoamides undergo regioselective photoreactions to produce β-lactams through the near exclusive operation of a sequential SET- destannylation route. Results and Discussion Four R-ketoamides, containing N-trimethylsilylmethyl or N-tributylstannylmethyl substituents, were prepared in order to investigate the issues discussed above. Reactions of the in situ prepared benzoylformic acid chloride with commercially available N-benzyl- (13) and N-methyl-N-trimethylsilylmethylamine (14) were used to produce the corresponding R-silylketoamides 15 and 16 (Scheme 4). An alternate approach, involving N-alkylation of the anion, formed by sodium hydride treatment of the secondary ketoamides 17 and 18, with tributylstannylmethyl iodide,20 was employed to prepare the analogous tin-containing substrates 19 and 20 (Scheme 5). Preparative photochemical reactions of the ketoamides were explored next. Solutions of 15, 16, 19, and 20 in MeOH and MeCN were irradiated with Pyrex glass filtered light (λ > 290 nm) for time periods required to (20) Ahman, J.; Somfai, P. Synth. Commun. 1994, 24, 1117.
β-Lactam-Forming Photochemical Reactions of R-Ketoamides SCHEME 5
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TABLE 1. Products and Yields of Photoreactions of r-Ketoamides 15, 16, 19, and 20 ketoamide
Solvent
products (% yield)
15
MeOH
15
MeCN
16 16 19 19 20 20
MeOH MeCN MeOH MeCN MeOH MeCN
21 (6), 22 (14), 23 (3), 27 (3), 28 (10), 29 (7), 32 (11), 33 (5), 37 (10) 21 (10), 22 (22), 23 (6), 27 (3), 38 (8), 39 (4), 40 (22) 24 (4), 25 (3), 30 (5), 35 (6), 35 (10) 36 (23) 24 (14), 25 (7), 31 (6), 41 (6), 42 (22) 21 (37), 26 (3), 27 (10), 32 (25), 37 (8) 21 (42), 26 (3), 27 (7) 24 (40, 31 (9), 36 (3) 24 (44), 31 (5)
promote >90% conversion of the substrates. Products were separated from each of the mixtures by using preparative TLC. Structural assignments to all previously uncharacterized photoproducts were made by using a combination of spectrophotometric and X-ray crystallographic techniques. As can be seen by viewing the data in Table 1, the mass balances of products generated in these photochemical reactions are less than 100%. In each case, the remaining material is comprised of a mixture of very minor (
