An Unusual Isotope Effect in the Reactions of the Naphthylcarbenes Fengmei Zheng, Michael L. McKee, and Philip B. Shevlin* Department of Chemistry, Auburn UniVersity Auburn, Alabama 36849-5312 ReceiVed May 24, 1999 Atomic carbon reacts with benzene and substituted benzenes on the singlet energy surface by an initial C-H insertion to generate a phenylcarbene which can either be trapped by an intramolecular reaction with a substitutent or ring expand to a cycloheptatetraene which can subsequently be trapped. 1-4 We have now used this method to generate the 1- and 2-naphthyl- carbenes, 1 and 2, and have found a remarkable isotope effect upon their reactivity. 5 Co-condensation of arc-generated carbon with naphthalene 3 at 77 K generates cyclobuta[de]naphthalene, 4, the 1- and 2-methylnaphthalenes, 5 and 6, along with compounds whose molecular weight indicates that they are products of the reaction of 1 and 2 with 3, and other uncharacterized products of the reactions of C 2 and C 3 with 3. Carbenes 1 and 2 have been well characterized in low-temperature matrixes, 6 in solution, 7 in the gas phase, 8 and computationally. 9 Both 1 and 2 rearrange to 4 at elevated temperatures, 8 and it is clear that the carbenes intercon- vert under these conditions. Calculations indicate that 4 is the thermodynamic sink and is accessible by rearrangement of 1 (E a ) 23.8 kcal/mol) and from 2 via 1 (E a ) 24 kcal/mol). 9 It thus appears that 4 is formed in our system from chemically activated singlet 1 which is generated in a reaction exothermic by roughly 117 kcal/mol. 10 Possible routes to the methylnaph- thalenes include H abstraction by triplet 1 and 2 and/or the addition of CH 2 (from consecutive H abstractions by C) to 3. That the formation of 4 requires thermal activation is indicated by the fact that raising the temperature of the surface upon which C + 3 is condensed increases the ratio of 4:(5 + 6). When 1 and 2 were generated independently by the C atom deoxygenation of the 1- and 2-naphthaldehydes, 7 and 8, 12 4 was formed in both reactions (Table 1), and we conclude that the exothermicity of the deoxygenation (ΔH )-112 kcal/mol) 10 brings about the interconversion of 1 and 2. 12 Addition of HBF 4 to the C + 3 low-temperature matrix does not trap a benzocycloheptatetraene 13 as benzotropylium ion, and addition of styrene does not trap the strained double bond in a benzobicyclo[4.1.0]heptatriene. 6a We thus conclude that singlets 1 and 2 either lose their excess energy and react intermolecularly or find their way to the thermodynamic sink, 4. The reaction of C with naphthalene-d 8 reveals a dramatic new dimension of reactivity in the naphthylcarbenes produced. As illustrated in Table 1, substitution of 3-d 8 for 3-d 0 increases the 4:(5 + 6) ratio by a factor of 26 at 77 K. Moreover, co- condensation of C with a 1:1 mixture of 3-d 0 and 3-d 8 generates 4-d 0 and 4-d 8 in a 0.12 ratio. These results appear to indicate an impossibly large inverse deuterium isotope effect on the formation of 4 in this system. Since both the formation of 1 and its rearrangement to 4 involve breaking a C-H bond in nonlinear transition states, modest normal primary isotope effects are expected. 14,15 However, a normal isotope effect upon the rate of intersystem crossing (ISC) in 1 would explain these results. In this scenario, 1 1-d 8 decays to the triplet much slower that 1 1-d 0 , and more 4-d 8 is generated in the reaction of C with 3-d 8 than in the reaction of C with 3-d 0 . A large k H /k D on the rate of ISC predicts more triplet products from 1-d 0 and 2-d 0 than from 1-d 8 and 2-d 8 . If we assume that 5 and 6 are triplet products, this is certainly the case (Table 1). However, as indicated above, 5 and 6 can also result from the addition of methylene to 3, and more definitive evidence for triplet products of carbenes 1 and 2 is needed. To further assess the role of triplets in this reaction, we have reacted C with 3-d 0 and 3-d 8 in the presence of E- and Z-2-butene and examined the stereochemistry of the carbene adducts as a function of deuterium content. 17 The results of these experiments, shown in eq 1 for Z-2-butene, are quite striking, with 4-18% of the trapped adducts (10 and 12) formed with loss of stereochemistry in the case of (1) Armstrong, B. M.; Zheng F.; Shevlin, P. B. J. Am. Chem. Soc. 1998, 120, 6007. (2) Gaspar, P. P.; Berowitz, D. M.; Strongin, D. R.; Svoboda, D. L.; Tuchler, M. B.; Ferrieri, R. A.Wolf, A. P. J. Phys. Chem. 1986, 90, 4691. (3) For a report of the reaction of triplet carbon with benzene in crossed beams experiment, see: Kaiser, R. I.; Hahndorf, I.; Haung, L. C. L.; Lee, Y. T.; Bettinger, H. F.; Schleyer, P. v. R.; Schaefer, H. F., III; Schreiner, P. R. J. Chem. Phys. 1999, 110, 6091. (4) For reviews of the chemistry of atomic carbon, see: (a) Skell, P. S.; Havel, J. J.; McGlinchey, M. J. Acc. Chem. Res. 1973, 6, 97. (b) MacKay, C. In Carbenes; Moss, R. A., Jones, M., Jr., Eds.; Wiley-Interscience: New York, 1975; Vol. II, pp l-42. (c) Shevlin, P. B. In ReactiVe Intermediates; Abramovitch, R. A., Ed.; Plenum Press: New York, 1980; Vol. I, pp 1-36. (5) The experimental setup is described in ref 1. Carbon atoms and substrate were co-condensed at 77 K, and products were anaylzed by GC, GC/MS, and NMR. (6) (a) Albrecht, S. W.; McMahon, R. J. J. Am. Chem. Soc. 1993, 115, 855. (b) Senthilnathan, V. P.; Platz, M. S. J. Am. Chem. Soc. 1981, 103, 5503 and references cited. (7) Platz, M. S.; Maloney, V. M. In Kinetics and Spectroscopy of Carbenes and Biradicals; Platz, M. S., Ed.; Plenum: New York, 1990; pp 242-262 and references cited. (8) (a) Becker, J.; Wentrup, C. J. Chem. Soc., Chem. Commun. 1980, 190. (b) Engler, T. A.; Shechter, H. Tetrahedron Lett. 1982, 23, 2715. (9) Xie, Y.; Schreiner, P. R.; Schleyer, P. v. R.; Schaefer, H. F. J. Am. Chem. Soc. 1997, 119, 1370. (10) This value has been estimated from the heats of formation of C ( 1 D), the organic precursor, and phenylcarbene. 11 (11) Poutsma, J. C.; Nash, J. J.; Paulino, J. A.; Squires, R. R. J. Am. Chem. Soc. 1997, 119, 4686. (12) Deoxygenation of aryl aldehydes is known to generate chemically activated carbenes: Rahman, M.; Shevlin, P. B. Tetrahedron Lett. 1985, 26, 2959. (13) Coburn, T. T.; Jones, W. M. J. Am. Chem. Soc. 1974, 96, 5218. (14) A kH/kD ) 1.77 has been observed for insertion of carbon into the aromatic C-H bond of tert-butylbenzene. 1 A kH/kD > 1 for the rearrangement of 1 to 4 was calculated using the method of Bigeleisen and Mayer: Bigeleisen, J.; Mayer, M. G. J. Chem. Phys. 1947, 15, 261. The calculations used the program QUIVER (Saunders, M.; Laidig, K. E.; Wolfsberg, M. J. Am. Chem. Soc. 1989, 111, 8989). (15) A reviewer has made the point that the observed isotope effects could arise from a normal kH/kD on the decomposition of 4. While this reaction has not been studied, byproducts observed when 4 is generated by FVP indicate that the rate-determining step would be cleavage of a C-C bond in the four- membered ring. 16 The kH/kD for this step would be small. (16) Engler, T. A.; Shechter, H. J. Org. Chem. 1999, 64, 4247 and references cited. 11237 J. Am. Chem. Soc. 1999, 121, 11237-11238 10.1021/ja9917013 CCC: $18.00 © 1999 American Chemical Society Published on Web 11/18/1999