J . Am. Chem. SOC.1993, 115,901-905 We acknowledge the use of a VG 7 0 4 mass spectrometer purchased through funding from the National Institutes of Health, S10-RR-02478, and 300-MHz and 360-MHz N M R instruments purchased through funding from the National Science Foundation, NSF CHE-85-16614 and NSF CHE-8206103, respectively. Some
90 1
mass spectral determinations were made at the Midwest Center for Mass Spectrometry with partial support by the National Science Foundation, Biology Division (Grant No. DIR9017262). We thank Mr. John Miller and Ms. Stacey Johnson for their preliminary efforts on the project described in this manuscript.
Mechanism of the Reaction of Methylene with Benzene: A Study of Kinetic Hydrogen Isotope Effects and Theoretical Calculations Nikolai Hartz, G. K.Surya Prakash, and George A. Olah* Contribution from the Donald P . and Katherine B. Loker Hydrocarbon Research Institute and Department of Chemistry, University of Southern California, L o s Angeles, California 90089- 1661. Received February 19, 1991. Revised Manuscript Received November 10, 1992
Abstract: The reaction mechanism of singlet and triplet methylene with benzene and related aromatic compounds was investigated by kinetic isotope effects (KIEs), solvent effects, and product studies. The results are further rationalized by a series of ab
and UMP2/6-31G*//UHF/6-31G* levels of theory. The proposed IC initio calculations at MP2/6-31G*//RHF/6-31G* intermediate for the triplet reaction was found by means of the calculations, whereas no singlet analog 1 could be found.
Introduction The reaction of diazomethane with aromatic compounds is well-known in organic chemistry.’ Various ratios of cycloheptatriene (via norcaradiene) and toluene are obtained depending on the reaction conditions. Although the reactivity of benzene with electrophiles and the mechanism of many methylene reactions are well established in the literature,2 no detailed study of this particular reaction mechanism has been reported so far.
:CH2
/ \
6-0
In general, two possible pathways are conceivable for the attack of singlet methylene on the aromatic ring: (i) direct C-H and C - C bond reaction giving products by two independent concerted mechanisms (similar to the singlet methylene reaction with aliphatic and olefinic compounds),3 and (ii) a multistep reaction mechanism involving a distinct intermediate that subsequently rearranges to the products. The intermediate in the latter path would resemble the Wheland intermediate of the typical electrophilic aromatic substitution mechanism. It can be depicted as a neutral zwitterionic species, 1 (i.e., benzeniummethylide). (1) (a) Meerwein, H.; Disselnkotter, H.; Rappen, F.; v. Rintelen, H . ; van de Vloed, H. Liebigs Ann. Chem. 1957, 151-154. (b) Lemmon, R. M.; Strohmeier, W. J . Am. Chem. SOC.1959, 81, 106-108. (c) Doering, W . v. E.; Knox, L. H . J . Am. Chem. SOC.1950, 72, 2305-2306. (d) Doering, W . v. E.; Knox, L. H . J . Am. Chem. SOC.1953, 75, 297-303. (2) Lowry, T. H.; Schueller Richardson, K. Mechanism and Theory in Organic Chemistry, 3rd ed.; Harper & Row: New York, 1987; pp 553-562, 623-639. (3) (a) Doering, W . v. E.; Prinzbach, H . Tetrohedron 1959,6, 24-30. (b) Dobson, R. C.; Hayes, D. M.; Hoffmann, R. J . Am. Chem. SOC.1971, 93, 6188-6192. (c) Hoffmann, R. J . Am. Chem. SOC.1968, 90, 1475-1485. (d) Skell, P. S.; Woodworth, R . C . J . Am. Chem. SOC.1956, 78, 4496-4497.
The effect of different solvents on the reaction of methylene with a variety of substrates has been studied4 previously. The changes in product distribution in aromatic and aliphatic solvents were discussed in terms of the formation of complexes between the carbene and the solvent. Schoeller5 has calculated structures analogous to 1 as possible transition states for the 1,5-walk rearrangement of norcaradienes, but so far no stable intermediate with a comparable structure has been reported. At this point it should be mentioned that the concerted thermal walk rearrangement does not show toluene derivatives as significant side products.4b However, experimental evidence for a distinct intermediate was found in pyrolysis studies of norcaradiene6 and norbornadiene,’ where toluene was predominantly formed. The pyrolysis reactions carried out at temperatures around 475 OC in the gas phase suggest the presence of a diradical intermediate.* We wish to report now an investigation of the reaction between methylene and benzene as well as some other aromatics. Kinetic isotope effects (KIEs), reaction product ratios for different substrates and spin states, and solvent effects were studied to in(4) (a) Russell, G . A.; Hendry, D. G.J . Org. Chem. 1963, 28, 1933-1934. (b) Neugebauer, S. M.; DeLuca, J. P. Tetrahedron Lerr. 1989, 30, 7169-7172. (c) Tomioka, H.; Ozaki, Y.; Izawa, Y . Tetrahedron 1985, 41, 4987-4993. ( 5 ) Schoeller, W. W . J . Am. Chem. SOC.1975, 97, 1978-1980. (6) Klump, K. N.; Chesick, J . P. J . Am. Chem. SOC.1963, 85, 130-132. (7) Woods, W . G.J . Org. Chem. 1958, 23, 110-112. (8) Wentrup, C . Reakriue Zwischenstufen; Georg Thieme Verlag: Stuttgart, 1979; Vol. 1 , p 55.
0002-7863 /93/ 1515-901%04.00/0 0 1993 American Chemical Society
902 J . Am. Chem. SOC.,Vol. 115, No. 3, 1993
Harts et al. ile I. Product Ratios of Singlet Methylene Reaction with Ferent Aromatic Substrates Substrate
C-H addition product
C=C additton product
4 1
I
I
1
0.2
0.4
0.6
I
OB
--
t I
.".,.BX
Figure 1. Plot of mole fraction of benzene in different halogenated solvents versus the observed reaction product ratio.
vestigate the existence of a benzeniummethylide intermediate. A series of ab initio calculations were also carried out in order to find structures similar to 1 as stationary points on both the singlet and the triplet surface of the system. On the basis of these results a mechanism is proposed that best fits the available data.
Results and Discussion Isotope effects were measured by photolysis of diazomethane in equimolar solutions of deuteriated and nondeuteriated solvents at 25 "C (see Experimental Section for details). Diazomethane was generated by hydrolysis of N-nitroso-N-methylurea and subsequently dried over KOH pellets. Triplet methylene was generated by photolysis of diazomethane with 3 13-nm light in the presence of benzophenone as sensitizer. The quantitative measurement of the isotope effects as well as the product ratios was made by integration of the G C peaks. The response factors for the correctly labeled and unlabeled toluene were determined with independently obtained reference compounds. In the reaction of benzene with methylene, the kinetic isotope effect (kH/kD) for toluene was measured to be 1.34 f 0.04. For the formation of cycloheptatriene (CHT), the kH/kDwas 1.05 f 0.04. Investigation of the isotope effects for the reaction of methylene with toluene gave the following results: kH/kD(ethylbenzene)= 1.3 1 f 0.04, kH/kD(O-Xylene) = 1.16 f 0.04, kH/kD(m,p-XyleneS) = 1.46 f 0.04, and kH/kD(methylated cycloheptatriene) = 1.02 f 0.04. The kinetic isotope effect for the formation of cycloheptatriene from benzene is not conclusive since none of the C-H bonds need to be broken. The observed effect can thus only be secondary in nature. Assuming that the formation of 1 occurs in the ratedetermining step, the isotope effect would be secondary for the formation of toluene. However, previous investigations suggest that the transformation of a trigonal carbon into a tetrahedral center in the transition state should lead to a reverse kinetic isotope e f f e ~ t . ~This is not observed in this case. The relatively small value of the measured primary isotope effect can be rationalized in terms of an unsymmetrical nonlinear transition state due to the high reactivity of methylene. Comparably small primary KIES were already reported for methylene reactions with substrates that are not able to form distinct intermediates.1° The rate-determining formation of a single intermediate from singlet methylene and benzene can be further excluded since two different isotope effects are being observed for the two products. The effect of different solvents on the reaction was investigated by measuring the product ratio ([CHT]/[toluene]) as a function of the benzene cmcentration in different halogenated solvents. (9) (a) Streitwieser, A,, Jr.; Jagow, R. H.; Fahey, R. C.; Suzuki, S. J . Am. Chem. SOC.1958, 80, 2326-2332. (b) Olah, G. A. Arc. Chem. Res. 1971, 4, 240-248.
(IO) (a) Cheswick, J. P. J . Am. Chem. SOC.1962,84, 2448. (b) Kirmse, W. Carbene Chemisrry; Academic Press: New York, 1964; Vol. I, p 23. (c) Seyferth, D.; Cheng, Y. M. J. Am. Chem. SOC.1973,95,6763-6770. (d) Su, D. T. T.; Thornton, E. R. J . Am. Chem. SOC.1978, 100, 1872-1875. (e) Barcus, R. 1.;Hadel, L. M.; Johnston, L. J.; Platz, M. S.; Savino, T. G.; Scaiano, J. C. J . Am. Chem. Sor. 1986, 108, 3928-3937. (f) Hadel, L. M.; Platz, M. S.; Scaiano, J. C. J . Am. G e m . SOC.1984, 106, 283-287.
2 1
C=C addition
