J . Phys. Chem. 1986, 90, 4691-4694
4691
Reactions of Recoiling "C Atoms with Toluene Peter P. Gaspar,* David M. Berowitz, Daniel R. Strongin, Daniel L. Svoboda, Matthew B. Tuchler, Department of Chemistry, Washington University, St. Louis, Missouri 631 30
Richard A. Ferrieri, and Alfred P. Wolf Department of Chemistry, Brookhaven National Laboratory, Upton, New York 11973 (Received: September 30, 1985; In Final Form: April 14, 1986)
Formation of benzocyclobutene,a characteristic rearrangement product of tolylmethylenes, strongly suggests that these species are formed as intermediates in the reactions of recoiling carbon-1 1 atoms and toluene. Degradation and additive studies indicated that both singlet and triplet carbon atoms can lead to benzocyclobutene formation through insertion into ring C-H bonds. Styrene is also formed, predominantly by insertion of singlet C- 11 atoms into a methyl C-H bond, followed by rearrangement of the resulting benzylmethylene.
Introduction The Ir-electrons and C-H bonds of benzene both offer attractive targets for electron-deficient species like free carbon atoms. Investigations over a 3-decade period1-I3 have led to the following conclusions: 1. High-energy as well as thermal-energy carbon atoms can react with aromatic molecules.6 2. The large variety of products formed from simple substituted benzenes points to a complex reaction mechanism.'*I2 3. The reactivity of carbon atoms toward aromatic systems is quite high, but the yields of fragmentation products (such as acetylene) are This suggested that various carbon atom adducts might be formed as reactive intermediates. 4. Symmetrical intermediates such as carbon atom-benzene *-complexes are not important contributors to the formation of the observed product^.^^'^ Despite various suggestions of possible reactive intermediates?b*'' the site of initial attack by a carbon atom on an aromatic ring has remained an unanswered question. The present experiments were prompted by the recognition that the two most likely modes of attack, addition to a carbon-carbon *-bond and insertion into a C-H bond of a suitable substrate, could reveal themselves in a new reaction product, heretofore unobserved. While the same product could result from *-addition and C-H insertion, the distribution of a labeled carbon atom in chemically stable end products can reveal the extent to which each process contributes to their formation. Toluene was chosen as the reaction substrate so that the methyl group could act as a trap to convert reactive intermediates into chemically stable products by a pathway that would preserve the distinction between various modes of initial attack. Insertion of a carbon into a ring C-H bond of toluene would give rise to the three isomeric tolylmethylenes shown in Scheme I. Addition to a carbon-carbon ?r-bond can give three isomeric ~~
I
I
..
FH
" P
~
(1) Giacamello, G. Ric. Sci. 1951, 21, 1211. (2) Giacamello, G.; Croatto, U.; Maddock, A. G. Ric. Sci. 1951,21, 1598. (3) Zifferero, M. Ann. Chim. (Rome) 1954, 21, 1211. (4) Wolf, A. P.; Redvanly, C. S.;Anderson, R. C. Nature (London) 1955, 176, 831. (5) Schrodt, A. G.; Libby, W. F. J. Am. Chem. SOC.1956, 78, 1267. (6) Suryanarayana, B.; Wolf, A. P. J. Phys. Chem. 1958, 62, 1369. (7) Visser, R.; Redvanly, C. S.; Sixma, F. L. J.; Wolf, A. P. Rec. Trav. Chim. Pays-Bas 1961,80, 533. (8) Sprung, J. L.; Winstein, S.;Libby, W. F. J. Am. Chem. SOC.1%5,87, 1812. (9) Rose, T.; MacKay, C.; Wolfgang, R. J. Am. Chem. SOC.1967, 89, 1530. (10) Williams, R. L.; Voigt, A. F. J. Phys. Chem. 1969, 73, 2538. (1 1) Lemmon, R. M. Acc. Chem. Res. 1976,80, 2094, and earlier refer-
ences contained therein. (12) Brinkman, G. A.; Gerritsen, G. A. V.; Visser, J. Radiochem. Radioanal. Lett. 1979, 41, 383. (13) Wolf, A. P., unpublished work.
bicyclic carbenes expected to undergo rapid valence tautomerization to isomeric cyclic allenes. The isomeric tolylmethylenes and cyclic allenes can all undergo interconversion by a reaction sequence in which benzocyclobutene and styrene are formed as stable products.14J5 The key feature
0022-3654/86/2090-469lSOl .50/0 0 1986 American Chemical Society
4692
The Journal of Physical Chemistry, Vol. 90, No. 19, 1986
SCHEME I11
Gaspar et al. SCHEME IV U
r i n g C-H
SCHEME V
*C i-
@5 T
rddit ion
C
@J- H
3
Experimental Section
Materials. Toluene used in the target system was purchased from Mallinckrodt and was further purified by conventional degassing techniques on a vacuum line. Additive gases included research grade oxygen (>99.98% purity), neon (99.998% purity), and xenon (99.98% purity) which were purchased from the Matheson Gas Co. Styrene used in chemical degradation and gas chromatography calibrations was purchased from Mallinckrodt and used without further purification. Benzocyclobutene was used for the same purposes and was synthesized according to previously described methods. I (14) Baron, W. J.; Jones, M., Jr.; Gaspar, P. P. J. Am. Chem. SOC.1970, 92, 4739. (15) Gaspar, P. P.; Hsu,J.-P.; Chari, S.; Jones, M., Jr. Tetruhedron 1985, 41, 1479. (16) The essence of the rearrangements depicted in Scheme I1 is the interchange of the roles of all but one of the ring carbon atoms of the phenylmethylene formed by carbon atom insertion into a ring C-H bond. The degenerate process that effects the interconversion is a Wolff rearrangen~ent:'~
..
co2
t
a A
of this rearrangement process is that it does not interchange a labeled carbon introduced via C-H insertion with one resulting from 9-addition. The specific nature of the rearrangement may be seen by following a carbon atom that inserts into a toluene para C-H bond through each step to the stable products, as shown in Scheme II.I6 A reaction sequence such as that of Scheme I1 can be written for each of the primary products of ring C-H insertion and aaddition shown in Scheme I. These can be summarized by the prediction that ifa reaction of a labeled carbon atom with toluene in the gas-phase" produces either a tolylmethylene via ring C-H insertion, or a cyclic allene by *-addition and rearrangement, benzocyclobutene and styrene will be the products of intramolecular rearrangements. The labeling pattern of these products will reveal the contributions to product formation from primary insertion and addition processes. Primary insertion into the methyl group is expected to give exclusively styrene, with its own characteristic labeling pattern. The predicted labeling patterns are shown in Scheme 111.
W
C02H
.. Y
Note that the roles of V and Z and of W and Y have been interchanged. The unique carbon whose identity is preserved in this rearrangement is X, corresponding in Scheme I1 to the carbon atom whose hydrogen was attacked by insertion. In n-bond addition (Schemes I and 111) the unique carbon is the attacking atom, corresponding to X in the Wolff rearrangement shown here. (17) In solution, intermolecular reactions will be competitive with rearrangement. (18) Sanders, A.; Giering, W. P. J . Org. Chem. 1973, 38, 3055.
Target Preparation and Irradiations. Neat toluene samples were prepared for irradiation by sealing 0.1 mL of toluene liquid into an evacuated 30-mL quartz irradiation vessel equipped with a Teflon-brand stopcock. The toluene pressure within the vessel was maintained at a constant 20 Torr. For mixed toluene-neon, toluene-oxygen, and toluene-xenon samples, the vessel was first loaded with toluene as described above and then reequilibrated with higher pressures of additives. Samples with additives were always irradiated at ambient temperature. The irradiations were carried out with a 33-MeV proton beam from the Brookhaven 60-in. cyclotron, where recoil carbon-1 1 atoms were generated via the '*C(p,pn)"C nuclear transformation. Typical beam intensities were 1 pA with exposure times of between 60 and 100 s. The radiation dose was 1.1 1 X eV molecule-' PA-] s-l as determined by acetylene to benzene d 0 ~ i m e t r y . I ~ Radioassay of " C Activity. Following irradiation, the volatile 'IC activity was determined by counting a 1-mL aliquot of the gaseous portion of the target using a well-type NaI(T1) scintillation crystal. This was accomplished by freezing the 0.1 mL of toluene into a cold finger on the vessel at -196 OC while simultaneously heating the vessel walls to draw high boiling components into the matrix. As the matrix was warmed to ambient temperature, a Teflon-brand sealed gas syringe (Precision Sampling Corp.) was inserted into the vessel through a silicone septum, and a 1-mL gas aliquot was withdrawn for subsequent scintillation counting. The target was then washed with 1 mL of petroleum ether. The 1.1-mL mixture was extracted from the vessel. Aliquots withdrawn from this mixture were subjected to radiogas chromatographic analysis and total "C assay by using the above scintillation counter. Additional washings were performed on the vessel to ensure complete removal of ["Clproducts. The total carbon-1 1 activity (TA) was determined by summing the combined volatile and wash activities after appropriate radioactive decay and fraction corrections were made. The samples subjected to radiogas chromatography provided a measure of the ["C]benzocyclobutene and ["Clstyrene yields along with a number of other products which will not be reported here. The radioactive compounds eluted from the column were measured by gas effluent counting.20 Product identification was made on the basis of retention index relative to authentic samples on four columns: (i) a 25-ft column of 20% SE-30 on 50-80 mesh Anakrom ABS; (ii) a 20-ft column of 30% 1,2,3-tris(2-cyanoethoxy)propane on 80-100 mesh Chromosorb P; (iii) a 20-ft column of 5% bentone 34 5% diisodecylpthalate on 60-80 mesh Chromosorb P; and (iv) a 20-ft column of 10% Apiezon L grease on 80-100 mesh Chromosorb P. Individual product yields were calculated by dividing the decay-corrected integrated peak activities obtained from the effluent counter by the TA. Degradation of ["CJBenzocyclobuteneand ["C]Styrene. Both ['IC] benzocyclobutene and ["Clstyrene were purified with authentic carrier by preparative gas chromatography. In Scheme IV, a degradation of benzocyclobutene is shown that allowed the distribution of label between the methylene groups and the aromatic ring to be determined. In the first stage, benzocyclobutene was catalytically oxidized to phthalic anhydride over alumina supported V , 0 5 (19 wt %; Chemical Dynamics Corp.). The
+
(19) Finn, R. F.; Ache, H.J.; Wolf, A. P. J. Phys. Chem. 1970, 74, 3194. (20) Welch, M. J.; Withnell, R.; Wolf, A. P. Chem. Instrum. (New York) 1969, 2, 177.
The Journal of Physical Chemistry, Vol. 90, No. 19, 1986 4693
Reactions of Recoiling l l C with Toluene
TABLE I: Absolute Yields of Styrene and Benzocyclobutene from the Reactions of Recoiling "C Atoms with Toluene" % additiveb
product
% neon
standard sample, 20 Torr
97.0
3.2 f 0.5 3.8 f 0.8
0.6 f 0.1 0.6 f 0.1
benzocvclobutene
styrene
% oxygen
% xenon
98.1 0.4 f 0.1
99.oc
28.3
90.0
