1653
Singlet Methylene Reactions with Cycloalkenes
Selectivity of Singlet Methylene Reactions with Cycloalkenesla T. L. Rose,* A. E. Haas,Ib T. R. Powers, and J. M. WhitneyIb Department of Chemistry, Texas A&M University, College Station, Texas 77843 (Received January 29, 1976) Publication costs assisted by the Robert A. Welch Foundation
The gas phase reactions of singlet methylene with cyclohexene and cyclopentene over a wide pressure range gave the expected products from C-H insertion and C=C addition; no subsequent decomposition or rearrangement was observed. The product ratios gave intramolecular, per bond, relative insertion rates into the vinylic, allylic, and nonallylic C-H bonds which are the same as for acyclic olefins. Competition reactions with isobutene, however, revealed that the absolute magnitude of these rates is about 50-80% higher than that of the previously studied olefins. The C=C addition rate, on the other hand, is in the same range as that observed for straight chain 1-alkenes. A dynamic effect is proposed to account for the selectivity of both the inter- and intramolecular effects in C-H insertion. The question of selectivity in the reactions of methylene and stored in a butylphthalate matrix at liquid nitrogen has been a topic of interest ever since 1956 when Doering et temperature. Cyclopentene (99.9%)and cyclohexene (99.9%) al. reported a study of methylene reactions with liquid hywere used as received from Chemical Samples, Inc. Matheson drocarbons and commented that “Methylene must be clasoxygen (99.6%),perfluoropropane (~WO), and isobutene (9Wo) sified as the most indiscriminate reagent known in organic were used directly from the containers. chemistry.”2 In the work that followed, the importance of The sample preparation and irradiation procedures have phase effects, singlet and triplet methylene reactivity, and been described previously.12To increase the surface to volume methylene precursors were elucidated. In the gas phase, sinratio for some of the runs, the pyrex irradiation vessel was glet methylene reacts with primary, secondary, and tertiary packed with 6 mm pyrex tubes into which slits had been cut. C-H bonds in the ratio of 1:1.31.4. This selectivity is less Irradiations were done at 436 and 366 nm using a 200-W pronounced in the liquid phase.3 For acyclic alkenes, the inhigh-pressure Hg lamp and narrow bandpass filters ( f 1 0 nm). sertion rate into vinylic C-H bonds is about 65% as fast as into All single compound irradiations were done at room temperallylic and paraffinic bond^.^^^ The per bond methylene adature and lasted, in general, about 3 h. The competitive redition to the C=C bond is about eight to ten times as fast as actions were carried out using a 100-W lamp and a 6-h irraC-H insertion. diation time. Cyclopentene and cyclohexene are ideal systems to measure The product spectrum was analyzed by FID gas chromathe selectivity of methylene reactions. They contain three tography. Separation of the products was accomplished using types of C-H bonds as well as the C=C. They are large enough a % in. X 40 f t stainless steel column with 10%polypropylene that subsequent reactions of the bicyclic compounds or the glycol on Anakrom C-22A. 1,l-Dimethylcyclopropaneand methylcycloalkenes formed can be stabilized a t easily acces3-methylbutene-1 were not separated, but since they both sible pressures. Methylene reactions in liquid cyclohexene arise from C=C addition to isobutene, their total yield was were studied in Doering’s early work2 and later by Kopecky, used. For the cyclopentene system the relative retention Hammond, and leer maker^.^ This liquid work showed general volumes of the products to cyclopentene at 25 “C were 1.57 agreement with the acyclic alkenes for C-H insertion and a (3-methylcyclopentene), 1.65 (4-methylcyclopentene), 2.42 slightly reduced ( ~ 5 0 %reactivity ) at the C=C. Our work (1-methylcyclopentene), and 3.06 (bicyclo[3.1.0]hexane).At represents the first study in the gas phase of cycloalkene 60 “C, the relative retention volumes of the CTH12 products systems.6 In order to compare the reactivity of the cyclic and to cyclohexene were 1.49, 1.52, 1.96, and 2.62 for 3-, 4-, and acyclic alkenes, competition studies with isobutene were done 1-methylcyclohexene, and bicyclo[4.1.O]heptane, respectively. as well. The resolution of the 3- and 4-methyl isomers was not comThe mechanism of addition of methylene to C=C and C-H plete, but was adequate to determine accurately their yields bonds has received a good deal of attention by theoreticians by electronic integration. Some of the early cyclopentene data recently.%ll The prediction of both detailed c a l c u l a t i ~ n s ~ ~ ~were J ~ obtained with an in. tandem column of 10 f t P,P’-oxyand MO followinglo is that methylene addition to the C=C dipropionitrile, 10 f t dimethylsulfolane, 5 f t silicone oil, and bond proceeds by an unsymmetrical pathway which is sym6 ft AgN03 and yielded results identical with the single metry allowed. In cycloalkenes where the geometry is more polypropylene glycol column. rigid than acyclic hydrocarbons, the effects of this unsymThe products were identified by comparison of their remetrical mechanism might be apparent. The C-H bond intention volumes with authentic samples and for the cyclosertion reaction should be less sensitive to the structure of the pentene products by comparison of their mass spectra with molecule if it involves an end-on approach to the H atom. As published values13 or mass spectra of authentic samples will be seen, however, the results from the cycloalkene studies measured in our laboratory. The product yields were calcushow that the C-H bond reactions are more sensitive to the lated from the peak areas measured by an electronic intecyclic structure than is the C=C addition pathway. grator. The FID sensitivity was measured for all the products and found to be directly proportional to the number of carbon Experimental Section atoms in the molecules within the precision of the measureDiazomethane was prepared from N,N’-nitrosomethylurea ments. The Journal of Physical Chemistry, Vol. 80, No. 15, 1976
1654
T. L. Rose, A. E. Haas, T. R. Powers, and J. M. Whitney
TABLE I: Reactions of 'CH, with Cyclopentenea Products Total pressure, Torr 296 199 201b 204" 96d 25 25 25 17 15 10
cb 0.12 0.12 0.12
37.0 36.1 39.1 40.1 * 0.4 38.1 38.7 39.4 39.3 38.4
w
'Q
9.8 8.8 10.3 9.6 t 0.2 9.3 10.9 10.6 8.3 11.7 12.2 11.8 11.7 12.9 10.6 t 1.4
35.3 17.9 38.6 16.4 34.4 16.2 0.10 34.7 t 0.3 15.5 i. 0.2 0.13 35.8 16.7 0.29 33.8 16.5 0.13 33.7 16.4 0.06 36.1 16.4 0.11 33.3 16.7 0.13 40.2 31.7 15.9 0.13 39.9 31.1 17.1 lob 0.13 39.6 30.1 18.5 1Od 0.12 39.3 31.2 16.5 Av 38.9 * 1.2 33.8 ?: 2.4 16.7 i: 0.8 a All reactions run using diazomethane as methylene precursor. Diazomethane:hydrocarbon ratio varied between 1:17 and 1:7. Irradiation times 3 h at 436 nm unless otherwise noted, b Packed reaction vessel. Surface to volume ratio increased by a factor of 22 over empty cell. C Averages of three samples extracted at 2, 4, and 6 h. d Irradiation wavelength 366 nm. TABLE 11: Reactions of 'CH, with Cyclohexenea Products Total pressure, from
600 0.10 603b 0.09 0.10 600d 304 0.10 84c 0.16 0.14 78 60 0.10 0.10 60b 60 (2 runs)d 0.10 54 0.05 25 0.34 12 0.13 Av a , b , c , d Same footnotes as in Table I.
35.2 34.4 34.0 34.6 32.0 t 0.8 30.7 31.3 30.9 37.3 t 0.6 31.4 32.5 32.8 33.1 * 2.0
Results The results for the reactions of methylene with cyclopentene are given in Table I and with cyclohexene in Table 11. Experiments were run over a pressure range of 10-300 Torr for cyclopentene and 10-600 Torr for cyclohexene. Perfluorpropane was added as an inert gas for experiments run a t pressures above the vapor pressures of the hydrocarbon. The products observed are the three methylcycloalkene isomers expected from C-H insertion and the bicyclic hydrocarbon resulting from C=C addition. Methylenecyclopentane was sought but was not observed at our limits of detectability (0.5%). Methylenecyclohexane would appear under the 4methylcyclohexene peak. However, from the absence of methylenecyclopentane in the cyclopentene system and the nearly equivalent yields of 3- and 4-methylcyclohexene, the methylene isomer yield is assumed to be very small. A small yield of cyclohexene (
