5546 Acknowledgments. W e thank the National Research Council of Canada for a scholarship to D.A.S. and Professors V. H. Smith and A. R. Norris for helpful discussions. References and Notes (1) G. W. Parshall and J. J. Mrowca, Adv. Organornet. Chem., 7, 157 (1968). (2) 0. A. Reutov and I. P. Beletskaya, "Reaction Mechanisms of Organometallic Compounds", North-Holland Publishing Co., Amsterdam, 1968. (3) F. R. Jensen and B. Rickborn, "Electrophilic Substitution of Organomercurials", McGraw-Hill, New York, N.Y., 1968. (4) M. H. Abraham, in "Comprehensive Chemical Kinetics", Vol. 12, C. H. Barnford and C. F. H. Tipper, Ed., Elsevier, Amsterdam, 1973. (5) D. S.Matteson, "Organometallic Reaction Mechanisms", Academic Press, New York, N.Y., 1974. (6) R. G. Pearson and W. R. Muir, J. Am. Chem. Soc., 92, 5519 (1970). (7) F. R. Jensen and D. D. Davis, J. Am. Chem. SOC.,93, 4048 (1971). (8)D. E. Applequist and G. N. Chmurny, J. Am. Chem. SOC., 69, 875 (1967). (9) W. H. Glaze, C. M. Selman, A. L. Ball, and L. E. Bray, J. Org. Chem., 34, 641 (1969). (10) H. C. Brown and C. F. Lane, Chem. Commun., 521 (1971). (11) D. R. Coulson, J. Am. Chem. Soc., 91, 200 (1969). (12) P. K. Wong and J. K. Stille. J. Organornet. Chem., 70, 121 (1974). (13) R. W. Johnson and R. G. Pearson, Chem. Commun., 986 (1970). (14) R. W. Johnson and R. G. Pearson, lnorg Chem., 10,2091 (1971). (15) D. Dodd and M. D. Johnson, Chem Commun., 571 (1971). (16) F. R. Jensen, V. Madan, and D. H. Buchanan, J. Am. Chem. SOC.,93,5283 (1971). (17) S. N. Anderson, D. H. Ballard, J. Z. Chrzastowski, D. Dodd, and M. D. Johnson, J. Chem. SOC.,Chem. Commun., 685 (1972). (18) I. Levitin, A. L. Sigan, and M. E. Vol'pin, J. Chem. SOC.,Chem. Commun., 469 (1975). (19) J. Halpern, M. S.Chan, J. Hanson, T. S. Roche, and J. A. Topich, J. Am. Chem. Soc., 97, 1606 (1975). (20) J. H. Espenson and D. A. Williams, J. Am. Chem. SOC., 96, 1008 (1974). (21) J. A. Labinger. D. W. Hart, W. E. Seibert. and J. Schwartz, J. Am. Chem. SOC.,97, 3851 (1975). (22) H. L. Fritz, J. H. Espenson, D. A. Williams, and G. A. Molander, J. Am. Chem. Soc., 96, 2378 (1974). (23) G. M. Whitesides and D. Boschetto, J. Am. Chem. SOC., 93, 1529 (1971). (24) P. L. Bock, D. J. Boschetto, J. R. Rasmussen, J. P. Demers, and G. M. Whitesides, J. Am. Chem. SOC.,96, 2814 (1974). (25) S. N. Anderson, C. W. Fong, and M. D. Johnson, J. Chern. SOC.,Chem. Commun., 163 (1973). (26) K. M. Nicholas and M. Rosenblum, J. Am. Chem. SOC., 95, 4449 (1973). (27) T. G. Attig and A. Wojcicki, J. Am. Chem. Soc., 96, 262 (1974). (28) N. A. Dunham and M. C. Baird, J. Chem. SOC., Dalton Trans., 774
(1975). D. Slack and M. C. Baird, J. Chern. SOC.,Chem. Cornrnun., 701 (1974). D. Slack and M. C. Baird, unpublished results. J. A. Osborn, F. H. Jardine, J. F. Young, and G. Wilkinson, J. Chern. SOC. A, 1711 (1966). S. Mamedov and D. N. Khydyrov, Zh. Obsch. Khirn., 32, 1427 (1962); T. lnoue and S. Tsutsumi, Bull. Chern. SOC.Jpn., 38, 661 (1965). "Handbook of Physics and Chemistry", The Chemical Rubber Co., Cleveland, Ohio. P. A. Wisernan, S. Betras and B. Lindley, J. Chern. Educ., 51, 348 (1974). W. H. Saunders, S. Asperger, and D. H. Edison, J. Am. Chem. SOC.,80, 2421 (1958). R. F. Nystrom and W. G. Brown, J. Am. Chem. SOC.,69, 2548 (1947). R. B. King, "Organometallic Syntheses", Vol. 1, Academic Press, New York, N.F., 1965, p 151. D. J. Ehntholt, G. F. Emerson, and R. C. Kerber, J. Am. Chem. Soc., 91, 7547 . .. 11969) H. Bodot, A. Leray, and L. Pujol, C. R. Hebd. Seances Acad. Sci., Ser. C, 265, 842 (1967). R. J. Jablonski and E. I. Snyder, J. Am. Chern. SOC.,91, 4445 (1969). C. L. Liotta, E. E. Grisdale, and H. P. Hopkins, Tetrahedron Lett., 4205 (1975). T. C. Flood and F. J. DiSanti. J. Chem. SOC., Chem. Commun., 18 (1975). P. M. Treichel, K. P. Wagner, and H. J. Mueh, J. Organornet. Chern., 86, C13 (1975). J. (19731. A. Labinger. A. V. Kramer, and J. A. Osborn. J. Am. Chem. Soc.,95,7908 - -, K. S. Y. Lau, R. W. Fries, and J. K. Stille, J. Am. Chem. SOC.,96, 4983 (1974). C. J. Lancelot, D. J. Cram, and P. v. R. Schleyer in "Carbonium Ions", Vol. ill, G. A. Olah and P. v. R. Schleyer, Ed.. Wiley-Interscience, New York, N.Y., 1972, p 1347. F. R. Jensen and R. J. Ouellette, J. Am. Chem. Soc., 85,367 (1963). R. J. Ouellette and B. G, v. Leuwen, J. Am. Chem. SOC., 90, 7061 (1968). C. A. Tolman, Chem. SOC.Rev., 1,337 (1972). R. Foster, "Organic Charge-Transfer Complexes", Academic Press, New York, N.Y., 1969. R. G. Pearson, ACC. Chem. Res., 4, 152 (1971). F. A. Cotton and G. Wilkinson, "Advanced Inorganic Chemistry". 3rd ed, Interscience, New York. N.Y., 1972, Chapter 20. D. A. Symon and T. C. Waddington, J. Chem. SOC.,Dalton Trans., 2140 (1975). D. L. Lichtenberger and R. F. Fenske. J. Am. Chern. SOC., 98, 50 (1976). J. R. Chipperfield. J. Ford, and D. E. Webster, J. Organornet. Chem., 102, 417 . . . 11975) G. Distefano. S. Pignataro, L. Szepes, and J. Borossay, J. Organornet. Chem., 104, 173 (1976). (57) D. L. Lichtenberger and R. F. Fenske, lnorg. Chem., 13, 486 (1974). (58) J. R. Chipperfield, J. Ford, and D. E. Webster, J. Chem. Soc., Dalton Trans., 2042 (1975). I
\ . - . - I .
The Isomerization of Bicyclo[2.1 .O]pent-2-enes William E. Farneth, Michael B. D'Amore, and John I. Braurnan* Contributionfrom the Department of Chemistry, Stanford Uniuersity, Stanford, California 94305. Received December 22, 1975
Abstract: The isomerization of 1- and 2-methylbicyclo[2.1 .O]pent-2-enes has been studied in the gas phase and in solution. The rates of decomposition, product ratios, and rates of product interconversion have been measured. The results are well explained by a mechanism in which central bond cleavage produces a chemically activated cyclopentadiene, which then undergoes competitive hydrogen shifts and collisional deactivation. This model is supported by RRKM calculations.
The isomerization of bicyclo[2.1 .O] pent-2-enes to cyclopentadienes is a mechanistic problem that has been under investigation for several years. The bicyclo[2.1 .O]pent-2-enes are the lowest homologues of the fused ring cyclobutene series, but geometric constraints in this system prevent a symmetryallowed ua2 sS2electrocyclic ring opening analogous to that observed for cyclobutenes.' Therefore, the valence bond isomerization is forced to proceed via an alternative mechanism, and several possible ones have been tested and excluded using deuterium or methyl-substituted bicyclopentenes.2
+
Journal of the American Chemical Society
/
98:18
+
Arguments for two of the remaining candidates, a us2 ua2 symmetry-allowed concerted process involving the 1,5 and 3,4 or 1,2 and 4,5bonds in the starting bicyclopentene, or central (1,4) bond cleavage by some nonallowed pathway, have been p r e ~ e n t e d .These ~ . ~ arguments are based on experiments that determine the relative amounts of 1- and 2-methylcyclopentadienes (3 and 4) produced by the decomposition of either 1or 2-methylbicyclo[2.1 .O]pent-2-ene (1 and 2); see Scheme I. We have investigated the isomerization of the methylbicy-
/ September I , 1976
5547 Scheme I
Table 1. Rate Constants for Isomerization of 2Methylbicyclopentene and Equilibration of 1 - and 2Methylcyclopentadiene in THF T, OC
k d , S-' a
1.3 x 10-4
50
x 10-4)d 1.1 x 10-5 (1.07 x lO-5)d
(1.01
30 2
4
~~
clopentenes both in solution and in the gas phase. At the time these experiments were undertaken, there appeared to be a direct conflict between the experimental results from two different laboratories on the solution-phase thermolysis of 2-methylbicq)clo[2.1 .O]~ent-2-ene,~ and one of our motives in undertaking this investigation was to clarify the solution-phase picture. It now appears that a consistent picture has emerged. In the gas phase, interpretation of product ratios in mechanistic terms may be complicated by the possibility of extensive rearrangement due to chemical activation effects, as pointed out by Flowers and F r e ~W . ~e have directed our gas-phase studies toward the demonstration that chemical activation is indeed occurring in this system. The proposed mechanism is shown in Scheme 11. Scheme I1
1
2
Results A. Preparation. 1- and 2-methylbicyclo[2.1 .O]pent-2-ene were prepared by'photolysis of an equilibrium mixture of the methylcyclopentadienes in a method analogous to that reported for the parent compound by van Tamelen, Brauman, and Ellisn6 The methylcyclopentadienes used were freshly prepared by cracking of the commercially available dimer. The product bicycl!pentenes were separated from cyclopentadienes by treatment with N-phenylmaleimide (NPM) in tetrahydrofuran for short periods of time, and from each other by preparative VPC on a 15% P,P-oxydipropionitrile on non-acid-washed Chromosorb P column. Gas chromatographic analysis of the collected samples showed both to be free of the other bicyclopentene isomer and to contain less than 1% methylcyclopentadiene. For storage, the bicyclic compounds were vacuum transferred to gas storage bulbs containing a 10% solution of NPM in tetraglyme. The bulbs were subsequently maintained a t -78 'C for the lifetime of the sample. This method of storage was found to be quite convenient, permitting easy access to small amounts of material by vacuum transfer and continuous purification by trapping of the cyclopentadienes with N-phenylrnaleimide. B. Solution Results. Since the ratio k23/k24 (Scheme I) was not inferred from product ratios, but instead determined by a fitting procedure, products were not trapped as in previous investigations but analyzed directly by VPC. Rate constants for disappearance of 2-methylbicyclopentene (k23 k24) (Scheme I) and equilibration of the methylcyclopentadienes, (k34, k43), were measured independently in dry tetrahydrofuran in 5% EDTA-washed sealed tubes. The results are shown in Table I, and for comparison, Baldwin's concurrently published) results in hexane are included in parentheses. Both the
+
b
6.5 x 10-5 (3.59x 10-5)d 5.9 x 10-6 (3.2 X 10-6)d
KC
1.32 (1.38)d
1.38
(1.44)d
' kcisappearance e k23 + k24 in Scheme 1. kequlilbratlon = k 3 4 + k43 in Scheme I. C K = [2-methylcyclopentadiene]/ [ l-methylcyclopentadiene]. From ref 3. disappearance and the equilibration data gave good linearity of the appropriate (first-order decay or approach to equilibrium) plots (correlation coefficients >0.99) over at least 3 half-lives. Internal standardization showed that all products (& 1%) were accounted for as 1- and 2-methylcyclopentadienes in both cases. Finally, addition of crushed glass to the ampules, increasing the exposed surface area by about a factor of 3, caused no appreciable alteration in the rates. In the case of the equilibration, prewashing the tubes with 10% N a O H or HCI similarly gave rates within experimental error of the normal data. These results indicate that surface catalysis did not contribute significantly to the observed rates. The disparity between the rate constants measured in this work and those of Baldwin (Table I) for the equilibration between the methylcyclopentadienes may be entirely the result of the solvent difference. W e have found a t 50 'C (k43 k34 in pentane)/ (k34 k43 in T H F ) z 0.66, yielding a value of 3.9 X s-' for k34 k43 in pentane, in good agreement with Baldwin and co-workers' value of 3.59 X 10-5 s-' in hexane. Furthermore, the ratio (k34 k43) 50 OCl(k34 k43) 30 O C is identical for the two investigations. The other rate constants are seen to be in good agreement with those determined by Baldwin and c o - ~ o r k e r sNo . ~ solvent effect on the rate of decomposition of 2-methylbicyclopentene for the solvents pentane, T H F , and methanol was observed. One point rate constants for the disappearance of 1-methylbicyclopentene at 50 'C indicated that it was eight-ten times faster than the 2-methyl compound, and somewhat smaller portions of rearranged methylcyclopentadiene were formed. C. Gas-Phase Results. The system was investigated in the gas phase in the presence of unreactive bath gases a t total pressures ranging from
