Deruan, Uyehara, Santilli
/ 1,I-Biradical Intermediates from Diazenes and Cyclobutanes
2069
Experimental Determination of the Relative Rates of Rotation, Cleavage, Closure, and 1,5-Hydrogen Shift for 3-Methyl- 1,4-pentanediyl. Evidence for 1,4 Biradicals as Common Intermediates from Different Precursors, and 3,4-(and 3,6-)Dimethyl-3,4,5,6-tetrahydropyridazines 1,2-Dimethylcyclobutanes,2 Peter B. D e r ~ a n , *Tadao ~ Uyehara, and Donald S. Santilli Contribution No. 5860from Crellin Laboratory of Chemistry, California Institute of Technology, Pasadena, California 91125. Received August 30, 1978
Abstract: Thermal decomposition of cis- and trans-3,6-dimethyl-3,4,5,6-tetrahydropyridazine (11 and 12, respectively) affords propene, cis- and trans- 1,2-dimethylcyclobutanes,and 1-hexene. The stereochemistry of the products is consistent with a 1,4-biradical intermediate(s) 2,5-hexanediyl, which has the properties k(rotation) k(c1eavage) k(closure). At 439 O C the retention/inversion (r/i) ratios for the 1,2-dimethylcyclobutane products are 1.7 and 1.7 from 11 and 12, respectively. At 306 OC, these ratios are 1.9 and 2.2, respectively. The results indicate that when the thermal reactions of cyclic azo compounds and cyclobutanes of similar substitution are compared at similar temperatures, the stereospecificities are similar. We conclude that stereoretention is dependent on both substitution and temperature, Le., stereospecificity increases as substitution at the radical center increases and as the temperature is lowered. The thermal decomposition of cis- and trans-3,4-dimethyl-3,4,5. tetrahydropyridazine (19 and 20, respectively) allows a dissection of direct vs. 1,4-biradical pathways in six-membered cyr azo decompositions, e.g., 36% direct/64% 1,4 biradical from cis- 19 and 32% direct/68% 1,4 biradical from trans-20. Thr tive rates of rotation, cleavage, and closure for azo-generated 1,4 biradicals, 3-methyl-1,4-pentanediyl(ST and SC), w _. determined. For ST, k(cleavage)/k(closure) = 1.6 and k(closure)/k(rotation) = 1.9. For SC, k(cleavage)/k(closure) = 1.8 and k(closure)/k(rotation) = 0.7. These relative rates of rotation, cleavage, and closure generate similar trans-/cis-2-butene ratios as found in the pyrolyses of cis- and trans- 1,2-dimethylcyclobutanesat the same temperature and phase. We conclude they pass the stereochemical test for identity, indicating evidence for common 1,4-biradical intermediates from two different precursors (1,2-diazenes and cyclobutanes). Finally, the data are compared with the recent literature values reported for the dimerization of ethylene and 2-butenes.
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Introduction Experimental work has shown that many thermal and photochemical reactions do not take advantage of cyclic transition states, but apparently proceed via simple bond broken species or b i r a d i c a l ~ .In ~ , this ~ paper we test whether certain biradicals generated from different precursors are common intermediates on some energy surface, Le., have identical stereochemical behavior. Variables such as temperature, phase, and substituents affect the behavior of biradical intermediates. In tests for commonality of biradical intermediates where these variables are not the same, the outcome usually shows different stereochemical behavior. Because of its size, simplicity, and theoretical importance, we have chosen to study the 1,4 biradicaL6-I2 Hoffmann's extended Huckel (EH) calculation for the energy surface between cyclobutane and two molecules of ethylene revealed a rather flat hypersurface.6b Segal concluded from an ab initio calculation (SCF at STO-3G level) that there are two well-defined potential energy minima for the gauche and trans conformations of tetramethylene.6fThe barriers to cleavage and closure for gauche-tetramethylene are 3.6 and 22.0 kcal/mol, respectively. Benson's thermochemical estimates predict similar differences between the heats of formation of the transition states for cleavage and closure from tetramethylene but a deeper well.5
1 gauche
2 trans
In 1961, Gerberich and Walters studied the pyrolyses of cisand trans- 1,2-dimethylcyclobutanesin the gas phase at 425 0002-7863/79/ 150 1 -2069$01 .OO/O
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f 25 "C.'lb The products were consistent with the intermediacy of 1,4 biradicals where rotation is competitive with cleavage and c l o s ~ r e Cyclic .~ azo compounds (or 1,2-diazenes) are considered good routes for the generation of b i r a d i ~ a l sIn .~ 1969, Bartlett and Porter studied the thermal decomposition of meso- and dl- 3,6-diethyl-3,6-dimethyltetrahydropyridazines (9) in solution at 150 0C.9aThe highly stereospecific
3 9
+; CD3 IO
formation of the isomeric cyclobutanes was consistent with a 1,4-biradical where /?(rotation)
