Role of spin-orbit coupling and symmetry in triplet carbenic addition chemistry

Journal of Physical Chemistry

Volumn 100, Issue 11, 1996, Pages 4339-4349

  Su, Ming Der ^a  

^a DARESBURY LABORATORY   (United Kingdom)

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EID: 0001492432     PISSN: 00223654     EISSN: None     Source Type: Journal    
DOI: 10.1021/jp950290j     Document Type: Article

References (169)

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47. On the other hand, however, a conventional interpretation of such stereospecific additions is that the chemistry of phenylcarbene involves a very low-lying, thermally accessible, and highly reactive singlet and a much less reactive ground triplet state (see ref 8c,e). We do not agree with this point of view and assume that spin-orbit coupling should play an important role in determining the stereospecificity of the triplet carbenes, which is extensively studied in the present work,
48. (e) In terms of the "reactivity" aspect, in Su's work it was found that the singlet carbene (whose ground state is triplet) is much more reactive in carbenic chemistry than the triplet carbene due to the energy difference between those two states of a carbene (i.e., AEst = Elripiei -Esingiei). See: Su, M.-D. Inorg. Chem. 1995, 34, 3829-3831.
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64. Indeed, it has been pointed out that all nonstereospecific additions need not be due to triplets and that all triplets need not add in a I nonstereospecific manner; see: (a) DeMore, W. B.; Benson, S. \V. Adv. \ Photochem. 1964, 2, 219-261.
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66. Reference 7. What is more important is that some of the mechanisms proposed in this work can perform intersystem crossing without passing through a rotatory and a fast equilibrium diradical as traditionally explained; see the text.
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95. For instance, consider a SO matrix element connecting a singlet state So with a triplet state TI, where So is derived from a configuration [ ]a- and where TI is derived from a configuration [ ja'i1. The brackets denote filled MOs and a and b are valence MOs. In this case, one can obtain
96. For .T approach, the triplet excited state [ ]|/>avj is (A')(A')(A') = A' where the bracket ([ ]) denotes filled MOs and ij> and Vare valence MOs. "-
97. Both HOMO and LUMO can be expressed as a function of the rotation and bending angles; see also ref 27 and: Trindle, C.; Pamuk, H. O. Tetrahedron 1978, 34, 747-752.
98. It must be noted that since, any contributions from s Orbitals on the nucleus will be annihilated by the / part of the operator,23 we will therefore consider only the action on p Orbitals in future.
99. Strictly speaking, in the extreme case, the MO polarization should make 2ab zero while (a2 + b2) should approach unity. Therefore, as polarity increases (a2 + b-) becomes larger than lab. See ref 27b,c.
100. Theoretically, this SO expression (eq 22) may reach an extreme at = 45°. But actually, is geometrically constrained to -19.47° in order to satisfy the tetrahedral angle requirement; see ref 27b.
101. According to Salem's work, the intersystem crossing via SO coupling is controlled both by an "ionic" factor and by an "orientational" factor. For details see ref 26.
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105. Ref 17.
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107. Our prediction for the stereospecific trapping of -'CHi by ethylene was also supported by Halevi and .Trindle's work using the "orbital correspondence analysis in maximum symmetry" theory; see refs 22 and 24.
108. For ab initio studies of trimethylene diradical in its triplet electronic state, see: (a) Doubleday, C., Jr.; Mclver, J. W., Jr.; Page, M. J. Am. Cliem. Soc. 1982, 104, 6533-6542.
109. (b) Yamaguchi, Y.; Schaefer, H. F. J. Am. Cliem. Soc. 1984,106,5115-5118.
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113. (f) Doubleday, C., Jr.; Mclver, J. W., Jr.; Page, M. J. Phys. Chem. 1988, 92, 4367-4371.
114. (a) However, according to one recent ab initio study, it was found that the SO coupling constant for CHi is calculated to be ~10 cm"1 using the 4s3p2dlf basis set at the MCSCF level. See: Vahtras, O.; Agren, H.; Jorgensen, P.; Jensen, H. J. Aa.; Helgaker, T.; Olsen, J. J. Cliem. Ph\s. 1992, 90, 2118-2126.
115. (b) Eder, T. W.; Carr, R. W. J. Chem. Phys. 1970, 53, 2258-2266.
116. Woodward, R. B.; Hoffmann, R. Tlie Conservation of Orbital Symmetry', Verlag Chemie: Weinheim, 1970.
117. According to a theoretical study (see ref 27d), it was proved that whenever both orbitalsymmetry and spin-inversion requirements are met along the same reaction coordinate, the reaction can be stereospecific.
118. For nonpolar solvent cases see: (a) Reference 10a.
119. (b) Reference 12a.
120. Reference 19f.
121. Reference 16h.
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123. (f) Jones, M., Jr.; Kulczycki, A., Jr.; Hummel, K. F. Tetrahedron Lett. 1967, 183-187.
124. (g) Moss, R. A.; Young, C. M. J. Am. Chem. Soc. 1983, 705, 5859-5865.
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127. For details, see: (a) Salem, L. Ace. Chem. Res. 1979, 12, 87-92.
128. (b) Albright, T. A.; Burdett, J. K.; Whangbo, W.-H. Orbital Interactions in Chemistry; John Wiley and Sons: New York, 1985; Chapter 10.
129. Experimentally, it is reported that a triplet ground state carbene is largely susceptible to solvent polarity, and the results are consistent with stabilization of the zwitterionic singlet state in solvents of high polarity. See: Garcia-Garibay, M. A.; Theroff, C.; Shin, S. H.; Jernelius, J. Tetrahedron Lett. 1993, 34, 8415-8418.
130. In addition, it was suggested that the triplet ground state of carbene XYC: is expected for substituents which are electropositive (with respect to carbon) and/or are bulky. This, in turn, can enhance the formation of stereoretention cyclopropanes. See: (a) Muller, P. H.; Rondan, N. G.; Houk, K. N.; Harrison, J. F.; Hooper, D.; Willen, B. H.; Liebman, J. F. J. Am. Chem. Soc. 1981, 103, 5049-5052.
131. (b) Carter, E. A.; Goddard, W. A., III. J. Chem. Phys. 1988, 88, 1752-1763.
132. Furthermore, according to one recent ab initio study (see ref 23a), it was suggested that the triplet addition reaction would become more unfavorable as the electron-releasing character of substituents of the carbene is increased.
133. Nevertheless, there is a related theoretical study using the extended Huckel method to interpret such phenomena; see: Hoffmann, R.; Levin, C. C.; Moss, R. A. J. Am. Chem. Soc. 1973, 95, 629-631.
134. For the heavy atom effect, see: (a) Turro, N. J. Modem Molecular Photochemistry; The Benjamin/Cummings Co.: Menlo Park, CA, 1978; Chapter 6.
135. (b) El-Sayed, M. A. Ace. Chem. Res. 1968, /, 8-16;
136. J. Chem. Phys. 1963, 38, 2834-2838.
137. Other factors which are known to affect singlet-triplet intersystem crossing include the following: (1) the magnitude of the singlet-triplet energy difference between the two states; (2) the configuration of the initial and of the final states; and (3) vibronic or Franck-Condon factors. See: (a) Reference 26a.
138. (b) Robinson, G. W.; Frosch, R. P. J. Chem. Phys 1962, 37, 1962-1973;
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142. (b) Sandhu, H. S.; town, E. M.; Strausz, O. P.; Gunning, H. E. J. Am. Chem. Soc. 1966, 88, 254-263.
143. Lown, E. M.; Dedio, E. L. J. Am. Chem. Soc. 1967, 59, 1056-1062.
144. (a) However, extended Huckel calculations suggested that triplet sulfur addition reactions are -stereospecific because the reactants correlate with an excited state of the product thiirane which retains CC bonding. See: Hoffmann, R.; Wan, C. C.; Neagu, V. Mol. Pins. 1970, 19, 113-120.
145. (b) For a similar explanation based on orbital correlation diagrams and ab initio SCF study refer to refs 50 and 51, respectively.
146. Leppin, E.; Gollnick, K. Tetrahedron Lett. 1969, 3819-3824.
147. Strausz, O. P.; Gunning, H. E.; Denes, A. S.; Csizmadia, I. G. J. Am. Chem. Soc. 1972, 94, 8317-8321.
148. (a) McConaghy, J. S.; Lwowski, W. J. Am. Chem. Soc. 1967, 89, 2357-2364;
149. (b) J. Am. Chem. Soc. 1967, 89, 4450-4456.
150. For the ab initio study, see: Fueno, T.; Bonacic-Koutecky, V.; Koutecky, J. J. Am. Chem. Soc. 1983, 105, 5547-5557.
151. (a) Cvetanovic, R. J. Adv. Photochem. 1963, 1, 115-145.
152. (b) Cvetanovic, R. J. Phys. Chem. 1970, 74, 2730-2732.
153. Scheer, M. D.; Klein, R. J. Phys. Chem. 1970, 74, 2732-2733.
154. Huie, R. E.; Herron, J. T. Prog. React. Kinet. 1975, S, 1-17.
155. Mcbee, E. T.; Sienkowski, K. J. J. Org. Chem. 1973, 38, 1340-1344.
156. (a) Blomstrom, D. C.; Herbig, K.; Simmons, H. E. J. Org. Chem. 1965, 30, 959-964.
157. (b) Kopecky, K. R.; Hammond, G. S.; Leermaakers, P. A. J. Am. Chem. Soc. 1962,84, 1015-1019.
158. Wittig, G.; Wingler, F. Chem. Ber. 1964, 97, 2139-2145;
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161. (b) 1959,81,4256-4264.
162. Blanchard, E. P.; Simmons, H. E.J.Am. Chem. Soc. 1964,86,1337-1346.
163. Simmons, H. E.; Blanchard, E. P.; Smith, R. D. J. Am. Chem. Soc. 1964,86, 1347-1356.
164. (e) Skell, P. S.; Valenty, S. J.; Humer, P. W. J. Am. Chem. Soc. 1973. 95, 5041-5043.
165. (a) Friedman, L.; Berger, J. D. J. Am. Chem. Soc. 1961, 83, 492-493.
166. (b) Doering, W. v. E.; Roth, W. Tetrahedron 1963, 19, 715-723.
167. Reference le, p 184.
168. However, there are some questions as to whether the cyclopropanes are formed by addition of free méthylène or whether some other intermediates, such as an excited méthylène iodide or iodomethyi radical, is involved in the méthylène transfer reaction. Nevertheless, one may expect that the presence of méthylène iodide and iodine in solution would considerably enhance spin relaxation (heavy atom effect).
169. It must be emphasized that those spin-inversion mechanisms suggested in this work (case A-case D) and their associated interpretations were derived first and justified by Shaik and Epiotis in ref 27b,c. The author wishes to thank professor Sason Shaik for pointing out this important fact.