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Reactive Intermediate Chemistry

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324SINGLET CARBENES

55.N. J. Turro, J. A. Butcher, Jr., R. A. Moss, W. Guo, R. C. Munjal, and M. Fedorynski,

J.Am. Chem. Soc. 1980, 102, 7576.

56.J. J. Zupanic and G. B. Schuster, J. Am. Chem. Soc. 1980, 102, 5958.

57.See R. A. Moss and N. J. Turro, in Ref. 9, pp. 213ff.

58.D. P. Cox, I. R. Gould, N. P. Hacker, R. A. Moss, and N. J. Turro, Tetrahedron Lett. 1983, 24, 5313.

59.R. A. Moss, H. Fan, L. M. Hadel, S. Shen, J. Wlostowska, M. Wlostowski, and K. KroghJespersen, Tetrahedron Lett. 1987, 28, 4779.

60.R. A. Moss, W. Lawrynowicz, L. M. Hadel, N. P. Hacker, N. J. Turro, I. R. Gould, and

Y.Cha, Tetrahedron Lett. 1986, 21, 4125.

61.R. A. Moss, C.-S. Ge, J. Wlostowska, E. G. Jang, E. A. Jefferson, and H. Fan,

Tetrahedron Lett. 1995, 36, 3083.

62.R. A. Moss, S. Shen, L. M. Hadel, G. Kmiecik-Lawrynowicz, J. Wlostowska, and

K.Krogh-Jespersen, J. Am. Chem. Soc. 1987, 109, 4341.

63.For further discussions of C6H5CX reactivity, see I. R. Gould, N. J. Turro, J. Butcher, Jr.,

C.Doubleday, Jr., N. P. Hacker, G. F. Lehr, R. A. Moss, D. P. Cox, W. Guo, R. C. Munjal,

L.A. Perez, and M. Fedorynski, Tetrahedron 1985, 41, 1587.

64.J. E. Chateauneuf, R. P. Johnson, and M. M. Kirchhoff, J. Am. Chem. Soc. 1990, 112, 3217.

65.J. E. Jackson, N. Soundararajan, M. S. Platz, and M. T. H. Liu, J. Am. Chem. Soc. 1988, 110, 5595. See J. E. Jackson and M. S. Platz, in Ref. 10, pp. 89ff; M. S. Platz, in Ref. 13, pp. 27ff.

66.N. J. Turro, G. F. Lehr, J. A. Butcher, Jr., R. A. Moss, and W. Guo, J. Am. Chem. Soc. 1982, 104, 1754.

67.(a) K. N. Houk, N. G. Rondan, and J. Mareda, J. Am. Chem. Soc. 1984, 106, 4293;

(b) Tetrahedron 1985, 41, 1555.

68.B. Giese and C. Neumann, Tetrahedron Lett. 1982, 23, 3357. B. Giese, W.-B. Lee, and

C.Neumann, Angew. Chem. Int. Ed. 1982, 21, 310.

69.R. A. Moss, W. Lawrynowicz, N. J. Turro, I. R. Gould, and Y. Cha, J. Am. Chem. Soc. 1986, 108, 7028.

70.N. J. Turro, M. Okamoto, I. R. Gould, R. A. Moss, W. Lawrynowicz, and L. M. Hadel,

J.Am. Chem. Soc. 1987, 109, 4973.

71.W. R. Moore, W. R. Moser, and J. E. LaPrade, J. Org. Chem. 1963, 28, 2200.

72.B. Zurawski and W. Kutzelnigg, J. Am. Chem. Soc. 1978, 100, 2654; J. F. Blake,

S.G. Wiershke, and W. L. Jorgensen, J. Am. Chem. Soc. 1989, 111, 1919; A. E. Keating,

M.A. Garcia-Garibay, and K. N. Houk, J. Am. Chem. Soc. 1997, 119, 10805.

73.A. E. Keating, S. R. Merrigan, D. A. Singleton, and K. N. Houk, J. Am. Chem. Soc. 1999, 121, 3933.

74.Structures 20 and 21 reproduced from Ref. 73 with permission from A. E. Keating,

S.R. Merrigan, and K. N. Houk, J. Am. Chem. Soc., 1999, 121, 3933. Copyright #1999 the American Chemical Society.

75.See Ref. 35 (Hoffmann, Hayes, and Skell), in which such geometries are also analyzed.

76.G. L. Closs and L. E. Closs, Angew. Chem. 1962, 74, 431; G. L. Closs and R. A. Moss,

J.Am. Chem. Soc. 1964, 86, 4042.

77.G. Boche and J. C. W. Lohrenz, Chem. Rev. 2001, 101, 697.

REFERENCES 325

78.W. von E. Doering and P. LaFlamme, J. Am. Chem. Soc. 1956, 78, 5447.

79.M. Jones, Jr., W. Ando, M. E. Hendrick, A. Kulczycki, Jr., P. M. Howley, K. R. Hummel, and D. S. Malament, J. Am. Chem. Soc. 1972, 94, 7469.

80.N. J. Turro, Y. Cha, and I. R. Gould, J. Am. Chem. Soc. 1987, 109, 2101.

81.K. R. Kopecky, G. S. Hammond, and P. A. Leermakers, J. Am. Chem. Soc. 1962, 84, 1015.

82.M. Jones, Jr. and K. R. Rettig, J. Am. Chem. Soc. 1965, 87, 4013; M., Jr., Jones and K. R. Rettig, J. Am. Chem. Soc. 1965, 87, 4015.

83.(a) J. Weber and U. H. Brinker, Angew. Chem. Int. Ed. Engl. 1997, 36, 1623. (b) R. A. Wagner and U. H. Brinker, Chem. Lett. 2000, 3, 246. See also N. C. Yang and T. A. Marolewski, J. Am. Chem. Soc. 1968, 90, 5644, who ﬁrst proposed this mechanism.

84.(a) J. E. Jackson, G. B. Mock, M. L. Tetef, G.-x. Zheng, and M. Jones, Jr., Tetrahedron 1985, 41, 1453; (b) J. Weber, L. Xu, and U. H. Brinker, Tetrahedron Lett. 1992, 33, 4537.

85.J. M. Cummins, T. A. Porter, and M. Jones, Jr., J. Am. Chem. Soc. 1998, 120, 6473; J. M. Cummins, I. Pelczer, and M. Jones, Jr., J. Am. Chem. Soc. 1999, 121, 7595.

86.Singlet carbene O H reactions have been very thoroughly reviewed by Kirmse: W. Kirmse, in Ref. 10, pp.1ff.; W. Kirmse, in Ref. 12, pp. 1ff.

87.Excellent reviews of early research on C H insertion reactions of carbenes may be found in Refs. 5 and 6.

88.W. von E. Doering and H. Prinzbach, Tetrahedron 1959, 6, 24.

89.W. Kirmse and M. Buschoff, Chem. Ber. 1969, 102, 1098.

90.G. L. Closs and L. E. Closs, J. Am. Chem. Soc. 1969, 91, 4549; G. L. Closs and A. D. Trifunac, J. Am. Chem. Soc. 1969, 91, 4554.

91.H. D. Roth, J. Am. Chem. Soc. 1972, 94, 1761.

92.I. Tabushi, Z.-i. Yoshida, and N. Takahashi, J. Am. Chem. Soc. 1970, 92, 6670.

93.K. Steinbeck, Chem. Ber. 1979, 112, 2402; K. Steinbeck and J. Klein, J. Chem. Res. (S)

1980, 94.

94.K. Steinbeck and J. Klein, J. Chem. Res. (S) 1978, 396.

95.D. Seyferth, V. M. Mai, and M. E. Gordon, J. Org. Chem. 1970, 35, 1993.

96.D. Seyferth, J. M. Burlitch, K. Yamamoto, S. S. Washburne, and C. J. Attridge, J. Org. Chem. 1970, 35, 1989.

97.D. Seyferth and Y. M. Cheng, J. Am. Chem. Soc. 1973, 95, 6763.

98.R. A. Pascal, Jr. and S. Mischke, J. Org. Chem. 1991, 56, 6954.

99.H. M. L. Davies, Q. Jin, P. Ren, and A. Yu. Kovalevsky, J. Org. Chem. 2002, 67, 4165.

100.D. Seyferth, R. Damrauer, J. Y.-P. Mui, and T. F. Jula, J. Am. Chem. Soc. 1968, 90, 2944.

101.H. D. Roth, J. Am. Chem. Soc. 1971, 93, 1527.

102.I. R. Likhotvorik, K. Yuan, D. W. Brown, P. A. Krasutsky, N. Smyth, and M. Jones, Jr.,

Tetrahedron Lett. 1992, 33, 911.

103.L. Xu, W. B. Smith, and U. H. Brinker, J. Am. Chem. Soc. 1992, 114, 783. This reference contains an excellent bibliography of dihalocarbene C H insertion reactions.

104.M. P. Doyle, J. Taunton, S.-M. Oon, M. T. H. Liu, N. Soundararajan, M. S. Platz, and J. E. Jackson, Tetrahedron Lett. 1988, 29, 5863.

105.J. E. Jackson, N. Soundararajan, M. S. Platz, M. P. Doyle, and M. T. H. Liu, Tetrahedron Lett. 1989, 30, 1335.

326SINGLET CARBENES

106.R. A. Moss and S. Yan, Tetrahedron Lett. 1998, 39, 9381.

107.R. A. Moss and S. Yan, Org. Lett. 1999, 1, 819.

108.See Ref. 5b, pp. 236 ff.

109.W. J. Baron, M. R. De Camp, M. E. Hendrick, M. Jones, Jr., R. H. Levin, and M. B. Sohn, in Ref. 6, pp. 19 ff.

110.See R. A. Moss, in Ref. 10, pp. 59 ff.

111.See R. Bonneau and M. T. H. Liu, in Ref. 11, pp. 1 ff.

112.See M. Jones, Jr., in Ref. 11, pp. 77 ff.

113.See M. S. Platz, in Ref. 11, pp. 133 ff.

114.See D. C. Merrer and R. A. Moss, in Ref. 12, pp. 53 ff. The Appendix to this reference contains extensive tables of rates and activation parameters for the intramolecular insertions of many alkylcarbenes.

115.See G. Szeimies, in Ref. 12, pp. 269 ff.

116.(a) R. Bonneau and M. T. H. Liu, J. Am. Chem. Soc. 1989, 111, 5973. (b) E. J. Dix, M. S. Herman, and J. L. Goodman, J. Am. Chem. Soc. 1993, 115, 10424; J. A. LaVilla and

J.L. Goodman, J. Am. Chem. Soc. 1989, 111, 6877.

117.J. W. Storer and K. N. Houk, J. Am. Chem. Soc. 1993, 115, 10426.

118.A. E. Keating, M. A. Garcia-Garibay, and K. N. Houk, J. Phys. Chem. A 1998, 102, 8467.

119.T. V. Albu, B. J. Lynch, D. G. Truhlar, A. C. Goren, D. A. Hrovat, W. T. Borden, and

R.A. Moss, J. Phys. Chem. A 2002, 106, 5323.

120.S. Celebi, S. Levya, D. A. Modarelli, and M. S. Platz, J. Am. Chem. Soc. 1993, 115, 8613;

R.Bonneau, M. T. H. Liu, and M. T. Rayez, J. Am. Chem. Soc. 1989, 111, 5893.

121.M. T. H. Liu and R. Bonneau, J. Am. Chem. Soc. 1996, 118, 8098.

122.A. Nickon, Acc. Chem. Res. 1993, 26, 84.

123.J. D. Evenseck and K. N. Houk, J. Phys. Chem. 1990, 94, 5518.

124.M. H. Sugiyama, S. Celebi, and M. S. Platz, J. Am. Chem. Soc. 1992, 114, 966.

125.M. T. H. Liu, Acc. Chem. Res. 1994, 27, 287.

126.D. C. Merrer, R. A. Moss, M. T. H. Liu, J. T. Banks, and K. U. Ingold, J. Org. Chem. 1998, 63, 3010.

127.(a) M. T. H. Liu and R. Bonneau, J. Am. Chem. Soc. 1990, 112, 3915; (b) R. Bonneau,

M.T. H. Liu, and M. T. Rayez, J. Am. Chem. Soc. 1989, 111, 5973.

128.M. T. H. Liu, R. Bonneau, S. Wierlacher, and W. Sander, J. Photochem. Photobiol., A: Chem. 1994, 84, 133.

129.S. Wierlacher, W. Sander, and M. T. H. Liu, J. Am. Chem. Soc. 1993, 115, 8943.

130.M. T. H. Liu and R. Bonneau, J. Phys. Chem. 1989, 93, 7298.

131.R. A. Moss, G. J. Ho, S. Shen, and K. Krogh-Jespersen, J. Am. Chem. Soc. 1990, 112, 1638. See also G.-J. Ho, K. Krogh-Jespersen, R. A. Moss, S. Shen, R. S. Sheridan, and

R.Subramanian, J. Am. Chem. Soc. 1989, 111, 6875.

132.R. A. Moss, G.-J. Ho, and W. Liu, J. Am. Chem. Soc. 1992, 114, 959.

133.B. T. Hill, Z. Zhu, A. Boeder, C. M. Hadad, and M. S. Platz, J. Phys. Chem. A 2002, 106, 4970.

134.(a) J. P. Pezacki, P. Couture, J. A. Dunn, J. Warkentin, P. D. Wood, J. Lusztyk, F. Ford, and M. S. Platz, J. Org. Chem. 1999, 64, 4456; (b) D. A. Modarelli, S. Morgan, and M. S. Platz, J. Am. Chem. Soc. 1992, 114, 7034.

REFERENCES 327

135.I. R. Likhotvorik, E. Tippmann, and M. S. Platz, Tetrahedron Lett. 2001, 42, 3049.

136.(a) Z. Zhu, T. Bally, L. L. Stracener, and R. J. McMahon, J. Am. Chem. Soc. 1999, 121, 2863; Y. Wang, T. Yuzawa, H. Hamaguchi, and J. P. Toscano, J. Am. Chem. Soc. 1999, 121, 2875; J.-L. Wang, I. Likhotvorik, and M. S. Platz, J. Am. Chem. Soc. 1999, 121, 2883; (b) F. Kaplan and G. K. Meloy, J. Am. Chem. Soc. 1966, 88, 950.

137.(a) H. M. Frey, Pure Appl. Chem. 1964, 9, 527. (b) A. M. Mansoor and I. D. R. Stevens, Tetrahedron Lett. 1966, 1733; (c) K.-T. Chang and H. Shechter, J. Am. Chem. Soc. 1979, 101, 5082.

138.M. S. Platz, in Ref. 11, p. 133ff.

139.J. M. Fox, J. E. Gillen Sacheri, K. G. L. Jones, M. Jones, Jr., Jones, P. B. Shevlin,

B.Armstrong, and R. Sztyrbicka, Tetrahedron Lett. 1992, 33, 5021.

140.G. Wu, M. Jones, Jr., W. von E. Doering, and L. H. Knox, Tetrahedron, 1997, 53, 9913.

141.(a) H. Glick, I. R. Likhotvorik, and M. Jones, Jr., Tetrahedron Lett. 1995, 36, 5715;

(b) B. M. Armstrong, M. L. McKee, and P. B. Shevlin, J. Am. Chem. Soc. 1995, 117, 3689.

142.W. Kirmse, B. G. von Bulow, and H. Schepp, Justus Liebigs Ann. Chem. 1966, 41, 691.

143.H. Huang and M. S. Platz, J. Am. Chem. Soc. 1998, 120, 5990.

144.R. A. Moss and N. J. Turro, Ref. 9, p. 213ff.

145.M. S. Platz, Ref. 11, p. 164.

146.M. Nigam, M. S. Platz, B. Showalter, J. Toscano, R. Johnson, S. C. Abbot, and M. M. Kirchhoff, J. Am. Chem. Soc. 1998, 120, 8055.

147.(a) P. E. Eaton and K. L. Hoffman, J. Am. Chem. Soc. 1987, 109, 5285; (b) P. E. Eaton and

R.B. Appell, J. Am. Chem. Soc. 1990, 112, 4055.

148.W. R. White, M. S. Platz, N. Chen, and M. Jones, Jr., J. Am. Chem. Soc. 1991, 113, 4981;

D.A. Hrovat and W. T. Borden, J. Am. Chem. Soc. 1996, 118, 1535.

149.See also R. T. Ruck and M. Jones, Jr., Tetrahedron Lett. 1998, 39, 4433.

150.R. T. Ruck and M. Jones, Jr., Tetrahedron Lett. 1998, 39, 2277.

151.M. I. Kahn and J. Goodman, J. Am. Chem. Soc. 1995, 117, 6635.

152.R. A. Moss, S. Yan, and K. Krogh-Jespersen, J. Am. Chem. Soc. 1998, 120, 1088;

K.Krogh-Jespersen, S. Yan, and R. A. Moss, J. Am. Chem. Soc. 1999, 121, 6269.

153.D. S. Wulfman, B. Poling, and R. S. McDaniel, Jr., Tetrahedron Lett. 1975, 4519.

154.J. P. Toscano, Ref. 11, p. 231ff.

155.See Reference 79.

156.R. C. Joines, A. B. Turner, and W. M. Jones, J. Am. Chem. Soc. 1969, 91, 7754.

157.H. Staudinger and R. Endle, Chem. Ber. 1913, 46, 1437; F. O. Rice and J. D. Michaelsen,

J.Phys. Chem. 1962, 66, 1535.

158.J. A. Myers, R. C. Joines, and W. M. Jones, J. Am. Chem. Soc. 1970, 92, 4740.

159.G. G. Vander Stouw, A. R. Kraska, and H. Shechter, J. Am. Chem. Soc. 1972, 94, 1655.

160.W. J. Baron, M. Jones, Jr., and P. P. Gaspar, J. Am. Chem. Soc. 1970, 92, 4739.

161.For a nice review of the early work, and provocative mechanistic suggestions, see: P. P. Gaspar, J.-P. Hsu, S. Chari, and M. Jones, Jr., Tetrahedron, 1985, 41, 1479.

162.O. L. Chapman, J. W. Johnson, R. J. McMahon, and P. R. West, J. Am. Chem. Soc. 1988, 110, 501.

328SINGLET CARBENES

163.P. R. West, O. L. Chapman, and J.-P. LeRoux, J. Am. Chem. Soc. 1982, 104, 1779; R. J. McMahon, C. J. Abelt, O. L. Chapman, J. W. Johnson, C. L. Kreil, J.-P. LeRoux, M. Mooring, and P. R. West, J. Am. Chem. Soc. 1987, 109, 2459; O. L. Chapman and C. J. Abelt, J. Org. Chem. 1987, 52, 121.

164.R. Warmuth, Eur. J. Org. Chem. 2001, 423.

165.R. Warmuth and M. A. Marvel, Chem. Eur. J. 2001, 7, 1209.

166.P. C. Venneri and J. Warkentin, J. Am. Chem. Soc. 1998, 120, 11182.

167.(a) N. Merkley, M. El-Saidi, and J. Warkentin, Can. J. Chem. 2000, 78, 356. (b) N. Merkley and J. Warkentin, Can. J. Chem. 2000, 78, 942.

168.R. A. Moss, B. K. Wilk, and L. M. Hadel, Tetrahedron Lett. 1987, 28, 1969.

169.R. A. Moss, Acc. Chem. Res. 1999, 32, 969.

170.R. A. Moss, L. A. Johnson, S. Yan, J. P. Toscano, and B. M. Showalter, J. Am. Chem. Soc. 2000, 122, 11256.

171.S. Yan, R. R. Sauers, and R. A. Moss, Org. Lett. 1999, 1, 1603.

172.M. A. Kesselmayer and R. S. Sheridan, J. Am. Chem. Soc. 1986, 108, 99, 844.

173.R. A. Moss, Y. Ma, F. Zheng, R. R. Sauers, T. Bally, A. Maltsev, J. P. Toscano, and B. M. Showalter, J. Phys. Chem. A, 2002, 106, 12280.

174.R. A. Moss, F. Zheng, R. R. Sauers, and J. P. Toscano, J. Am. Chem. Soc. 2001, 123, 8109.

175.R. C. Bingham and P. v. R. Schleyer, J. Am. Chem. Soc. 1971, 93, 3189.

176.R. A. Moss, F. Zheng, J.-M. Fede´, Y. Ma, R. R. Sauers, J. P. Toscano, and B. M. Showalter, J. Am. Chem. Soc. 2002, 124, 5258.

CHAPTER 8

Stable Singlet Carbenes

GUY BERTRAND

UCR-CNRS Joint Research Chemistry Laboratory, (UMR 2282)

Department of Chemistry, University of California, Riverside, CA

1.	Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	329
2.	Singlet versus Triplet Ground State?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	330
3.	How the First Two Families of Stable Carbenes Were Discovered . . . . . . . . . .	332
	3.1. From the Curtius Bis(carbene)–Acetylene
	Analogy to (Phosphino)(silyl)carbenes . . . . . . . . . . . . . . . . . . . . . . . . . .	332
	3.2. From Wanzlick Equilibrium to Diaminocarbenes . . . . . . . . . . . . . . . . . .	334
4.	Synthesis and Structural Data for Stable Singlet Carbenes. . . . . . . . . . . . . . . .	335
	4.1. Carbenes with a p-Electron-Donating and a p-Electron-Withdrawing
	Heteroatom Substituents (D-C-W). . . . . . . . . . . . . . . . . . . . . . . . . . . . .	335
	4.2. Carbenes with Two p-Electron-Donating Heteroatom Substituents (D-C-D)	338
	4.3. Carbenes with One Electronically Active Heteroatom Substituent . . . . . . .	340
5.	Reactivity of Stable Singlet Carbenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	347
	5.1. Dimerization. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	347
	5.2. Cyclopropanation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	350
	5.3. Reactions with Lewis Bases and Acids . . . . . . . . . . . . . . . . . . . . . . . . .	354
	5.4. Stable Carbene–Transition Metal Complexes: Applications in Catalysis. . .	358
	5.4.1. Electronic Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	359
	5.4.2. Catalytic Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	362
6.	Conclusion and Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	365
Suggested Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		366
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		366

1. INTRODUCTION

As early as 1835, attempts to prepare the parent carbene (CH2) by dehydration of methanol had been reported.1 It is interesting to note that at that time the

Reactive Intermediate Chemistry, edited by Robert A. Moss, Matthew S. Platz, and Maitland Jones, Jr. ISBN 0-471-23324-2 Copyright # 2004 John Wiley & Sons, Inc.

329

330 STABLE SINGLET CARBENES

tetravalency of carbon was not established, and therefore the existence of stable carbenes was considered to be quite reasonable. At the very beginning of the twentieth century, Staudinger and Kupfer demonstrated that carbenes, generated from diazo compounds or ketenes, were highly reactive species.2 It quickly became clear that their six valence electron shell, which deﬁed the octet rule, was responsible for their fugacity. As a consequence, the quest for stable carbenes became an unreasonable target, and indeed remained so for quite some time! In the 1950s, Breslow3 and Wanzlick,4 realized that the stability of a carbene could be dramatically enhanced by the presence of amino substituents, but they were not able to isolate a ‘‘monomeric’’ carbene. It was only in 1988 that our group reported the synthesis of a stable carbene, namely, a (phosphino)(silyl)carbene.5

Since this discovery, a few persistent triplet carbenes6 have been prepared and other types of stable singlet carbenes have been isolated. Triplet carbenes are discussed in Chapter 9 by H. Tomioka in this volume, and therefore this chapter will be focused on singlet carbenes.

2. SINGLET VERSUS TRIPLET GROUND STATE?

The divalent carbon atom of carbenes has only six electrons in its valence shell. Four are used to make s bonds with the two substituents, and therefore two nonbonding electrons remain. To understand, the difference between the singlet and the triplet state, let us consider a prototypal carbene. The carbon atom can be either linear or bent, each geometry is describable by a speciﬁc hybridization. The linear geometry implies an sp hybridized carbene center with two nonbonding degenerate orbitals ( px and py). Bending the molecule breaks this degeneracy and the carbon atom adopts an sp2-type hybridization: the py orbital remains almost unchanged (it is usually called pp), while the orbital that starts as a pure px orbital is stabilized because it acquires some s character (it is therefore called s) (Fig. 8.1). The carbene ground-state multiplicity is related to the relative energy of the s and pp orbitals. The singlet ground state is favored by a large s-pp separation; Hoffmann assumed

E	py	pπ
	px
		σ
	py	p
	px	π
		σ

linear bent

Figure 8.1. Relationship between the carbene bond angle and the nature of the frontier orbitals.

SINGLET VERSUS TRIPLET GROUND STATE?

331

that a value of at least 2 eV is necessary to impose a singlet ground state, whereas a value <1.5 eV leads to a triplet ground state.7

Given these conditions, the inﬂuence of the substituents on the carbene groundstate multiplicity can be easily analyzed in terms of steric and electronic effects.

The electronic stabilization of the triplet state is at a maximum when the carbene frontier orbitals are degenerate, so that a linear geometry will favor the triplet state. A way to force a linear geometry is to increase the steric bulk of carbene substituents.8 Dimethylcarbene has a bent singlet ground state (111 ),9 while the di(tert- butyl)-10 and diadamantyl-11 carbenes are triplets. In contrast, cyclopropenylidene12 and even cyclopentylidene13 have singlet ground states due, at least in part, to angular constraint.

Does that mean that all carbenes featuring a broad carbene bond angle will have a triplet ground state? The answer is clearly no. Indeed, steric effects dictate the ground-state spin multiplicity as far as the electronic effects are negligible, which

is rarely the case.14 The inﬂuence of the substituents’ electronegativity on the carbene multiplicity was recognized relatively early on,15,16 and reexamined more

recently.17 It is now well established that s-electron-withdrawing substituents favor the singlet versus the triplet state. In particular, Harrison et al.15a,c showed that the ground state goes from triplet to singlet when the substituents are changed from electropositive lithium (triplet favored by 23 kcal/mol) to hydrogen (triplet favored by 11 kcal/mol) to electronegative ﬂuorine (singlet favored by 45 kcal/mol), although mesomeric effects certainly also play a role for the latter element. Indeed, inductive effects play a minor role on the ground-state spin multiplicity compared to mesomeric effects, which involve of the interaction of the carbon orbitals (s, pp) and appropriate p or p orbitals of one or two carbene substituents. Substituents interacting with the carbene center can be classiﬁed into two types, namely, D (for p-electron-donating groups such as F, Cl, Br, I, NR2, PR2, OR,SR, etc.), and W (for p-electron-withdrawing groups such as COR, CN,BR2, SiR3, PRþ3 , etc.). Importantly, D-C-D, most D-C-W, and some W-C-W carbenes are predicted to have singlet ground states; however, their electronic

structure and geometry can vary a lot, as shown in Figure 8.2.

(D,D)-Carbenes are predicted16,17 to be bent and the donation of the D substituent lone pairs results in a polarized four-electron three-center p system. The C D

	δ-	W C W	δ+	D C W	δ+ δ-
C	C	W C W	W C W	D C W	D C W
D D	D D		1/2δ- 1/2δ-
	1/2δ+ 1/2δ+

Figure 8.2. Electronic effects of the substituents for D-C-D, W-C-W, and D-C-W carbenes (D p-electron-donating substituent; W p-electron-withdrawing substituent).

332 STABLE SINGLET CARBENES

bonds acquire some multiple bond character, which implies that (D,D)-carbenes are best described by the superposition of two ylidic structures with a negative charge at the carbene center. The most representative carbenes of this type are the transient dimethoxy-18 and dihalocarbenes,19 and the stable diaminocarbenes that will be

described in detail in this chapter.

Most of the (W,W)-carbenes are predicted to be linear16,17 and this substitution pattern results in a polarized two-electron three-center p system. Here also, the C W bonds have some multiple bond character; these (W,W)-carbenes are best described by the superposition of two ylidic structures featuring a positive charge at the carbene carbon atom. The most studied carbenes of this type are the transient dicarbomethoxycarbenes20 and the ‘‘masked’’ diborylcarbenes.21 Since no carbenes of the latter type have yet been isolated, they are not included in this chapter. Lastly, the quasilinear (D,W)-carbenes combine both types of electronic interaction. The D substituent lone pair interacts with the py orbital, while the W substituent vacant orbital interacts with the px orbital. These two interactions result in a polarized allene-type system with DC and CW multiple bonds. Good examples of this type of carbene are given by the transient halogenocarboethoxycarbenes22 and by the stable (phosphino)(silyl)- and (phosphino)(phosphonio)carbenes (see below).

3. HOW THE FIRST TWO FAMILIES OF STABLE CARBENES WERE DISCOVERED

3.1. From the Curtius Bis(carbene)–Acetylene Analogy to (Phosphino)(silyl)carbenes

The ﬁrst carbene ever isolated was Ia, which was prepared using the most classical route to transient carbenes, namely, the decomposition of diazo compounds. The [bis(diisopropylamino)phosphino](trimethylsilyl)diazomethane precursor (1a) was obtained by treatment of the lithium salt of trimethylsilyldiazomethane with 1 equiv of bis(diisopropylamino)chlorophosphine.23 Dinitrogen elimination occurs by photolysis (300 nm) or thermolysis (250 C under vacuum)5 affording carbene Ia as a red oily material in 80% yield (Scheme 8.1). Carbene Ia is stable for weeks at room temperature and can even be puriﬁed by ﬂash distillation under vacuum (10 2 Torr) at 75–80 C.

We have to confess that when we ﬁrst carried out the decomposition of the diazo derivative 1a, we did not hope that the carbene Ia would be stable. In fact, based on

(i-Pr)2N

∆ or hν

(i-Pr)2N

SiMe3

- LiCl

(i-Pr)2N

- N2

(i-Pr)2N

SiMe3

Scheme 8.1

HOW THE FIRST TWO FAMILIES OF STABLE CARBENES WERE DISCOVERED

333

N2 N2

(i-Pr)2N

hν

(i-Pr)

(i-Pr)2N

–N2

(i-Pr)2N

x 2

(i-Pr)2N

N N(i-Pr2N)

(i-Pr)2N N

N(i-Pr2N)

Scheme 8.2

a very well-known reaction described at the end of the nineteenth century by Curtius24 and on our previous work concerning phosphinonitrenes (A),25 we were trying to demonstrate that a phosphinocarbene would behave as a phosphaacetylene (Scheme 8.2). Indeed, Curtius had shown that a,a0-bis(diazo) derivatives spontaneously lost two molecules of N2 giving rise to the corresponding alkynes. As diazo compounds are precursors of carbenes, this result clearly showed that an a,a0-bis (carbene) is an alkyne. Like carbenes, tricoordinated-trivalent phosphorus atoms possess a lone pair of electron and, to some extent, an accessible vacant orbital of the s type. Therefore, it was reasonable to believe that the phosphorus–carbon bond of a-phosphinocarbenes would feature some multiple-bond character. This hypothesis was reinforced by our own work concerning phosphinonitrenes. Indeed, we had shown that the decomposition of phosphine azides led to transient phosphinonitrenes (A) featuring a strong multiple-bond character, as shown by the obtention of the 2 þ 2 dimer, namely, a cyclodiphosphazene (B, Scheme 8.2).25

When we carried out the photolysis of 1a in the presence of trimethylchlorosilane and dimethylamine, as trapping agent, clean reactions occurred giving the corresponding adducts formally resulting from a 1,2-addition across the polarized PC-multiple bond.23 We only recognized later5a that these adducts more likely resulted from a carbene insertion into the A X bonds of the trapping agent, followed by 1,2-shifts (Scheme 8.3).

(i -Pr)2N

h ν

(i -Pr)2N

SiMe3

−N2

(i -Pr)2N

(i -Pr)2N A

= Me2N

H or Cl

SiMe3

Scheme 8.3

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