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738 STRAINED HYDROCARBONS: STRUCTURES, STABILITY, AND REACTIVITY

REFERENCES

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2.J. B. Pedley, R. D. Naylor, and S. P. Kirby, Thermochemical Data of Organic Compounds, 2nd ed., Chapman and Hall, London, 1986.

3.J. L. Jensen, Prog. Phys. Org. Chem. 1976, 12, 189.

4.U. Burkert and N. L. Allinger, Molecular Mechanics, American Chemical Society, Washington, DC, 1982. N. L. Allinger and P. v. R. Schleyer, Eds., J. Comput. Chem. 1996, 17, pp. 488ff.

5.M. Saunders, J. Comput. Chem. 1991, 12, 645.

6.W. J. Hehre, L. Radom, P. v. R. Schleyer, and J. A. Pople, Ab Initio Molecular Orbital Theory, John Wiley & Sons, Inc., New York, 1986.

7.W. Koch and M. C. Holthausen, A Chemist’s Guide to Density Functional Theory, 2nd ed., Wiley-VCH, Weinheim, 2001.

8.L. A. Curtiss, K. Raghavachari, G. W. Trucks, and J. A. Pople, J. Chem. Phys. 1991, 94, 7221.

9.J. W. Ochterski, G. A. Petersson, and J. A. Montgomery, J. Chem. Phys. 1996, 104, 2578.

10.K. B. Wiberg, J. Comput. Chem. 1984, 5, 197. K. B. Wiberg, J. Org. Chem. 1985, 50, 5285. M. R. Ibrahim and P. v. R. Schleyer, J. Comput. Chem. 1985, 6, 157. L. R. Schmitz and Y. R. Chen, J. Comput. Chem. 1994, 15, 1337.

11.M. W. Chase, Jr., C. A. Davies, J. R. Downey, Jr., D. J. Frurip, R. A. MacDonald, and

A.N. Syverud, J. Phys. Chem. Ref. Data 1985, 14, Suppl. 1.

12.For an example of the use of heats of atomization, see J. W. Ochterski, G. A. Petersson, and K. B. Wiberg, J. Am. Chem. Soc. 1995, 117, 11299.

13.Compare K. B. Wiberg and J. W. Ochterski, J. Comput. Chem. 1997, 18, 108.

14.J. L. Franklin, Ind. Eng. Chem. 1949, 41, 1070. Compare K. M. Engler, J. D. Andose, and

P.v. R. Schleyer, J. Am. Chem. Soc. 1973, 95, 8005.

15.S. J. Blanksby and G. B. Ellison, Acct. Chem. Res. 2003, 36, 255.

16.W. F. Maier and P. v. R. Schleyer, J. Am. Chem. Soc. 1981, 103, 1891.

17.W. R. Roth, F.-G. Klarner, and H.-W. Lennartz, Chem. Ber. 1990, 113, 1818. W. Fang and D. W. Rogers, J. Org. Chem. 1992, 57, 2294.

18.C. A. Coulson and W. E. Mofﬁtt, Philos. Mag. 1949, 40, 1.

19.C. A. Coulson, Valence, Clarendon Press, Oxford, 1952, p. 200.

20.Unpublished calculations.

21.K. B. Wiberg and F. H. Walker, J. Am. Chem. Soc. 1982, 104, 5239. K. B. Wiberg, W. P. Dailey, F. H. Walker, S. T. Waddell, L. S. Crocker, and M. Newton, J. Am. Chem. Soc. 1985, 107, 7247.

22.K. B. Wiberg and S. T. Waddell, J. Am. Chem. Soc. 1990, 112, 2194.

23.J. D. Kemp and K. S. Pitzer, J. Chem. Phys. 1936, 4, 749.

24.R. M. Moriarty, Top. Stereochem. 1974, 8, 270.

25.M. K. Leong, V. S. Mastryukov, and J. E. Boggs, J. Mol. Struct. 1998, 445, 149.

26.E. L. Eliel and S. H. Wilen, Stereochemistry of Organic Compounds, John Wiley & Sons, Inc., New York, 1994, pp. 762ff.

27.F. A. L. Anet, Top. Curr. Chem. 1974, 45, 169.

REFERENCES 739

28.F. Vogtle, Cyclophane Chemistry: Synthesis, Structure and Reactions, John Wiley & Sons, Inc., New York, 1993.

29.D. J. Cram, R. B. Hornby, E. A. Truesdale, H. J. Heich, M. H. Dalton, J. M. Cram,

Tetrahedron, 1974, 30, 1757.

30.D. J. Cram and R. A. Reeves, J. Am. Chem. Soc. 1958, 80, 3094.

31.G. J. Abruscato and T. T. Tidwell, J. Org. Chem. 1972, 37, 4151.

32.Compare M. S. Newman, Steric Effects in Organic Chemistry, John Wiley & Sons, Inc., New York, 1963.

33.D. W. Rogers, H. von Voitherberg, and N. L. Allinger, J. Org. Chem. 1978, 43, 360.

34.M. Squillacote, A. Bergman, and J. De Felippis, Tetrahedron Lett. 1989, 30, 6805. G. M. Wallraff and J. Michl, J. Org. Chem. 1986, 51, 1794. Y. Inoue, T. Ueoka, T. Kuroda, and

T.Hakushi, J. Chem. Soc. Chem. Commun. 1981, 1031. E. J. Corey, F. A. Carey, R. A. E. Winter, J. Am. Chem. Soc. 1965, 87, 934.

35.R. G. Solomon, K. Folting, W. E. Streib, and J. K. Kochi, J. Am. Chem. Soc. 1974, 96, 1145. trans-1-Phenylcyclohexene has been observed spectroscopically (R. Bonneau, J. Joussot-Dubien, L. Salem, and A. J. Yarwood, J. Am. Chem. Soc. 1976, 98, 4329) and its strain energy has been measured via photoacoustic calorimetry (J. L. Goodman, K. S. Peters, H. Misawa, and R. A. Caldwell, J. Am. Chem. Soc. 1986, 108, 6803. It is 47 3 kcal/mol less stable than the cis isomer.)

36.A. T. Blomquist, R. E. Burge, Jr., and A. C. Sucsy, J. Am. Chem. Soc. 1952, 74, 3636.

37.M. Traetteberg, Acta. Chem. Scand. 1975, B29, 29.

38.R. C. Haddon, J. Am. Chem. Soc. 1987, 109, 1676.

39.K. B. Wiberg, Acc. Chem. Res. 1996, 29, 229.

40.A. E. Reed, L. A. Curtiss, and F. Weinhold, Chem. Rev. 1988, 88, 899.

41.P. M. Warner, Chem. Rev. 1989, 89, 1067.

42.R. Keese and E. P. Krebs, Angew. Chem. Int. Ed. Engl. 1971, 10, 262. Compare C. M. Geise and C. M. Hadad, J. Am. Chem. Soc. 2000, 122, 5861 and E. Kohl, T. Stroter,

C.Siedschlag, K. Polborn, and G. Szeimies, Eur. J. Org. Chem. 1999, 3057.

43.K. Lukin and P. E. Eaton, J. Am. Chem. Soc. 1995, 117, 7652.

44.M. Hare, T. Emrick, P. E. Eaton, and S. R. Kass, J. Am. Chem. Soc. 1997, 119, 237.

45.D. A. Hrovat and W. T. Borden, J. Am. Chem. Soc. 1988, 110, 4710. W. T. Borden, Synlett 1996, 711.

46.K. B. Wiberg, N. McMurdie, J. V. McClusky, and C. M. Hadad, J. Am. Chem. Soc. 1993, 115, 10653.

47.F. H. Walker, K. B. Wiberg, and J. Michl, J. Am. Chem. Soc. 1982, 104, 2056. K. B. Wiberg, F. H. Walker, W. E. Pratt. and J. Michl, J. Am. Chem. Soc. 1983, 105, 3683.

48.C. J. Doubleday, Am. Chem. Soc. 1993, 115, 11968.

49.K. B. Wiberg, J. J. Caringi, M. G. Matturro, J. Am. Chem. Soc. 1990, 112, 5854.

50.P. Eaton, G. Temme, III, J. Am. Chem. Soc. 1973, 95, 7508. The compound that was studied had an N,N-dimethylcarboxamide group.

51.W.-D. Stohrer and R. Hoffmann, J. Am. Chem. Soc. 1972, 94, 779 (cf. Ref. 39).

52.P. B. Shevlin and A. P. Wolf, J. Am. Chem. Soc. 1970, 92, 406.

740STRAINED HYDROCARBONS: STRUCTURES, STABILITY, AND REACTIVITY

53.G. Maier, S. Pfriem, U. Schafer, and R. Matusch, Angew. Chem. Int. Ed. Engl. 1978, 17, 520. R. Notario, O. Castano, J. L. Andres, J. Elguero, G. Maier, and C. Hermann,

Chem.—A Eur. J. 2001, 7, 342.

54.For a review of orbital symmetry, see R. B. Woodward and R. Hoffmann, The Conservation of Orbital Symmetry, Academic Press, New York, 1970.

55.R. Criegee and K. Noll, Ann. Chem. 1959, 627, 1. R. E. K. Winter, Tetrahedron Lett. 1965, 1207.

56.R. Breslow and P. Dowd, J. Am. Chem. Soc. 1963, 85, 2729.

57.M. S. Baird, H. Hussain, and W. Clegg, J. Chem. Soc. Res. Synop. 1988, 110.

58.Compare W. E. Billups, W. Luo, G-A. Lee, J. Chee, B. E. Arney, Jr., K. B. Wiberg, and D. R. Artis, J. Org. Chem. 1996, 61, 764.

59.K. B. Wiberg, M. G. Matturo, P. J. Okarma, and M. E. Jason, J. Am. Chem. Soc. 1984, 106, 2194.

60.K. B. Wiberg and G. Bonneville, Tetrahedron Lett. 1982, 5385.

61.G. Szeimies, in Strain and its Implications in Organic Chemistry, A. de Meijere and S. Blechert, Eds., Kluwer Academic Publishers, Dordrecht, The Netherlands, 1988, p. 361.

62.M. D. Levin, P. Kaszynski, and J. Michl, Chem. Rev. 2000, 100, 169.

63.K. B. Wiberg and N. McMurdie, J. Am. Chem. Soc. 1991, 113, 8995.

64.R. Baird and A. A. Aboderin, J. Am. Chem. Soc. 1964, 86, 252, 2300.

65.M. Saunders, P. Vogel, E. L. Hagen, and J. Rosenfeld, Acc. Chem. Res. 1973, 6, 53.

66.K. B. Wiberg and S. R. Kass, J. Am. Chem. Soc. 1985, 107, 988.

67.D. M. Adams, J. Chatt, G. Guy, and N. Sheppard, J. Chem. Soc. 1961, 738.

68.L. Cassar, P. E. Eaton, and J. Halpern, J. Am. Chem. Soc. 1970, 92, 3535.

69.B. Rybtchinski and D. Milstein, Angew. Chem. Int. Ed. Engl. 1999, 38, 871. K. C. Bishop, III, Chem. Rev. 1976, 76, 461.

70.J. J. Burke and P. C. Lauterbur, J. Am. Chem. Soc. 1964, 86, 1870.

71.D. J. Patel, M. E. H. Howden, and J. D. Roberts, J. Am. Chem. Soc. 1963, 85, 3218. C. D. Poulter, R. S. Boikess, J. I. Brauman, and S. Winstein, J. Am. Chem. Soc. 1972, 94, 1191.

72.H.-O. Kalinowsi, S. Berger, and S. Braun, Carbon-13 NMR Spectroscopy, John Wiley & Sons, Inc., Chichester, 1988.

73.M. B. Robin, Higher Excited States of Polyatomic Molecules, Vol. 1, Academic Press, New York, 1974, pp. 140 ff.

74.V. A. Walters, C. M. Hadad, Y. Thiel, S. D. Colson, K. B. Wiberg, P. M. Johnson, and J. B. Foresman, J. Am. Chem. Soc. 1991, 113, 4782.

75.K. B. Wiberg and R. E. Rosenberg, J. Phys. Chem. 1992, 96, 8282. K. B. Wiberg, R. E. Rosenberg, and S. T. Waddell, J. Phys. Chem. 1992, 96, 8293.

CHAPTER 16

Arynes

MICHAEL WINKLER, HANS HENNING WENK, and WOLFRAM SANDER

Lehrstul fu¨r Organische Chemie II der Ruhr-Universitat Bochum, 44780 Bochum, Germany

1.	Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	741
2.	The Parent Benzynes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	742
	2.1. The o-Benzyne Story. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	742
	2.2. The m-Benzyne Story . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	747
	2.3. The p-Benzyne Story. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	752
3.	Substituted Arynes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	759
	3.1. o-Benzynes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	759
	3.2. m-Benzynes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	760
	3.3. p-Benzynes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	762
4.	Annellated Arynes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	764
	4.1. Naphthynes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	764
	4.2. Didehydroindenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	769
5.	Heterocyclic Arynes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	773
6.	Related Systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	782
	6.1. Tridehydrobenzenes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	782
	6.2. Benzdiynes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	782
	6.3. Cyclo[6]-carbon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	784
7.	Conclusions and Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	784
Suggested Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		786
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		786

1. INTRODUCTION

Arynes were postulated as reactive intermediates more than 100 years ago.1 First indications for the existence of such molecules were reported by Stoermer and Kahlert2 in 1902. The isolation of 2-ethoxybenzofuran (3) after

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.

741

742	ARYNES
	Br
		KOH/EtOH		KOH/EtOH	OEt
		− HBr		+ EtOH	OEt
	O	− HBr	O	+ EtOH	O
	O		O		O
	1		2		3

Scheme 16.1. The reaction 1 ! 3 gave the ﬁrst hint on the existence of an aryne, o-didehydrobenzofuran (2).2

treatment of 3-bromobenzofuran (1) with bases in ethanol suggests that o-dide- hydrobenzofurane (2) is formed in the course of this reaction (Scheme 16.1).2

Despite occasional speculation about arynes in the following years,3 it took several decades until the groups around Roberts,4 Huisgen,5 and Wittig6 could obtain deﬁnitive evidence for the existence of these reactive intermediates. The discovery of the endiyne cytostatics7 has initiated a renaissance in aryne chemistry and many research efforts in this area have been reported during the last 20 years.1,8 Furthermore, benzynes have been recognized as important reaction intermediates in combustion processes,9 and they are expected to play a key role in the transformations of aromatics on transition metal surfaces.10 This chapter will focus primarily on direct spectroscopic and theoretical investigations of arynes, whereas indirect evidence, as, for example, from trapping reactions, is only dealt with incidentially. Biological aspects and research directed toward the reactivity of endiynes with desoxyribonucleic acid (DNA), though very important, are not discussed in this chapter, because they have been reviewed elsewhere in more detail recently.1

2. THE PARENT BENZYNES

2.1. The o-Benzyne Story

o-Benzyne (4) was ﬁrst suggested as a reactive intermediate in 1927,11 and the existence of this species was further elaborated by Wittig in the following years.6 In a classic investigation in 1953, Roberts et al.4a found compelling evidence for the intermediate formation of 4 in the reaction of 14C labeled chlorobenzene with sodium amide (Scheme 16.2).

Formula 16.1

In 1956 Wittig was able to trap o-benzyne in various Diels–Alder reactions.6c Nonetheless, he remained skeptical about his own hypothesis of the existence of

	THE PARENT BENZYNES		743
Cl	NH2
+ KNH2	+ NH3		NH2
+ KNH2	+ NH3	+
− KCl, NH3		+
− KCl, NH3
4	50 %	50 %
= 14C

Scheme 16.2. In a classic experiment, Roberts et al.4a demonstrated the involvement of 4 in the nucleophilic aromatic substitution by isotopic labeling experiments. The product distribution can only be explained by the assumption of a symmetric intermediate.

4,6d until Huisgen and Knorr5c could demonstrate in an elegant work that the same intermediate is formed from four different precursors of 4 (Scheme 16.3). At approximately the same time, Fisher and Lossing12 investigated the pyrolysis of the three isomeric diiodobenzenes using mass spectrometry (MS) and identiﬁed 4

			O
Li
			O
F			k1
F
MgBr
N+
2
COO−		4
COO−			k2
k	/ k	= const	+ ...
1	2

SO2

Scheme 16.3. In a very elegant trapping experiment, Huisgen and Knorr demonstrated that the same intermediate is formed from four different precursors.5c If 4 is trapped by a mixture of furan and cyclohexadiene, the same product ratio is observed in all four reactions.

on the basis of the measured ionization potential. Berry et al.13 studied the photoinitiated decomposition of benzenediazonium carboxylates and characterized 4 by its ultraviolet (UV) and mass spectrum in the gas phase.

The ﬁrst direct infrared (IR) spectroscopic detection of o-benzyne was accomplished by Chapman et al.,14 using matrix isolation spectroscopy at very low temperatures to generate 4 starting from phthaloyl peroxide (5) and benzocyclo-

744 ARYNES

		O	O
	hν	O
O	hν
O	− CO2	O	O
			O

− CO2

∆

Scheme 16.4.

hν		hν (308 nm) + CO
hν
− 2 CO		hν (248 nm) − CO
4			8
4		hν (248 nm)
		hν (248 nm)	O
			O
hν (308 nm)	− CO
	− CO2
	O
		hν (308 nm)
	O	− CO2	O
		− CO2

Photochemistry of 4 and the corresponding precursors.

butenedione (6, Scheme 16.4). High yields of 4 were also obtained using phthalic anhydride (7) as a precursor. The comparatively complex reactions of the molecules involved in this photochemistry led to some controversy about the frequency of the C C stretching vibration, surely the most interesting feature in the IR spectrum of 4. Chapman et al.14 assigned a band at 2085 cm 1 to the C C stretching vibration and this ﬁnding was conﬁrmed in a later investigation by Dunkin and MacDonald.15 Wentrup et al. found this band at 2080 cm 1 in a phthalic anhydride matrix at 77 K. On annealing of the matrix a second overlapping absorption at the same position was observed, which was assigned to cyclopentadienylideneketene (8).16 The assignment of the 2085-cm 1 trace to the C C stretching vibration in 4 was further supported theoretically.17 Doubts arose when Leopold et al.18 inferred a frequency of 1860 cm 1 from the gas-phase PE spectrum. In order to clarify this discrepancy, Schaefer and co-workers19 calculated the IR spectrum of 4 and proposed that the absorption should be found in the range from 1965 to 2010 cm 1. In 1990 Schweig and co-workers20 were able to demonstrate that an absorption at 2087 cm 1 belongs to cyclopentadienylideneketene rather than 4. This issue was ﬁnally settled in 1992

		THE PARENT BENZYNES	745
	1
by Radziszewski et al.,21 who could deﬁnitively identify the frequency of the C C
stretching vibration at 1846 cm		by thorough analysis of the IR spectra of various
isotopomers of 4.

As expected, the formal C C triple bond in benzyne is signiﬁcantly weaker than in unstrained alkynes, the C C stretching vibrations of which usually fall in the

region 2150 cm 1. Nevertheless, o-benzyne is better described as a strained alkyne rather than a biradical, which is evident from the large singlet–triplet splitting of 37.5 0.3 kcal/mol22 as well as the alkyne-like reactivity (e.g., in Diels– Alder reactions). The enthalpy of formation of 4 was determined to be 106.6 3.0 kcal/mol by Wenthold and Squires.23 For the C C bond length a value of 124 2 pm was found experimentally,24 which comes closer to a typical C C triple bond (120.3 pm in acetylene) rather than a C C double bond (133.9 pm in ethylene).

In remarkable work, Warmuth et al.25 were able to isolate o-benzyne in a hemi- carcerand—a ‘‘molecular container’’—and to measure its NMR spectrum in solution. Whereas previous theoretical studies26 concerning the structure of 4 had to rely more or less solely on IR spectra,27 there were now additional experimental data available to judge the quality of the theoretical predictions.27b The 1H and 13C chemical shifts were most accurately reproduced using the B3LYP/6-311G** structure [r(C1C2) ¼ 124.5 pm], while calculations on the basis of CASSCF(8,8)/ DZP or CCSD(T)/6-31G** geometries led to poorer agreement with experiment.27b Furthermore, it was concluded that 4 more closely resembles a cyclic alkyne rather than a cumulene.

The important role of o-benzyne in combustion processes28 has been realized only recently. In 1997, Lin and co-workers9a suggested that loss of a hydrogen atom from the phenyl radical, which is well known to be a key intermediate in the formation of polycyclic aromatic hydrocarbons (and soot) in ﬂames, is a dominant process in the combustion of lead-free gasoline (which contains up to 20–30% of small aromatics). A deeper understanding of combustion processes thus requires a knowledge of the high-temperature chemistry of arynes. In this context, a series of elegant studies by R.F.C. Brown et al.29–31 deserves special attention. Flash vacuum pyrolysis (FVP) of phthalic anhydride (7) (T ¼ 850 C), partly (5%) 13C labeled in the 1 and 6 position, gave a pyrolysate containing biphenylene in which one 13C label was distributed between two quaternary positions (Scheme 16.5).29a This rearrangement was explained in terms of a ring contraction of benzyne (4) leading to the exocyclic carbene (9), a reaction analogous to the acetylene–methylenecar-

bene (vinylidene) rearrangement. Different precursors (6,					T ¼ 650–830 C; 11,
T	¼	400–850 C) have been used, all of which contain a CO moiety in the leaving
			29b,29c	The interpretation of the labeling	experiments has been
group, however.

questioned by Wentrup et al., who proposed that the observed scrambling may better be explained by a Wolff type ring contraction of the intermediate ketenecarbene (12, Scheme 16.5).32 Thus, the mechanism of the thermal decomposition of 7 has not been fully clariﬁed, yet although the aryne contraction pathway has been established for some benzannellated derivatives of 4.29 A number of theoretical studies has been devoted to rearrangements on the C6H4 potential energy surface.9b,33 At

746 ARYNES

COOH

N N NMe2	= 13C
11

FVP

FVP	12	O	9

			..

Scheme 16.5. The FVP of isotopically labeled phthalic anhydride (7) indicates a scrambling

of carbon atoms. Yet, it is not clear whether a reversible ring contraction of 4 is responsible for this exchange or if it takes place already at the stage of ketenecarbene (12).29,30,32

According to recent calculations, the barrier for the rearrangement 4 ! 9 is only 32 kcal/ mol, while the back-reaction proceeds with a very small or even vanishing barrier.33 Thus, at 850 C the ring contraction of 4 to 9 should at least energetically be possible.

the G2M level of theory the reaction 4 ! 9 is endothermic by 31 kcal/mol and the barrier is only slightly larger (32 kcal/mol), thus rendering this reaction of o- benzyne possible at elevated temperatures.33

o-Benzyne also plays an important role in benzene transformations and decomposition on transition metal surfaces and thus in heterogenous catalysis.10 An

THE PARENT BENZYNES

747

ordered overlayer of adsorbed benzyne on Ir{100} has recently been prepared by thermal decomposition of adsorbed benzene, and characterized by low-energy electron diffraction (LEED). It has been demonstrated that 4 is di-s-bonded to the surface with its ring plane at 47.2 to the surface normal.10a In a theoretical study, it has been shown that this tilt arises from back-bonding interactions of the aromatic p system with the surface d orbitals.10b

This short outline of the history of o-benzyne, one of the classic reaction intermediates, already shows how difﬁcult apparently easy things (assigning IR absorptions or investigating elementary rearrangements) can be. Progress in aryne chemistry generally comes only slowly; despite the long history, many studies remain to be done in this challenging ﬁeld of research.

2.2. The meta-Benzyne Story

Early attempts to generate and characterize m-benzyne (13) comprise the pyrolysis of 1,3-diiodobenzene (14)12 and the ﬂash photolysis of benzenediazonium- 3-carboxylate (15).34 While in the former case only enediynes (16) were in accordance with the ionization potentials determined (pyrolysis temperature: 960 C), the results of the latter work remain unclear. A transient with m/z ¼ 76 and a lifetime of 400 ms was observed, however, it was not possible to unambiguously identify this species as 13 due to the complexity of the product spectrum (Scheme 16.6). Indirect evidence for the existence of 13 stems from trapping experiments.35–37

I		I	.		N2+
			.
	960 °C	960 °C	.	hν
I	− I	. − I	.	− N2, CO2	COO−
14			13		15
			960 °C

960 °C

(E )-16

(Z )-16

Scheme 16.6. Early attempts to generate m-benzyne did not give conclusive evidence for existence and structure of 13.

In 1975, Washburn et al.38 investigated the dehydrohalogenation of 2,6-dibromo- bicyclo[3.1.0]hex-3-ene (17) with bases and postulated the intermediate formation of bicyclo[3.1.0]hexatriene (18, Scheme 16.7).

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