5723
The Formation and Reactions of Monovalent Carbon Intermediates. I I I. The Reaction of Carbethoxymethyne with Olefins 0. P. Strausz,* G. J. A. Kennepohl,’ F. X. Garneau,’ Thap DoMinh,’ B. Kim,’ S. Valenty,2 and P. S . Skel12 Contribution from the Departments of Chemistry, University of Alberta, Edmonton, Alberta, Canada, and Pennsylvania State University, University Park, Pennsylvania. Received March 26, 1974
Abstract: Carbethoxymethyne (EtOCOc :) has been generated by the short wavelength photolysis of diethylmercury bisdiazoacetate, (N2CC02Et),Hg+ hv + 2EtOCOC : 2N2 Hg, and its reactions with cyclohexene,
+
+
cis-2-butene,and trans-2-butene were investigated. The carbyne, formed in its TI ground state, adds to the olefinic x bond in a spin allowed but, according to INDO MO calculations, orbital symmetry forbidden concerted step with retention of the geometrical configuration of the parent olefin to yieid a cyclopropyl radical. Parallel to addition, preferential insertion into the allylic C-H bonds also occurs yielding an alkyl radical. Thus, the basic chemical behavior of doublet ground state carbynes appears to bear a close resemblance to that of singlet carbenes. The cyclopropyl and alkyl radicals undergo free radical combination and hydrogen abstraction reactions with solvent molecules. The hydrogen abstraction reaction of cyclopropyl radicals is stereoselective and occurs from the sterically less hindered side of the ring, resulting in the preponderant formation of the less stable endo product. In the longer wavelength photolysis of the source compound N2is eliminated in a stepwise fashion and a singlet state mercury carbene, which is capable of insertion and stereospecific addition, is formed. Cleavage of the C-Hg linkage in the source compound results in the formation of ethyl diazoacetate which was detected and monitored during the photolysis by infrared spectroscopy. The effect of conversion and wavelength of photolysis on product yields and the relative importance of the various competing primary modes of decomposition were investigated and the overall mechanism is discussed.
arbon free radicals constitute the most common and frequently encountered class of organic intermediates. The chemistry of the trivalent, divalent, and to a lesser extent of the zerovalent carbon radicals, the simplest representatives of which are H3C-,H2C:, and the free carbon atom, respectively, comprises one of the fundamental pillars of modern organic chemistry. Little, however, is known about the family of monovalent carbon radicals, the carbynes. The simplest of these is CH, which has been detected spectroscopically in extraterrestrial matter 3 , 4 and under laboratory conditions in numerous energetic reactions including hydrocarbon flame^,^'^ discharges,’ and photochemical systems.* Its intermediacy has also been postulated in the reactions of “hot” carbon atomsgwith hydrocarbons, etc. lo
C
(1) University of Alberta.
(2) The Pennsylvania State University.
(3) G. Herzberg, “Molecular Spectra and Molecular Structures,” Vol. I, Van Nostrand, Princeton, N. J., 1967, p 491. (4) P. C. Keenan and W. W. Morgan, Astrophys. J., 94,501 (1941). ( 5 ) A. G. Gaydon, “Spectroscopy of Flames,” Chapman and Hall, London, 1959; H. F. Calcote, S . C. Kurzins, and J. W. Miller, S y m p . (Znt.) Combust.. IProc.1. 10th. 605 (1965): A. Fontiin. W. J. Miller. and J. M. Hogan; ibid.,.’545 (1965); G . P.’Glass,G. B,’Kistiakowsky, J: V. Michael, and H. Niki, ibid., 513 (1965). (6) D. R. Safrany, R. R. Reeves, and P. Harteck, J . Amer. Chem. SOC.,86, 3160 (1964). (7) P. S. Skell and R. R. Engel, J . Amer. Chem. SOC..87.1135 (1965): J. L. Sprung, S . Winstein, and W. F. Libby, J . Amer,. Chem. Sdc., 87, 1812 (1965); P. S . Skell and R. R. Engel, J . Amer. Chem. Soc., 93,2699 (1971); K. H. Homann, W. Lange, and H. G. Wagner, Ber. Bunsenges. Phys. Chem., 75,121 (1971). (8) L. J. Stief and V. J. De Carlo, J . Chem. Phys., 43, 2552 (1965); D. E. Milligan and M. E. Jacox, ibid., 48, 2265 (1968); W. Braun, A. M. Bass, D. D. Davis, and J. D. Simmons, Proc. R o y . SOC. Ser. A , 312, 417 (1969); E. Tschuikow-Roux and S . Kodama, J . Chem. Phys., 5 0 , 5292 (1969); R. G. W. Norrish, G. Porter, and B. A. Thrush, Proc. R o y . SOC.,Ser. A , 216, 165 (1953); J. H. Callomon and R. A. Ramsay, Can. J . Phys., 35, 129 (1957); G. Herzberg and J. W. Johns, Astrophys. J . , 158,399 (1969).
The basic objectives of the present study were twofold: to develop a suitable photochemical source for generating carbynes under conditions amenable to quantitative kinetic-mechanistic studies, and to examine their chemistry. Diethyl mercurybisdiazoacetate (I) appeared to be a suitable source material. The synthesis of I was described by Buchner“ in 1895, and in the past decade several additional diazomercurial structures were prepared. The first study of the chemistry of CH was reported by Safrany, Reeves, and Harteck6 in 1964. Early results on the photolysis of I have been reported in two preliminary communications by Strausz and cow o r k e r ~along ~ ~ with some observations on the reactions of carbethoxymethyne. Preceding and concurrent to the latter studies were two articles by McNesby, Braun, Welge, and Bass14*15 who described the detection of CH in flash photolyzed methane by kinetic spectroscopy and derived rate constants for some of its reactions. In the intervening period a few flash photolysis(9) J. Nicholas C. MacKay, and R. Wolfgang, J. Amer. Chem. Soc., 88, 1065 (1966); C. MacKay, J. Nicholas, and R. Wolfgang, ibid., 89, 5758 (1967). (10) F. Martinotti, M. J. Welch, and A. P. Wolf, Chem. Commun., 115 (1968); A. R. Fairbairn, J. Quant. Spectrosc. Radiat. Transfer,9, 943 (1969); G. W. Spangler, S . K. Lott, and M. J. Jonich, Chem. Commun., 842 (1966); G. M. Meaburn and D. Perner, Nature (London), 212, 1042 (1966). (11) E. Buchner, Chem. Ber., 28,215 (1895). (12) P. Yates and F. X.Garneau, Tetrahedron Lett., 71 (1967); H. D . Hartzler, Abstracts of the 155th National Meeting of American Chemical Society, San Francisco, Calif., 1968, No. P205. (13) T. DoMinh, H. E. Gunning, and 0. P. Strausz, J . Amer. Chem. SOC.,89, 6785 (1967); 0. P. Strausz, T. DoMinh, and J. Font, ibid., 90, 1930 (1968). (14) W. Braun, K. H. Welge, and J. R. McNesby, J . Chem. Phys., 45, 2650 (1966). (15) W. Braun, J. R. McNesby, and A. M. Bass, J . Chem. Phys., 46. 2071 (1967).
Strausz, et al.
Reaction of Carbethoxymethyne with OIeJins
5724
kinetic spectroscopic studies on carbynes have also been reported from other laboratories. l6- l8 Still, apart from our preliminary reports13 no detailed mechanistic study of a carbyne reaction has appeared to date in the literature. Results Photolysis of I in Cyclohexene Solution. Upon photolysis, nitrogen evolution, bleaching of the yellow color, and mercury precipitation became evident. In smaller diameter photolysis cells metallic mercury plated out in mirror form. In addition to N B , mercury, and ethyl diazoacetate (II), the following retrievable products were identified
6
where R is a COzEt group, and 3,3’-bicyclohexenyl (W. Gc retention times and mass spectra of these compounds are compiled in Table I and molecular weights from high resolution mass spectra and the principal peaks of the ir and nmr spectra in Table 11. The data are fulIy consistent with the proposed structures. Also, compounds IIIa-c, IV, V, and IX were synthesized independently and found to be identical with those resulting from the photolysis of I in cyclohexene. Examination of the nmr and mass spectra of the mother liquor after the isolation of VIII indicated the presence of at least one, and possibly two, mercury compounds in addition to residual VIII. They are tentatively assigned the structures
IIIa, ethyl (1’-cyclohexeny1)acetate
CHPR I
IIIb, ethyl (3‘-cyclohexeny1)acetate
?H2R I
IIIc, ethyl (4’-cyclohexeny1)acetate
IV, ethyl endo-norcarane-7-carboxylate
V, ethyl exo-norcarane-7-carboxylate R I
VI, ethyl norcarane-7-(3’-cyclohexenyl)-7-carboxylate R
m I
VII, ethyl norcarane-7-cyclohexyl-7-carboxylate
VIII, diethyl bis(7-norcaranyl)mercury-7,7’-dicarboxylate (16) M. W. Bosnali and D. Perner, 2. Naturforsch. A , 26, 1768 (1971). (17) W. J. R. Tyerman, Trans. Faraday SOC.,65, 2948 (1999); J . Chem. SOC. A , 2483 (1969). (18) R. McDaniel, R. Dickson, H. E. Gunning, and 0. P. Strausz, to b: submitted for publication.
Journal of the American Chemical Society / 96:18
The yields of products III-VI11 are dependent on the wavelength of the exciting radiation and were determined employing four different cut-off filters. The results are given in Table 111. Each figure reported represents the average of three determinations. The scatter among parallel runs in sdme cases was large and some trend appeared with the size of the photolysis cell. Nonetheless, it is clear that the total yield of metallic mercury and retrievable mercury free products decreases and that of the mercury adducts increases with increasing wavelength, Tn order to establish the mechanism of the formation of products 111 to VIII, the photolysis of VIII, the thermal stability of I, and the effect of dissolved oxygen upon the reactions were examined in auxiliary studies. The photolysis of VI11 was carried out under identical conditions and for the same time of irradiation as that of I. As in the photolysis of I, the photolysis of VI11 at X >210 nm affords products 111, IV, and V. At 53 and 93% yields of mercury the product ratio IV to V is about 20. The rate of photolysis of VI11 is much slower than the rate of photolysis of J, and its occurrence during the photolysis of I is insignificant. The results of the thermal studies are presented in Table IV. As seen from the data, I is thermally stable in oxygen-free cyclohexene solution up to 145’ but tends t o decompose even at room temperature at a measurable rate when dissolved oxygen or air is present. The decomposition yields mercury oxide and 11, in yields of up to 73% of the theoretical, as well as typical oxidation products of cyclohexene including cyclohexanol, cyclohexenol, cyclohexenone, bicyclohexenyl, etc. For the elucidation of the role of I1 in the photolysis of I, auxiliary studies were performed in which the concentrations of both species were continuously monitored using the sharp, characteristic infrared diazo bands of I1 and I at 2110 and 2075 cm-I, respectively. The rate of disappearance of I and the rate of formation of J T were determined from the initial slopes of the time profiles of the concentration curves shown for the photolysis with Vycor filtered light in Figure 1. The intervention of I1 in the early stages of the reaction was estimated to have upper limits of 8 % (A >210 nm) and 12 %(X >240 nm).
1 September 4, 1974
5725 Table I. Gc Retention Times and Mass Spectra of Products 111-IX from the Photolysis of I in Cyclohexene ~
Compd
tR,
111
180
IV
20.5"
V
24a
VI
21b
VI1
18b
VI11
9c
IX
min
260
m/e(P)
--m/e and (% of
I
168 (14) 168 (18) 168 (40) 248
94 (44) 122 (56) 95 (70) 81
250 (31) 536 (2100 A >2400A >2800A >3200A (Vycor) (Corex) (Pyrex) (uranium) 9 14 2 3 3 31 16 47 -35
8 7 1 3 2 21 27 48
-
30
5 2 1 2 1 11 39 50 -16
3 1 0.3 0.6 0.1 5 38 43 -4
Average of three experiments.
mination of the extent of conversion. Photolysis of I was carried out for different exposure times in cyclohexene, the solvent evaporated, trans-2-butene added, and the irradiation continued until no diazo band could be detected in the ir spectrum. After butene evaporaStrausz, e t a[.
1 Reaction of Carbethoxymethyne with Olefins
5726 Table IV. Thermolysis of I in Cyclohexene in the Absence and Presence of Oxygen -Concn, Temp, "C
Amount of oxygen
Reaction time, hr
25 80 145
Nil Nil Nil Nil 4 X mol 7 x 10-8 mol Open to air Open to air Open to air Open to air
24 8 3 1.5 1 0.5 4 4 4 4
165*
20 80 20 40 60 70 a
Per cent I1 produced from decomposed I.
7 -
Initial-
I
I1
12.1 12.1 12.1 11.9 12.1 12.0 12.0 12.0 12.0 12.0
0.5 0.8 1 .o 1 .o 0.5 1 .o 0.25 0.25 0.25 0.25
M X 10-8 -lanFi----I
Per cent0 I1 produced
I1
12.1 0.5 12.1 0.9 12.0 1.2
