2398
J. Am. Chem. SOC.1983, 105, 2398-2408
fore did not interfere with the analysis. A typical run consisted of eight to nine points plus an infinity sample. Deuterium exchange reaction kinetics were run with 250-300 mL of substrate in 10 mL of alcoholic alkoxide solution. Aliquots (-1 cm’) were syringed into a separatory funnel containing 50 mL of ca. 1 N aqueous HCI and 0.5 mL of CCI,. The substrate was isolated from the CCI, by preparative gas chromatography and then submitted for mass spectral analysis. A typical run consisted of six to eight points plus an infinity sample. Detritiation kinetics were run with 10-40 r L of substrate in 15 mL of alcoholic sodium alkoxide. Aliquots were taken by a I-cm’ glass Luer-lok syringe equipped with a 4.5-in. stainless steel needle. Nine points and an infinity sample was taken for each run. The same size sample (0.90-1.00 mL) was taken with a separate syringe for each point. The aliquot was syringed into a 125” separatory funnel (with a Teflon stopcock) that contained 50.0 mL of 0.2-0.3 N aqueous HCI and 15.0 mL of toluene. The funnel was shaken for a set time for each run (usually 30-60 s) and then allowed to sit for ca. 10 min prior to draining the lower water layer. The toluene layer was then poured into a 25“ Erlenmeyer containing Drierite. After the solution dried, 10.0 mL of the toluene solution was added to a scintillation vial containing 10.0 mL of scintillation cocktail (4 g of PPO, 1 g of POPOP in 1.0 L of toluene).
After dark adapting for 30-60 min, the samples were counted. The results and calculated activation parameters are reported in Table I. The temperature range and number of total runs used for the calculations are also given. Normally we would measure rate constants at IO-deg intervals and a reported range of 50-80 OC with 4 runs would translate to single kinetic runs at 50, 60, 70, and 80 ‘C. Occasionally a kinetic run would be off the Arrhenius plot and was therefore not included in the calculations.
Acknowledgment. We are grateful to the donors of Petroleum Research Fund, administered by the American Chemical Society, and to the Research Corporation for a Cottrell College Science Grant for support of this research. We thank Jerry Kresge and William Tumas for helpful suggestions and discussions, and J. van Thuijl, W . C. M. M. Luyten, and J. J. van Houte for the mass spectral data used to calculate the hydrogen and deuterium exchange rates. Registry No. D,, 7782-39-0; H2, 1333-74-0; T,, 10028-17-8; EtOD, 925-93-9; MeOD, 1455-13-6;C6HSCHC1CF3,384-65-6; C6HSCH(CF),, 3 142-78-7.
Photolytic Cleavage of Remote Functional Groups in Polyfunctional Molecules. Photolysis of exo- and endo-2-Benzonorbornenyl Chlorides and Methanesulfonatesla,b Harry Morrison,*lf Alan Miller,le and B. Bigotld Contributionfrom the Department of Chemistry, Purdue University West Lafayette, Indiana 47907, and Laboratoire de Chimie Organique ThPorique, UniversitP Pierre et Marie Curie, Paris Cedex 75230, France. Received July 12, 1982
Abstract: The photochemistry and photophysics of the title compounds are reported. The exo chloride exhibits a facile cleavage of the C-Cl bond upon excitation of the aryl chromophore (eq 1-3). The reaction has a rate >700-fold larger than that observed with the endo isomer, derives from the singlet excited state, and leads to the formation of “hot” carbocations as well as free radicals. The exo mesylate shows similar characteristics (eq 4). A mechanism (Scheme 11) is suggested which incorporates both heterolytic fission as well as sequential homolytic fission (to a polarized radical pair), followed by electron transfer to form an ion pair. The stereoelectronically controlled activation of the C-Cl and C-OMs moieties is explained by a correlation diagram (Figure 5) which depicts the T,T* state as correlating with the diradical product state through an avoided crossing with the C-X u,u* state. Calculations indicate that this avoided crossing is considerably larger (and the barrier to reaction therefore considerably smaller) when the C-X unit in the 2-benzonorbornenyl series is exo.
Introduction The photochemical and spectroscopic consequences of intramolecular interactions between nonconjugated functionalities remains a subject of considerable interest.2 There is now ample evidence that only by a systematic investigation of model bifunctional substrates can the organic photochemist expect to understand and utilize the diverse chemistry to be found in complex multifunctional compounds. Toward these ends we have earlier (1) (a) Organic Photochemistry. 54. Part 53: Morrison, H.; Giacherio, D.; Pandey, G. Tetrahedron Lett. 1982,3437. Part 52: Morrison, H.; Miller, A.; Pandey, B.; Pandey, G.; Severance, D.; Strommen, R.; Bigot, B. Pure Appl. Chem. 1982,54, 1723. Part 51: Morrison, H.; Giacherio, D. J. Org. Chem. 1982,47, 1058. (b) Abstracted, in part, from the Doctoral Dissertation of A. M., Purdue University, 1980. (c) Purdue University. (d) Universite Pierre et Marie Curie. (2) (a) For a review and leading references, see: Morrision, H. Acc. Chem. Res. 1979, 12, 383. (b) Additional examples include Lin, C-I; Singh, P.; Maddox, M., Ullman, E. F. J. Am. Chem. Soc. 1980,102,3261. Studebaker, J.; Srinivasan, R.; Ors, J. A.; Baum, T. Ibid. 1980,102,6872. McCullough, J. J.; MacInnis, W. K.; Lock, C. J. L., Faggiani, R. Ibid. 1980, 102,7782. Manning. T.D. R.; Kropp, P. J. Ibid. 1981, 103, 889. Calcaterra, L. T.; Schuster, D. I. Ibid. 1981, 103, 2460. Castellan, A,; Desvergne, J.-P.; Bouas-Laurent,H. Chem. Phys. Lett. 1980,76, 390. Cristol, S. J.; Opitz, R. J.; Bindel, T. H. Dickenson, W. A. J . Am. Chem. SOC.1980, 102, 7977.
0002-1863/83/ 1505-2398$01.50/0
presented a preliminary report3 on an unusual, stereoelectronically controlled, photolytic cleavage of the chloro and methanesulfonyl (mesyl) groups in the benzonorbornenyl series. In this paper, we summarize our detailed studies on these interesting examples of the photoactivation of remote functional groups.
Results A. Photochemistry and Spectroscopy of ex0 -2-Benzonorbornenyl Chloride (exoC1BNB). Photochemistry of exoClBNB in Cyclohexane. The title compound was prepared by hydrochlorination of benz~norbornadiene.~Irradiation of a 0.02 M solution in cyclohexane at 254 n m produces HCl and two volatile photoproducts which have been identified as benzonorbornene (1) and bicyclohexyl (2) by comparison of spectra with authentic samples.5 Quantum efficiencies were determined in low conversion runs and are given in eq 1.6 (3) Morrison, H.; Miller, A. J . Am. Chem. SOC.1980, 102,372. (4) Wilt, J. W.; Gutman, G.; Ranus, W. J., Jr.; Zigman, A. R. J . Org. Chem. 1961,32,893. (5) Several high molecular weight products are also detectable, the principal two being tentatively assigned as exo- and endo-2-cyclohexylbenzonorbornene on the basis of ‘HNMR and mass spectral data.
0 1983 American Chemical Society
Photolysis of 2- Benzonorbornenyl Chlorides
J. A m . Chem. SOC.,Vol. 105, No. 8, 1983 2399 Table I. Effect of (E)-2-Heptene on the exoClBNB/MeOH Photoreactiona
@,
1 0.07
L
1
Q2 0.04
(1)
HCI
hc10.18
Photosensitization of exoClBNB in Cyclohexane. Photolysis of 0.015 M solutions of exoClBNB in cyclohexane with 3.3 M p-xylene at 254 nm results in only a trace of 2 and a 98% reduction in the formation of 1. Since p-xylene’ and exoClBNB* have triplet energies of about 80 kcal/mol, the efficiency of energy transfer in this experiment could be considered suspect; however, we have observed that triplet transfer from p-xylene to exoClBNB in methanol is relatively efficient (see below). Xenon Perturbation Study of exoClBNB in Cyclohexane. The addition of xenon to freeze-pumpthaw degassed exoClBNB resulted in a 65% quenching of the formation of 1 and a 48% quenching of the formation of 2. Photochemistry of exoClBNB in Methanol. Irradiation of 0.02 M exoClBNB in methanol a t 254 nm results in six primary photoproducts (in addition to HCI). The reaction and quantum efficiencies are given in eq 2.
3
1
@, 0.12
I CH20H
Q3
5 @s0.07
0.079
6
O6 0.13
4 G4 0.034
A
OCH3
@,
7 0.027
Products 4 (endo-benzonorbornen-2-ylmethanol)and 5 (benzonorbornadiene) are known compounds and were identified by spectal comparison with authentic samples. Product 3 (exobenzonorbornen-2-ylmethanol)was identified by its spectral data (cf. Experimental Section) and by conversion into the known exo-benzonorbornen-2-ylcarboxylicacids9 The ethers 6 (exobenzonorbornen-2-yl methyl ether) and 7 (1,3-methanonaphthalen-4-yl methyl ether) were identified by their spectral data (cf. Experimental Section). An additional photoproduct begins to appear a t very low conversions (-4% disappearance of exoC1BNB) and has been identified (VPC retention time vs. an independently synthesized sample) as 8 (tetracyclo-
8
[5.4.0.02~4.03~6]undeca-l (7),8,10-triene). This is the di-*-methane, triplet-sensitized, rearrangement product of 5 (known not to be formed by direct photolysis of 51°). The ratio of 5 and 8 varies ~
%change in 6
~~
( 6 ) The &,CI was determined in n-hexane. A previously reported3 value for # I represented an average of data obtained in cyclohexne and n-hexane. (7) Murov, S. L. “Handbook of Photochemistry”; Marcel Dekker: New York, 1973. (8) Estimated from the onset of the 0band in the phosphorescence emission spectrum at 77 K (1:l pentane/cyclopentane glass). (9) We are grateful to Professor J. W. Wilt for a sample of this acid.
a
+21
3
4
-32
-32
5
8
(5 + 8 1
+71
-63
0
6 -8
7 -7
0.1 m (Ei-2-heptene.
Table 11. Effect of PrSH on the exoClBNB/MeOH Photoreaction
70 change in @
[PrSH]
(m) 0.04 0.12
1
3
4
5
8
6
7
+92 +75
-86 -88
-100 -100
-100 -100
-100 -100
-23 -23
-31 -38
Table 111. Effect of Oxygen on the exoClBNB/MeOH Photoreaction
1 %change in @
-100
3
4
5
8
(5 + 8)
6
7
-86
-90
-33
-71
-49
-36
-33
inversely with irradiation time and 8 is assigned as a secondary photoproduct.’ I Photolysis of ExoClBNB in the presence of exo-norborn-2-yl chloride at 254 nm in methanol, to 90% loss of the aryl chloride, gave no loss of the aliphatic substrate (nor, in fact, did the direct irradiation of a methanolic solution of norbornyl chloride with 254-nm light). Photosensitization of exoClBNB in Methanol. A photolysis of 0.015 M exoClBNB in methanol with 3.3 M p-xylene (the latter absorbing all the incident light) was conducted simultaneously with a direct photolysis of the halide. Product formation was normalized to the formation of 1 by direct excitation; the data are presented as the ratios for direct vs. sensitized photolysis: 1, 1.00:0.13; 3, 0.55:trace; 4, 0.26:trace; 5, 0.38:trace; 6,2.01:1.16; 7, 0.1 1:trace. Photolysis of 0.02 M exoClBNB in 10% (by volume) of acetone in methanol with 300-nm light resulted in a 72% loss of the starting material and the following products: 1, 1.9%; 3, trace; 4, trace; 5, 12.6%; 6,57%; 7, not detected. It may be noted that neither aryl (Le., toluene) nor acetone sensitization leads to fragmentation of exo-norborn-2-yl chloride. Effect of (E)-2-Heptene on exoClBNB Photochemistry in Methanol. Irradiation of 0.018 M exoClBNB in methanol with 0.1 M (E)-2-heptene a t 254 nm gave the results shown in Table I. The principal observations are the following. (1) There is an increase in 1 (A+ = +0.025) which approximates the total diminution in 3 and 4 (A+ = -0.036). We presume this is due to hydrogen abstraction from the heptene by the benzonorbornenyl radical before the latter can couple with hydroxymethylene radicals.12 (2) The di-*-methane reaction (5 8) is quenched, but there is no change in the sum of 5 8.13 (3) Ether formation is not much affected by the heptene, and, in fact, there is little net quenching; Le., the sum of 1, 3, and 4 decreases by 5% and that of the ethers by -8%. Effect of n-Propyl Mercaptan on exoClBNB Photochemistry in Methanol. Irradiation of 0.022 M exoClBNB in methanol a t
+
-
(10) Edman, J. R. J . Am. Chem. SOC.1969, 91, 7103. (1 1) The $’s given in eq 2 were measured at 9% loss of exoClBNB, whence over 50% of 5 has rearranged to 8. Since an accurate G5 could not be determined at the conversions (54%) needed to prevent formation of 8, and since 8 is the only sensitized photoproduct of 5,’O d5 was taken as the sum of 5 + 8. (12) trans-Piperylene (0.04 M) eliminates all the radical products but has no effect on 5, 6, or 7; the other multiplicity data make it apparent that the diene is reacting chemically with the norbornyl radical. (13) Since heptene (ET 75 kcal/mol)14 quenching of 5 (ET 70 kcal/mol)1° should be endothermic, and kinetically not competitive with the intramolecular di-a-methane reaction, we believe heptene is quenching the triplet which sensitizes 5 (is., exoC1BNB). (14) By comparison with similar olefins in ref 7.
-
-
2400 J . Am. Chem. SOC.,Vol. 105, No. 8, 1983
Morrison et al.
Table IV. Effect of Water on the exoClBNB/MeOH Photoreaction
Table VI. Photolysis of exoClBNB in MeOH, i-PrOH, and r-BuOH
7% change in @ % H , O I
10 20 30 40
-27 -52 -64 -74
quantum efficiencies“ (5
+
3
4
5
8
8
-38 -58 -74 -82
-44 -59 -76
-18
-36
-34 -52
-50
-70
-85
-64
-82
-29 -47 -64 -76
)
6
7
-4
+9
-1 -5 -5
+21
+9 +4
Table V. Product Ratios for Silver-Ion-Assisted Ground-State Solvolysis vs. Photolysis of exoClBNB in Methanol/Water Mixtures
0.55
MeOH i-PrOH t-BuOH
0.48 0.38
0.079
0.034 0.010 0.017
0.027 0.050
photolvsis
10
0.50
20 30
0.76 1.36
0.18 0.40 0.75
254 nm, with two concentrations of mercaptan, gave the results shown in Table I1 (data are corrected for partial light absorption by the mercaptan). &e for the chloride is virtually unchanged by added mercaptan; G., (0.04 M PrSH) = 0.33, &is (0.12 M PrSH) = 0.55. As observed with trans-2-heptene, (+0.09,0.12 M PrSH) ~ lo), though the mercaptan is even more approximates A c $ ~ +(-0. efficient in trapping the norbornyl radical. By contrast with the heptene data, 5 8 are now eliminated, presumably because of reaction of PrSH with 5. There is also a reduction in ether formation which may be attributed to some reaction of carbocation(s) with the mercaptan.l5 Effect of Oxygen on exoClBNB Photochemistry in Methanol. Irradiation of 0.018 M exoClBNB in oxygen-saturated methanol a t 254 nm gave the results shown in Table 111. The formation of exo-benzonorbomen-2-01in the oxygenated solution is detectable by VPC. Note that there is virtually complete quenching of 1, 3, and 4, an inhibition of the di-?r-methane reaction (5 8), and partial quenching of (5 8), 6, and 7. We also observe that for the chloride is 0.37 in the presence of oxygen (Le., -33%), and fluorescence measurements, with and without oxygen, show that there is a reduction in 4f in the presence of oxygen of -37%. Effect of Water on exoClBNB Photochemistry in Methanol. Irradiation of 0.015 M exoClBNB in methanol/water mixtures at 254 nm gave the results shown in Table 1V. In addition, the formation of exo-benzonorbornen-2-o1(9)was assayed, and the data for 49 are: 10% H 2 0 , 49 0.022; 20% H 2 0 , 49 0.052% 30% HzO, d9 0.093; 40% H 2 0 , I#J~ 0.15.16 The I#J& value for the chloride is virtually unchanged by the added water, i.e., 10% H 2 0 , 4& 0.57; 20% H20, c$& 0.55; 30% H20, $& 0.53; 40% H20, 4& 0.50. Thus, there is extensive quenching of 1, 3, 4, and (5 + 8), a minimal effect on 6 or 7, and formation of 9 which is appreciable but does and must therefore be coming at the expense not seem to effect ddiS of the quenched products. In a separate experiment, photolyses in aqueous methanol were carried out with 0.06 M n-propyl mercaptan. The results were qualitatively similar to those given in Table 11. i.e., enhancement of 1, large diminution in 3, 4, 5, and 8, and small reductions in 6 and 7. Alcohol 9 was virtually unaffected by added mercaptan. Ground-State Solvolysis of exoClBNB in Methanol/Water Mixtures. Solvolyses of 0.015 M substrate were conducted in methanol/water mixtures containing 0.035 M silver nitrate for 20 h a t 25 OC.” The formation of the alcohol 9 and ether 618 was measured by VPC, and the ratios are presented in Table V, together with corresponding data from the photolyses. The se-
+
0.13
0.037 0.019
hexane r-BuOH i-PrOH EtOH MeOHb CH,CN/l% H,O EtOH/20% H,O
@f
Ea
0.065
1.90 12.50 19.41 24.33 32.62 36.02 -35.1
0.010
0.010 0.007 0.007 0.004 0.002 Dielectric constants from reference 19; the values for the mixed media were approximated by assuming a linear relationship between solvent composition and dielectric constant. The @f in MeOH was unchanged by the addition of lo-’ n HC1 or NaHCO, (to saturation).
EO0 1000
t1
AESOI v)
w 60
(16) No new product which could be attributed to rearranged alcohol was detected, but there are differences between the sum of the 4’s for product formation vs. qbdir which could reflect the presence of such a product. (17) There is essentially no loss of the chloride over this time period in the absence of the silver salt. (18) There is very little (
