187
J. Am. Chem. SOC.1984, 106, 187-193
Photochemical Reactions of Arenecarbonitriles with Aliphatic Amines. 1. Effect of Arene Structure on Aminyl vs. a-Aminoalkyl Radical Formation Frederick D. Lewis,* Beth E. Zebrowski, and Paul E. Correa Contribution from the Department of Chemistry, Northwestern University, Eaanston, Illinois 60201. Received June 20, I983
Abstract: The photochemical reactions of several singlet arenecarbonitriles with aliphatic amines have been investigated. The reactions of 9-phenanthrenecarbonitrile with diethylamaine and its N- and a-C-deuterated derivatives result in exclusive N-H atom transfer to yield the diethylaminyl and 9-cyan0-9,lO-dihydrophenanthren-9-ylradicals in benzene solution. Increased solvent polarity results in the formation of both diethylaminyl and 1-(ethy1amino)ethyl radical. Similar results are obtained with 9-anthracenecarbonitrile. The competition between aminyl vs. a-aminoalkyl radical formation in this and other reactions is reviewed. Aminyl radical formation is characteristic of relatively nonpolar heteroexcimers in which hydrogen bonding may favor N-H transfer. Pure charge-transfer exciplexes, like alkoxy1 radicals, yield predominantly the thermodynamically more stable a-aminoalkyl radical. The dominant reaction of the aminyl-phenanthren-9-yl radical pair in nonpolar solvent is combination to yield arene-amine adducts, which quantitatively lose HCN upon heating or chromatography. In contrast, the a-aminoalkyl-phenanthren-9-yl radical pairs formed in polar solvent disproportionate to yield reduced arene and oxidized amine, plausibly via radical pair electron transfer followed by proton transfer.
Introduction The interaction of electronically excited organic molecules with tertiary amines has been extensively investigated and provides much of the basis for the present understanding of exciplex and radical ion pair The observation of exciplex fluorescence from numerous arene-amine exciplexes has allowed detailed spectroscopic investigations of these species. Addition of the a-C-H bond of trialkylamines to some arenes is attributed to an electron-transfer, proton-transfer mechanism (Scheme Recent investigations of the mechanism of reactions of triplet benzophenone* and singlet trans-stilbene7e with trialkylamines indicate that proton transfer occurs via an exciplex (contact radical ion pair) intermediate and not a solvent-separated radical ion pair, confirming an earlier proposal by Bryce-Smith and Gilbert.4b The interaction of singlet arenes with secondary amines results in the formation of short-lived, nonfluorescent exciplexes. Picosecond laser spectroscopic investigations of pyrene-secondary amine exciplexes indicate that the amine N-H group is hydrogen bonded to the pyrene a-orbitals in nonpolar solvent and that intersystem crossing is much more rapid than in the case of pyrene-tertiary amine e x c i p l e ~ e s . ~Chemical reactions of arene-secondary amine exciplexes normally result in 1,2- or 1,4addition of the amine N-H bond to the arene or arene reduction via a mechanism analogous to that proposed for tertiary (1) (a) Beens, H.; Weller, A. ‘Organic Molecular Photophysics”; Birks, J. B., Ed.; Wiley: London, 1975; Vol. 2. (b) Weller, A. “The Exciplex”; Gordon, M., Ware, W. R.,Ed.; Academic Press: New York, 1975; Chapter 2. (c) Weller, A. 2.Phys. Chem. (Wiesbaden) 1982, 130, 129-138. (2) (a) Mataga, N.; Ottolenghi, M. “Molecular Association”: Foster, R., Ed.; Academic Press: London, 1979; Vol. 2. (b) Masuhara, H.; Mataga, N. Acc. Chem. Res. 1981, 14, 312-318. (3) Davidson, R.S. Adu. Phys. Org. Chem. 1983, 19, 1-126. (4) (a) Bellas, M.; Bryce-Smith, D.; Gilbert, A. Chem. Commun. 1967, 862-863. (b) Bellas, M.; Bryce-Smith, D.; Clarke, M. T.: Gilbert, A,; Klunkin, G.; Krestonosich, S.; Manning, C.; Wilson, S. J . Chem. Soc., Perkin Trans. I 1977 , 2571-2580. (c) Gilbert, A,; Krestonosich, S . Ibid. 1980, 2531-2534. (d) Cookson, R.C.; Costa, S. M. de B.; Hudec, J. Chem. Commun. 1969,753-754. (e) Ruzo, L. 0.;Bunce, N.J.; Safe, S. Can. J . Chem. 1975, 53, 688-693. (0 Bunce, N. J. J . Org. Chem. 1982, 47, 1948-1955. (5) Barltrop, J. A. Pure Appl. Chem. 1973, 33, 179-195. (6) Ohashi, M.; Miyake, K.; Tsujimoto, K. Bull. Chem. SOC.Jpn. 1980, 53, 1683-1688. (7) (a) Lewis, F. D.; Ho, T.-I. J . Am. Chem. SOC.1977, 99, 7991-7996. (b) Lewis, F. D. Ace. Chem. Res. 1979, 12, 152-158. (c) Lewis, F. D.; Ho, T.-I.; Simpson, J. T. J . Am. Chem. SOC.1982,104, 1924-1929. (d) Hub, W.; Schneider, S.; Dorr, F.; Simpson, J. T.; Lewis, F . D. Ibid. 1982, 104, 2044-2045. ( e ) Hub, W.; Schneider, S.; Dorr, F.: Oxman, J. D.; Lewis, F. D., unpublished results. (8) Simon, J. D.; Peters, K. S. J . Am. Chem. SOC.1981, 103, 6403-6406. (9) Okada, T.; Karaki, I.; Mataga, N.J . Am. Chem. SOC. 1982, 104, 7 191-7 195.
0002-7863/84/1506-0187$01.50/0
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Scheme I. Mechanism of Addition of Benzene and T r i e t h ~ l a m i n e ~ ~ IC,%* + E$N
-
‘(C&:
+
EtN:)’
(C,”
t
EtJbHCH,)
__c
D ; H N m ,
Me
Table I. Kinetic Data for the Reaction of 9-Phenanthrenecarbonitrile and 9-Anthracenecarbonitrile with
Die thvlainine arene 9-PCN
amine Et,”
solvent C6H6
~o-~k,,;~ kasTsf M-’ M-’s-
119
7.9
(103:‘ 8 S d ) 205 (238‘) 107
9-PCN Et,” CH,CN 8.5 9-PCN Et,ND C,H, 7.1 CH,CN 201 8.4 9-PCN Et,ND 106 7.1 9-PCN (CH,CD,),NH C,H, 7.0 9-PCN (CH,CD,),NH CH,CN 168 9-ACN Et,” C6H6 188 11.0 a Least-squares slopes of fluorescence-quenchingStern-Volmer plots, except as noted. bCalculated from Stern-Volmer slope and measured singlet lifetimes.18 ‘Intercept slope ratio from Figure 1 for formation of I. dIntercept slope ratio from Figure 1 for formation of 111. Addition of the a-C-H bond of secondary or primary amines to arenes is less common, having been reported only for reactions of benzenedicarbonitriles,6pyridine,4c and 1phenylcy~lohexene.~~ The observation of both N-H and a-C-H addition of secondary amines to excited arenes suggests that either dialkylaminyl or a-aminoalkyl radical formation can occur, depending upon the reaction conditions (eq I ) . Detailed investigations of the pho-n. R,CHNHR’ R,CHNR’ or R,CNHR’ (I)
-
toreduction of benzophenone by secondary and primary amines by Cohen and co-workers” indicate that both N-H and a-C-H (10) (a) Yang, N. C.; Libman, J. J. Am. Chem. SOC.1973,95, 5783-5784. (b) Grandclaudon, P.; Lablache-Combier, A,; Parkanyi, C. Telrahedron 1973, 29, 651-658. (c) Sugimoto, A,; Sumida, R.; Tamai, N.; Inoue, H.; Otsuji, Y . Bull. Chem. SOC.Jpn. 1981, 54, 3500-3504. (11) (a) Inbar, S.; Linschitz, H.; Cohen, S. G. J . Am. Chem. SOC.1981, 103, 1048-1054. (b) Stone, P. G.; Cohen, S. G. J . Phys. Chem. 1981, 85, 1719-1725.
0 1984 American Chemical Society
188 J. Am. Chem. SOC..Vol. 106, No. 1. 1984
Lewis, Zebrowski, and Correa
Table 11. Quantum Yields for Product Formation from 9-Phenanthrenecarbonitrile and Diethylamine in Various Solventsa
solvent pentane cyclooctane benzene diethyl ether
,b
QI
fl
%Ia
1.84 0.240 0.0030 0.033 2.12 2.12 0.0032 0.049 2.28 0.645 0.0032(0.005)d 0.026(0.029)d 4.34 0.242 0.0018 0.0082 ethyl acetate 6.02 0.455 0.00094 0.0016 tetrahydrofuran 7.58 0.55 0.0013 0.00079 acetonitrile 38.8 0.345 0.0031(0.003)d e sulfolane 43.3 10.29 0.0013 e pentane-ethyl acetate 4:l 2.34 0.0025 0.024 pentane-ethyl acetate 3:2 2.84 0.0026 0.019 pentane-ethyl acetate 2:3 3.34 0.0029 0.018 pentane-ethyl acetate 1:4 3.84 0.0021 0.012 ethyl acetate-acetonitrile 4:1 12.6 0.001 2 0.00056 ethyl acetate-acetonitrile 3:2 19.2 0.0017 0.00034 ethyl acetate-acetonitrile 2:3 25.1 0.0021 0.00016 ethyl acetate-acetonitrile 1 :4 32.2 0.0030 e aValues for deoxygenated solutions of 9-PCN(0.01 M) and diethylamine (0.12M). Limits of error for relative values are small (
