Laser Flash Photolysis of [3,n]Paracyclophan-2-ones. Direct Observation and Chemical Behavior of 4,4′-(1,n-Alkanediyl)bisbenzyl Biradicals Enrique Font-Sanchis,† Miguel A. Miranda,*,‡ Julia Pe´rez-Prieto,*,§ and J. C. Scaiano*,† Department of Chemistry, University of Ottawa, Ottawa, Ontario, Canada K1N6N5, Instituto de Tecnologı´a Quı´mica UPV-CSIC/Departamento de Quı´mica, Universidad Polite´ cnica de Valencia, Camino de Vera s/n, Valencia 46071, Spain, and Departamento de Quı´mica Orga´ nica/Instituto de Ciencia Molecular, Universidad de Valencia, Vicent Andre´ s Estelle´ s s/n, Burjassot, 46100 Valencia, Spain
[email protected] Received March 30, 2002
The 4,4′-(1,n-alkanediyl)bisbenzyl biradicals (2b-d) have been generated from the Norrish type-I reaction of [3,n]paracyclophan-2-ones (1b-d) giving the paracyclophanes 3b-d as the only reaction products. The behavior of biradicals 2b-d has been studied in detail and compared with the previously reported biradical 2a. The lifetimes increase as the chain length decreases and are affected by the solvent viscosity, thus showing the effect of the length of the chain on the conformations of the biradicals. Quenching with persistent radicals such as TEMPO resulted in length-dependent rate constants. Finally, the study of the magnetic field effects on the biradical lifetimes suggest that ISC control determines biradical lifetimes for long-chain systems. Introduction Biradicals have been postulated as reactive intermediates in numerous thermal and photochemical reactions.1-3 Their direct detection by time-resolved techniques has allowed the study of their spectral properties and chemical behavior. The lifetimes of triplet biradicals are usually controlled by intersystem crossing (ISC), which is sensitive to the different biradical conformations.4-9 While ISC can occur by two major pathways, spin-orbit coupling (SOC) and electron-nuclear hyperfine coupling (HFC), the latter is only expected to be important when the triplet-singlet energy gap is very small, as for example, when there is large separation between the two radical centers. Recent interest has been focused on the study of interactions between the radical termini in long-chain acyl-alkyl biradicals.10,11 One of the main limitations in the study of biradicals is the availability of suitable †
University of Ottawa. Universidad Polite´cnica de Valencia. Universidad de Valencia. (1) Wilson, R. M. In Organic Photochemistry; Padwa, A., Ed.; Marcel Dekker Inc.: New York, 1985; Vol. 7, pp 339-465. (2) Johnston, L. J. Chem. Rev. 1993, 93, 251-266. (3) Scaiano, J. C.; Johnston, L. J. In Organic Photochemistry; Padwa, A., Ed.; Marcel Dekker: New York, 1989; Vol. 10, pp 309-356. (4) Barton, D. H. R.; Charpiot, B.; Ingold, K. U.; Johnston, L. J.; Motherwell, W. B.; Scaiano, J. C.; Stanforth, S. J. Am. Chem. Soc. 1985, 107, 3607-3611. (5) Johnston, L. J.; Scaiano, J. C.; Sheppard, J. W.; Bays, J. P. Chem. Phys. Lett. 1986, 124, 493-498. (6) Scaiano, J. C. Tetrahedron 1982, 38, 819-823. (7) Tanaka, R.; Kuriyama, Y.; Itoh, H.; Sakuragi, H.; Tokumaru, K. Chem. Lett. 1993, 8, 1447-1450. (8) Zimmt, M. B.; Doubleday: C.; Gould, I. R.; Turro, N. J. J. Am. Chem. Soc. 1985, 107, 6724-6726. (9) Miranda, M. A.; Font-Sanchis, E.; Perez-Prieto, J.; Scaiano, J. C. J. Org. Chem. 1999, 64, 7842-7845.
photochemical precursors, since unambiguous assignment of the reaction products is only possible when they can be clearly distinguished from starting substrates. In this respect, alkanediyl biradicals have been generated from acyl alkyl biradicals obtained in the photolysis of the readily accessible cycloalkanones through thermal or photochemical loss of carbon monoxide.9,12,13 It is known that their lifetimes increase with increasing distance between the radical centers, showing the SOC contribution to ISC.14 We have previously reported the generation of 4,4′-(1,2ethanediyl)bisbenzyl biradical (2a) from [3,2]paracyclophan-2-one (1a).13 This methodology could be extended to (1,n-alkanediyl) homologues with longer tethers whose chemical behavior has not been reported (Scheme 1). Further, to the best of our knowledge, the synthesis and photochemical behavior of [3,n]paracyclophan-2-ones 1b-d have not been described. Results
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Ketones 1b-d were synthesized by the reaction of p-toluenesulfonylmethyl isocyanide with the corresponding 1,n-bis(4-bromomethylphenyl)alkanes15 under phase(10) Suzuki, I.; Tanaka, R.; Yamaguchi, A.; Maki, S.; Misawa, H.; Tokumaru, K.; Nakagaki, R.; Sakuragi, H. Bull. Chem. Soc. Jpn. 1999, 72, 103-113. (11) Tsentalovich, Y. P.; Morozova, O. B.; Avdievich, N. I.; Ananchenko, G. S.; Yurkovskaya, A. V.; Ball, J. D.; Forbes, M. D. E. J. Phys. Chem. A 1997, 101, 8809-8816. (12) Weiss, D. S. In Organic Photochemistry; Padwa, A., Ed.; Marcel Dekker Inc.: New York, 1981; Vol. 3, pp 347-420. (13) Miranda, M. A.; Font-Sanchis, E.; Perez-Prieto, J.; Scaiano, J. C. J. Org. Chem. 2001, 66, 2717-2721. (14) Salem, L.; Rowland, C. Angew. Chem., Int. Ed. Engl. 1972, 11, 92-111.
10.1021/jo025776s CCC: $22.00 © 2002 American Chemical Society
Published on Web 07/30/2002
J. Org. Chem. 2002, 67, 6131-6135
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Font-Sanchis et al. SCHEME 1
transfer conditions followed by acid hydrolysis. Lamp irradiations of deaerated solutions of 1b-d were performed at 254 nm. Analysis of the photolyzate by 1H NMR and GC/MS shows the presence of the decarbonylated cyclophanes (3b-d) as the only reaction products (Scheme 1).16,17 No disproportionation or fragmentation products are detected. Laser flash photolysis (LFP) of deaerated 1 mM solutions of 1c,d in cyclohexane at 266 nm (Nd:YAG laser, fourth harmonic,
