J. Org. Chem. zyxwvutsr 1991,56,6399-6403 6399 and a-methylnaphthalene. a-Isopropylnaphthalene was prepared in two steps via reaction of the Grignard reagent of a-bromo- naphthalene with acetone, followed by reduction of the resulting carbinol according to a published literature procedure.'6 A sample of 1,9-ethyleneanthracene was provided by Prof. Laren Tolbert (Georgia Institute of Technology). Carbon tetrachloride was slurried with potassium hydroxide for 24 h, decanted, and frac- tionally distilled from phosphorus pentoxide. The middle portion was stored over molecular sieves. zyxwvutsrq NBS zyxwvut (Aldrich) was recrystallid from water and dried in vacuo before use. Gas chromatographic analyses were performed on a Hewlett-Packard HP 5890A in- strument equipped with both FID and TCD detectors and an HP 3393A reporting integrator. Nuclear magnetic resonance spectra were recorded on a 270-MHz Bruker FT NMR spectrometer. Competition Experiments. Competitive brominations (NBS/CCl,) were carried out as described earlier.' Reaction mixtures were analyzed by GLC (vs chlorobenzene as internal standard) in triplicate. Relative rate constants were calculated by k,,/kB zyxwvutsrqponm = In (A,/A)/ln (B,/Bf), where the subscripts zyxwvut *om and "f" refer to the initial and final concentrations of substrate, re- spectively. Mass balances were nearly quantitative, although elimination products were detected by NMR and GCMS in the (15) Calved, D. J.; De La Mare, P. B. D.; Ogawa, T.; Yamamoto, H.; Suzuki, H. Cazz. Chim. Ital. 1987,117, 357-61. reactions involving a-isopropylnaphthalene, l,&ethylene- naphthalene, and 1,9-ethyleneanthracene. Theoretical. SemiempiricalMO calculations were performed using the AM1 approximation developed by Dewar et al.' and implemented through MOPAC Version 5.0 (QCPE 455). Full geometry optimizations were performed on the parent hydro- carbon. For the open-shell species, geometries were optimized using UHF, followed by a single-point calculation using the half-electron approximation.lB*l7 Acknowledgment. We gratefully acknowledge and thank the Thomas F. Jeffress and Kate Miller Jeffress Memorial Trust Fund for financial support. We also thank Prof. Laren Tolbert (Georgia Tech) for the sample of 1,9-ethyleneanthracene and for helpful suggestions. Registry No. 6,496-11-7;7,83-32-9; 8,641-48-5; Br, 10097-32-2; PhEt, 100-41-4; a-methylnaphthalene, 90-12-0; a-ethyl- naphthalene, 1127-76-0; a-isopropylnaphthalene, 6158-45-8; 9- ethylanthracene, 605-83-4. (16) Dewar, M. J. S.; Rzepa, H. S. J. Am. Chem. SOC. 1978,100,784. (17) Clark, T. Handbook zyxwv of Computational Chemistry: A Practical Guide to Chemical Structure and Energy Calculations; Wiley: New York, 1985. Rate Constants and Arrhenius Parameters for the Reactions of Some Carbon-Centered Radicals with Tris(trimethylsily1)silane C. Chatgilialoglu,*J J. Dickhaut, and B. Giese* Institute of Organic Chemistry, University zyxwvuts of Easel, St. Johanns-Ring 19, CH-4056 Easel, Switzerland Received April 2, 1991 Rate constanta for the reactions of some carbon-centered radicals with (Me3Si)3SiH have been measured over a range of temperatures by using competing unimolecular radical reactions as timing devices. For example, the rate constants (at 298 K) are 3.7,1.4, and 2.6 X 106 M-' s-l from primary, secondary, and tertiary alkyl radicals, respectively. Comparison of the radical trapping abilities of tri-n-butylstannane and tris(trimethylsily1)silane is discussed. The use of l,l-dimethyl-5-hexenyl cyclization as a radical clock has been recalibrated by using new data and data from the literature. Introduction Free radicals are of considerable importance in the de- velopment of organic chemistry, and many methodologies in radical-based synthesis employ tributyltin hydride.24 It has recently been shown that tris(trimethylsily1)silane is a valuable reducing agent for a variety of organic sub- stratesPi6 This reagent has proved to be an attractive alternative to tributyltin hydride for the majority of these reactions although in a few cases the two reagents can complement each other. The key step in these straight- Scheme I kc U* - R* (1 UH RH (1) Viiting Scientiet at the University of Base1 (FeSJuly 1990). Permanent address: I.Co.C.E.A., Consiglio Nazionale delle Ricerche, 40064 Ozzano Emilia (Bologna) Italy. (2) Gieee, B. Radicals in Organic Synthesis: Formation of Carbon- Carbon Bonds; Pergamon Press: Oxford, 1986. (3) Neumann, W. P. Synthesis 1987,665 and references cited therein. (4) Curran, D. P. Synthesis 1988,417 and 489. (5) Chatgilialoglu, C. in Free Radicals in Synthesis and Biology; Minisci, F., Ed.; Kluwer: Dordrecht, 1989; pp 115-127. (6) Chatgilialoglu, C.; Griller, D.; Lasage, M. J. Org. Chem. 1988,53, 3641. Ballestri, M.; Chatgilialoglu,C.; Clark, K. B.; Griller, D.; Gieae, B.; Kopping, B. J. Org. Chem. 1991,56, 678. 0022-3263/91/ 1956-6399$02.50/0 0 1991 American Chemical Society forward radical-chain reduction reactions?ss as well as in those processes where their use as a mediator for the formation of carbon-carbon bonds' via an inter- or intra- molecular addition, is (1) R' + X3MH - RH + X3M' (7) Chatgilialoglu, C.; Giese, B.; Kopping, B. Tetrahedron Lett. 1990, 31, 6013. Giese, B.; Kopping, B.; Chatgilialoglu, C. Tetrahedron Lett. 1989, 30, 681.