Organometallics 1995, 14, 2672-2676
2672
Rate Constants and Arrhenius Parameters for the Reaction of Acyl Radicals with BusSnH and (Me3Si)sSiH C. Chatgilialoglu,*JC . Ferreri,2M. L ~ c a r i n iP. , ~ Ped~ielli,~ and G. F. Pedulli3 I.Co.C.E.A., Consiglio Nazionale delle Ricerche, Via P. Gobetti 101, 40129 Bologna, Italy, Dipartimento di Chimica Organica e Biologica, Universita di Napoli “Federico II”, Via Mezzocannone 16, 80134 Napoli, Italy, and Dipartinento di Chimica Organica “A. Mangini”, Universita di Bologna, Via S. Donato 15, 40127 Bologna, Ztaly Received January 3, 1995@ The kinetics of hydrogen abstraction from BuaSnH and (MesSi)3SiH by a variety of acyl radicals have been studied in nonpolar solvents over a range of temperatures by using competing decarbonylation reactions. The relative preexponential factors are essentially independent of the nature of the acyl radical and of the hydride; Le. logdAd/AMH) = 4.8. On the other hand, the relative activation energies, i.e. E d - EMH, decrease by ca. 2 kcal mol-l on going from primary to secondary and also on going from secondary to tertiary substituents at the carbonyl moiety for a particular hydride. As one goes from Bu3SnH to (Me3Si)sSiH for a particular acyl radical, the E d - EMHalso decreases by ca. 2 kcal mol-l. Combining our relative kinetic data with the gas-phase Arrhenius parameters for the propanoyl radical decarbonylation allows for the evaluation of the absolute Arrhenius expressions for the reactions of acyl radicals with Bu$3nH and (Me3Si)3SiH and for the decarbonylation of secondary and tertiary acyl radicals. Rate constants a t 80 “C for the reactions of (Me3Si)aSi’ and Bu3Sn’ radicals with acyl derivatives have also been estimated.
Introduction In recent years free-radical reactions have proven to be valuable in synthetic organic chemistry, due to their selectivity and the mild conditions r e q ~ i r e d .In ~ particular, the reduction of several functional groups and the formation of new carbon-carbon bonds have successfully been accomplished. Tri-n-butyltin hydride6 and tris(trimethylsilyl)silane6are the most widely used reagents for these procedures, which utilize radical chain reactions. For a synthetically useful radical chain reaction the intermediates must be disciplined. The concept of discipline in free-radical reactions is strictly connected with the kinetic information in each individual step, although upon first consideration, the importance of kinetic knowledge might be less apparent in planning a synthetic strategy. Thus, for the design of new radical reactions one is faced with the problem of simultaneously occurring carbon-centered radicals which must have different duties. Therefore, it is of considerable importance t o know how rapidly different classes of carbon-centered radicals abstract hydrogen from different hydride^.^ In 1981 and 1991, rate constants and Arrhenius parameters for the reaction of some carbon-centered @Abstractpublished in Advance ACS Abstracts, April 1, 1995. (1) Consiglio Nazionale delle Ricerche. (2) Universith di Napoli. (3) Universitl di Bologna. (4) For example, see: (a) Curran, D. P. Synthesis 1988, 417, 489. (b) Jasperse, C. P.; Curran, D. P.; Fevig, T. L. Chem. Rev. 1991, 91, 1237. (c) Curran, D. P. In Comprehensive Organic Synthesis; Trost, B. M., Fleming, I., Eds.; Pergamon Press: Oxford, U.K., 1991; Vol. 4, Chapters 1 and 2. (d) Motherwell, W. B.; Crich, D. Free Radical Chain Reactions in Organic Synthesis; Academic Press: London, 1992. (5) Neumann, W. P. Synthesis 1987, 665. (6) (a) Chatgilialoglu, C. Acc. Chem. Res. 1992, 25, 188. (b) Chatgilialoglu, C. Chem. Rev., in press. (7) For a recent review, see: Newcomb, M. Tetrahedron 1993, 49, 1151.
radicals with Bu~SnH8and (Me3Si)3SiHg(henceforth (TMS)3SiH)were determined. The numerous citations of the former article over the last decade undoubtedly indicate the importance of these widely used data in understanding and programming free-radical react i o n ~ Furthermore, . ~ ~ ~ ~ ~ rate constants and Arrhenius parameters for the reaction of benzyl radicals with Bu3SnHIOJ1 as well as absolute rate constants for hydrogen transfer reactions of the perfluoro-n-heptylradical with BusSnH and (TMS)3SiH have also been determined.13 Acyl radicals are very important intermediates in chemical synthesis, as demonstrated by the large number of related papers which have appeared in the last (8)Chatgilialoglu, C.; Ingold, K. U.; Scaiano, J. C. J . Am. Chem. SOC. 1981, 103, 7739. (9) Chateilialoelu. C.: Dickhaut., J.:, Giese. B. J . O w . Chem. 1991. I
I
I
1
-
56, 6399. (10)Franz, J . A.; Suleman, N. K.; Alnajjar, M. S.J . O g . Chem. 1986, 51, 19. (11)Rate constants and Arrhenius parameters for the reaction of phenyl, vinyl, cyclopropyl, and neopentyl radicals with BusSnH using the corresponding acyl peroxides with light as the source of radical formation have also been reported.l2 However, recent measurements on the decarboxylation processes indicated that some of these data are not reliable.7 (12) Johnston, L. J.;Lusztyk, J.;Wayner, D. D. M.; Abeywickreyma, A. N.; Beckwith, A. L. J.; Scaiano, J. C.; Ingold, K. U. J. Am. Chem. Soc. 1986,107,4594. (13) (a) Rong, X. X.;Pan, H.-Q.; Dolbier, W. R., Jr.; Smart, B. E. J . Am. Chem. Soc. 1994,116,4521.(b)Avila, D. V.; Ingold, K. U.; Lusztyk, J.; Dolbier, W. R. Jr.; Pan, H.-Q.; Muir, M. J. Am. Chem. SOC.1994, 116, 99. (14) Acyl radical chemistry is an extremely active field at present. For some recent examples, see: (a)Penn, J. H.; Liu, F. J . Org.Chem. 1994,59, 2608. (b) Chen, L.; Gill, B.; Pattenden, G. TetrahedronLett. 1994,35, 2593. (c) Mendenhall, G. D.; Protasiewicz, J . D.; Brown, C. E.; Ingold, K. U.; Lusztyk, J. J. Am. Chem. Soc. 1994, 116, 1718. Id) Ryu, I.; Yamazaki, H.; Ogawa, A,; Kambe, N.; Sonoda, N. J.Am. Chem. Soc. 1993, 115, 1187 and references cited therein. (e) Ryu, I.; Hasegawa, M.; Kurihare, A.; Ogawa, A.; Tsunoi, S.; Sonoda, N. Synlett 1993, 143. (0 Boger, D. L.; Mathvink, R. J. J . Org. Chem. 1992, 57, 1429 and references cited therein. (g) Bachi, M. D.; Bosch, E. J. Org. Chem. 1992, 57, 4696 and references cited therein. (h) Chen, C.; Crich, D.; Papadatos, A. J . Am. Chem. Soc. 1992,114,8313and references cited therein.
0276-733319512314-2672$09.00/00 1995 American Chemical Society
Organometallics, Vol. 14, No. 6, 1995 2673
Reaction of Acyl Radicals with BuSSnH and (MesSi)sSiH few years.14 However, not much is known as t o how rapidly this class of carbon-centered radicals abstracts hydrogen from various hydrogen donors. To our knowledge, the only previously reported rate constants are limited to the reaction of a benzoyl radical with benzenethiol15J6 and to our preliminary report of the present work.17 In order t o overcome this gap of information, we have used competing decarbonylation reactions as timing devices (free-radical clocks7J8) to investigate the rates of primary, secondary, and tertiary acyl radicals as a function of temperature with the above common reducing agents.
Table 1. Reaction of Phenyl Selenoester Derivatives with U"MS)&iHa ~
~
(15)Neville, A. G.; Brown, C. E.; Rayner, D. M.; Lusztyk, J.; Ingold, K. U. J. Am. Chem. SOC.1991,113, 1869. (16) Recently the reaction of propanoyl radical with tri-n-butyltin deuteride has also been measured by using time-resolved infrared spectroscopy and found to be 3.0 x lo6 M-' s-l at 296K (Brown, C. E.; Neville, A. G.; Rayner, D. M.; Ingold, K. U.; Lusztyk, J. Aust. J. Chem. 1995,48,363).We thank Dr. J. Lusztyk for kindly informing us of his article prior to publication. (17) Chatgilialoglu, C.; Lucarini, M. Tetrahedron Lett. ISM, 36, 1299. (In this communication, tert-butylbenzene should be read as toluene.) (18)Griller, D.; Ingold, K. U. Acc. Chem. Res. 1980,13, 317. (19) For reviews, see: (a) Kupchik, E. J. In Organotin Compounds; Sawyer, A. K., Ed.; Marcel Dekker: New York, 1971; Vol. 1, pp 2833. (b) Kuivila, H. G. Synthesis 1970, 499. (20) (a) Lusztyk, J.; Lusztyk, E.; Maillard, B.; Ingold, K. U. J. Am. Chem. SOC.1984,106,2923. (b) Lusztyk, J.; Lusztyk, E.; Maillard, B.; Lunazzi, L.; Ingold, K. U. J.Am. Chem. SOC.1983, 105, 4475. (21j(a) Kuivila, H. G.; Walsh, E. J., Jr. J . Am. Chem. SOC.1966, 88, 571. (b) Walsh, E. J., Jr.; Kuivila, H. G . J . Am. Chem. SOC.1966, 88, 576. (22) Walsh, E. J., Jr.; Stoneberg, R. L.; Yorke, H.; Kuivila, H. G. J. Org. Chem. 1969,34, 1156. (23) (a)Grieco, P. A.; Jaw, J. Y.; Claremon, D. A,; Nicolaou, K. C. J. Org. Chem. 1981, 46, 1215. (b) Nicolaou, K. C.; Claremon, D. A.; Barnette, W. E. J. Am. Chem. SOC.1979,101, 3704. (24) Pfenninger, J.; Heuberger, C.; Graf, W. Helu. Chim. Acta 1980, 63, 2328. (25) (a) Batty, D.; Crich, D. Tetrahedron Lett. 1992, 33, 875. (bj Batty, D.; Crich, D.; Fortt, S. M. J. Chem. SOC.,Perkin Trans. 1 1990, 2875. (c) Batty, D.; Crich, D. Synthesis 1990, 273. (26) Ballestri, M.; Chatgilialoglu, C.; Cardi, N.; Sommazzi, A. Tetrahedron Lett. 1992, 33, 1787.
~
RC(0)SePH
~~
~
~
yield of
yield of RH,b %
RC(O)H,b %
35 75
40 7
79
