J. Am. Chem. Soc. 1986, 108, 4993-4998 4993 peptides in CHCl3 solvents cannot be done successfully due to solvent interference in the far-UV. Our goal in this work is to develop the basis set of conformationally characteristic peptide spectra for the newest technique, VCD, to a level where it will truly develop into a practical tool. Acknowledgment. We thank the National Institutes of Health (Grant GM-30147, T.A.K.) for partial support of this research and the Research Board of the University of Illinois for partial purchase of the FT-IR spectrometer. Registry No. LA2, 103003-66-3; ALA2, 103003-67-4; A,LA,, 103003-68-5; A,LA,, 103003-69-6; A,LA,, 103003-70-9; ASLA,, 1008 17-47-8. On the Addition of Silyl Radicals to Unsaturated Carbonyl Compounds: Regioselectivity of the Attack and 1,3 Carbon to Oxygen Silicon Migration Angelo Alberti,*' Chryssostomos Chatgilialoglu,+ Gian Franco Pedulli,*t and Paolo Zaniratos Contribution from the Istituto dei Composti del Carbonio Contenenti Eteroatomi e loro Applicazioni, CNR, 40064 Ozzano Emilia, Italy, Istituto di Chimica Organica dell'Universita', 401 36 Bologna, Italy. and Istituto di Chimica Organica dell' Universita', 09100 Cagliari, Italy. Received January 22, 1986 Abstract: The addition reactions of silyl radicals with 2,6-di-tert-butyl- (l), 2,6-dimethoxy- (2), and 2,6-dimethyl-p-benzoquinone (3), 3,6-dimethylthieno[3,2-b] thiophene-2,Sdione (4), 9-methyleneanthrone (5), and 5-benzylidene-3,6-dimethylthieno[3,2- blthiophen-2-one (6) have been studied by EPR spectroscopy. With substrates 5 and 6 only the adducts resulting from attack at the exocyclic olefinic double bond were detected up to 400 K. With the quinones and dithiolactone 4 the nature of the adducts depended on the experimental temperature: thus at low temperature preferential addition of silyls to the less hindered ring carbon atom was evident, while at higher temperature only the adducts to a carbonyl oxygen were detected. Experimental evidence has been obtained that the kinetically favored carbon adduct converts to the thermodynamically more stable oxygen adduct via a 1,3 carbon to oxygen internal migration of the organometallic group involving a four-membered cyclic transition state. The kinetics of the rearrangement has been followed for the triphenylsilyl adduct of 1 and the Arrhenius equation log (kls-') = [(I33 & 0.3) - (18.2 & 0.3)]/0 was determined. The rearrangement of the corresponding adduct of 4 was much slower and its kinetic analysis was complicated by a fast equilibration with its dimer. A kinetic scheme is outlined according to which the observed rate constants should refer to the rearrangement of the dimer rather than of the rddical. An alike behavior is also reported for the triphenylgermyl adducts of compounds 1 and 4. It is well established that for molecules undergoing radical addition and containing both olefinic and carbonylic double bonds the preferred site of attack by carbon-centered radicals is the weaker C=C (rBDE' ca. 60 kcal/mol) rather than the C=O bond which is some 15 kcal/mol stronger.2 Silyl radicals, in view of the exothermicity of their reactions with unsaturated derivatives, should attack preferentially C=C double bonds. Actually, in recent kinetic investigations, Ingold and co-workers showed that the rates of addition of trialkylsilyl radicals to olefins (ca. lo6 to for the addition to structurally related ketones (ca. lo5 to lo6 M-I Indeed, the low-temperature reaction of silyl radicals with isopropenyl acetate* or maleic anhydride6 led to the EPR detection of species resulting from the addition to the olefinic double bonds; 107 M-1 s-l ) 3 were about an order of magnitude higher than those s-1) .4 Istituto CNR, Ozzano Emilia. * Universita' di Cagliari. 8 Universita' di Bologna. (1) (a) Citterio, A.; Minisci, F.; Vismara, E. J. Org. Chem. 1982, 47, 81-88. (b) Giese, B.; Kretzschmar, G. Chem. Ber. 1982,115,2012-2014 and references cited. (2) Golden, D. M.; Benson, S. W. Chem. Rea. 1969, 69, 125. (3) Chatgilialoglu, C.; Ingold, K. U.; Scaiano, J. C. J. Am. Chem. SOC. 1983, 105. 3292-3296. (4) Chatgilialoglu, C.; Ingold, K. U.; Scaiano, J. C. J. Am. Chem. SOC. 1982. 104. 5119-5123. (5) Bowles, A. J.; Hudson, A.; Jackson, R. A. J. Chem. SOC. B 1971, 1947-1949. (6) Alberti, A.; Hudson, A.; Pedulli, G. F. Terrahedron 1982, 38, 3749-3752. yet, in the large majority of reactions between silyls and unsat- urated carbonyl compounds, only addition to C=O double bonds was ~bserved.~ Although the preferential formation of oxygen adducts was j~stified'~ on thermodynamic basis with the greater strength of the 0-Si bond thus formed (1 19-128 k ~ a l / m o l ) ~ * ~ if compared with that of the C-Si bond (89 k~al/mol),~ it remains in contradiction with expectation. This behavior can, however, be accounted for by two alternative reaction pathways involving as a first step the kinetically favored addition of R,Si' to the C=€ double bond. Subsequently, the so formed primary radicals may evolve to the thermodynamically more stable oxygen adducts either by cleavage of the C-Si bond followed by readdition to the car- bonyl group or by an intramolecular carbon to oxygen migration of silicon through a cyclic transition state. The former mechanism implies reversibility of the addition of silyls to unsaturated carbons which is known to be unimportant for the adducts to simple olefins.I0 The latter mechanism seems more reasonable in view (7) (a) Cooper, J.; Hudson, A,; Jackson, R. A. J. Chem. SOC., Perkin Trans. 2 1973, 1933-1937. (b) Alberti, A.; Hudson, A. J. Chem. SOC., Perkin Trans. 2 1978, 1098-1102. (c) Chen, K. S.; Foster, T.; Wan, J. K. S. Ibid. 1979, 1288-1292. (d) Adeleke, B. E.; Wan, J. K. S., Ibid. 1980, 225-228. (e) Alberti, A,; Hudson, A,; Pedulli, G. F.; Zanirato, P. J. Organomet. Chem. (8) Jackson, R. A. Spec. Pub1.-Chem. SOC. 1970, No. 24, 295-321. (9) Walsh, R. Acc. Chem. Res. 1981, 14, 246-252. (10) (a) Bennett, S. W.; Eaborn, C.; Jackson, R. A,; Pearce, R. J. Orga- nomet. Chem. 1968, 15, P17-Pl7. (b) Jackson, R. A. J. Chem. SOC., Chem. Commun. 1974, 573-574. (c) Chatgilialoglu, C.; Woynar, H.; Ingold, K. U.; Davies, A. G. J. Chem. SOC., Perkin Trans. 2 1983, 555-561. 1980, 198, 145-154. 0002-7863/86/ 1508-4993$01.50/0 0 1986 American Chemical Society