941 Aug 1972, ORGN 133, for a brief disclosure of these results. Subsequent experiments (manuscript in preparation) have shown the quoted yields of minor products to be in error but in the direction to further substantiate the above arguments. (3) H. L. Goering and D. L. Towns, J. Am. Chem. SOC., 85, 2295 (1963), and references cited therein. (4) N. A. LeBel and J. E. Huber, J. Am. Chem. SOC., 85, 3193 (1963); R. R. Fraser and S.O'Farrell, Tetrahedron Lett., 1143 (1962). (5) (a) For a discussion of the "Goering series" and "LeBel series" of products and their relation to the interconversion of carbonium ions within the bicyclooctenyl framework see J. A. Berson. J. J. Gajewski, and D. S. Donald, J. Am. Chem. SOC., 91, 5550 (1969); (b) Cf.. also J. A. Berson, Angew. Chem., 80, 765 (1968); Angew. 779 (1968). In the language of this author proto 1 in methanol' operates with almost perfect "preservation of memory." (6) Such electrophiles have been previously termed "uniparticulate electrcphiles" by L. A. Paquette. G. R. Allen, Jr., and M. J. Broadhurst (J. Am. Chem. SOC.,93, 4503 (1971)). In that attack on a strained ring bond is contemplated the term "tanycyclophile," as introduced by P. G. Gassman (Acc. Chem. Res., 4, 128 (1971)), woukl also apply. (7) That only dipolar intermediates are represented in this scheme is purely a consequence of our focus on the electrophilically available pathways for reaction of 1. Depending on the nature of the 7-eiectrophile and the preponderance of evidence concerning its mode of nonconcerted, e.g. (2 2), cycloaddition, diradical intermediates may be preferable. (8) The implication that the first ionic intermediate of path b has a classical secondary structure is for the purposes of discussion only and should not be construed as having any supportive basis. In fact the evidence from the acid-catalyzed ring opening of 1 in methanol,' as well as the TCNE results presented here, strongly argue against a classical intermediate at this stage of the reaction. (9) The complex proton spectrum was partially unraveled with the help of decoupling experiments and inspection of molecular models. The assigned long-range coupling constants are of the W-type. The subscripts a (anti), s (sun), x (exo), and n (endo) stereolabel the respective methylene bridge protons at C9 and C10 in accordance with accepted practice for bicyclo[3.2.l]oct-2-ene derivatives. (10) See paragraph at end of paper regarding supplementary material. (11) (a) K. B. Wiberg and W. J. Bartley. J. Am. Chem. SOC.,82, 6375 (1960). (b) G. L. Closs and K. D. Krantz, J. Org. Chem., 31, 638 (1966). (c) Olefin 1 can be conveniently prepared in 30-40 g lots by a slightly modified version of the pubiishedllb procedure. (12) H. W. Whitlock, Jr., and M. W. Siefken. J. Am. Chem. Soc.,90, 4929 (1968). (13) H.E. Simmons, J. Am. Chem. SOC.,83, 1657 (1961). (14) Spectral data (ir, NMR, MS) were in good accord with the assigned structure. (15) R. Graf, Angew. Chem., 80, 179 (1968); Angew. Chem., Int. Ed. Engl.! 7, 172 (1968); E. J. Moriconi, Mech. React. SuIfur Compd., 3, 131 (1968).
+
reaction to go to completion a t widely varying rates to yield HRe(C0)dL and HRe(C0)jLz. With PPh3, HRe(C0)dL was formed a t similarly erratic rates. Similar results were obtained in THF as solvent, except that the reactions were generally much faster. Exposure to air or hydroquinone in low concentration retards reaction. Contrary to an earlier report,6 HRe(C0)5 alone in either solvent does not react with dissolved oxygen at room temperature. These observations suggest a radical pathway involving adventitious radicals as initiators. The following mechanism accounts for the observations.
-
+ HRe(CO)5 Re(CO)5 + L R
+
Re(C0)4L
f
L
Re(C0)4L + HRe(CO)5
Re(CO),L, + HRe(CO)5 Re(CO),,L,
RH + Re(C0);
(1)
Re(CO)IL + CO Re(CO),L, + CO
(2)
(3)
Re(CO)5 + HRe(CO)4L (4) Re(CO), + H R e ( C 0 ) A
-
+ Re(CO),-,,,L,,,
-
(5)
Rep(CO)lOn.mL+m (6)
Chain termination steps involving formation of RRe(CO)S, Re2(C0)9L, and Rez(C0)gLz (but probably not Re2(CO)&') are all possible. The following results support the proposed mechanism. A solution containing loT3 M HRe(C0)5 with A4 P(nC4H9)3 in hexane was irradiated with a 1000-W mercuryxenon lamp filtered through an interference filter centered a t 31 1 nm. There resulted a slow substitution to form initially HRe(C0)4L. After 2 hr the reaction was only about 10% complete. Continued irradiation yielded HRe(C0)3L2 in addition to HRe(C0)dL. After 12 hr of irradiation the reaction was only about 66% complete. Substitution in this system is presumably due to CO photodissociation from HRe(C0)5 and later from HRe(C0)AL. Our proposed mechanism requires that R e ( C 0 ) j be laMerle A. Battiste,* James M. Coxon, Robert G . Posey bile toward substitution, since it probably has a rather short Roy W. King, M. Mathew, Gus J. Palenik lifetime in solution. The absorption of Rez(C0)lo at 310 Department of Chemistry, University of Florida nm is ascribed to the u-o* transition of the Re-Re b ~ n d . ~ . ~ Gainesville, Florida 3261 1 Photochemical studies suggest that irradiation of Received October 18, I974 Re2(CO)lo a t this wavelength produces Re(CO)s radicals.I0 A 5 X M solution of Re2(CO)io with excess P(n-C4H9)3 in hexane does not undergo substitution at room temperature over a period of several hours. IrradiaTransition Metal Carbonyl Substitution via a tion a t 3 1 1 nm for a period of 150 min results in about 15% Radical Chain Pathway' loss of Re2(CO)lo, with Re*(C0)9P(n-C4H9)3 and Rez(CO)s[P(n-C4H9)3]2 as products along with several other Sir: as yet unidentified substitution products. These results are Recognized pathways for substitution at a transition suggestive of a rapid substitution of (probably) diffusively metal carbonyl center include rate-determining CO dissoseparated R e ( C 0 ) s radicals, followed by recombination of ciation, displacement of CO in an associative step, and ligsubstituted radica1s.l M HRe(C0)5, and m i g r a t i ~ n . ~We . ~ report here the first evidence for a A hexane solution containing heretofore unrecognized radical chain pathway for substituM P(n-C4H9)3, and M Re2(CO)lo shows no evidence tion, which may prove to be of considerable generality and of reaction in the dark over a period of several hours. The importance. reaction solution was irradiated at 31 1 nm. Typically, the Although substitution of HRe(C0)5 by triphenylphosparent hydride disappeared very slowly during the first 6phine (PPh3) and other similar ligands has been r e p ~ r t e d , ~ 10 min, during which time only HRe(C0)4L was formed as no kinetics studies have been reported. We found that it was product. After this initial induction period,' the reaction exceedingly difficult to obtain reproducible kinetics results. proceeds very rapidly under irradiation; reaction is essenUnder the most rigorous attainable conditions of solvent tially complete after about 5-6 min. Both HRe(C0)4L and and reagent purity, with exclusion of light, the thermal HRe(C0)3L2 are formed concurrently, in roughly 3.1 reaction in hexane, under N2, of M H R e ( C 0 ) s with ratio. l 4 . l 5 M tributylphosphine, P(n-C4H9)3, exhibited no In all the above experiments the flux of 31 1 nm photons reaction after 60 days a t 25'. Thus HRe(CO)5 is extraordiis constant. The results thus indicate that, whatever the narily inert toward substitution via CO dissociation or hyquantum yield for photosubstitution of Rez(CO)lo, the dride migration pathways, in comparison with quantum yield for substitution of HRe(C0)5 with photocaH M I I ( C O ) ~Exposure .~ to light, failure to exhaustively purtalysis by Re2(CO) 10 or Mn2(CO)lo is enormously higher, ify the reagents, and various other circumstances caused the consistent with the proposed mechanism. Communications t o the Editor
948 Inhibition of the reaction by dissolved 0 2 is consistent with the proposed mechanism. Formation of 0 2 complexes of carbonyl radicals has been demonstrated in ESR experiments with c O ( c 0 ) 4 and Mn(C0)5.16 In Re(C0)502 the unpaired spin could be localized on 02, as suggested by the ESR hyperfine data for Mn(C0)502, so that the metal possesses a coordinatively saturated (i.e., 18 e-) configuration. The metal is thus probably not substitutionally labile nor capable of hydrogen abstraction from HRe(C0)S. Additional tests of the radical chain hypothesis are in progress. The radical chain process for substitution should be applicable to many other transition metal systems. Extension to other metal hydride systems is especially appealing. Deliberate photochemical initiation with Re2(CO)io or other suitable source of radicals may provide a convenient route to substituted hydrides, both mononuclear and polynuclear. Radical chain processes may be of importance in cobalt carbonyl hydride chemistry. Studies of this and several other systems, including metal carbonyl halides and other substituted metal carbonyl compounds susceptible to radical attack, are also in progress.
References and Notes This research was supported by the National Science Foundation through Contract DMR-7203026 with the Materials Research Laboratory, University of Illinois, and research Grant MPS 71-03201. F. Basolo and R. G. Pearson. "Mechanisms of Inorganic Reactions," 2nd ed, Wiley, 1967, Chapter 7. (a) D. A. Brown, lnorg. Chirn. Acta Rev., 1, 35 (1967); (b) R. J. Angelici. Organomet. Chem. Rev., 3, 173 (1966); (c) H. Werner, Angew. Chem., Int. Ed. Enol.. 7. 930 (1968). N. Flitcrof
