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C6H11]+, loo), 457 ([M - C6Hll - PhjP]+, 39). Anal. Calcd for C43H4103F3P2SPt:C, 54.26; H, 4.34. Found: C, 54.40; H, 4.36. Carbon monoxide was bubbled through the mother liquors for 2 h, leading to a white precipitate. Workup as above afforded 313 mg (50%) of carbonyl product 4b as white microcrystals. Single-Crystal X-ray Diffraction Analysis of Sb. Colorless single crystals of Sb were obtained as thin plates by diffusion crystallization from CH2C12/Et20. X-ray data were collected on a CAD4 automated diffractometer as summarized in Table I. The structure was solved by treating the phenyl rings of the PhJP as rigid bodies and by other standard heavy-atom techniques using the UCLA Crystallographic Package. NMR Observation of $-Allyl Complex Sc Formation. Complex 1 (1 I .3 mg, 1.51 X IF2mmol) was placed in a thin-wall 5-mm NMR tube equipped with a rubber septum. Previously degassed C6D6(0.6 mL) was added via syringe along with toluene ( I . 1 mg, I .2 X 1W2 mmol) as an internal standard (6 2.1 ppm). A IH NMR spectrum was obtained at room temperature, and then 3,3-dimethyl-2-butenyl triflate was added via syringe. The tube was inverted five times to mix the reagents, and then the reaction was monitored by 'H NMR. The arrayed spectra were recorded with preacquisition delays of 0, 212, 212, 212, 512, 512, 512, 512, 1112, 1112, 1112, 1112, 1712, 1712, 1712s,andtheacquisitiontime for each spectrum was 48 s. Reaction of 6 with Et,N. Complex 6 (13.3 mg, 1.40 X 10-2 mmol) was placed in a thin-wall 5-mm NMR tube and charged with 0.45 mL of THF-d,. The tube was sealed with a rubber septum and Et,N (3.9 pL, d 0.726 g/mL, 2.8 X mmol) was then added via syringe. The mixture was heated in a 65 OC oil bath for 6 h, leading to a yellow heterogeneous solution. The 'H NMR showed the clean production of 2,3-dimethyl-1,3-butadiene(16). Reaction of Sb with Et3N in the Absence of Ph,P. Complex Sb (12.8 mg, 1.35 X IO-' mmol) was weighed into a thin-wall 5-mm NMR tube and 0.50 mL of THF-d8 was added via syringe. The tube was sealed with a rubber septum and Et,N (2.0 rL, d 0.726 g/mL, 1.5 X 10-2 mmol) was added via syringe. The septum was wrapped with liberal amounts of parafilm, and the mixture was heated at about 50 OC in an oil bath for 4 h. The clear solution turned orange within 5-10 min. At the end of the reaction, a precipitate was present. The 'H NMR spectrum of the heterogeneous mixture showed the formation of 2-methyl-l,3-pentadiene
(17) and trace impurities with resonances between 6.5 and 6.8 ppm. Reaction of Sb witb Et3N in the Presence of Ph,P. The reaction was performed according to the above procedure with 5b (1 3.1 mg. 1.38 X mmol), Et,N (2.1 pL, d 0.726 g/mL, 1.5 X mmol), and Ph3P
mmol) in 0.50 mL of THF-d8. The 'H NMR (4.0 mg, 1.5 X spectrum of the heterogeneous mixture revealed the clean formation of 17. Isomerization Reaction of 2-Methyl-1,3-pentadiene (17) and 4Methyl-1,fpentadiene (18) in the Presence of Et3Nand Ph,P. Ph3P (6.3
mg, 2.4 X mmol) was placed in a thin-wall 5-mm NMR tube. THF-d8 (0.50 mL) was then added and the tube was sealed with a rubber septum. Et,N (3.3 pL, d 0.726 g/mL, 2.4 X mmol) was added via syringe followed by the pure diene 17 (2.6 pL, d 0.718 g/mL, 2.4 X IF2 mmol). The septum was then wrapped with liberal amounts of parafilm. The solution was placed in a 60 OC oil bath for 24 h. The 75/25 mixture of 17 and 18 was also subjected to the same conditions. No isomerization of the pure diene or the mixture dienes had occurred according to 'H NMR spectroscopy. Isomerization Reaction of 2-Methyl-1,3-pentadiene(17) and 4Methyl-1,3-pentadiene (18) in the Presence of ( P ~ I , P ) ~ P ~ ( C ~(H I ),,) Ph3P, and Et3N. Platinum ethylene complex 1 (11.8 mg, 2.4 X
mmol) and Ph,P (6.3 mg, 2.4 X mmol) were weighed into a thin-wall 5-mm NMR tube. THF-d8 (0.50 mL) was added and the tube was sealed with a rubber septum. EtlN (3.3 pL, d 0.726 g/mL, 2.4 X mmol) was added via syringe followed by the pure diene 17 (2.6 rL, d 0.718 g/mL, 2.4 X mmol). The septum was wrapped with liberal amounts of parafilm. The resulting yellow solution was then heated in a 60 OC oil bath for 24 h. The 75/25 mixture of 17/18 was also subjected to the same conditions. The 'H NMR spectra of both solutions were obtained. No isomerization of dienes was observed. Acknowledgment. W e are grateful to the NSF for financial support ( C H E 8802622 and CHE 9101767) and to JohnsonMatthey, Inc., for the generous loan of K2PtC14. Supplementary Material Available: Details of the single-crystal X-ray structure of 5b (32 pages); observed and calculated structure factors for 5b (19 pages). Ordering information is given on any current masthead page.
Thianthrene 5-Oxide as a Mechanistic Probe for Assessing the Electronic Character of Oxygen-Transfer Agents Waldemar Adam,* Wolfgang Haas,+ and Braj B. Lobray' Contributionfrom the Institute of Organic Chemistry, University of Wurzburg, Am Hubland, 0-8700 Wiirzburg, Germany. Received December 11, 1990 Abstract: Thianthrene 5-oxide (SSO) was employed to assess the electronic nature of oxygen-transfer reagents: Those oxidants that attack preferentially the sulfide "S" site to give the bis(su1foxide) SOSO are electrophilic in their reactivity; those that predominantly react at the sulfoxide 'SO" site to give the sulfone SSOz are nucleophilic. The X , parameter was introduced, defined as the mole fraction of SS02 product (SO attack), for which strongly electrophilic oxygen-transfer agents (typically
acidified hydroperoxides and hypochlorite) take near-zero values and strongly nucleophilic ones (typically basified hydroperoxides and superoxide) near-unity. On the XSOscale, ozone and peroxy acids are as expected electrophilic oxidants and dioxiranes are significantly more nucleophilic but more electrophilic than carbonyl oxides. The latter exhibit pronounced nucleophilic reactivity toward SSO, which is in agreement with their observed reactivity. Free radicals, e.g., t-BuOO', display very high electrophilicity in their oxygen-transfer propensity by reacting essentially exclusively at the S site. Control experiments have established that such radicals do not act through electron transfer to afford the SSO" radical cation, although the latter, generated either by photosensitized or chemical oxidation, behaves toward dioxygen strongly electrophilic. While the SSO probe provides a realistic measure of the electronic nature of oxygen-transfer agents, caution should be excercised when preferential complexation of the reagent at the S or SO site of SSO takes place or when electron transfer is involved with SSO to produce the SSO*+or SSO' radical ions. Also, during the in situ generation of transient oxidants, several species of different electronic character might act simultaneously and the composite Xsovalue erroneously express the reactivity of the oxidant in question. In such suspicious cases, control experiments are obligatory to acquire meaningful Xso data with SSO. Oxygen-transfer reactions are of wide interest in peroxide chemistry' due to their importance in biological oxidations as well 'Present address: Dr. W.Haas, Consortium fur Elektrochemische Industrie, GmbH, Zielstattstraae 20, W-8OOO Munchen 70, Germany. *Present address: Dr. B. B. Lohray, Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139.
as in industrial applications. For example, in the recent past, model studies have been made to understand the mechanism of oxygen (1) Kropf, H., Ed. Organische Peroxo-Verbindungen, Methoden der Organischen Chemie (Houben-Weyl); Georg Thieme Verlag: Stuttgart, 1988; Vol. E13, Parts 1 and 2.
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Thianthrene 5-Oxide transfer of flavin monooxygenase enzyme^,^ carbonyl oxides, or Criegee's zwitterion in the ozonolysis of alkenes4 or by the photooxygenation of diazo compounds? hydroperoxides generated by singlet oxygenation of alkenes,6 dioxiranes,' dialkylperoxonium ions: and many others. Of particular value have been spectroscopic studies in combination with the matrix isolation technique, which have helped to assess the electronic structure of some of these reactive species, especially carbonyl oxides and d i o ~ i r a n e s . ~ Despite intensive work on oxygen-transfer reactions,lOJl no definitive and consistent view exists to date on the electronic nature of these oxygen-transfer reagents, particularly what concerns the nucleophilic and electrophilic reactivity toward a given substrate such as olefinic, aromatic, and heteroatom-containing molecules (amines, phosphines, sulfides). A few years ago we have reported thianthrene 5-oxide (SSO) as a useful mechanistic tool to assess the electronic character of a variety of oxidants.I2 Earlier studies using SSO include work by Oae" and by Matsui.14 Several recent applications underscore the interest of the scientific community to employ our novel p r ~ b e ; however, ~J~ conflictive results have been reported in regard to the reliability of the acquired data. Thus, MurraylSaquestions the nucleophilic nature of dioxiranes in view of a negative p value (-0.8) obtained in the oxidation of substituted aryl sulfides, which is unexpected of an electron-rich oxidant. Bloodworth8 used SSO to assess the electronic nature of gem-dialkylperoxonium ions, which again revealed too high a nucleophilic reactivity for these presumed electrophilic oxidants, while T o m a ~ e l l iimplied '~~ radical-type activity (electron transfer) of several neutral and anionic metal peroxo complexes to rationalize their more electrophilic nature than determined by us with SS0.'2c Yet for carbonyl oxides, Sander1" has shown that infrared frequencies and MIND 0 / 3 calculations correlate well with our data determined with the SSO probe. Furthermore, Ortiz de MontellanoIM used successfully the SSO probe to assess the electronic nature of biological oxidants. In this paper, we present the complete details of our previous workI2 by defining the concept of the nucleophilicity parameters X, for oxygen-transfer agents, the experimental method, and its application to a wide range of common oxidants. Besides typical stable nucleophilic and electrophilic oxidants, also intermediary species, particularly radical-type oxygen-transfer agents were included in this study. The usefulness of the SSO probe and its validity and limitations as a mechanistic tool to assess the electronic character of oxidizing reagents will be discussed. It is shown that (2) Ingraham, L. L.; Meyer, D. L. Biochemistry. of Plenum . Dioxygen; .Press: Niw York, 1985. (3) (a) Branchand, 9. P.; Walsh, C. T. J. Am. Chem. Soc. 1985,107,2153. (b) Oae. S.:Asada. K.: Yoshimura. T.Tetrahedron Lett. 1983.24. 1265. IC) ., Ball, S.; Bruice, T.C. J . Am. Chem. SOC.1979, 101, 4017. 14) Crienee. R. Anpew. Chem.. Inr. Ed. E n d . 1975. 14. 745. (5) Kirmse,'W.; Horner, L.; Hoffmann, HPJusrus Lie6igs Ann. Chem. 1958. 614. 19.
( 6 ) Frimer, A. A., Ed. Singlet 02;CRC Press, Inc.: Boca Raton, FL,1985; VOIS. 1-4. (7) (a) Adam, W.; Curci, R.; Edwards, J. 0. Acc. Chem. Res. 1989, 22, 205. (b) Murray, R. W. Chem. Rev. 1989, 89, 1187. (c) Curci, R. In Advances in Oxygenated Processes; Baumstark, A. L., Ed.; JAI Press: Greenwich CT, 1990; Vol. 2, Chapter 1. (8) Bloodworth, A. J.; Melvin, T.;Mitchell, J. C. J. Org. Chem. 1988, 53, 1078. (9) Sander, W. Angew. Chem., Inr. Ed. Engl. 1990, 29, 344. (10) Ando, W.; Moro-Oka, Y.; Eds.The Role of Oxygen in Chemistry and Biochemistry; Elsevier: Amsterdam, 1988. ( I 1) (a) Di Furia, F.; Modena, G. Pure Appl. Chem. 1982,54, 1853. (b) Mimoun, H . Angew. Chem., Inr. Ed. Engl. 1982, 21, 734. (12) (a) Adam, W.; Haas, W.; Sieker, G. J. Am. Chem. SOC.1984, 106, 5020. (b) Adam, W.; Dllrr, H.; Haas, W.; Lohray, B. B. Angew. Chem., Int. Ed. Engl. 1986, 25, 101. (c) Adam, W.; Lohray, 9. B. Angew. Chem., Inr. Ed. Engl. 1986, 25, 188. ( 1 3) (a) Oae, S.; Takata, T.;Kim, Y. H . Bull. Chem. Soc. Jpn. 1981,54, 2712. (b) Oae, S.; Takata, T. Tetrahedron Lett. 1980, 21, 3213. (14) Matsui, M.; Miyamoto, Y.;Shibata, K.; Tokase, Y. Bull. Chem. Soc. Jpn. 1984, 57, 2526. (15) (a) Murray, R. W.; Jcyaraman, R.; Pillay, M. K. J . Org. Chem. 1987, 52,746. (b) Tomaselli, G. Pnvate communication. (c) Cremer, D.; Schmidt, T.; Sander, W.; Bischof, P. J. Org. Chem. 1989. 54. 2515. fd) Ortiz de ~
Montellano, P. R. Private communication.
Scheme I
I:
Rvs-
f "s" W n
u , A J \ 0
"so" (hddn
the Xso parameter reflects reliably the electron demand of an oxidant, provided that no complexation occurs with the sulfur sites in SSO,no electron transfer takes place between the oxygen atom donor and acceptor, and no multicomponent oxidizing systems are used.
Tbianthrene 5-Oxide as Mechanistic Probe The oxidation of organic substrates by oxygen-transfer agents has been extensively studied. Viewed from the perspective of the oxidant, such oxidations may proceed by either an electrophilic or a nucleophilic pathway.I6 As already proclaimed above, so far no clear-cut chemical method has been devised to assess the electronic character of oxygen-transfer agents; no detailed comparison of the relative oxygen-transferring ability of a variety of oxidants to set up a common scale is available. In this context, thianthrene 5-oxide (SSO)may serve as a suitable oxygen atom acceptor in view of its following unique features. (a) SSO has both a sulfide and a sulfoxide site; the former, being electron-rich, is expected to undergo facile electrophilic oxidation, whereas the latter should preferably be oxidized by a nucleophilic oxidant. Intramolecular competition between the two sites offers the advantage of assessing the relative rates of oxygen transfer of an oxidant by merely determining the amount of sulfide attack to afford the bis(su1foxide) SOSO and sulfoxide attack to lead to the sulfone S S 0 2 in one and the same molecule. A similar approach has been adopted by Oae13 in his study of the oxygen transfer to 3-methyl-l,2-dithiane 1-oxide by H202or NaIO, under acidic conditions or by m-CPBA. (b) Unlike Oae's system,I3 the SOSO and S S 0 2 products do not readily interconvert and hence there is no ambiguity regarding the electronic nature of the oxygen-transfer agent. (c) SSO has a fixed geometry, the folded conformation shown in eq 1, with a well-defined distance of 3.8 A between the two functional groups." Thus, steric factors in the approach of the oxidant toward the sulfide and sulfoxide sites should be nearly constant.
(d) X-ray analysisi7 of SSO reveals the boat-type structure, and oxygen transfer to the sulfide site would give rise to cis and trans diastereoisomers, which should enable one to examine the stereochemical course of the oxygen transfer and hence elucidate (1 6 ) Stewart, R. Oxidation Mechanisms: Application to Organic Chemistry; Benjamin: New York, 1984. (17) Hosoya, S . Acta Crystallogr. 1966, 21, 21.
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Table I. X,
entry 1
2 3 4 5 6 7 8 9
IO
Adam et al.
Values of Oxygen-Transfer Agents Assessed by the Thianthrene 5-Oxide (SSO) Probe total product composition (%a). reaction conditions' vield (%)b SSO, SOSO SOSO, 100
