0 Copyright 1995 American Chemical Society
Volume 14, Number 4, April 1995
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L'ommuntcattons Oxidation of Coordinated Alkynes by Dimethyldioxirane. Conversion to a-Keto Carbene Complexes Shouheng Sun, J. 0. Edwards, and D. A. Sweigart" Department of Chemistry, Brown University, Providence, Rhode Island 02912
Lucia D'Accolti and Ruggero Curci* C N R Centre MISO, Department of Chemistry, University of Bari, Via Amendola 173,Bari, Italy 70126 Received November 28,1994@ Summary: Dimethyldioxirane (DMDO) oxidizes the coordinated alkyne ligand in (RCp)Mn(CO)z(diarylaEkyne) (3) to give moderate yields of the corresponding a-keto carbene complex (RCp)Mn(CO)z(C(Ar)C(O)(Ar))(8,thus illustrating the synthetic potential of dioxiranes for the controlled oxidation of coordinated ligands. The transformation 3 5 might involve initial alkyne epoxidation to give a n intermediate containing a coordinated oxirene ligand.
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Recent research has shown1 that dioxiranes (1) rapidly oxidize a wide range of organic functionalities. Dimethyldioxirane (DMDO; la), which can be easily prepared as a solution in acetone, and methyl(trifluor0methy1)dioxirane (lb)are the most commonly employed oxidants.2 These reagents have been used in a variety of synthetic transformations, including the oxyfimctionalization of saturated hydrocarbons (C-H bond activation).2a,cHowever, applications of these oxidants to inorganic and organometallic systems are quite limited to date. It is known that DMDO oxidizes Cp*Re@Abstractpublished in Advance ACS Abstracts, March 15, 1995. (1)(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: Greenwich, CT, 1990;Vol. 2,Chapter 1. (2)(a) Mello, R.; Fiorentino, M.; Fusco, C.; Curci, R. J.Am. Chem. Sot. 1989,111, 6749.(b) Mello, R.; Cassidei, L.; Fiorentino, M.; Fusco, C.; Hiimmer, W.; Jiiger, V.; Curci, R. J. Am. Chem. SOC.1991, 113, 2205.(c) Kuck, D.;Schuster, A,; Fusco, C.; Fiorentino, M.; Curci, R. J . A m . Chem. SOC.1994,116,2375.
(COI3 to Cp*ReOS and transfers an oxygen to certain metal(I1) porphyrins t o produce metal oxo s p e ~ i e s .It~ has been shown that DMDO oxidizes organometallic thiolates to sulfinato complexes4and induces the oxidative decomplexation of the arene from (arene)Cr(CO)s and the carbene (as the ketone) from Fischer carbene c~mplexes.~ In this paper we address the reaction of DMDO (la) with alkynes coordinated to a manganese center. The oxidation of free alkynes R C W R with DMD06 and with (3) (a)Herrmann, W. A.; Kiprof, P.; Rypdal, R; Tremmel, J.; Blom, R.; Alberto, R.; Behm, J.; Albach, R. W.; Bock, H.; Solouki, B.; Mink, J.; Lichtenberger, D.; Gruhn, N. E. J.Am. Chem. Sac. 1991,113,6527. (b) Wolowiec, S.;Kochi, J. K. Inorg. Chem. 1991,30,1215. (4) (a) Schenk, W. A,; Frisch, J.; Adam, W.; Prechtl, F. Inorg. Chem. 1992, 31, 3329. (b) Perez-Encado, A,; Perrio, S.; Slawin, A. M. Z.; Thomas, S. E.; Wierzchleyski, A. T.; Williams, D. J . J. Chem. SOC., Perkin Trans. 1 1994,629. ( 5 ) (a) Lluch, A.-M.; Sanchez-Baeza, F.; Camps, F.; Messeguer, A. Tetrahedron Lett. 1991,32,5629.(b) Lluch, A.-M.; Jordi, L.; SanchezBaeza, F.; Ricart, S.; Camps, F.; Messeguer, A,; Moret6, J. M. Tetrahedron Lett. 1992,33,3021. (6)(a)Curci, R,; Fiorentino, M.; Fusco, C.; Mello, R.; Ballistreri, F. P.; Failla, S.; Tomaselli, G . A. Tetrahedron Lett. 1992,33, 7929. (b) Murray, R. W.; Singh, M. J. Org. Chem. 1993,58, 5076.
0276-7333/95/2314-1545$09.00/00 1995 American Chemical Society
1546 Organometallics, Vol. 14,No.4, 1995
Communications
Table 1. Spectroscopic Data for a-Keto Carbene Complexes (5) Obtained by the Oxidation of Coordinated Alkynes with Dimethyldioxirane
H H
H Me
Me Me Me Me Me Me
Me H OMe Me OMe
H Me H Me OMe H OMe Me
H
30 42 25 41 27d d 29d d
1989, 1931, 1641 1985, 1929, 1642 1985, 1927, 1641 1983. 1923, 1642 1983, 1925, 164od d 1977, 1919, 163gd d
7.38-7.70 7.15-7.63 7.31-7.75 7.15-7.63
'HNMR (by (m,Ph), 4.98 (s, Cp) (m, Ph), 4.94 (s, Cp), 2.35 (s, Me), 2.28 (s, Me)
(m, Ph),4.78-4.90(m, Cp), 1.81 (s, Me) (m, Ph), 4.75-4.84 (m, Cp), 2.35 (s, Me), 2.28 (s, Me), 1.82 (s. Me) 6.80-8.10(m,Ph),4.71-4.78(m,Cp),3.81 (s,OMe), 1.81 @,Me) 6.80-8.10 (m,Ph), 4.81-4.87 (m, Cp), 3.83 (s, OMe), 1.83, (s, Me) 6.83-7.74 (m,Ph), 4.70-4.85 (m, cp),3.81 (s, OMe), 2.35 (s, Me), 1.81 (s, Me) 6.83-7.74 (m, Ph), 4.70-4.85 (m, Cp), 3.83 (s, OMe), 2.28 (s, Me), 1.82 (s, Me)
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' I Yield based on total amount of starting complex. In hexanes. In CD?CI?. Product consisted of a mixture of two isomers with Rz and R1 interchanged (see text).
Scheme 1 R
$
Q
R3
3
4 ?1
5
'4
peroxyacids7 produces a variety of products which are believed to be derived from an oxirene intermediate (2) formed in an initial epoxidation step. The oxirene ring is a 4n antiaromatic system and is predicted8 theoretically to undergo facile ring opening to the a-keto carbene, which then undergoes subsequent reactions. Direct evidence for the existence of oxirene 2 (R = CH3, CF3) has resultedg from trapping experiments using rare-gas matrices and from IR spectra of intermediates formed during the photolysis of a-diazo ketones. It occurred to us that in situ generation of a coordinated oxirene via epoxidation of a coordinated alkyne may provide the stability required for the generation of an oxirene complex (e.g., the transformation 3 41, which might then evolve to the a-keto carbene complex 5 (Scheme 1). A series of alkyne complexes 3 were synthesized by published procedures.1° Reaction of 3 with DMDO typically proceeded as follows: under nitrogen, 3 (0.080
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(7) For example, see: (a) Ciabattoni, J.;Campbell, R. A.; Renner, C. A.; Concannon, P. W. J . A m . Chem. SOC.1970, 92, 3826. (b) IbneRasa, K. M.; Pater, R. H.; Ciabattoni, J.; Edwards, J. 0. J . Am. Chem. SOC.1973, 95, 7894. ( 8 )(a) Strausz, 0. P.; Gosavi, R. K.; Denes, A. S.; Csizmadia, I. G. J . A m . Chem. SOC.1976, 98, 4784. (b) Strausz, 0. P.; Gosavi, R. K.; Gunning, €3. E. J . Chem. Phys. 1977, 67, 3057. (c) Fowler, J. E.; Galbraith, J. M.; Vacek, G.; Schaefer, H. F. J . A m . Chem. SOC.1994, 116, 9311. (9) (a)Debti, F.; Monnier, M.; Verlaque, P.; Davidovics, G.; Pourcin, J.; Bodot, H.; Aycard, J.-P.C. R. Acad. Sci. Paris, Ser. 2 1986,303, 897. (b) Torres, M.; Bourdelande, J. L.; Clement, A,; Strausz, 0. P. J . A m . Chem. SOC.1983, 105, 1698.
mmol) was dissolved in 10 mL of anhydrous HPLC grade methylene chloride and cooled to -20 "C. A solution of DMDO was prepared as previously described2and stored at -78 "C; assays showed that these solutions typically contained DMDO at 0.07 M. A volume of the DMDO solution containing 0.4 mmol (ca. 6 mL) was quickly added to the solution of 3,and the reaction mixture was stirred for ca. 20 h at -20 "C.The solvent was then stripped to leave a viscous oil that was dissolved in benzene and chromatographed through deactivated neutral alumina. Hexanes eluant removed unreacted starting material, and hexaneddiethyl ether (4/1 to 10/1) removed the product as a yellow-green band. IR and lH NMR spectra (Table 1)suggested that the products were a-keto carbene complexes (5). This was verified by obtaining the X-ray structure of one of the products (5; R1= R2 = R3 = Me), which is illustrated in Figure l . l l (Selected bond lengths and angles are provided in Figure 1.) The yields of 5 were modest (Table 1) and were found to depend markedly on the temperature and the molar ratio of DMDO to complex 3. Along with product 5, the final reaction mixture generally contained some starting material, unidentified insoluble brown metal-containing species (probably oxides), and a-diketones (from the further oxidation of 5). It is likely that the reactions with DMDO could be optimized to provide better yields than have been obtained so far.12 It is pertinent to note that an attempt to epoxidize 3 in CH2C12 with the common oxidant m-chloroperbenzoic acid led to rapid and complete decomposition, with no evidence for species 4 or 5. In contrast, the dioxirane DMDO effected the controlled oxidation of the coordinated alkyne. (10)(a) Strohmeier, W.; Laporte, H.; Hobe, D. V. Chem Ber. 1962, 95,455. (b) Strohmeier, W.; Hellmann, H. Chem. Ber. 1965,98, 1598. (c) Butler, I. S.; Coville, N. 3 . ; Cozak, D. J. Organomet. Chem. 1977, 133, 59. (d) Cash, G. G.; Pettersen, R. C. J. Chem. Soc., Dalton Trans. 1979, 1630. (e) Caulton, K. G. Coord. Chem. Rev. 1981, 38, 1. (D Alt, H. G.; Engelhardt, H. E. J . Organomet. Chem. 1988, 342, 235. (g) Coughlan, S. M.; Yang, G. K. J . Organomet. Chem. 1993, 450, 151. (11)A single crystal of 5 (R1 = Rz = R3 = Me) of dimensions 0.50 x 0.44 x 0.40 mm was grown by ether diffusion from a soluiion of 5 in pentanelether (2/1). Crystal data: triclinic, space group P1, with a = 8.071(2) A, b = 8.373(2)A, c = 15.909(3)A, a = 77.64(3)",p = 86.87". y = 72.25", V = 1000.1(4)A3,2 = 2, Dealed = 1.369 g ~ m - data ~ ; collected at 25 "C with Mo K a radiation, fi = 6.81 cm-', 0 range 2.61-26.00", 256 variables refined with 3905 independent reflections (2 > 2 d I ) ) to R = 0.0427, wR2 = 0.01114, and GOF = 0.981. Refinement based on F was carried out using the SHELXL 93 package. (12) The reaction of 3 with DMDO was attempted in three solvents (CHzC12, MeZCO, C ~ H M )at, five temperatures (-78, -40, -20, 0, 25 "C), and with six 3iDMDO ratios ~1.0,0.50,0.33,0.25,0.20,0.10). The best results were obtained in CHZC12 at -20 "C with a 3iDMDO ratio of 0.20. (Indenyl)Mn(C0)2(alkyne)analogues of 3 were also found to react with DMDO to give a-keto carbene complexes, although the yields were low.
Communications
Organometallics, Vol. 14,No. 4, 1995 1547
Figure 1. Molecular structure and labeling scheme for complex 5 (R1= Rz = RB= Me). Selected bond lengths (A): Mn-C(7), 1.887(2);C(7)-C(8), 1.499(3);C(7)-C(9), 1.466(3); C(8)-0( l), 1.224(3); C(8)-C(16), 1.483(3). Selected bond angles (deg): Mn-C(7)-C(9), 132.2(2); Mn-C(7)C(8), 114.9(2);C(9)-C(7)-C(8), 112.8(2);C(7)-C(8)-C(16), 118.4(2);C(7)-C(8)-0(1), 121.2(2);C(l)-C(8)-C(l6), 120.6(2). The a-keto carbene complex 5 (R1 = Me, Rz = R3 =
H)has been previously synthesized in 37% yield,13aand a low-resolution X-ray structure of 5 (RI = Rz = R3 = H)has been described.13b The reported synthesis of 5 consisted of the direct reaction of (MeCp)Mn(CO)z(THF) with the appropriate a-diazo ketone carbene precursor 5 is fun(e.g., diazodeoxybenzoin). The reaction 3 damentally different in that carbene formation is the consequence of alkyne oxidation. It is quite likely that a n intermediate oxirene complex 4 is formed, which rapidly undergoes ring opening to give 5. When Rz t:
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(13)(a) Herrmann, W.A. Angew. Chem. 1974,86,556. (b) Redhouse, A. D. J . Organomet. Chem. 1975, 99, C29.
R3 in 3,oxidation with DMDO gave a n isomeric mixture of 5 (as determined from lH NMR data). Thus, the isomers (Rz = Me; R3 = OMe) and (Rz = OMe; R3 = Me) were formed in a 1:l ratio when 3 had the substituent set (Me, OMe) on the alkyne ligand. The isomeric ratio of 5 formed a t -20 “C with the alkyne substituent set (H, OMe) was 4:l in favor of the complex having (Rz = OMe; R3 = H); however, the isomer ratio was reversed at low temperatures. I t might become possible to stabilize (and hence detect) an oxirene intermediate in the epoxidation of coordinated alkynes by increasing the barrier to metal slippage via increased rigidity a t the metal-alkyne bond. Experiments along this line are planned using caged alkynes as well as alkynes anchored to the metal via appended phosphine ligands. In conclusion, we have shown that dimethyldioxirane oxidizes the coordinated alkyne ligand in 3 to generate moderate yields of the corresponding a-keto carbene complex 5. It is speculated that this reaction involves initial alkyne epoxidation to give a n intermediate containing a coordinated oxirene ligand. Whether or not 5 illustrates that this is the case, the reaction 3 dioxiranes are potentially useful reagents for the controlled oxidation of coordinated ligands.
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Acknowledgment. This work was supported by a grant from the National Science Foundation (Grant No. CHE-9400800) and by the CNR (Rome, Italy), Progetto Strategic0 “Tecnologie Chimiche Innovative”. We are grateful to Professor G. B. Carpenter for valuable expert assistance with the X-ray structure determination. Supplementary Material Available: Crystallographic data for complex 5, includingtables of atomic coordinates,bond lengths, bond angles, and anisotropic displacement coefficients and figures giving additional views of the structure (8 pages). Ordering information is given on any current masthead page. OM940905J
