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A New Vinyl Ether Synthesis Organometallics 1982, 1, 1467-1473 1467 Reactions of Ketals and Acetals with (CO),MnSi(CH,),. A New Vinyl Ether Synthesis Marianne Marsi and J. A. Gladysz"' Department of Chemistry, University of California, Los Angeles, California 90024 Received April 14, 1982 The reaction of (CO)5MnSi(CH3)3(1; 1.0-2.4 equiv, 2-4 h, 50 "C, acetonitrile)with ten different methyl ketals(Tab1e I) gives methyl enol ethers in 56% to yields. Easily removed byproducta CH30Si(CH3), and (C0)5MnH(or >%YOalso form. When regio- and/or geometric isomers are possible, thermodynamicMn2(CO)9(CH3CN)/Mn2(CO)lo) mixtures are obtained. The reaction of 1 with acetals is more complex, but when conducted under 200 psi of CO, manganese acyls (CO)5MnCOCH(OR)R'(derived from alkyl intermediates) can be isolated. A general mechanism is proposed in which a ketal or acetal oxygen is initially silylated , by 1. Also, 1 rapidly converts cyclohexanone ethylene glycol ketal to (CH3)3SiOCH2CH20C=CHCH2- -CH2CH2CH2 (7) and slowly transforms cyclohexanone diallyl ketal to 1-cyclohexenyl allyl ether. Ortho ester CH3C(OCH3),and 1 react to give principally CH3C02CH3and CH30Si(CH3),. Introduction compatible with thermally labile and acid sensitive groups, Reactions of transition-metal trialkylsilanes such as eq 1 appeared to offer distinct advantages over existing (C0)5MnSi(CH3)3( 1)2 and ~is-(C0),Fe[Si(CH,)~1,~with methodology. In this paper, we report (a) reactions of 1 oxygen-containing organic molecules have been the subject with a variety of dimethyl ketals which give methyl vinyl of intensive study in our laboratory.e7 Two broad classes ethers in good to excellent yields, (b) the high yield con- of useful transformations have been found: (1) new version of cyclohexanone ethylene glycol ketal to a ring- metal-carbon bond forming reactions which give isolable opened, silylated vinyl ether, (c) related, more complex organometallic products46and (2) organic functional group reactions of 1 with acetals, cyclohexanone diallyl ketal, and transformation^.^^^^^ ortho esters, and (d) mechanistic data on these transfor- Previously, we noted that (C0)5MnSi(CH3)3(1) cleanly mations, including the use of CO to trap manganese alkyl converts ketones with a-hydrogens to their trimethylsilyl intermediates. A portion of this study has been commu- enol ether^.^ In contrast to conventional silyl enol ether ni~ated.~ syntheses, no acid or base was required. The byproduct RO OR CH$N was the mildly acidic (CO)&InH (pK, N 7): Mechanistic + ICO)~M~SI(CH~)~ considerations led us to predict that 1 might similarly 1 2-4 h transform ketals to enol ethers according to eq 1. Enol ethers are conventionally synthesized by the reaction of ketals and acetals with protic acids at temperatures in the 100-200 OC range.+ll Since such conditions are not Results The methyl ketals listed in Table I were treated with (1)Address correspondence to this author at the Department of Chemistry, University of Salt Lake City, 84112. Fellow of 1.0-2.4 equiv of 1 in CH3CN (or CD3CN) for 2-4 h at 50 the Alfred P. Sloan FoundationUtah, andUtah Camille and Henry (1980-1984) f 2 OC (eq 1). Methyl enol ethers formed in 56 to >95% Dreyfw Teacher-Scholar Grant Recipient (1980-1985). yields, determined by GLC and IH NMR spectroscopy. (2) (a) A. D.; MacDiarmid, A. G. Znorg. Nucl. Chem. Lett. as Berry, 1969, In three representative cases (entries 6-8), yields of iso- 5,601. (b) Mhh,W.; Kuhn, M. Chem. Ber. 1974,107,979. (c) Gladysz, J. A.; Williams, G. M.; Tam, W.; Johnson, D. L.; Parker, D. W.; Selover, lated, distilled products were obtained. Identities of the J. C. Znorg. Chem. 1979, 18, 553. enol ethers in Table were confirmed by comparison with (a) Jetz, W.; Graham, W. A. G. J. Organomet. Chem. I (3) 1974,69,383. independently prepared authentic samples. Volatile by- (b) Vancea, L.; Bennett, M. J.; Jones, C. E.; Smith, R. A.; Graham, W. A. G. Znorg. Chem. 1977, 16, 897. (c) Blakeney, A. J.; Johnson, D. L.; product CH30Si(CH3),formed in all reactions. Donovan, P. W.; Gladysz, J. A. Zbid. 1981,20,4415. Hydride (CO)5MnHwas the initial inorganic product of (a) Johnson, L.; Gladysz, J. A. Am. Chem. (4) D. J. SOC.1979, 101, eq 1, as evidenced by a 6 -7.9 'H NMR resonance.12 6433. (b) Johnson, D. L.; Gladysz, J. A. Znorg. Chem. 1981, 20, 2508. (5) (a) Brinkman, K. C.; Gladysz, J. A. J. Chem. Soc., Chem. Commun. However, (CO)5MnH decomposed during the reaction to 1980,1260. (b) Brinkman, K. C.; Gladysz, J. A., manuscript in prepara- Mn2(CO)9(CH,CN)(IR (cm-l, hexane) 2094 (w), 2026 (s), tion. 2005 (s), 1996 (vs), 1974 (m), 1954 (m))13 and small (6)Blakeney, A. J.; Gladysz, J. A. J. Organomet. Chem. 1981,210,303. (7)Marsi, M.; Gladysz, J. A. Tetrahedron Lett. 1982, 23, 631. amounts of Mn2(CO)10.Side reactions of some products (8)Shriver, D. F. Acc. Chem. Res. 1970,3,231. with (CO)5MnH were observed, so an excess of 1 was (9)For some representative syntheses, see: (a) Meerwein, H. In Houben-Weyl 'Methoden der Organischen Chemie"; Muller, E., Ed.; utilized with the less reactive dimethyl ketals to increase the rate of initial reaction (Experimental Section). The Georg Thieme Verlag: Stuttgart, Vol. 6/3, p 97, p 199 ff. (b) Taskinen, E. Acta Chem. Scad., Ser. B 1974,B28,357. (c) Newman, M. S.; Vander GLC yield of a-methoxystyrene (entry 8) reached a max- Zwan, M. C. J. Org. Chem. 1973,38,2910. (d) Wohl, A. Synthesis (e) House, 0.; Kramer, V. J. Or$. Chem. R. imum (93%) at 11.6:l.O 1:acetophenone dimethyl ketal 1974,38. H. 1963,28,3362.(f) ratios. Norris, R. 0.; Verbanc, J. J.; Hennion, G. F. J. Am. Chem. SOC.1938,60, 1159. The trapping of byproduct (C0)5MnHwas attempted. (10)There are many specialized procedures for vinyl ether synthesis Reactions conducted in the presence of 1.0 equiv of PPh, such (a) Kluge, A. F.; Cloudsdale, I. S. J. Org. Chem. as: 1979,44,4847. or Ph2PCH2CH2PPh2gave phosphine-substituted man- (b) Pine, S. H.; Zahler, R.; Evans, D. A.; Grubbs, R. H. J. Am. Chem. SOC. 1980,102,3270. (c) Gilbert, J. C.; Weerasooriya, U.;Wiechman, B.; Ho, L. Tetrahedron Lett. 1980,21,5003. (d) Barbot, F.: Mieiniac,- P. HeEu. Chim. Acta 1979, 62,1451. (12)Whitesides, G. M.; Maglio, G. J. Am. Chem. SOC.1969,91,4980. (11)Miler, R. D.;McKean, D. R. Tetrahedron Lett. 1982, 23, 323. (13)Ziegler, M. L.; Haas, H.; Sheline, R. K. Chem. Ber. 1965,98,2454. 0276-7333/82/2301-1467$01.25/0 0 1982 American Chemical Society 1468 Organometallics, Vol. 1, No. 11, 1982 Marsi and Gladysz Table I. Vinyl Ethers Synthesized from Dimethyl Ketals and 1 yield datab % entry starting ketal product GLC NMR is01 1 CH306 OCH) 97 95 CH30 3CHj 6 2 98 98 e Ed?38 62 3 CH3OK OCH) 93 98 4 56 60 78 77 85 95 79 88 95 74 25(E):75(2)" 8 93 97 81 cH3% OCH) 78 84 97 93 2 (71) 3 (4(2):25(E)" a See Experimental Section for reaction conditions and geometric isomer assignments. Yields are based upon ketal and are estimated to be accurate within c 5%. ganese hydrides, as evidenced by 31P-coupled 'H NMR 1 gave regioisomeric enol ethers (Table I, entry 10) 2 and resonances (6 -7.4 (d, &&31p = 33 Hz) and -7.9 (t,&+31p 3 (mixture of Z/E isomers). After 1hr, the ratio of 2:3 was = 46 Hz)). These hydrides were more stable than (C- (81 f 2):(19 f 2). After 24 h, the ratio of 2:3 was (71 f O)5MnH to the reaction conditions, but higher yields of 2):(29 f 2) thermodynamic mixture.14 A thermodynamic enol ether products were not obtained. Base additives mixture of geometric isomers was obtained from 1 and (~Z-C~H~)~N,2,6-di-tert-butylpyridine, DBU, 4-(di- 5-nonanone dimethyl ketal (Table I, entry 7). methylamino)pyridine, and 2,2,6,6-tetramethylpiperidine The possible influence of (C0)5MnHupon reaction re- either reacted with starting 1% or failed to deprotonate giochemistry was examined. A (17 f 2):(86 f 2) mixture ( C0)5MnH. of 1-methoxy-Zmethylcyclohexene and 2-methoxy-3- The disappearance of (C0)6MnHduring the course of methylcyclohexene was treated with (C0)6MnH (0.32 eq 1 was probed. No reaction occurred when equivalent equiv) at 50 "C in CH3CN. After 0.4 h, the regioisomer amounts of 1 and (CO)5MnH were heated at 50 "C in ratio was (24 f 2):(76 f 2). At 1.5 and 3.5 h, ratios were CD3CN for 24 h. Similarly, (C0)5MnHwas unreactive (29 f 2):(71 f 2) and (35 f 2):(65 f 2), respectively. toward 5-nonanone dimethyl ketal and acetophenone di- Reactions of 1 with certain functionalized dimethyl methyl ketal (2 h, 50 "C, CD3CN). However, (C0)6MnH ketals were not as clean as those in Table I. Ketal ether slowly converted a-methoxystyrene and 5-methoxy-4- 1,3,3-trimethoxybutane and 1 (1.86 equiv) gave only ca. nonene to the corresponding saturated ethers (GLC 25% of 2,4-dimethoxybutene, as determined by 'H NMR identified); Mn,(CO)JCH,CN) formed concurrently. and GLC coinjection. Traces of 3,3-dimethoxy-l-butene The regio- and stereoselectivity of eq 1was investigated. were present, but 2-methoxy-1,3-butadiene was absent.
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