1761
J. Am. Chem. SOC.1981, 103, 1761-1765
Simple Enols. 1. l The Generation of Vinyl Alcohol in Solution and Its Detection and Characterization by NMR Spectroscopy2 Brian Capon,* David S. Rycroft, Thomas W. Watson, and Cdsar Zucco Contribution from the Department of Chemistry, University of Glasgow, Glasgow GI 2 8QQ, Scotland, U.K. Received September 30, 1980 Abstract: Vinyl alcohol has been generated by hydrolysis of methoxy(viny1oxy)methyl chloroacetate and acetate (1 and 3), bis(viny1oxy)methyldichloroacetateand trichloroacetate(7 and 6 ) , dimethyl vinyl orthoformate (4), dimethyl vinyl orthoacetate (5), and ketene methyl vinyl acetal (8) in aqueous [2H3]acetonitrileor aqueous [2H7]dimethylformamide.It was characterized by ‘H and 13CN M R spectroscopy and by its conversion into acetaldehyde. On replacement of OH by OD the 13C N M R spectrum shows isotope shifts of -0.12 and -0.09 ppm for the a-and &carbons under conditions where no such shifts were detectable for the corresponding carbons of ethyl vinyl ether. Vinyl alcohol with an HO group was generated under conditions of slow hydroxyl-proton exchange with 8 as the precursor in CD3COCD3(99 v %)/H20 (1 v %) at -10 OC and the following coupling constants were evaluated: 3Jtrans = +14.0, 3J6, = +6.3, 2Jgem= -0.8, 3J(HOC,H) = +9.8, 4J(HOCBHanti)= +0.4, 14J(HOCoH,)I < 0.2 Hz. Under the conditions used vinyl alcohol was converted into acetaldehyde ca.100 times faster than ethyl vinyl ether but could nevertheless be kept in solution for several hours below ca. -10 O C. The postulated intermediate, methyl vinyl hemiorthoformate could not be detected in the hydrolysis of 1,3,and 4 nor could methyl vinyl hemiorthoacetate be detected in the hydrolysis of 5 and 8. However divinyl hemiorthoformate was easily detected in the hydrolysis of 6 and 7. It was concluded that this resulted from the weaker “push” of the vinyloxy group which remained attached to the pro-acyl carbon atom compared to a methoxyl group.
Introduction Vinyl alcohol, the simplest enol and a thermodynamically very unstable s e c i e ~ , has ~ - ~been detected on several occasions. Thus CIDNF*’,g and NMR9 spectroscopy were used to detect it (usually admixed with acetaldehyde and other species) in various photochemical reactions, microwave spectroscopy1° was used in the gas-phase dehydration of ethylene glycol, and N M R and I R spectroscopy” was used in the flash thermolysis of its [2 + 41 cyclo-addition adduct with anthracene. Estimates of the rate constants for the ketonization of other enols have been reported. Thus, Lienhard and Wang12 estimated that the rate constant for the H30+-catalyzed ketonization of 1-hydroxycyclohexenein H 2 0 is 56 M-’ s-l at 25 OC, and Toullec and Dubois” estimated that for isopropenyl alcohol to be 1700 M-’ s-l. Both sets of authors emphasized that the rate constants for the acid-catalyzed ketonization of enols should be similar to those for the hydrolysis of the corresponding enol ethers. On this basis the rate constant for the H30+-catalyzed ketonization of vinyl alcohol should be similar to that for the hydrolysis of methyl vinyl ether,5 Le., 0.76 M-’ s-* at 25 OC.14 Therefore it was thought that vinyl alcohol should be detectable in any reaction in which it is formed at a (1) For an excellent review on “Simple Enols” see Hart, H. Chem. Reu. 1979, 79, 515-528. (2) A preliminary report of some of this work has appeared: Capon, B.; Rycroft, D. S.; Watson, T. W. J. Chem. Sor., Chem. Commun. 1979, 724-7 25. (3) Bell, R. P.; Smith, P. W. J. Chem. SOC.B 1966, 241-243. (4) Hine, J.; Arata, K. Bull. Chem. SOC.Jpn. 1976, 49, 3085-3088, 3089-3092. (5) Guthrie, J. P.; Cullimore, P. A. Can. J. Chem. 1979, 57, 240-248. Guthrie, J. P. Ibid. 1979, 797-802, 1177-1185. (6) Blank, B.; Fischer, H. Helu. Chim. Acta 1973,56, 506-510. Blank, B.; Henne, A.; Laroff, G. P.; Fischer, H. Pure Appl. Chem. 1975,41,475-494. (7) Bargon, J.; Seifert, K.-G. Chem. Ber. 1975, 108, 2073-2079. (8) Sojka, S. A.; Poranski, C. F.; Moniz, W. B. J. M a g . Reson. 1976,23, 417-420. Moniz, W. B.; Sojka, S. A.; Poranski, C. F.; Birkle, D. L. J. Am. Chem. SOC.1978,100,7940-7945. (9) Henne, A.; Fischer, H. Angew. Chem., Inr. Ed. Engl. 1976, 15, 435. (10) Saito, S. Chem. Phys. Lett. 1976, 42, 399-402. Cf. Bouma, W. J.; Poppinger, D.; Radom, L. J. Am. Chem. Soc. 1977,99,6443-6444. Bouma, W. J.; Radom, L. Ausr. J. Chem. 1978,31,1167-1176, 1649-1660. J. Mol. Srrucr. 1978, 43, 267-271. (11) Ripoll, J.-L. Nouu. J. Chim. 1979, 3, 195-198. (12) Lienhard, G. E.; Wang, T.-C. J . Am. Chem. SOC. 1969, 91, 1146-1 153. (13) Toullec, J.; Dubois, J. E. Tetrahedron 1973, 29, 2851-2858, 2859-2866. J. Am. Chem. SOC.1974,96, 3524-2532. (14) Kresge, A. J.; Sagatys, D. S.; Chen, H. L. J. Am. Chem. SOC.1977, 99, 7228-7233.
0002-7863/81/1503-1761$01.25/0
Scheme Ia (6 531
/o-c=c
7
/H
(4321
H‘ (6
”-]\OcH3 OCOCH2CI
14581
l3 4 4 1
-
( 4 221
1 H
I I
H-C
,o-c=c O ‘ICH, OD
/H /
\H
-
16.451
(3.82)
H\ /H /CEC\~ DO
-
I!19.651
1
O=C-CHzD 12.121
(4.18)
-k 13 701
a The
figures in parentheses are the 6 values.
rate substantially greater than its rate of ketonization estimated on the basis of this rate constant and that simpler reactions than those reported above could be used for its generation. This paper reports an investigation by N M R spectroscopy of several such reactions.
Results and Discussion The first precursor chosen was methoxy(viny1oxy)methyl chloroacetate (1). This is analogous in structure to the precursors we have used to generate hemiorthoesters, the tetrahedral intermediates in 00-acyl-transfer rea~ti0ns.l~By analogy to our earlier work this should be hydrolyzed to yield methyl vinyl hemiorthoformate (2) which should break down to vinyl alcohol and methyl formate (or methanol and vinyl formate). The ‘H NMR spectrum of (1) (ca. 0.15 M ) in [2H3]acetonitrile at -20 “C (Figure la) shows an approximately first-order ABC system for the vinyl group with bA 6.53, bB 4.58, and bc 4.32 and JAB 14.7 Hz, JAC = 4.9 Hz, and Jec = 1.7 Hz. The signals for the C H , chloroacetate, and methoxy groups are as shown in Scheme I. On (15) Capon, B.; Gall, J. H.; Grieve, D. McL. A. J. Chem. Soc., Chem. Commun. 1976, 1034-1035. Capon, B.; Grieve, D. McL. A. J. Chem. Soc., Perkin Tram. 2 1980, 300-305.
0 1981 American Chemical Society
1762 J. Am. Chem.SOC.,Vol. 103,No. 7, 1981
Capon et al.
Table I. The 'HNMR Spectrum of Vinyl Alcohol (OD) Generated from Different Precursors temp, precursor solvent "C 6 ( J , Hz)" -20 6.45, 4.18, 3.82 (14.2,6.4, 1) methoxy(viny1oxy)methylchloroacetate (1) CD,CN/D,O (9: 1, v/v) methoxy(viny1oxy)methylacetate (3) CD,CN/D,O (l:l,v/v) -20 6.43,4.22, 3.94 (13.8, 6.3, 1.1) dimethyl vinyl orthoformate (4) CD,CN/D,O (9:1, v/v), lo+ M DCl 0 6.45, 4.17, 3.84 (14.0,6.4,0.9) dimethyl vinyl orthoacetate ( 5 ) CD,CN/D,O ( 1 9 : 1 , ~ / ~5)X, M DCl -10 6.44, 4.17, 3.83 (14.1, 6.6, 1.0) bis(viny1oxy)methyl trichloroacetate ( 6 ) DMF-d,/D,O (19~1,V/V) -20 6.55, 4.16, 3.83 (14, 6,
