Ketenes tautomers

Mtinchnones can undergo ring opening to ketene tautomers, which can then engage in chemistry. A few examples of this pathway have been described since the reviews by Potts (1) and Gingrich and Baum (10). For example, mtinchnones react... 

Robert and co-workers (239,240) discovered novel conversions of 2-amino-1,3-dithiolium-4-olates (348) into other mesoionic heterocycles. For example, reaction of 348 with carbon disulfide, phenyl isocyanate, or phenyl isothiocyanate affords l,3-dithiolium-4-thiolates (349), l,3-thiazolium-4-olates (350), and 1,3-thiazolium-4-thiolates (351), respectively. Some of these reactions proceed via the ring-opened ketene tautomer of 348 (240). 

Replacing one CO group in Cr(CO)3Cp with diazomethane costs 3.6 kcal/mol in terms of free energy. In the presence of free CO, the formation of a ketene complex was observed experimentally. The structures and harmonic vibrational frequencies of the possible ketene tautomers were also computed (see Figure 10.15). Since ketenes possess C=C as well as C=0 Jt-bonds, both the r 2-(C,C) and the r 2-(C,0) coordination modes had to be considered. As chromium is of rather oxophilic character, it is not surprising that the r 2-(C,0) tautomer (Figure 10.15c) was lower in energy by 1.3 kcal/mol. 

Cyclobutenones give ethyl 2-chloro-3-alkenoates, probably via the ketene tautomers and the confluent action of VO(OEt)Cl2 and CuCh on them. 

As presented in some detail by Gingrich and Baum (see their Table 4.31 and Scheme 4.51) azlactones react with ot,p-unsaturated imines to give ot-pyridones. Sandhu and co-workers continued the studies in this area, and they now view this reaction as involving initial electrophilic attack on the azlactone (munchnone) followed by cyclization to an ot-pyridone, rather than prior ring-opening to the corresponding ketene tautomer, as was originally proposed (Table 4.1). With azlactone 52 and A-aryl cinnamaldehyde anils 53, in the presence of acetic anhydride, the 4-substituted azlactones 54 are isolated (Fig. 4.15). Dalla Croce... 

Sain and Sandhu found that azlactones react with dienamine 101 to give cyclohexenone 102 in good yield (Fig. 4.35). Because these reactions occur at or below room temperature, the authors favor a pathway involving C-5 attack on a munchnone rather than thermal equilibration to the ketene tautomer and subsequent... 

This same group generated bicychc munchnones (e.g., 334) and trapped them with imines to afford either imidazoles 335 or spirocyclic p-lactams 336, depending on conditions, although mixtures are produced and yields of the p-lactams are invariably low (Fig. 4.113). The latter can arise by nucleophilic attack on the munchnone itself or the ring-opened ketene tautomer (vide infra). 

In contrast with the vast array of known alkyl or aryl alcohols/ethers, relatively scant attention has been paid to ynols and related compounds. This is due, in part, to the rapid conversion of formal ynols into ketene tautomers. It should be noted that metal ynolates can be readily generated and used for organic transformations [187]. For ynol ethers, a relatively small number of approaches are known for the preparation of these compounds. The following sections will highlight modifications made to this system as well as new methods for generating oxygen-carbon (sp ) bonds. 

Azirines254 in principle react analogously to Schiff bases the 2,3-diphenyl pyridones-4 403 obtained from diphenyl cyclopropenone may well result from a primary betaine 401, which reorganizes to the pyridone-4-system via its valence tautomer, the ketene 402. 

It is well known that l,3-thiazine-4,6-diones can exist in three tautomeric forms <1960CB671, 1976KGS1042>. Cycloaddition of chlorocarbonyl ketenes with thioamides has been reported to produce only the 4-hydroxy-l,3-thiazin-6-ones, whereas the same reaction with amides gives either 4-hydroxy-1,3-oxazin-6-ones or a mixture of tautomers depending on the substituents on the starting materials <2005ARK(xv)88>. 

Ketenes have been detected as secondary photoproducts from the keto tautomers of 86 and 45. Formation of the ketene, observed from 86, implies that the keto tautomer of phenol is present, although below the limits of detection, during photolysis of 86 at 77 K.135... 

The enol tautomers of many ketones and aldehydes, carboxylic acids, esters and amides, ketenes, as well as the keto tautomers of phenols have since all been generated by flash photolysis to determine the pH rate profiles for keto-enol interconversion. Equilibrium constants of enolization, KB, were determined accurately as the ratio of the rate constants of enolization, kE, and of ketonization, kK, Equation (1). 

These results prompted them to attempt the stereoselective synthesis of the IV-phenylsulfonyl substituted spiro- (3-lactams 150, 151 (Scheme 36) from the N-(phenylmethyIe ne )be n ze nesulfonamide and the ketene valence tautomer of the bicyclic mesoionic compounds such as (2 S,4/ )-4-acetyloxy or benzoyloxy-IV-acyl-prolines 149 in the presence of acetic anhydride [109]. The presence of the stereocenter in position 4 of the cyclic amino acid 149 was found to be sufficient to ensure complete stereoselectivity on the spiranic C-4. 

Some reactions of munchnones occur via acylamino ketenes, the covalent valence tautomers of the betaines. The ketenes are intermediates in the thermolysis (see Scheme 22) and in the formation of azetidinones from imines (equation 69) they are thought to be involved in the aminolysis of the mesoionic compounds, which results in amides of a-acylamino acids, and in the formation of the benzodioxin (247) by the combined action of acetic anhydride and tetrachloro-o-benzoquinone on Af-benzoylalanine (equation 70). 

RCH=C=0, except when these are generated in situ by decomposition of a diazo ketone (the Wolff rearrangement, 18-8) in the presence of the imine. It has been done with ketene, but the more usual course with this reagent is an addition to the enamine tautomer of the substrate. Thioketenes ° (R2C=C=S) give p-thio-Imines also form p-lactams when treated with (i) zinc (or another... 

The most studied of the mesoionic compounds are the mtinchnones (90), which are 3-substituted-5(4i/)-oxazolones with at most one substituent on the 4 position. They react in such a 1,3-dipolar form or in a ring-opened ketene form. Reactions of 3-unsubstituted-5(4/f)-oxazolones may also proceed through their mesoionic (91) or ketene (92) tautomers (Scheme 29). 

Scheme 193). On heating the mixture, preferably in the absence of triethylamine, the hexahydro-a-quinolone 185 was obtained directly, together with the enamide 184 and amide 186. Since the enamide could not be cyclized to the hexahydro-a-quinalone under thermal or acid-catalysed conditions, then clearly the enamide is not an intermediate in the formation of the heterocycle which must therefore arise by. /V-acylation of the enamine tautomer, followed by a [3,3]sigmatropic rearrangement and cyclization of the ketene intermediate (Scheme 194). Significantly vide infra), when the imine of 2-methylcyclohexanone was used, the rearrangement to the ketene occurred at the more substituted Cj position. 

Hydroxy tautomers of some types of 1,3-oxazinediones have been encountered in the previous section. Here, compounds normally represented as diketo compounds are described. Thus, aroyl-ketenes, generated through the thermolyses of 6-aryl-2,2-dimethyl-l,3-dioxin-4-ones, can be trapped by ethyl carbamate to give 7V-aroylacetylcarbamates (208). These products on heating give 6-aryl-... 

For a less-complicated consideration, all the tautomers, rotamers, as well as resonance structures are not specifically considered here. For example, although these equilibriums can be included in the kinetic analyses, the outcome reaction rate results in the same kinetic effects on [HA] or [A] as described in the text. In order to have more focused discussion in the text, detailed kinetic analyses of all other possible mechanisms, which can be done as described in the text, are not hsted here. For recent reviews, see (a) Tidwell, T.T. Ketenes lohn Wiley Sons New York, 1995. (b) Wentrup, C. Heihnayer, W. KoUenz, G. Synthesis 1994, 1219. (c) Tidwell, T.T. Acc. Chem. Res. 1990, 23, 273. For leading references, see (a) Gong, L. McAUister, M.A. Tidwell, T.T. J. Am. Chem. Soc. 1991, 113, 6021. (b) Lien, M.H. Hopkinson, A.C. J. Org. Chem. 1988, 53, 2150. (c) Armitage, M.A. Higgins, M.J. Lewars, E.G. March, R.E. J. Am. Chem. Soc. 1980, 102, 5064. 

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