Observable Enols of Anhydrides: Claimed Literature Systems, Calculations, and Predictions

The literature describing the observation of enols of carboxylic anhydrides and mixed carboxylic‐sulfuric anhydrides was examined. In the phenylbutyric anhydride system, the alleged enol was shown to be ethylphenylketene, and the monoenol EtC(Ph)=C(OH)OC(=O)CH(Ph)Et ( 5 ) and the dienol ( 6 ) should not be observed according to calculations. Calculations also show that the claimed enols H 2 C=C(OH)OSO 2 Y, Y=SO, Ac ( 15 ) and the enol of 2 H ‐pyran‐2,6(3 H )‐dione ( 7 ) are too unstable to be observed. The bulky enols of β , β ‐ditipylacetic formic ( 35a ) or trifluoroacetic ( 35b ) anhydride were calculated to be unstable with p K Enol =7.7 (6.2). The suggestion that compounds with the 3‐acyl or 3‐aroyl‐2 H ‐pyran‐2,6(3 H )‐dione skeleton are enolic was examined. In the solid state, all the known structures show that enolization takes place on C(5)=O. However, B3LYP/6‐31G** calculations show that, for 3‐acetyl‐4‐methyl‐2 H ‐pyran‐2,6(3 H )‐dione ( 10 , R 1 =Me, R 2 =H), which is completely enolic, the enol on the acetyl group ( cf. 12 ) is only 0.9 kcal/mol more stable than the enol on the anhydride ( cf. 11 ). Calculations also revealed that 3‐(trifluoroacetyl)‐2 H ‐pyran‐2,6(3 H )‐dione ( 28 ) should exist in nearly equal amounts of the enol of anhydride ( cf. 30 ) and the enol of the acyl group ( cf . 29 ), whereas the enol of anhydride ( cf . 32 ) is the only stable species for 3‐(methoxycarbonyl)‐2 H ‐pyran‐2,6(3 H )‐dione ( 31 ). Furan‐2,5‐diol ( 27 ) and 5‐hydroxyfuran‐2‐one ( 26 ) are calculated not to give observable isomers of succinic anhydride ( 25 ) (p K Enol =30 and 18, resp.) in spite of the expected aromatic stabilization of 27 . Surprisingly, the calculations reveal that the enol (NC) 2 C=C(OH)OCHO ( 38 ) is less stable than its tautomeric anhydride ( 37 ) (p K Enol =1.6). Comparison of calculated p K Enol values for (NC) 2 CHC(=O)X ( 41 ) and MeC(=O)X indicates that the assumption that substitution by two β ‐CN groups affects similarly all the systems regardless of X is incorrect. A p K Enol ((NC) 2 CHC(=O)X) vs . p K Enol (MeC(=O)X) plot is linear for most substituents with severe and mild negative deviations, respectively, for X=NH 2 and MeO. Appropriate isodesmic reactions have shown that the β , β ‐(CN) 2 substitution increases the stabilization of the enol of amide (X=NH 2 ) by 14.6 kcal/mol over that for the anhydride (X=OCHO), whereas the amide form is 7.1 kcal/mol less destabilized than for the anhydride. The p K Enol value for (MeOCO) 2 CHCOOCHO ( 43 ) is 3.6, i.e. , stabilization by these β ‐electron‐withdrawing groups is insufficient to make the enols observable.