Reaction of isocyanates XNCO (X = Ar, i-Pr, t-Bu) with CH(2)(Y)CO(2)CH(2)CCl(3) (Y = CO(2)Me, CO(2)CH(2)CCl(3), CN) gave 15 amides XNHCOCH(Y)CO(2)CH(2)CCl(3) (6) or enols of amides XNHC(OH)=C(Y)CO(2)CH(2)CCl(3) (5) systems. The amide/enol ratios in solution depend strongly on the substituent Y and the solvent and mildly on the substituent X. The percentage of enol for group Y increases according to Y = CN > CO(2)CH(2)CCl(3) > CO(2)Me and decreases with the solvent according to CCl(4) > C(6)D(6) > CDCl(3) > THF-d(8) > CD(3)CN > DMSO-d(6). With the most acidic systems (Y = CN) amide/enol exchange is observed in moderately polar solvents and ionization to the conjugate base is observed in DMSO-d(6). The solid-state structure of the compound with Y = CN, X = i-Pr was found to be that of the enol. The reasons for the stability of the enols were discussed in terms of polar and resonance effects. Intramolecular hydrogen bonds result in a very low delta(OH) and contribute to the stability of the enols and are responsible for the higher percentage of the E-isomers when Y = CO(2)Me and the Z-isomers when Y = CN. The differences in delta(OH), delta(NH), K(enol), and E/Z enol ratios from the analogues with CF(3) instead of CCl(3) are discussed.

Twenty open-chain mono-, di-, and trialkyl and aryl-N-substituted cyanomalonamides R2R1NCOCH(CN)CONHR3 were prepared. In solution, signals for both amide and a single enol are mostly observed, despite the potential for E and Z isomeric enols. The equilibrium (KEnol) values between the amides and the enols were determined in different solvents by NMR spectra. They decrease on increasing the polarity of the solvent in the order CDCl3 approximately C6D6>THF-d8>(CD3)2CO>CD3CN>DMF-d7>DMSO-d6. For the R1R2NCOCH(CN)CONHR3 system when R1=R2=H, Me or R1=H, R2=Me, KEnol for R3 follows the order: C6F5>Ph>or=An>or= i-Pr>or= t-Bu, and for R1, R2:H, H>Me, H>Me, Me in all solvents. A unique feature is the appreciable % enol in DMSO-d6 when R1=R2=H, in contrast with enol systems with other electron-withdrawing groups (EWGs). Calculations (B3LYP/6-31G**) corroborate the higher KEnol values for less alkyl-substituted systems, showing that in the most stable conformer when R1=H, R2=R3=Me the N-hydrogens are closer to the CN group. The order of promoting substituents for enol of amide formation is CONH2>CO2CH2CF3>CO2Me>CONHMe. The solid-state structures of the isolated species, determined by X-ray crystallography, were either amides or enols, and a higher KEnol(CDCl3) value does not ensure a solid enol structure. In no system were both solid species isolated. The X-ray structures of the enols were temperature-dependent. In most cases, the difference between the O-H and O...H bond lengths at low temperature were appreciable, but they become closer at the higher temperature. Similar tendency for either the C=C/C-C or the C-O/C=O bonds was observed. This is ascribed to a hydrogen shift between two regioisomeric enols in an asymmetric double-well potential, which becomes faster at a higher temperature. Calculations show that the enol structures are nonsymmetrical, resembling the lower temperature structures, even when they are chemically symmetrical, but the energy differences between the two regioisomers are <1 kcal. The hydrogen bonds in the enol moiety are strong, with O...O distances <2.45 A, and are resonance-assisted hydrogen bonds. IR spectra in solution and the solid state qualitatively corroborate the NMR and X-ray structure determination.

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The strained double bond of cyclopropenylcarbinols undergoes a facile oxidation reaction to lead to unsaturated carbonyl derivatives. The distribution of the formed products depends on the relative stability of carbon-centered radical species, and the Sharpless kinetic resolution leads to enantiomerically pure Baylis-Hillman enal adducts.

SummaryCarbocupration of α-heterosubstituted alkynes leads to the formation of stereodefined functionalized vinyl copper species as single isomer. Recent advances in the field show that a simple pre-association of the organometallic derivative with an additional polar functional group in the vicinity of the reaction center may completely change the stereochemical outcome of the reaction. Representative examples are given in this mini-review.