Nucleophilic attack on ketene, 6‐methylene‐2,4‐cyclohexadienylideneketene, 6‐oxo‐2,4‐cyclohexadienylideneketene and 4‐oxo‐2,5‐cyclohexadienylideneketene: an ab initio study

Abstract

AbstractNucleophilic attack of cyanide anion at the carbonyl carbon was studied computationally at the MP2/6–311+G** level for ketene and at the MP2/6–31+G* and MP2/6–31+G*//HF/6–31+G* levels for all four title compounds. Heats of addition were computed for formation of in‐plane adducts and perpendicular adducts where the terms ‘in‐plane’ and ‘perpendicular’ indicate the direction of nucleophilic attack relative to the ketene plane. Heats of activation were also computed for the two modes of attack. For ketene (1) the in‐plane adduct is favored by 37.6 kcal mol−1 and the in‐plane transition state by 18.8 kcal mol−1. For the 6‐methylene compound (2) the in‐plane adduct is favored by 20.1 kcal mol−1 and the in‐plane transition state by 7.3 kcal mol−1. For the 6‐oxo compound (3) the in‐plane adduct is more stable by 11.2 kcal mol−1 whereas the in‐plane transition state is preferred by 8.8 kcal mol−1. For the 4‐oxo compound (4) the in‐plane adduct is favored by 19.3 kcal mol−1 but its transition state by only 2.8 kcal mol−1. The in‐plane adduct for ketene is the enolate of acetyl cyanide while the perpendicular adduct is a pyramidalized carbanion attached to a cyanocarbonyl group. Calculated NPA charges support this conclusion. The perpendicular adduct is actually the transition state for rotation about the CC bond of the ketene moiety in the in‐plane adduct, and the 37.6 kcal mol−1 enthalpy difference is closely matched by the calculated activation enthalpy for rotation about the CC bond of acetaldehyde enolate: ΔH‡ = 33.6 kcal mol−1. The smaller differences found for in‐plane vs perpendicular adduct formation for the 6‐methylene, 6‐oxo and 4‐oxo compounds are correlated with decreases in bond length alternation within the six‐membered rings which occur upon addition of the nucleophile. Less alternation may be associated with an approach to aromaticity, hence an increase in stability. Both modes of attack lead to reduced bond length alternation, but the amount of alternation is lowered more for the perpendicular adducts than for the in‐plane adducts. Changes in the CO bond lengths are also diagnostic. For all four ketenes the ketenyl CO length in the in‐plane adducts is greater than in the perpendicular adducts reflecting more single bond (enolate) character in the former and more double bond (carbonyl) character in the latter. On the other hand the lengths of the 6‐oxo and 4‐oxo CO bonds in their in‐plane adducts are smaller than in the perpendicular adducts, consistent with increased conjugation with the oxo substituent in the latter. The greater NPA charge on the 6‐oxo and 4‐oxo oxygens of the perpendicular adducts supports this conclusion. Calculated enthalpies of activation are in the order ketene > 6‐methyleneketene > 6‐oxoketene, in qualitative agreement with experimentally determined rates of hydration. Copyright © 2004 John Wiley & Sons, Ltd.