Participation of acetylenic bonds in pericyclic reactions. Thermal cleavage of .beta.-hydroxyacetylenes

Abstract

The ability of acetylenic systems to participate in intramolecular reactions proceeding via six-membered cyclic transition states has been established by thermolyses of a number of substituted /3-hydroxyacetylenes, in both the vapor and the liquid states. Thermolyses products consisted solely of those alienes and carbonyl compounds derivable from a 1,5-hydrogen transfer. The homogeneous reactions followed the first-order rate law. Activation energies with a variety of alkyl substituents were essentially within experimental error.

Elimination kinetics of β-hydroxynitriles in the gas phase

“…3 In addition, the elimination process of Eqn (1) by the nucleophilic attack of the more polar C=N bond of 2-(2-hydroxyethyl)pyridine was found, as expected, to be faster than that for the corresponding 2-hydroxyethylbenzene. 5 The faster rates due to the greater nucleophilicity of the CC with respect to the C=C bond and the C=N with respect to the C=C bond for the abstraction of the hydrogen of the OH group led us to examine the reactivity differences in elimination reaction (1) between CC and CN, that is, the b-hydroxyacetylenes and their corresponding b-hydroxynitriles.…”

“…As in the pyrolyses of b-hydroxyalkenes 1,2 and b-hydroxyalkynes, 3 the C(OH)-CH 2 bond polarization, in the direction of C(OH) … CH 2 À , is the limiting factor (structures 1-4).…”

“…Several studies on the pyrolyses of b-hydroxyalkenes 1,2 and b-hydroxyalkynes 3 have shown that they proceed via a six-membered cyclic transition state as pictured in Eqn (1). Moreover, the replacement of the double bond with the 1,2-aromatic p-bond of benzene 4 and of pyridine 5 was also found to eliminate in a similar manner [Eqn (1)].…”

Theoretical study of the thermolysis reaction of β‐hydroxynitriles in the gas phase

“…It has been recently reported 1 that β‐hydroxynitriles decompose thermally to give a mixture of aldehydes or ketones and nitriles as depicted in Figure 1. This reaction seems to be similar to that thought to be involved in related thermal decompositions such as for β‐hydroxyketones 2–6, β‐hydroxyolefins 7–10, β‐hydroxyesters 4, 11–14, and β‐hydroxyalkynes 15–18. In these reactions, calculated activation energies ( E a ≈ 150–180 kJ mol −1 ) and activation entropies (Δ S ≠ ≈ −19 to −60 Jmol −1 K −1 ) have been used in support of reaction mechanisms via a six‐membered cyclic transition state (Fig.…”

“…Further, the decomposition rates of β‐hydroxynitriles, as is the case for the above‐mentioned β‐hydroxy‐related compounds, increase from primary to tertiary carbon containing the hydroxyl group. Indeed, relative rates of decomposition for the β‐hydroxynitriles were found to be slower than those for the corresponding β‐hydroxyalkyne analogs 7–9, 15, i.e., 1:178, 1:87, and 1:26 for the primary ( 1 ), secondary ( 2 ), and tertiary ( 3 ) compounds, respectively, at 683.15 K and 0.06 atm. However, for β‐hydroxynitriles experimental values of log A in the range of 13.7–14.4 seems to point out a reaction pathway that does not involve a cyclic six‐membered transition state.…”