Stereoselectivity of the Thermally Initiated Free‐Radical Chain Addition of Cyclohexane to 1‐Alkynes

Alkanes can be added to alkynes in a thermally initiated free‐radical chain reaction (“ane reaction”). The addition of cyclohexane to 1‐alkynes 1a–1 yields a mixture of (Z)‐ and (E)‐2‐cyclohexyl‐1‐alkenes 3a–1. An essential step in this reaction is the addition of cyclohexyl radicals to the alkynes to give 2‐cyclohexyl‐1‐alkenyl radicals 2a–1 which abstract hydrogen from cyclohexane to yield the products 3a–1. The stereoselectivity of the addition has been measured in the temperature range of 160–260°C. It strongly depends on the substituent X of the radical center and varies over a range of almost four orders of magnitude from [(Z)‐3a]:[(E)‐3a] = 33 (X = OMe) at 160°C to [(Z)‐31]:[(E)‐31] = 0.012 (X = tBu) at 260°C. The stereoselectivity is further influenced by the temperature and in most cases by the concentration of the hydrogen donor cyclohexane. The reaction is discussed in terms of the stereoselectivity of the addition of cyclohexyl radicals to the alkyne, of the structure of the 1‐alkenyl radical (σ and π radical, respectively), the rate of inversion in the case of σ alkenyl radicals, and the relative rates of syn and anti hydrogen transfer.