ROTATIONAL BARRIERS OF ALLYL ANIONS IN SOLUTION

Barriers to rotation about the carbon-carbon bonds of allylalkali metal compounds in THF have been measured by NMR exchange rates of the terminal allyl protons: ΔG‡, kcal/mol (coalescence temperature, °C), = 10.7 ± 0.2 (-51) for allyllithium, 16.7 ± 0.2 (68) for allylpotassium, and 18.0 ± 0.3 (68) for allylcesium. The barrier for allylcesium is a new experimental lower limit to the rotational barrier of a free allyl anion in solution. Barriers to rotation about carbon-carbon bonds of 2-methylallylpotassium, (Z)-l-methylallylpotassium, and (Z)- and (E)-l-isopropylallylpotassium also have been measured. The (Z)-1-alkyl groups have little effect on C2-C3, but the (E)-1-isopropyl group decreases the C2-C3 barrier by 2.7 kcal/mol. Both (Z)- and (Z)-l-alkyl groups increase markedly the barrier of C1-C2 Mechanisms of bond rotation are discussed in terms of the role played by the cation. © 1979, American Chemical Society. All rights reserved.