Synthesis and dynamic behaviour of diethyl(1-indenyl)borane

Sigmatropic [1,3] B shift was observed by 2D ¹H EXSY NMR experiments in diethyl(1-indenyl)borane 1i prepared in 83% yield from indenyl lithium and chlorodiethylborane. The activation barrier of the rearrangement (E_A = 17.7±0.2 kcal mol^-1, lnA = 28.6±0.5) was determined from a series of 2D ¹H EXSY spectra recorded in the temperature interval 25-80°C. After prolonged storage at ambient temperatures or short heating at 100°C an equilibrium between 1i, diethyl(3-indenyl)borane 4i, and diethyl(2-indenyl)borane 5i is established (the equilibrium ratio 1i:4i:5i is 1:2.3:0.2). Addition of an excess of pentadeuteropyridiine leads to a new equilibrium between corresponding complexes (the ratio 1j:4j:5j is 1:3.2:0.2).