Nitrogen Base Adducts of Tetraorganodiboroxanes

The tetraorganodiboroxanes R2BOBR2 [R = C2H5 (1); R2 = 1,5‐C8H14 (2)] react with N‐bases [pyridine (P); 4‐methylpyridine (4‐MeP), 4‐EtP, 4‐tBuP, 3,5‐Me2P, DMAP; quinuclidine (Q); 1,5‐diazabicyclo[4.3.0]non‐5‐ene (DBN)] to form stable liquid (N‐base‐1) and solid (N‐base‐2) 1:1 adducts (e.g. P‐2, Q‐2, and DBN‐2). In solution, the rates of fluctuation of the N‐base between the two boron atoms of the R2B groups depend on the Lewis acidities of 1 and 2, the basicities of the bases, as well as the steric factors of the donor‐acceptor pairs. At low temperatures (e.g. ≤ −20°C for P‐2 and ≤ −50°C for P‐1), bonding of the base to only one of the boron atoms is indicated. When Q‐2 is dissolved in CDCl3 or [D8]toluene at room temperature it dissociates completly (11B and 13C NMR). Below −60°C interaction between the base and the borane was observed. Low‐temperature (‐50°C) 11B‐NMR data indicate that in the presence of excess base (Do) a species of the type (Do)2‐1 may be present.