Chloroboration and allied reactions of unsaturated compounds. Part IV. Boration of di-p-tolylcarbodi-imide

Abstract

Di-p-tolylcarbodi-imide reacted with a number of boranes [BCl₃,BBr₃, MeOBCl₂, (MeO)₂BCl, o-C₆H₄O₂B·SBuⁿ, o-C₆H₄O₂B·NEt₂, (Et₂N)₂BCl, PhBCl₂, PhB(OMe)₂, PhB(NEt₂)SBuⁿ, Ph₂BCl, Ph₂B·OMe, Ph₂B·SBuⁿ, Ph₂B·NEt₂, and Ph₃B] under mild conditions to give high yields of novel borylenebisamidines, boryl-amidines and -guanidines and related compounds, (I), (II), (III), and (IV). These reactions are insertions into the B–X (X = Cl, Br, Ph, OMe, NEt₂, or SBuⁿ) bond. The synthesis of (III) and (IV) shows that under more forcing conditions it is possible to effect the insertion of more than 1 mol. of a 1 : 2 dipole into the B–X bond. Aminoboration with amine elimination provides a route to the 1,3,5,2-triazaborine (V).

By examining the structures of reaction products obtained from the carbodi-imide and mixed boranes, the relative migratory aptitudes (r.m.a.) of the various groups attached originally to boron and ultimately to carbon were established. For example from the reaction Ph₂BNEt₂+ TolNCNTol → Ph₂B·NTol·C(:NTol)·NEt₂ the r.m.a. order is NEt₂ > Ph. Similar experiments led to the r.m.a. orders (a) NHR > NR₂ > SR > Ph > Cl, (b) OR > Ph, and (c) Cl > OR, and it is suggested that these borations are thermodynamically- rather than kinetically-controlled processes. The results are discussed in the context of metallations generally.