Guanidinophosphazenes:Design,Synthesis,and Basicity in THF and in the Gas Phase

摘要

A principle for creating a new generation of nonionic superbases is presented.It is based on attachment of tetraalkylguanidino,1,3-dimethylimidazolidine-2-imino,or bis(tetraalkylguanidino)carbimino groups to the phosphorus atom of the iminophosphorane group using tetramethylguanidine or easily available 1,3-dimethylimidazolidine-2-imine.Seven new nonionic superbasic phosphazene bases,tetramethylguani-dino-substituted at the P atom,have been synthesized.Their base strengths are established in tetrahydrofuran (THF) solution by means of spectrophotometric titration and compared with those of eight reference superbases designed specially for this study,P_2- and P_4-iminophosphoranes.The gas-phase basicities of several guanidino- and N',N',N''N''-tetramethylguanidino (tmg)-substituted phosphazenes and their cyclic analogues are calculated,and the crystal structures of (tmg)_3P=N-t-Bu and (tmg)_3P=N-t-Bu centre dot HBF_4 are determined.The enormous basicity-increasing effect of this principle is experimentally verified for the tetramethylguanidino groups in the THF medium:the basicity increase when moving from (dma)_3P= N-t-Bu (pK_(alpha)=18.9) to (tmg)_3P=N-t-Bu (pK_(alpha)=29.1) is 10 orders of magnitude.A significantly larger basicity increase (up to 20 powers of 10) is expected (based on the high-level density functional theory calculations) to accompany the similar gas-phase transfer between the (dma)_3P=NH and (tmg)_3P=NH bases.Far stronger basicities still are expected when,in the latter two compounds,all three dimethylamino (or tetramethylguanidino) fragments are replaced by methylated triguanide fragments,(tmg)_2C=N-.The gas-phase basicity (around 300-310 kcal/mol) of the resulting base,[(tmg)_2C=N-]_3P=NH,having only one phosphorus atom,is predicted to exceed the basicity of (dma)_3P=NH by more than 40 powers of 10 and to surpass also the basicity of the widely used commercial [(dma)_3P=N]_3P=N-t-Bu (t-BuP_4) superbase.