Design of non-ionic carbon superbases: second generation carbodiphosphoranes.

Sebastian Ullrich,Björn Koch,Borislav Kovačević,Jörg Sundermeyer,Klaus Harms

doi:10.1039/c9sc03565f

Abstract

A new generation of carbodiphosphoranes (CDPs), incorporating pyrrolidine, tetramethylguanidine, or tris(dimethylamino)phosphazene as substituents is introduced as the most powerful class of non-ionic carbon superbases on the basicity scale to date. The synthetic approach as well as NMR spectroscopic and structural characteristics in the free and protonated form are described. Investigation of basicity in solution and in the gas phase by experimental and theoretical means provides the to our knowledge first reported pK BH + values for CDPs in the literature and suggest them as upper tier superbases.

Highlights

The stepwise expansion of the molecular scaffold in order to better delocalize the positive charge formed upon protonation, was applied to synthesize higher-order Nsuperbases of guanidines,[5,6] imidazolidine amines[7] and cyclopropeneimines.[8,9]
Other strategies for augmenting the intrinsic proton affinity have been investigated: in proton sponges, a second nitrogen basicity centre in close proximity to the rst one increases the pKBH+ value up to 16 orders of magnitude by intramolecular hydrogen bonding compared to corresponding non chelating bases.[10]
We present a synthesis for hexa(pyrrolidino) carbodiphosphorane ((pyrr)6-CDP) with its calculated rst and second proton affinity (PA) of 287.6 and 188.9 kcal molÀ1,34 which exceeds the PAs of (Ph)6-CDP (280.0 and 185.6 kcal molÀ1)[34] and6-CDP (279.9 and 174.9 kcal molÀ1).[34]

Summary

Introduction

Much theoretical and synthetical effort has been devoted to li non-ionic organic bases to the basicity level of common inorganic or metalorganic bases.[1,2] With his famous phosphazenes Schwesinger established a widely used and commercially available class of (organo-)superbases.[3,4] His homologization concept, the stepwise expansion of the molecular scaffold in order to better delocalize the positive charge formed upon protonation, was applied to synthesize higher-order Nsuperbases of guanidines,[5,6] imidazolidine amines[7] and cyclopropeneimines.[8,9] such basicity enhancement is accompanied by an unwanted growth of the bases' molecular weight. Other strategies for augmenting the intrinsic proton affinity have been investigated: in proton sponges, a second nitrogen basicity centre in close proximity to the rst one increases the pKBH+ value up to 16 orders of magnitude by intramolecular hydrogen bonding compared to corresponding non chelating bases.[10] Additional thermodynamic driving force comes from relief of strain of the aromatic backbone.[11] Many derivatives of such proton sponges were designed by combining aforementioned superbasic functionalities with the 1,8-diaminonaphthalene structural motif[12] or as proton pincers with different backbones.[13]. We present a synthesis for hexa(pyrrolidino) carbodiphosphorane ((pyrr)6-CDP) with its calculated rst and second proton affinity (PA) of 287.6 and 188.9 kcal molÀ1,34 which exceeds the PAs of (Ph)6-CDP (280.0 and 185.6 kcal molÀ1)[34] and (dma)6-CDP (279.9 and 174.9 kcal molÀ1).[34] we apply the homoligization concept to CDPs by introducing PR2R0 units bearing one intrinsically superbasic substituent R0 to access CDP.

Results and discussion

At carbon

Conclusions

Conflicts of interest