In light of our difficulties in obtaining a clean sample of a PC3 base, we also targeted the synthesis of a partially monocyclopropeniminylphosphazene (PC1) in order to fully characterize the first example of a molecule combining cyclopropenimine and phosphazene moieties. We observed that 1 was reluctant to add more than once to trichloroiminophosphorane 13, so reaction of these components in a 2:1 ratio at −78 °C followed by treatment with excess piperidine at 0 °C provided 2.71 g of PC1 base 14 as its HBF4 salt after the appropriate workup ().
Organic bases are important class of reagents for synthetic chemistry. Recent developments in optimizing the basicity and in reducing nucleophilicity provide various new synthetic methodologies. This review describes synthetic applications of two excellent organic superbases. One is phosphazene base developed by Schwesinger and the other is proazaphosphatrane base, which was first synthesized by Verkade. The bacisity of the Schwesinger's P 4 base is in the range of organolithium bases and 18 orders of magnitude more basic than conventional DBU. Verkade bases are also strong bases due to the protonation on the bridgehead phosphorus atom with a resultant transannulation to form the corresponding azaphosphatrane structure. Properties, applications, and synthetic benefits of these bases are discussed.
Superbases for Organic Synthesis. Guanidines, …
In addition to having unusually high loneâpair Lewis basicity, organic superbases are also generally characterized by good kinetic activity in proton exchange processes and by the ability of their protonated forms to delocalize their positive charge via conjugation over two or more bonds, thus stabilizing the conjugate acid.
↑ Superbases for Organic Synthesis Ed.
The nonmetallic character of superbases also lends itself well to meeting the increasing need for environmentally friendly organic catalysts and base reagents.
Guanidines, Amidines, Phosphazenes and Related Organocatalysts
The first two chapters cover general aspects and properties of the aforementioned superbases, the next four chapters emphasize their catalytic and stoichiometric applications in organic synthesis, the next two chapters describe applications of the related organocatalysts DMAN and derivatives of urea, the penultimate chapter reports the occurrence of amidine and guanidine moieties in natural products and pharmaceuticals, and the last chapter offers some thoughtâprovoking ideas regarding acid–base systems for asymmetric synthesis and for molecular recognition of substrates based on the formation of multiple intermolecular hydrogen bonds.
Chapter One General Aspects of Organosuperbases
With robust synthetic routes in hand, we sought to determine the basicities of these new higher-order super-bases. The pKBH+ values in acetonitrile are shown in . Notably, while the basicity trend of the parent su-perbases is guanidine 16, 17, and 18), the trend for the higher-order superbases is cyclopropenimine 11, 4, and 19 or 6, 2, and 12). These data indicate that the cyclopropene core is relatively less effective at communicating the basicity of the substituents to the head imino group. The greater insulating character of the cyclopropenimine functionality can be understood by consideration of the conjugate acid form, the core of which is a cyclopropenium ring. As a closed-shell aromatic, the cyclopropenium group gains less in stability from the donation of electron density than other cations, a trend long-established for the chemistry of the cyclopropenium unit.
03/03/2006 · A review with 136 refs
We recently introduced 2,3-diaminocyclopropenimines as a new class of organic superbase. These cyclopro-penimines are as basic as the P1 phosphazenes but have dramatically improved stability profiles in non-inert atmosphere. Given the growing recognition of the utility of cyclopropenimine bases, we sought to develop a number of higher-order cyclopropenimine superbases with the goal of realizing enhanced basicities and unique functional properties. In this Article, we describe the synthesis and characterization of four new classes of super-bases that incorporate cyclopropenimine functionality () as either the core group (blue box), the substituents (red box), or both (purple box). In addition, we report the first example of a bisphosphazenylguanidine higher-order superbase, as well as a monocyclopropeniminyl phosphazene base (not shown).