Replacing Sp2 Hybridized Carbon Centers with Phosphorus

Abstract

The striking parallels between the chemistry of molecular carbon and phosphorus compounds has fascinated chemists for decades and has even led to phosphorus being coined “the Carbon Copy.” A guiding tenet in organic chemistry is that carbon can be bonded to a maximum of four other atoms to satisfy the octet rule. By comparison, phosphorus, a heavier p-block element, can exist in hypervalent states and can form bonds with up to six other atoms. Apparent patterns in the bonding and reactivity of carbon and phosphorus compounds typically manifest when these elements are in lower coordination environments (bonded to few other atoms). Molecules featuring low-coordinate earth abundant p-block elements (C, P, Si, Al, etc) have generated considerable research interest as of late, with pivotal discoveries showing these systems can facilitate processes more traditionally distinctive of transition metal complexes. Such behavior includes the activation of thermodynamically strong chemical bonds, which in some cases is reversible, and the ability to directly perform catalytic transformations on organic molecules, overall pushing the needle towards more sustainable and cost-conscious chemistry. Low-coordinate phosphorus cations are one general group of compounds that show great promise in these domains. This thesis delineates the synthesis of cations that distinctly feature a sp2 hybridized phosphorus atom and are principally inspired from classic carbon motifs. Firstly, we explore a synthetic route to a phosphorus analog of a [3]cumulene (R2C=C=C=CR2). We found our target phospha-cumulene, also known as an allenylidene phosphonium cation, to be uniquely accessible when harnessing electron rich substituents known as N-heterocyclic imines (NHIs). The properties and reactivity of the allenylidene phosphonium cation are investigated, with one interesting reactivity pathway being a thermally reversible [2+2] cycloaddition which occurs between a P=C and C=C bond. This process is remarkable given that comparable [2+2] cycloadditions between two C=C functional groups typically requires photoexcitation. The NHI substituent platform is also used to prepare a series of phosphenium cations (the phosphorus analog of carbenes) and the reactivity of these species is subsequently disclosed.