THE CONFORMATION OF MEDIUM-SIZED HETEROCYCLES: SYNTHESIS AND SOLID-STATE CONFORMATION OF A 6-ARYLTHIO-SUBSTITUTED DIBENZO-[d,f][1,3,2]DIOXAPHOSPHEPIN

Abstract

Although the conformational analysis of both carbocyclic and small-membered heterocyclic compounds has played a key role in the development of mechanistic and synthetic chemistry, the stereochemistry of medium-sized organophosphorus heterocycles has received little attention until very recently.¹ Sterically congested bisphosphite ligands that incorporate a seven-membered dibenzo[d,f][1,3,2]dioxa-phosphepin ring have been claimed as superior ligands for transition-metal-cata-lyzed reactions.² Quite recently, the highly regioselective formylation of α-olefins by the rhodium(I) complexes of these bisphosphite ligands was communicated by Buchwald and Cuny.³ The observation of unprecedented eight-bond P—P J coupling in the ³¹P{¹H} NMR spectra of these bisphosphite ligands had been attributed to the proximity of the phosphorus atoms due to intramolecular steric interaction.⁴ Recent studies have shown that sterically hindered medium-sized phosphorus het-erocycles may adopt an unusual diequatorial placement in pentaoxyphosphoranes. Rapid advances have been made by Holmes and co-workers on establishing the factors that influence ring orientation and conformation in pentaoxyphosphoranes which incorporate these ring Our efforts to further elaborate the factors that dictate the conformation of the seven-membered dibenzo[d,f][1,3,2]dioxaphosphepin ring are reported herein.

Key Words:

• Dibenzo[d,f][1,3,2]dioxaphosphepin
• medium-sized heterocycles, dibenzo[d,f][1,3,2]dioxa-phosphepin, conformational analysis
• dibenzo[d,f][1,3,2]dioxaphosphepin, solid state conformation
• dibenzo[d,f][1,3,2]dioxaphosphepin, synthesis