Phosphinoboration of Diazobenzene: Intramolecular FLP Synthon for PN₂B‐Derived Heterocycles

Abstract

Phosphinoboration of diazobenzene with Ph2PBR′2 cleanly affords products of the form Ph2P(PhNNPh)BR′2 (2: R′2=catechol, cat; 4: R′2=phenanthrenediol, quin) and shows evidence of Ph2P(PhNNPh)Bpin 7 (pin: pinacol). The mechanism of these reactions was probed computationally and shown to proceed via intermediates involving a diazobenzene‐adduct of the boron center of the PB reagent. The resulting PNNB species 2 and 4 are shown to be frustrated Lewis pairs (FLPs). Despite the presence of weakly Lewis acidic boron centers, these species react with diazobenzene. Additionally, the FLP reactivity of 2 was further probed with 4‐phenyl‐1,2,4‐triazole‐3,5‐dione, 1,10‐phenanthroline‐5,6‐dione, and benzyl azide to give unique five‐, six‐, and eight‐membered heterocyclic rings. Thus, phosphinoboration provides a new avenue to FLPs in which donor and acceptor sites are linked by nitrogen atoms.

Organocatalytic trans Phosphinoboration of Internal Alkynes

“…Recent developments in phosphinoborations have focused on addition of PB species across C–heteroatom and N−N π‐bonds. In pioneering studies by Westcott and co‐workers, it was revealed that PBs Ph 2 P‐Bpin and Ph 2 P‐Bcat readily reacted with various aldehydes, ketones, and aldimines without any additives at room temperature to afford the corresponding 1,2‐addition products (Scheme a) .…”

The phosphinoboration of acyl chlorides

“…11 Indeed, we have recently reported on the application of the BNNP FLPs derived from phosphinoborane addition to diazobenzene chemistry. 12 Acyl phosphines are used in industry as photoinitiators for radical-induced polymerization reactions 13 for automotive coatings, adhesives, latex composition kits, and various dental and orthodontic materials. 14 Known methodologies to such photoinitiators exploit pyrophoric, toxic, expensive reagents that are often difficult to handle.…”

Synthesis, Structure and Reactivities of Pentacoordinated Phosphorus–Boron Bonded Compounds

“…More recently, phosphino‐boranes, bearing the lower coordinated bond between a tricoordinated phosphorus and a tricoordinated boron atom (R 2 P–BR 2 ), have regained attention as they have been shown to activate H 2 and other small molecules and to coordinate to metals in a symmetric side‐on fashion . Moreover, phosphinoboronate esters [R 2 P–B(OR) 2 ] have shown phosphinoboration reactivity to double‐bond compounds and N‐heterocycles , . The different reactivities and properties of phosphine‐boranes, phosphino‐boranes and phosphino‐boronate esters depends on the coordination number of the phosphorus and boron atoms as well as the presence or absence of the phosphorus lone pair and boron vacant 2p orbital.…”