Recent Advances in the Chemistry of Anellated Azaphospholes

Abstract

Azaphospholes are 6 aromatic heterocycles incorporating at least one 2 , 3 -phosphorus atom and one 3 , 3nitrogen atom. This review covers the synthesis, structural characterization, reactions and theoretical calculations of azaphospholes anellated to a six-or five-membered heterocyclic ring, reported during the last 10-12 years. Three synthetic methods, namely [4+1] cyclocondensation, [3+2] cyclocondensation and 1,5-electrocyclization make a variety of these phosphorus heterocycles available. The aromatic character of these compounds has been confirmed by carrying out density functional theory (DFT) calculations. Mainly 1 H, 13 C and 31 P NMR studies have been employed for the structural characterization. Almost all the functionalities present in the azaphosphole ring have been investigated and electrophilic substitution reactions of the 1-unsubstituted compounds and Diels-Alder reactions as well as 1,2-addition reactions to the >C=P-moiety have been accomplished successfully. The stereo-and regioselectivities observed in the Diels-Alder reactions have been explained on the basis of DFT calculations. Cheletropic cycloadducts formed from [4+1] cycloaddition of o-quinones on the 2 , 3 -P atom have been obtained. A variety of coordination compounds formed bycoordination of the 2 , 3 -P atom have been prepared.

Diels–Alder Reactions with the >C=P– Functionality of Annelated Azaphospholes

“…However, just to complete the story recounted in the previous section, general chemical equations showing the use of two methods for the syntheses of these compounds are giv-en in the next section; details of these methods can be found in the relevant research papers cited in our recent reviews. [11][12][13][14] Our comprehensive review entitled 'The Diels-Alder reactions involving >C=P-functionality' 15 includes our results published up to 2008, besides other work in this field. This account covers the literature right from the beginning up to our most recent results.…”

3 a,6 a‐Diaza‐1,4‐Diphosphapentalenes: Synthesis and Complexation with the Organic π‐Acceptor 1,2,4,5‐Tetracyanobenzene

“…On the other hand, we have found a high π‐donor ability of diazadiphosphapentalenes, which contrasts with their closest analogues, azaphospholes and diazaphospholes, [2] containing two‐coordinated phosphorus.…”

Electron‐Rich Aromatic 1,3‐Heterophospholes – Recent Syntheses and Impact of High Electron Density at σ2P on the Reactivity

“…1,2,4‐Diazaphospholes are similarly stable whereas N ‐bridging heteroaromatic anellated 1,3‐azaphospholes of the 2‐phosphindolizine type are susceptible to hydrolysis with ring opening. Alcohols, NH‐amines, and thiophenols add at the P=C bond of the latter set of compounds after activation by NaOR (ROH) or sulfur oxidation 10a–10c. Non‐anellated aromatic azaphospholes with one P–N bond and one to three N atoms are usually susceptible to addition of ROH and at least in part also to NH‐functional amines, except the lone‐pair donating σ 3 N atom is in the β‐position (conjugation) to σ 2 P. Such reactions have been reviewed in detail previously 9…”