Diazaphospholes

Keywords 1,3,2-diazaphospholes 1,3,2-diazaphospholenes 1,3,2-diazaphosphinines 2,3-dihydro-l,3,2-diazaphosplinines diketiminto-phosphenium ions N-heterocyclic phosphines N-heterocyclc phosphenium ions 1,2,3-diazaphospholenium ions... 

Construction of isolated or benzannulated five-membered rings of NHPs can be accomplished by means of various condensation or cycloaddition reactions all of which involve interaction of an electrophilic Pj and a nucleophilic C2N2 building block. Salts containing 1,3,2-diazaphospholide anions or 1,3,2-diazaphospholenium cations can be directly accessed by some of these reactions but the products are in most cases neutral 1,3,2-diazaphospholes or NHP. A particularly concerted effort has been directed toward the synthesis of P-halogen-substituted NHP which are capable of undergoing further reactions via halide displacement or halide abstraction and serve thus as entry points for the preparation of a wide variety of neutral and cationic NHP derivatives. 1,3,2-Diazaphospholide anions are normally accessed by deprotonation of suitable iV-H-substituted precursors. 

Diazaphospholes with an isolated ring have been prepared from diaminoma-leonitrile and PC13 [10, 11], Reacting both components in refluxing acetonitrile... 

Benzannulated 1,3,2-diazaphospholes 5 form readily upon condensation of monosubstituted o-phenylenediamines (R=H, alkyl, Ph) with P(NMe2)3 [13, 14] (Scheme 2). A -Alkylated products can be detected by NMR but evade isolation by... 

Benzannulated NHPs are straightforwardly accessible from AUV-disubsti luted o-phenylenediamines either via base-induced condensation with substituted dichlorophosphines [25] or PC13 [26], or via transamination with tris(dialkylamino) phosphines [13, 14, 27], respectively. An analogous NH-substituted derivative was obtained in low yield via transamination of o-phcnylcncdiaminc with ethoxy-bis(diethylamino)phosphine [28], and condensation of o-phenylenediamine with excess tris(diethylamino)phosphine furnished a l,3-bis(phosphino)-substituted heterocycle [29], Intermediates with one or two NH functions were detectable by spectroscopy but could not be isolated in pure form under these conditions. However, 2-chloro-benzo-l,3,2-diazaphospholene and the corresponding 1-phenyl derivative were prepared in acceptable yield via condensation of PC13 with o-phenylenediamine under microwave irradiation [30], or with A-phenyl-o-phenylenediamine under reflux [27], respectively, in the absence of additional base. The formation of tetrameric benzo-NHPs during transamination of A-alkyl-o-phenylenediamines with P(NMe2)3 has already been mentioned (cf. the section entitled 1,3,2-Diazaphospholes and 1,3,2-Diazaphospholides ). 

Although 1,3,2-diazaphospholenium cations are usually prepared from neutral NHPs or 1,3,2-diazaphospholes via Lewis-acid induced substituent abstraction or A-alkylation, respectively (cf. Sect. 3.1.2), the group of Cowley was the first to describe a direct conversion of a-diimines into cationic heterocycles by means of a reaction that can be described as capture of a P(I) cation by diazabutadiene via [4+1] cycloaddition [31] (Scheme 4). The P(I) moiety is either generated by reduction of phosphorus trihalides with tin dichloride in the presence of the diimine [31] or, even more simply, by spontaneous disproportionation of phosphorus triiodide in the presence of the diimine [32], The reaction is of particular value as it provides a straightforward access to annulated heterocyclic ring systems. Thus, the tricyclic structure of 11 is readily assembled by addition of a P(I) moiety to an acenaphthene-diimine [31], and the pyrido-annulated cationic NHP 12 is generated by action of appropriate... 

The synthesis of new heterocyclic derivatives under conservation of a preformed cyclic structure is not only of particular importance for the synthesis of ionic 1,3,2-diazaphosphole or NHP derivatives but has also been widely apphed to prepare neutral species with reactive functional substituents. The reactions in question can be formally classified as 1,2-addition or elimination reactions involving mutual interconversion between 1,3,2-diazaphospholes and NHP, and substitution processes. We will look at the latter in a rather general way and include, beside genuine group replacement processes, transformations that involve merely abstraction of a substituent and allow one to access cationic or anionic heterocycle derivatives from neutral precursors. 

Transformations through 1,2-addition to a formal PN double bond within the delocalized rc-electron system have been reported for the benzo-l,3,2-diazaphospholes 5 which are readily produced by thermally induced depolymerization of tetramers 6 [13] (Scheme 2). The monomers react further with mono- or difunctional acyl chlorides to give 2-chloro-l,3,2-diazaphospholenes with exocyclic amide functionalities at one nitrogen atom [34], Similar reactions of 6 with methyl triflate were found to proceed even at room temperature to give l-methyl-3-alkyl-benzo-l,3,2-diazaphospholenium triflates [35, 36], The reported butyl halide elimination from NHP precursor 13 to generate 1,3,2-diazaphosphole 14 upon heating to 250°C and the subsequent amine addition to furnish 15 (Scheme 5) illustrates another example of the reversibility of addition-elimination reactions [37],... 

Routine identification and analytical characterization of all types of NHPs was preferably carried out by multinuclear NMR spectroscopy. Of particularly high diagnostic value are 31P NMR spectra where rather specific chemical shift ranges for heterocycles with different types of P-substituents can be observed. The largest chemical shifts occur for anionic and neutral 1,3,2-diazaphospholes (220-300 ppm) and 1,3,2-diazaphospholenium cations (210-200 ppm). Chemical shifts of P-halogen-l,3,2-diazaphospholenes vary over an overall range of 200-110 ppm and decrease in the order I (205-190 ppm)>Br (194-185 ppm two compounds... 

This effect allows one to monitor the perturbation of the tt-c lection system by interaction of the electrophilic phosphorus atom with a Lewis base. Following the same rationale, the still larger chemical shifts of neutral 1,3,2-diazaphospholes and 1,3,2-diazaphospholide anions are considered to reflect predominantly a reduction in n-n transition energy due to destabilization of the n(P) orbital with an increasing number of lone-pairs on the NPN-moiety rather than differences in the charge densities or n-electron distribution in the heterocyclic ring [16]. 

Table 1 Typical ranges of bond distances (in A) in the rings of 1,3,2-diazaphosphole and 1,3,2-diazaphospholene derivatives...

During the last few years, both neutral and cationic 1,3,2-diazaphospholes and NHP have been studied extensively by computational methods. The best part of these studies focused on a discussion of n-electron delocalization and their implication on chemical reactivities and stabilities, the explanation of the unique ionic polarization of exocyclic P-X bonds noted for some species, and the evaluation of structural and spectroscopic properties with the aim of helping in the interpretation of experimental data. 

A large part of the chemical reactions of 1,3,2-diazaphosphole and NHP derivatives reported to date include transformations under substitution of functional substituents at the C, N, or P ring atoms. The interest in several of these displacement processes was mainly directed by the desire to develop synthetic pathways for specifically... 

Planarization of the P-substituted azaphospholes results in structures with similar aromaticity to the planar phosphole. Replacement of the CH units by N in these rings has some effect on the barrier to planarization. The MP2/6-31C(d) barrier of 277-1,3,2-diazaphosphole was 17.6, while that of 377-1,5,3-diazaphosphole was 13.5 kcal/mol (phosphole 17.2 kcal/mol at the same level). ... 

Quantum chemical calculations on different levels have been reported for 2//-1,2,3-diazaphospholes (G) <83X1507), for 4,6-diamino-l,3,5-triaza-2-phosphapentalenes derived from 1,3,2-diazaphospholes (I) <86CB3213), for the tetrazaphospholium ion (S) <93CB1513), for 1//-1,2,3-azadiphosphole (U) <89NJC309), and for l//-l,2,3-benzazadiphosphole <93PS(76)45). Planar and nonplanar structures of 2/7-1,3,2-diazaphosphole and its 2-boryl derivative as well as of 4//-1,2,4-diazaphosphole and its 4-boryl and 3,5-diboryl derivatives are compared by correlated ab initio calculations <95JPC586). 

The reaction of P(NMe2)j with DAMN (81ZN(B)1273 83TL2137) or DAMN anil (84CC183) gives the 1,3,2-diazaphosphole 148 (Scheme 55). 

Diazaphospholes 6 are colorless to pale yellow distillable liquids or crystalline solids that are stable to oxidation by air and do not react with elemental sulfur. 1,3,2-Oxazaphospholes 8 and 1,3,2-thiazaphospholes 9 are mainly known as benzo and fused derivatives. In CHEC-II(1996), only the chemistry of 4,5-dicyano-l,3,2-diazaphosphole, 1,3,2-benzo-diazaphospholes, 1,3,2-benzoxazaphospholes, and 1,3,2-benzothiazaphospholes is discussed <1996CHEC-II(4)771>. No significant new results on the chemistry of 1,3,2-oxaza- and 1,3,2-thiazaphospholes have been reported since the publication of CHEC-II(1996). 

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