4/7-1,2,4-Diazaphospholes, from

In the past, several general reviews on heterophospholes [2, 4-6] have covered the chemistry of diazaphospholes reported until early 1990s. Synthetic routes to different classes of diazaphospholes have been reviewed [7], Analogy between the synthesis of anellated diazaphospholes and their nonphosphorus analogs has also been reviewed [8], Some recent advances in the chemistry of anellated diazaphospholes made by our group have been included in a review on anellated azaphosp-holes [9], The reviews on the synthetic applications of phosphalkynes also describe the synthesis of diazaphospholes from these synthons [10-12], Present review aims to highlight and update important reports related to the chemistry of diazaphospholes which have mainly appeared during the last 15 years. 

Scheme 10 Synthesis of [l,2,4]diazaphosphole from transient phosphaalkene...

Scheme 19 Synthesis of monocyclic [l,4,2]diazaphospholes from [3+2] cyclocondensation...

The 3//-l,2,4-diazaphospholes formed from the reaction of diazomethane and its monosubstituted derivatives (R CH=N2 R = H, alkyl, aryl, acyl, phosphoryl) could not be isolated due to a rapid 1,5-H shift leading to 27/-l,2,4-diazaphospholes 227. When diazo(trimethylsilyl)methane or [bis(diisopropylamino)phosphino]dia-zomethane was used, the l,5-SiMe3 [or PR2, R = N(/-Pr)2] shift completely dominates over the H shift (289,290). In the case of open-chain or cyclic a-diazoketones, cycloadducts 228 cannot be isolated due to rapid acyl shifts giving 229 and ultimately 230 (289). This transformation offers a versatile method to prepare [h]-fused 1,2,4-diazaphospholes from cyclic a-diazoketones and phos-phaalkynes (289). 

Analogous to the synthesis of anellated imidazoles [48-50], a method has been developed for the preparation of anellated [l,4,2]diazaphospholes from [3 + 2] cyclocondensation of 2-aminocycloimines with chloromethyldichlorophosphine in the presence of Et3N (Scheme 12) [51]. 

Carbon-phosphorus double bonds are also formed in addition reactions of tris(trimethylsilyl)phosphine 1692 (which can be readily prepared from white phosphorus, sodium, and TCS 14 [13a,b,c]) to give oxazohum fluorides 1691 which then give the azaphospholes 1694, via 1693 [3, 14]. On addition of 1692 to 1695, the diazaphosphole 1696 [3, 15] is prepared, whereas l,3-azaphospholo[l,2a]pyridines 1698 [16] are formed from 1692 and 1697, and 1,3-thiaphospholes 1700 are formed from the dithiohum fluorides 1699 [17]. l,3-Benzodiphospholyl anions 1703 are generated by reaction of acid chlorides with the dihthium salts 1701, via 1702 [18] (Scheme 11.3). 

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

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],... 

Monocyclic 2H-[ 1,2,3 Idiazaphospholes (B) are easily accessible from the condensation of the four-membered chain incorporated in hydrazones or azoalkanes with phosphorus trichloride making available a large number of representatives that have been intensively studied [2, 4, 7], In contrast, their 1//-isomers (A) are less known and are obtained only as second minor product during the synthesis of 2//-[l,2,3]diazaphospholes in some cases. A facile synthesis for pyrido-anellated azaphospholes has been developed in our group by making use of 1,2-disubstituted pyridinium salts for condensation with phosphorus trichloride [8, 13-15], Accordingly, cyclocondensation of 2-alkyl-1-aminopyridinium iodides (1) with phosphorus trichloride in the presence of triethylamine affords pyrido-anellated l//-[l,2,3]diazaphospholes, i.e. l,2,3]diazaphospholo 1,5-a] pyridines (2) (Scheme 1) [16],... 

Baccolini et al. developed a novel method for the synthesis of 2-phenyl derivatives of 2//-[l,2,3 Idiazaphospholcs [17], which are otherwise difficult to obtain from the condensation of hydrazones and PC13. Fused benzothiadiphosphole 4 was used as a phosphorus furnishing reagent in its reaction with conjugated phenylazoalkenes 3 to obtain 2-phenyl-[l,2,3]diazaphospholes (6, R =Ph) via intermediacy of a spiro-cyclic adduct 5 (Scheme 2). 

On the other hand, 3,5-disubstituted [l,2,4]diazaphosphole 29 could be obtained from the reaction of tri-ferf-butylphenylphosphaethyne (25) with freshly prepared lithiated trimethylsilyldiazomethane (30) and subsequent treatment of the initially formed diazaphospholide ion (31) with trifluoroacetic acid (Scheme 9) [33],... 

Denis and coworkers [37] could trap transient simple phosphaalkene 33, generated by HC1 elimination from chlorophosphine 32 at low temperatures [38], by ethyl diazoacetate at -50°C. Aromatization of the initially formed isomeric E-/Z-cycloadducts 35 to [l,2,4]diazaphospholes 36 could be achieved via P-chlorination/ dehydrochlorination sequence at -30°C using /V-chIorosuccinimidc (Scheme 10). 

Previous attempts to synthesize benzo[l,3,2]diazaphosphole (59) in an analogous manner from benzene- 1,2-diimines by Malavaud et al. led to tetra-meric cyclo-phosphazanes, which existed in equilibrium with monomer only at... 

Diazaphospholes are known to undergo facile 1,3-dipolar cycloaddditions with a variety of dipoles [2, 4, 7, 98], During recent years, some interesting [2+3] cycloaddition reactions have been reported. 2-Acyl-[l,2,3]diazaphospholes 6 were reported to undergo [2+3] cycloaddition with diazocumulene 92, the minor equilibrium isomer of a-diazo-a-silyl ketones 91, to form a bicyclic cycloadduct 93 (Scheme 29). Thermolysis of the cycloadduct results in the formation of tricyclic phosphorus heterocycle 94, which can be explained due to the possibility of two parallel reactions of cycloadduct. On the one hand, extrusion of molecular nitrogen from 93... 

Recently, [2+3] cycloaddition reaction of 2-acetyl-[l,2,3]diazaphosphole (6) with 9-diazofluorenes (96) has been reported [105, 106], From the reaction in cyclohexane at rt, bicyclic phosphirane 97 was obtained as a result of the loss of nitrogen from the initial cycloadduct (Scheme 30). The cycloadduct, 3-spiro substituted 3H-[l,2,4]diazaphospholo-fused [l,2,3]diazaphosphole (98) could be isolated in good yield at room temperature in one case (R=/Bu) its stability was assigned to the presence of bulky fert-butyl group at 7-position. Use of polar solvent like dichloromethane led to the cyclic trimeric compound 99 (Scheme 30). 

Zheng et al. treated potassium [l,2,4]diazaphospholides, obtained from the reaction of 3,5-disubstituted-[l,2,4]diazaphospholes with metallic potassium in THF, with [Cp RuCl]4 to afford [(T75-dp)RuCp ] type pseudoruthenocene complex (106) (Scheme 33). Sandwich structure with almost eclipsed orientation of two n-bonded ligands has been confirmed by X-ray crystal structure determination [110], Catalytic application of [(rf-dp)RuCp ] complexes in the Heck reaction has also been investigated [111]. 

Scheme 35 Transition metal complexes from 2,5-dimethyl-4-phosphino-[l,2,3]diazaphosphole...

A great variety of heterophospholes have been developed during the last quarter of a century, from 1970 to 1995. They are covered in chapters 3.15 and 3.16, and in their majority in this chapter. The first representatives of heterophospholes, i.e. 2/f-l,2,3-diazaphospholes were found in 1967 <67CHE60i > and remained the only examples for seven more years. They are also the only heterophospholes in this chapter which are mentioned in the first edition of Comprehensive Heterocyclic Chemistry (CHEC-I) <84CH(l)523>. 

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). 

From the thiaza- and diazaphosphole molecular dimensions aromaticity indices were derived <90X5697). 

Table 2 Bond lengths (pm) in 2H- and l//-l,2,3-diazaphosphole rings (G, H) and bond angle(s) at the phosphorus atom derived from x-ray or electron diffraction.

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