3- Acyl- diazaphospholes

A -Unsubstituted 1,2,4-diazaphospholes (4) undergo A -alkylation by reaction with alkyl vinyl ether, sulfur ylides, and diazo compounds <95HAC403>. They react with acyl chlorides in a 2 1 molar ratio to give a mixture of the A -acylated diazaphosphole and the diazaphosphole hydrochloride. Preparative A -acyclation is achieved in presence of a tertiary amine. Sulfonyl chlorides and phosphorus trichloride also give A -substitution reactions (Scheme 2) <87TH 422-01 >. 

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],... 

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... 

Kerth and Maas <1999EJ02633> have studied the reaction between (/-diazo-2-oxoalkyl)silanes and 2-acyl-l,2,3-diazaphospholes. These form the [3+2] cycloadducts 124a-l. When heated to 80-100°C, they lose molecular... 

Similar to their reaction with phosphaalkenes, l-diazo-2-(oxoalkyl)silanes 29 react with various heterophospholes by [3 + 2] cycloaddition of the diazocumulene system 30 (which is in equilibrium with 29) across the P=C bond. With 2-acyl-1,2,3-diazaphospholes 119 (R = Ac, Bz no reaction with R = Me, Ph up to 60 °C), the expected cycloaddition products 120 (Scheme 8.27) could be isolated (186). Elimination of N2 from these bicyclic A -pyrazolines occurred upon heating at 100 °C and furnished the tricyclic systems 122 when SiRs was a trialkylsilyl group. Apparently, the thermolysis of 120 generates the 5-aIkenylidene-l,2,5-diazaphosphole 121 (by N2 extrusion) as well as diazaphosphole 119 (by a [3 + 2] cycloreversion process), which recombine in an intermolecular cycloaddition to furnish 122. When SiRa = SiPhaf-Bu, a formal intramolecular [3 + 2] cycloaddition of the C=P=C unit with an aromatic C=C bond occurs and the polycyclic compound 123 is obtained (187). 

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). 

The molecular geometries and the frontier orbital energies of heterophospholes 28-31 were obtained from density functional theory (DFT) calculations at the B3LYP/6-311- -G, level. The 1,3-dipolar cycloaddition reactivity of these heterophospholes in reactions with diazo compounds was evaluated from frontier molecular orbital (FMO) theory. Among the different types of heterophospholes considered, the 2-acyl-l,2,3-diazaphosphole 28, 377-1,2,3,4-triazaphosphole 30, and 1,3,4-thiazaphosphole 31 were predicted to have the highest dipolarophilic reactivities. These conclusions are in qualitative agreement with available experimental results <2003JP0504>. 

Kerth and Maas have reported reactions of 2-acyl-l,2,3-diazaphospholes 73 with diazo ketones 71 to form bicyclic compounds 74, the products of a 1,3-dipolar cycloaddition reaction of diazoalkenes 72, which are in equilibrium with diazo ketones 71 (Scheme 3) <1999EJ02633>. 

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