Pyridinium bromides 3 prepared by alkylation of 2-methyl-, 2-ethyl-, and 2-benzylpyridines 1 with methyl bromides 2 bearing an electron-withdrawing group (COPh, CN, C02Et, C6H4NO2) are condensed with PC1, in the presence of Et3N to give 2-phosphaindolizines 4. The 1-unsubstituted representatives 9 (prepared from 8) may undergo a substitution reaction at this position with an excess of PC13 1-and 2-phosphaindolizines are 1,3-azaphospholes with a pyridine ring a-annuAzaphospholes constitute a relatively recent group of fivemembered aromatic heterocycles'). Among these systems representatives with and without a nitrogen atom adjacent to the phosphorus atom in the ring may be distinguished. While the former can usually be prepared by a condensation reaction using PC1, or P(NMe2),, the latter (1,3-azaphospholes and 1,2,4-diazaphospholes) are obtained by condensation with phosphines or silylphosphines or by cycloaddition reactions. Thus, the monocyclic 1,3-azaphospholes as well as their benzo and pyrido derivatives B7-12) and D4) have been prepared. The latter may be viewed as 1 -phospha analogs of indolizines E.are not yet known. In this paper we report on their synthesis by means of a PC1, c~ndensationl~). This is the first example of a heterophosphole with carbon atoms on both sides of the two-coordinate phosphorus atom in the ring being formed in this way27). As the positions 1 and 2 of the indolizine ring are different in ~haracter'~) the phosphorus ring member in these positions and hence the systems D and C should differ characteristically, e.g. in their NMR spectra and in their reactivity. SynthesisThe PCI3 condensation of a suitable four-membered chain presents a facile route to heteropho~pholes'~). For the synthesis of 2-phosphaindolizines of type C 1,2-dialkylpyridilated to the 1,2 and 1,5 bond, respectively. The different types of annulation result in characteristic differences in charge distribution, 31P-NMR shift, and chemical behavior. 2-Phosphaindolizines 4 and 9 are stable in dry air and do not undergo alkylation. Compounds 9 hydrolyze to give zwitterionic ' (1-alkyl-2-pyridino)methylphosphinates 13. The phosphorus atom of a 2-phosphaindolizine can act as a ligand atom as shown by the example of a Cr(CO), complex.nium salts 3 offer themselves as the starting material. Their 2-methylene group is sufficiently reactive for a triethylarninemediated condensation; its deprotonation results in a stable enamine16'. In contrast, the 1-methylene group needs activation by an electron-withdrawing substituent R2. COPh, CN, C02Et, and C6H4N02-4 have been found to be effective for this purpose. The respective pyridinium salts 3 are readily obtained17-19) from 2-ethyl-or 2-benzylpyridine (1 a, c) and phenacyl bromide (2a), bromoacetonitrile (2 b), ethyl bromoacetate (2c), or 4-nitrobenzyl bromide (2d). For the condensation, equimolar amounts of 3 and PC1, are treated with a fourfold molar amount of Et3N in toluene at ambient temperature or preferably in acetonitrile at 0 -5°C. The reaction is usually complete within...
Regioselectivity / 31P-, lH-, 13C-NMR spectraThe [3 + 21 cyclocondensation of 2-amino-1,3-thiazoline, 2-aminopyridines, 2-and 4-aminopyrimidines, 2-aminopyrazine, and 2-aminoquinoline with chloromethyldichlorophos- [',*]. These syntheses often parallel those of azoles: When carboxylic acid derivatives are used for condensation to yield azoles, phosphorus trihalides or trisamides take their part in azaphosphole syntheses. When acetylenes are employed in the synthesis of azoles involving cycloaddition, phosphaalkynes are used instead for azaphospholes. A further route to azaphospholes by a condensation reaction may be understood as a parallel of the Hantzsch thiazole synthesis, with the part of the ahaloketone played by chloromethyldichlorophosphane. Starting from thioamides, we thus obtained 1,3,4-thiazaphospholes and starting from amidines, we obtained 1,4,2-diazaphospholes. Only this route does give access to representatives containing an unsubstituted carbon ring member adjacent to phosphorus (i.e. in the 5-or 3-position)r31. The present paper describes the synthesis of anellated 1,4,2-diazaphospholes starting from heterocycles containing an amidine system, i.e. 2-aminopyridines and 2-amino-l,3-thiazoles, and in one case a 2-aminothiazoline. SynthesesThe condensation of equimolar amounts of 2-amino-l,3-thiazoline with chloromethyldichlorophosphane in the presence of a threefold molar amount of triethylamine in acetonitrile proceeds regiospecifically to give 5,6-dihydro
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