2‐Phosphaindolizines

Abstract

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...

1.4.2-Diazaphospholo[4,5-a]pyridine / 1.4.2-Diazaphospholo[4,5-a]pyridines

“…Die 2-Am inopyridiniumbrom ide l a -d reagie ren mit einem Äquivalent PC13 und der vierfach molaren Menge N E t3 zu den Diazaphospholopyridinen 3 a -d : Wie bei anderen Azaphospholsynthesen [4,5,8], so ist auch hier für den Ringschluß ein elektronen ziehender Substituent R 1 erforderlich, wie der hier verwendete C 0 2Et-Rest. Für R 1 = Ph, 4-MeC6H 4 bleibt dagegen die Reaktion auf der Stufe der Aminodichlorphosphane 2e, f stehen [9], welche sich in guter Ausbeute in Reinsubstanz isolieren lassen.…”

“…Wie wir vor kurzem gezeigt haben, liefert die Cyclokondensation von 2-Alkylpyridinium-bzw. 2-Aminothiazoliumbromiden mit PC13 1,3-Azaphospholo[l,5-a]pyridine (2-Phosphaindolizine) A [4] bzw. Thiazolo [3,2-<i][l,4,2]diazaphosphole B [5] in guten Ausbeuten.…”

2-phosphaindolizines

“…The compounds represent heteroaromatic 10p-or, in the case of partly hydrogenated annulated rings, 6p-systems [3,4], as shown by characteristic NMR data and, for 2-phosphaindolizines, by MNDO calculations [1], *Present address: Department of Chemistry, University of Rajasthan, Jaipur, India.…”

“…The first synthesis of 2-phosphaindolizine by [4 ‫ם‬ 1]cyclocondensation of 1,2-dialkylpyridinium salts with PCl 3 [1,2] led to the development of an advantageous synthetic method for the preparation of a variety of annulated azaphospholes. The compounds represent heteroaromatic 10p-or, in the case of partly hydrogenated annulated rings, 6p-systems [3,4], as shown by characteristic NMR data and, for 2-phosphaindolizines, by MNDO calculations [1], *Present address: Department of Chemistry, University of Rajasthan, Jaipur, India.…”

Synthesis of [1,2,4,3]Triazaphospholo[1,5-a]pyridines+

“…A gen eral approach involves the [4 + l]-cyclocondensation of 1,2-disubstituted pyridinium salts with a phosphorus reagent like phosphorus trichloride or tris(dimethylamino)phosphine. Following this m ethod, we have synthesized 2-phosphaindolizines [2,3], l-aza-2-phosphaindolizines [4] and 3-aza-2-phosphaindolizines [5].…”