1,3‐azaphospholo[5,1‐a]isoquinolines

1,5‐electrocyclization versus 1,5‐proton shift in imidazolium allylides and 2‐phospha‐allylides: a DFT investigation

“…The advancement of the C8-C2 bond in TSE 7 is accompanied by C6-C7 and C7-C8 bonds acquiring partial double and single bond characters, respectively, which develop finally into the typical carbon-carbon double bond ($1.347 AE 0.001) and single bond ($1.494 AE 0.012) in the products. A similar trend is observed in the 1,5-electrocyclization of imidazolium 2-phosphallylides (8). The NBO results show that s(C8-C2) in the transition structures, TSE 7,8 is occupied by $1.8e and the rest is delocalized into the p * (C6-C7) or p * (C6-P7) orbital resulting in the second order perturbative ''donor-acceptor'' interactions.…”

“…Nevertheless, the formation of the intermediate 4 could be confirmed by a cross-reaction experiment. [8] In some cases, it has been possible to isolate the dihydro intermediate depending upon the nature of the substituents present in the allylic part. The intermediate so obtained changed in indolizine on oxidation.…”

“…The N-C bonds to the carbene center in PrC 7,8 are longer by 0.01-0.05 Å than in the corresponding imidazolium allylides and 2-phosphallylides, which agrees with the earlier results. [12] The NBO studies of TSC 7,8 show that except in TSC 7b (X ¼ F), when formation of the s(C8-H9) is already significantly advanced (occupancy 1.74e) accompanied by the development of the lone pair at C2 (occupancy 1.49e), in all other cases, s(C2-H9) is still occupied by $1.90e, although a strong second order perturbative ''donor-acceptor'' interaction develops between LP(C8) ($1.40e) and s * (C2-H9) (occupancy $0.30e).…”

“…[6,7] The above method could be extended subsequently to the isoquinolinium 2-phosphaallylides that produced [1,3]azaphospholo [5,1-a]isoquinolines. [8] 3-Imidazolium [9] and benzimidazolium [10] allylides have been found to show similar behavior and undergo intramolecular 1,5electrocyclization to produce pyrrolo [1,5-a]imidazole and -benzimidazole derivatives, respectively. In contrast to the generation of pyridinium 2-phosphaallylides and their subsequent 1,5-electrocyclization, our attempt to obtain 1,3-azaphospholo [1,5-a] imidazole from the reaction of 3-(ethoxycarbonylmethyl)-1-methylimidazolium bromide with phosphorus trichloride in the presence of triethylamine, however, failed, although the formation of a phosphorus-containing species was detected by 31 P NMR spectroscopy of the reaction mixture (d 31 P ¼ 178.2).…”

“…3-Imidazolium allylides (7) and 2-phosphaallylides (8), oxygen of the carbonyl function in the pyridinium phenacylides, [30] it might be expected that C2-H of the imidazolium allylides and 2-phosphaallylides would undergo intramolecular 1,5-shift leading to the generation of an imidazol-2-ylidene carbene (11) and its 2-phospha analogue (12), respectively (Path B, Scheme 3). Furthermore, the latter might react intramolecularly in S N 2 manner involving the attack of the nucleophilic carbene center on C8, if a suitable leaving group is present on it.…”