Science of Synthesis

Tobrman, T., Science of Synthesis Knowledge Updates, (2024) 1, 133.

The properties of 1H-phospholes differ significantly from those of the other classes of five-membered heterocycles. For example, furan, thiophene, and pyrrole readily undergo electrophilic aromatic substitution. The ease of electrophilic aromatic substitution correlates with the calculated nucleus-independent chemical shift (NICS) aromaticity for furan, thiophene, and pyrrole (‌Scheme 1‌).[‌9‌] However, 1H-phosphole has a low degree of aromaticity, as illustrated by significantly lower NICS values. This finding is consistent with the known reactivity of P-unsubstituted 1H-phosphole, which undergoes a [1,5]-H shift at room temperature to give the corresponding 2H-phosphole (‌Scheme 1‌).[‌10‌] The unusually low aromaticity of the 1H-phophole is caused by the trigonal pyramidal shape of the tricoordinate phosphorus atom, which prevents the effective overlap of the free electron pair on the phosphorus atom with the π-electrons of the diene part.

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References