We then explored the nucleophilic reactivity of 3a, 3b and 5 toward CO2 as an electrophile (Figure ). It is well-known that phosphine imides react with CO2 to give phosphine oxides (O=PR3) and isocyanates (R′-N=C=O). In such reactions,  5 -oxazaphosphetane species have been proposed as a key intermediate. However, there have been no reports on the direct observation and isolation of a  5 -oxazaphosphetane species from the reaction of a phosphine imide and CO2 (or other heterocumulenes). Thus, demonstrating direct evidence for such species in the transformation of CO2 is valuable for the fundamental understanding of the corresponding reaction mechanism. Treatment of 3a with CO2 (5 atm) at room temperature resulted in the quantitative formation of  5 -oxazaphosphetane 6a within 20 minutes, which was isolated in >99% yield. When 3b was subjected to the identical conditions, 6b was quantitatively obtained after a period of 1 h, and was also isolated in 95% yield. No reaction occurred between 5 and CO2 even at 60 °C after a period of 3 days. In these reactions, we used α,α,α-trifluorotoluene (TFT) as a solvent because 3a is sufficiently soluble in TFT, but the formation of 6a and 6b also proceeded in C6D6 and CD2Cl2.