Further, a series of control experiments with alkyne 10, which is unable to form corresponding allene or substrates not equipped with distal EWGs (11-13) proved that isomerization is critical to afford reactivity (Figure , middle). Similarly, alkene 14 gave no hydrocyanation product, underscoring chemoselectivity of alkyne hydrocyanation (Figure , middle). Drawing from the phosphine-based activation of EWG-conjugated allenes reported in the literature, we propose that the hydrocyanation of 2 proceeds through a cationic phosphine-allene adduct (Figure , bottom). This hypothesis is supported by an experiment in which this intermediate, prepared in the form of chloride salt (15), was used instead of the phosphine catalyst. In this case, the reaction delivered 3a in an almost quantitative yield (Figure , bottom). Additionally, P NMR monitoring under standard conditions supports the formation of such a species, accompanied by the decay of the PPh3 signal (see Figure in Supporting Informations). In turn, experiments conducted in the presence of D2O revealed deuterium incorporation during both the isomerization process and the dual-catalytic reaction (Figure ). The data indicated deuterium labeling at acidic proton sites in 1a and 2a, as well as in product 3a, specifically at the -and -positions, suggesting the involvement of proton transfer processes. A lack of labeling at other sites was observed.