A comparison of different phosphines showed that P(n-Bu)3 afforded 76% of 3a with the same Z/E ratio as PPh3 (Entry 2), whereas sterically congested P(t-Bu)3 proved ineffective, yielding only 12% of the product (Entry 3). In turn, different aromatic phosphines revealed no drastic effect on efficiency and selectivity (Entries 4-7) when compared to the superior catalyst (Entry 1). The reaction also proceeds in various solvents like acetonitrile, tetrahydrofuran, and toluene, however, with reduced yield in a range of 77-84% (Entries 8-10). Kinetic studies (Scheme 3c) revealed that the PPh3-catalyzed reaction is completed within 4 h, achieving the final ratio of Z/E stereoisomers, which remains constant over a longer period of time. Interestingly, the initial formation of the E isomer is faster than Z-alkene. However, the Z/E ratio increases during the reaction course (see Figure in Supporting Information), suggesting that the final composition of stereoisomers represents a thermodynamic equilibrium. Overall, the above findings demonstrate that phosphine-catalyzed allene hydrocyanation is not only a promising subprocess for our envisaged dual-catalytic system, but it is also an intrinsically valuable synthetic transformation.