posted on 2024-01-04, 07:13authored byMaría Gutiérrez-Blanco, Andrés G. Algarra, Eva Guillamón, M. Jesús Fernández-Trujillo, Mónica Oliva, Manuel G. Basallote, Rosa Llusar, Vicent S. Safont
Semihydrogenation
of internal alkynes catalyzed by the
air-stable
imidazolyl amino [Mo3S4Cl3(ImNH2)3]+ cluster selectively affords the
(Z)-alkene under soft conditions in excellent yields.
Experimental results suggest a sulfur-based mechanism with the formation
of a dithiolene adduct through interaction of the alkyne with the
bridging sulfur atoms. However, computational studies indicate that
this mechanism is unable to explain the experimental outcome: mild
reaction conditions, excellent selectivity toward the (Z)-isomer, and complete deuteration of the vinylic positions in the
presence of CD3OD and CH3OD. An alternative
mechanism that explains the experimental results is proposed. The
reaction begins with the hydrogenation of two of the Mo3(μ3-S)(μ-S)3 bridging sulfurs to
yield a bis(hydrosulfide) intermediate that performs two sequential
hydrogen atom transfers (HAT) from the S–H groups to the alkyne.
The first HAT occurs with a spin change from singlet to triplet. After
the second HAT, the singlet state is recovered. Although the dithiolene
adduct is more stable than the hydrosulfide species, the large energy
required for the subsequent H2 addition makes the system
evolve via the second alternative pathway to selectively render the
(Z)-alkene with a lower overall activation barrier.