Alkynes are useful building blocks in organic synthesis that allow for a plethora of transformations. Among them, the transition-metal catalyzed semi-hydrogenation of alkynes to alkenes is a very important reaction in organic chemistry. [1] It has been extensively used for the synthesis of biologically important molecules, such as natural products, pharmaceuticals, and fragrances, because many of these molecules have carbon-carbon double bonds with defined Z or E configurations. [2] Among the various catalytic (heterogeneous or homogenous) and non-catalytic methods [1,3] , hydrogenation by using the Lindlar catalyst [4] is probably the most facile procedure for obtaining Z-alkenes. The Z selectivity arises from the addition of the two hydrogen atoms of H 2 suprafacially to the p system of the alkyne by a sequence of hydrometalation/reductive elimination steps. However, the analogous transformation of the alkyne functionality to the Ealkene remains a major challenge, particularly in late-stage synthesis. In fact, all commonly practiced methods for the direct conversion of alkynes into E-alkenes are stoichiometric in nature. The Birch-type reduction of alkynes by alkali metals (Li, Na) in liquid ammonia is the traditional and powerful method for the synthesis of E-alkenes, [5] but major limitations are the low functional-group tolerance as a result of the harsh reaction conditions, and the stoichiometric amounts of waste generated. Although the use of overstoichiometric amounts of chromium reagents resulted in some improvement in the scope of the E-selective hydrogenation reaction, [6] the use of non-catalytic, toxic reagents, and the generation of copious toxic waste are major problems. In 1999, iridium-catalyzed selective hydrogenation of alkynes to E-alkenes using methanol as the hydrogen donor was described by Tani and co-workers. [7a] In 2002, Trost et al. reported an effective two-step method for the synthesis of Ealkenes, in which the alkyne is first subjected to a rutheniumcatalyzed trans-hydrosilylation [7b] followed by proto-desilylation of the resulting alkenylsilanes with stoichiometric amounts of a fluoride source. In 2005, Shirakawa et al.demonstrated reduction of alkynes with hexamethyldisilane and deuterium oxide to E-1,2-dideuterioalkenes. [7c] Cheng and co-workers [7d] described an efficient palladium-catalyzed reduction of alkynes by using HSiEt 3 and H 2 O as the hydrogen donors, with E-selectivity resulting from isomerization of the nascent Z-alkene to the more stable E-alkene by using a catalytic amount of CuSO 4 . Yin, Han et al. [7e] reported that in their palladium-catalyzed hydrogen-transfer reaction, changing the hydrogen donor from formic acid to 25 % aqueous formic acid also led to the isomerization of Zto E-alkenes. Bargon and co-workers reported an NMR study on ruthenium-catalyzed E-selective semi-hydrogenation that excluded the possibility of Z-E isomerism and confirmed direct E-hydrogenation, thus suggesting a bridged alkynedihydrogen dinuclear ruthenium complex. [7f] Very re...