Stereoselective Transfer Semi‐Hydrogenation of Alkynes to E‐Olefins with N‐Heterocyclic Silylene–Manganese Catalysts

Abstract

The synthesis and structures of the first Si -donor supported manganese(II) complexes [L1]MnCl , [L2]MnCl , and [L3] MnCl are reported, bearing a pincer-type bis(NHSi)-pyridine ligand L1, bidentate bis(NHSi)-ferrocene ligand L2, and two monodentate NHSi ligands L3 (NHSi = N-heterocyclic silylene), respectively. They act as unprecedented very active and stereoselective Mn-based precatalysts (1 mol % loading) in transfer semi-hydrogenations of alkynes to give the corresponding E-olefins using ammonia-borane as a convenient hydrogen source under mild reaction conditions. Complex [L1]MnCl shows the best catalytic performance with quantitative conversion rates and excellent E-stereoselectivities (up to 98 %) for different alkyne substrates. Different types of functional groups can be tolerated, except CN, NH , NO , and OH groups at the phenyl group of 1-phenyl substituted alkynes.

Recent Advances in Hydrometallation of Alkenes and Alkynes via the First Row Transition Metal Catalysis

“…[330] However, research of hydrometallation reactions based on manganese catalysts has been scare. [334] The stereoselectivity of the reaction was controlled by the combination of Manganese salts and ligands. [331] Scheme 120 Mn-catalyzed hydroboration of terminal alkenes Manganese carbonyl complex [Mn 2 (CO) 10 ] was ever used as catalyst for hydrogenation [332] and hydrosilylation [333] of alkenes.…”

Isolable Silylene Ligands Can Boost Efficiencies and Selectivities in Metal‐Mediated Catalysis

“…Chemie borylation of less sterically hindered benzene and toluene,r espectively.F urther analysis based on the Dewar-Chatt-Duncanson model suggests the trend of s donor strength of ligands is in line with the catalytic performance of corresponding Ir complexes, namely:SiCSi (10) > GeCGe (31) > PCP (32) > PCP (33). [32] Recently,asilylene promoted catalytic activity was also demonstrated in the Ni-catalyzed amination reactions reported by our group,u sing the Ni complexes 13 a and 13 b (Scheme 7) based on the 1,2-bis-NHSi-substituted ortho-carborane L15.T he CO stretching vibration modes of 15 b indicate that the Si II atoms in the ligand are even stronger s donors than the P III atoms in phosphine and C II atoms in analogous N-heterocyclic carbene complexes.M oreover, the strong donor character of the ligand enables corresponding Ni complex to act as an outstanding precatalyst (0.5 mol %l oading) in the catalytic aminations of chlorobenzene and morpholine (93 %), surpassing the activities of bis-phosphinyl ortho-carborane analogues 34 (66 %) and 35 (63 %), respectively (Scheme 12).…”

“…Very recently,t he manganese complexes 24, 25,a nd 26 (Scheme 9) stabilized by different silylene ligands were reported by us. [33] Complexes 24, 25,a nd 26 were the first manganese complexes catalyzing the (transfer) hydrogenation of unsaturated hydrocarbons.C omplex 24 gave the best performance in stereoselective transfer semi-hydrogenation of alkynes using ammonia-borane (NH 3 BH 3 ). Thec atalytic transfer semi-hydrogenation of different alkynes yielded corresponding (E)-alkenes as main products (Scheme 10 d; further details below).…”

“…Scheme 9d epicts the hitherto known silylene complexes which have been successfully applied in catalytic reduction reactions.The complexes 18 a, [14a] 20, [15] 21, [19] 24-28 [13,33] were synthesized by the straightforward ligand exchange reactions of the corresponding metal precursors with the respective isolable silylene ligands.C omplexes 18 b and 18 c were obtained through salt metathesis reactions of 16 with Li-[HBEt 3 ]a nd MeLi, respectively. [14a] Thec omplexes 19 a and 23 were synthesized by the reduction of the corresponding dihalide precursors in the presence of PMe 3 and C 6 H 6 , respectively.…”

“…[29] Very recently,t he Si II -donor supported manganese(II) complexes were found to act as stereoselective precatalysts (1 mol %loading) in transfer semi-hydrogenations of alkynes with ammonia-borane as hydrogen source.O ptimization experiments using 1-phenyl-1-hexyne suggest that complex 24 (Scheme 9) bearing apyridine based bis-silylene ligand led to the highest yield of E-olefins under mild reaction conditions.I tt urned out that the pincer-type diphosphine pyridine analogue 45 containing ligand 43 was also active but led to relatively low conversion rates (43 %) and E-selectivity (49 %; Scheme 18). [33]…”