An Annelated Mesoionic Carbene (MIC) Based Ru(II) Catalyst for Chemo- and Stereoselective Semihydrogenation of Internal and Terminal Alkynes

Abstract

The catalytic utility of [RuL1(CO)2I2] (1), containing an annelated π-conjugated imidazo-naphthyridine-based mesoionic carbene (MIC) ligand (L1), is evaluated for E-selective alkyne semihydrogenation. The precatalyst 1, in combination with 2 equiv of AgBArF, semihydrogenates a broad range of internal alkynes with molecular hydrogen (5 bar) in water. (E)-Alkenes are accessed in high yields, and a number of reducible functional groups are tolerated. A chelate MIC ligand and two cis carbonyls provide a well-defined platform at the Ru center for hydrogenation and isomerization. The loss of two iodides and the presence of two carbonyls render the Ru center electron deficient and thus the formation of metal vinylidenes with terminal alkynes is avoided. This is leveraged for the semihydrogenation of terminal alkynes by the same catalytic system in isopropyl alcohol. Reaction profile, isomerization, kinetic, and DFT studies reveal initial alkyne hydrogenation to a (Z)-alkene, which further isomerizes to an (E)-alkene via metal-catalyzed Z → E isomerization.

Mechanistic Investigations on Hydrogenation, Isomerization and Hydrosilylation Reactions Mediated by a Germyl‐Rhodium System

“…We decided to explore whether catalyst 2 could mediate cis / trans olefin isomerization in the absence of dihydrogen (Table 3). Heating a benzene solution of cis‐ stilbene in the presence of compound 2 (10 mol%) yielded indeed full conversion into the corresponding trans ‐olefin despite the absence of H 2 , which contrasts with most previously reported systems that require its presence [16f,g,18b,26] . Other cis ‐olefins were also isomerized with good yields and, contrary to our expectations, the introduction of a bulky tert ‐butyl group in CH 3 CH=CH( t Bu) compared to CH 3 CH=CHCH 2 CH 3 enhanced the isomerization rate (entries 3 and 4).…”

Enantiopure 2,9‐Dideuterodecane – Preparation and Proof of Enantiopurity

“…13 C{ 1 H} NMR (75 MHz, CDCl 3 ): δ =14.09 (CH 3 ), 22.34 (CH 2 ), 31.25 (CH 2 ), 33.65 (CH 2 ), 114.24 (CH 2 ), 139.37 (CH) ppm. All spectroscopic data were in accordance with the literature [41] . C 6 H 12 (84.16).…”

“…13 C{ 1 H} NMR (75 MHz, CDCl 3 ), ( E )‐isomer: δ =14.13 (2×CH 3 ), 22.36 (2×CH 2 ), 32.00 (2×CH 2 ), 32.45 (2×CH 2 ), 130.46 (2×CH) ppm; ( Z )‐isomer: δ =14.16 (2×CH 3 ), 22.51 (2×CH 2 ), 27.06 (2×CH 2 ), 32.13 (2×CH 2 ), 130.00 (2×CH) ppm. All spectroscopic data were in accordance with the literature [41,43] . C 10 H 20 (140.27)…”

Controlled selectivity through reversible inhibition of the catalyst: Stereodivergent semihydrogenation of alkynes

“…This translates into a TOF of more than 1000 h -1 , which, to the best of our knowledge, represents the most efficient trans-semihydrogenation of any alkyne reported to date. [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][46][47][48][49][50][51][52][53][54][55][56][57][58][59][60][61] As the reaction progressed, a clear color change from brown-red to yellow was observed, with the latter being that of Ru-1, and this visible change could be used as a reaction indicator. 7 Notably, only negligible over-reduction into alkane 3a was observed (<1 %), possibly due to the very mild conditions employed.…”

“…[1][2] For example, the trans-selective catalytic semihydrogenation of alkynes typically begins with cis-hydrogenation, but the generated (Z)-alkene is only a kinetic intermediate, which is then rapidly isomerized into the thermodynamically more stable (E)-alkene product (Figure 1b). [3][4][5][6][7][8][9] If a strategy can be devised to slow down a specific reaction step, such as the Z-to-E isomerization in the trans-semihydrogenation of alkynes, the reactive intermediates can be stabilized, and may even be isolable as end-products from the same system, which would be of great interest and is highly-advantageous. Nevertheless, the state-of-the-art methodologies to selectively access both (E)-and (Z)-alkenes rely on the transfer-semihydrogenation of alkynes using different catalysts.…”