Calcium Hydride Catalyzed Highly 1,2‐Selective Pyridine Hydrosilylation

Abstract

Reaction of the calcium hydride complex (DIPPnacnac-CaH⋅THF)2 with pyridine is much faster and selective than that of the corresponding magnesium hydride complex (DIPPnacnac = [(2,6-iPr2 C6 H3 )NC(Me)]2 CH). With a range of pyridine, picoline and quinoline substrates, exclusive transfer of the hydride ligand to the 2-position is observed and also at higher temperatures no 1,2→1,4 isomerization is found. The heteroleptic product DIPPnacnac-Ca(1,2-dihydropyridide)⋅(pyridine) shows fast ligand exchange into homoleptic calcium complexes and therefore could not be isolated. Calcium hydride reduction of isoquinoline gave well-defined homoleptic products which could be characterized by X-ray diffraction: Ca(1,2-dihydroisoquinolide)2 ⋅(isoquinoline)4 and Ca3 (1,2-dihydroisoquinolide)6 ⋅(isoquinoline)6 . The striking selectivity difference in the dearomatization of pyridines by Mg or Ca complexes could be explained by DFT theory and was utilized in catalysis. Whereas hydroboration of pyridine with pinacol borane with a calcium hydride catalyst gave only minor conversion, the hydrosilylation of pyridine and quinolines with PhSiH3 yields exclusively 1,2-dihydropyridine and 1,2-dihydroquinoline silanes with 80-90 % conversion. Similar results can be achieved with the catalyst Ca[N(SiMe3 )2 ]2 ⋅(THF)2 . These calcium complexes represent the first catalysts for the 1,2-selective hydrosilylation of pyridines.

Ring‐Shaped Phosphinoamido‐Magnesium‐Hydride Complexes: Syntheses, Structures, Reactivity, and Catalysis

“…Heating to 60 °C for 15 h lead to the clean rearrangement of 11 to the 1,4‐dihydropyridide derivative 13 [LMg(py) 2 (1,4‐dhp)] (Scheme ). An analogous rearrangement is not possible for 12 under these conditions; a similar observation has previously been noted for other Group 2 metal–hydride systems . The molecular structure of 12 (Figure ) shows a distorted tetrahedrally coordinated Mg 2+ ion, which binds to a terminal κ 1 ‐ N ‐phosphinoamide ligand, a 4‐dimethylamino‐1,2‐dihydropyridide ligand (from formal 1,2‐addition of an MgH unit onto the DMAP pyridine ring), and two neutral, coordinating DMAP molecules.…”

Lithium Dihydropyridine Dehydrogenation Catalysis: A Group 1 Approach to the Cyclization of Diamine Boranes

“…The research groups of Harder,8 Hill,9 Wright,10 and Okuda,11 amongst others,5 have reported main‐group d 0 complexes that are active in stoichiometric and catalytic dehydrocoupling processes; these complexes have contained mostly Group 2 or 13 metals (e.g., Mg, Ca, or Al) and bulky β‐diketiminato or (silyl)amide ligands. Despite this variety of active precatalysts, Group 1 based complexes have remained largely underexplored, which is surprising, since Group 1 precatalysts offer the advantage over Group 2 complexes that they are not involved in Schlenk‐type equilibria,3d though they can engage in aggregation phenomena.…”

Early Main Group Metal Catalysts for Imine Hydrosilylation

“…1 HNMR (400 MHz, C 6 D 6 ): d = 7.21-7.11( m, 10 H), 7.01 (m, J = 7.4 Hz, 2H), 6.96-6.86 (m, 1H), 6.49 (t, J = 7.1 Hz, 1H), 6.40 (m, J = 7.9 Hz, 2H), 4.95 (s, 1H), 4.10 (s, 2H), 3.26 (hept, J = 6.8 Hz, 4H), 2.96-2.79 (m, 4H), 1.77 (s, 6H), 1.22 (d, J = 6.8 Hz, 12 H), 1.17 (d, J = 6.9 Hz, 12 H), 0.99-0.92 (m, 4H)ppm. 13…”

“…[11] Although early main-group-metal catalysis has made substantialp rogress, [12] it is surprising that hitherto imine hydrosilylation has not been reported.T he closest to imine hydrosilylation is the recently described catalytic dearomatization of the C=Nb ond in pyridine by 1,2-selective hydrosilylation using a calcium catalyst. [13] In contrast, magnesium catalysts showed no activity in pyridine hydrosilylation but performed well in its hydroboration. [14,15] In this work we present the first investigations on imine hydrosilylationu sing early main group metal catalysts.…”