Rh(I)/(III)‐N‐Heterocyclic Carbene Complexes: Effect of Steric Confinement Upon Immobilization on Regio‐ and Stereoselectivity in the Hydrosilylation of Alkynes

Abstract

Rh(I) NHC and Rh(III) Cp* NHC complexes (Cp* = pentamethylcyclopentadienyl, NHC=N-heterocyclic carbene = pyrid-2-ylimidazol-2-ylidene (PyÀ Im), thiophen-2-ylimidazol-2ylidene) are presented. Selected catalysts were selectively immobilized inside the mesopores of SBA-15 with average pore diameters of 5.0 and 6.2 nm. Together with their homogenous progenitors, the immobilized catalysts were used in the hydrosilylation of terminal alkynes. For aromatic alkynes, both the neutral and cationic Rh(I) complexes showed excellent reactivity with exclusive formation of the β(E)-isomer. For aliphatic alkynes, however, selectivity of the Rh(I) complexes was low. By contrast, the neutral and cationic Rh(III) Cp* NHC complexes proved to be highly regio-and stereoselective catalysts, allowing for the formation of the thermodynamically less stable β-(Z)-vinylsilane isomers at room temperature. Notably, the SBA-15 immobilized Rh(I) catalysts, in which the pore walls provide an additional confinement, showed excellent β-(Z)-selectivity in the hydrosilylation of aliphatic alkynes, too. Also, in the case of 4aminophenylacetylene, selective formation of the β(Z)-isomer was observed with a neutral SBA-15 supported Rh(III) Cp* NHC complex but not with its homogenous counterpart. These are the first examples of high β(Z)-selectivity in the hydrosilylation of alkynes by confinement generated upon immobilization inside mesoporous silica.

The Influence of Silane Steric Bulk on the Formation and Dynamic Behavior of Silyl Palladium Hydrides

“…In hydrosilylation reactions, the steric hindrance of silane substituents is observed to impact the regio- and stereoselectivity of the reaction for a wide range of metal catalysts including Pt, − Pd, − Ni, Rh, − and other late transition metals. − Nonetheless, despite the proposed role of silane steric hindrance in transformations involving silyl metal hydride intermediates, focused studies on the impact of silane steric bulk in oxidative addition with late transition metals are scarce. In examples of oxidative addition using Ni, , Pd, − and Pt complexes, − tertiary silanes are observed to yield discrete silyl metal hydrides, while reacting the same complexes with secondary or primary silanes gives metal bis­(silyl) species; however, some silyl metal hydride complexes of Pd and Pt have been isolated from very bulky primary silanes. ,, In contrast, systematic variation of silane substituents in the oxidative addition of tertiary silanes to Rh complexes reveal a pronounced decrease in the rate of oxidative addition (and stability of the formed products) with increasing steric bulk at silicon. , Deeper understanding of the impact of silane steric bulk on the formation and stability of silyl metal complexes is necessary to more fully understand hydrosilylation reactions.…”

Hydrosilylation of Alkynes Under Continuous Flow Using Polyurethane‐Based Monolithic Supports with Tailored Mesoporosity

“…Since the covalently-binding DPPS permeates extensively across the microstructure of P5, it can be surmised that excess hydroxyl groups exist throughout the monolithic structure, and can thus offer sites for subsequent functionalization. The presence of excess hydroxyl groups in the micropore domain can also account for the substantial non-permanent (swelling) − ], containing a cationic Rh(I)-NHC complex, [41] onto the surface of a hydroxylcontaining polyurethane-based monolith. porosity observed due to the swelling of the monolith structure in "good" polymer solvents such as CHCl 3 (vide supra).…”

“…The hydrosilylation of both aromatic and aliphatic 1-alkynes, that is, phenylacetylene, 4-ethynyltoluene, 4-ethynylanisole, 1-hexyne, 1-octyne, and 1-nonyne, with HSiMe 2 Ph was performed under continuous biphasic conditions at 55 °C using [BMIM + ][BF 4 − ] as monolith-supported IL phase, methyl tert-butyl ether (MTBE) as the second liquid transport phase and the cationic Rh-NHC complex [1-(pyrid-2-yl)-3-mesityl)-imidazol-2-ylidene))(𝜂 4 -1,5-cyclooctadiene)Rh(I) tetrafluoroborate] [41] dissolved in the IL phase, applying a linear flow of 0.2 mL . min −1 .…”

“…As can be seen from Figure 3, the Z/E ratio (Z-content) of the hydrosilylation products increased substantially up to 3.26 for 4-ethynylanisole (62% Z-isomer) in case the reactions were performed under monolith-supported, biphasic continuous flow conditions compared to reactions carried out under biphasic conditions. This increase in Z-selectivity can be attributed to a confinement effect [41] created by the constrained geometry inside the small mesopores, which affects the transition state of the Rh-catalyst, favoring a Z-arrangement (Figure 4).…”

“…[21][22][23] Compared to poly(norbornene) or poly(styrene) based monoliths, polyurethane-based monoliths are more polar and offer the possibility of surface functionalization via excess hydroxyl or isocyanate groups. In course of our activities in the area of molecular heterogeneous catalysis in confined geometries [24][25][26][27][28][29][30] we were interested in synthesizing polyurethane-based monolithic supports that fulfill the general requirements for polymeric monoliths such as unitary structure, incompressibility, transport pores in the micrometer range, high linear flow (up to 20 mm s −1 ) at low back pressure (<10 bar), while having a tailored mesoporosity in the 2-10 nm range. These mesoporous monoliths were then surface functionalized with quaternary ammonium groups followed by immobilization of an ionic Rh-catalyst-containig IL for use in heterogeneous, biphasic, continuous catalysis using the mesopores as confinements [31][32][33] that ultimately govern the reactivity of the Rh-catalyst.…”