Catalyzing Aldehyde Hydrosilylation with a Molybdenum(VI) Complex: A Density Functional Theory Study

Abstract

[MoCl(2)O(2)] catalyzes the hydrosilylation reaction of aldehydes and ketones, as well as the reduction of other related groups, in apparent contrast to its known behavior as an oxidation catalyst. In this work, the mechanism of this reaction is studied by means of density functional theory calculations using the B3LYP functional complemented by experimental data. We found that the most favorable pathway to the first step, the Si--H activation, is a [2+2] addition to the Mo=O bond, in agreement with previous and related work. The stable intermediate that results is a distorted-square-pyramidal hydride complex. In the following step, the aldehyde approaches this species and coordinates weakly through the oxygen atom. Two alternative pathways can be envisaged: the classical reduction, in which a hydrogen atom migrates to the carbon atom to form an alkoxide, which then proceeds to generate the final silyl ether, or a concerted mechanism involving migration of a hydrogen atom to a carbon atom and of a silyl group to an oxygen atom to generate the silyl ether weakly bound to the molybdenum atom. In this Mo(VI) system, the gas-phase free energies of activation for both approaches are very similar, but if solvent effects are taken into account and HSiMe(3) is used as a source of silicon, the classical mechanism is favored. Several unexpected results led us to search for still another route, namely a radical path. The energy involved in this and the classical pathway are similar, which suggests that hydrosilylation of aldehydes and ketones catalyzed by [MoCl(2)O(2)] in acetonitrile may follow a radical pathway, in agreement with experimental results.

Dioxomolybdenum Complexes as Excellent Catalysts for the Deoxygenation of Aryl Ketones to Aryl Alkenes

“…This new methodology has the advantage of using the air-stable oxo complex MoO 2 Cl 2 (H 2 O) 2 as the catalyst, which has an easy and inexpensivep reparation by extraction from ah ydrochlorica cid solution of Na 2 MoO 4 with diethyl ether. [16] We propose that the mechanism for the deoxygenation of the aryl ketones to the corresponding alkenes with the catalytic system of PhSiH 3 / MoO 2 Cl 2 (H 2 O) 2 involves the formation of ah ydride species, which results from SiÀHb ond activation of the silane by the oxo molybdenumc omplex as demonstrated in our previousw ork, [17] followed by the reductiono ft he ketone to the corresponding alcohol. If phenylsilane is used as the reducing agent, the major product obtained in the reduction of the ketone is the alcohol and not www.chemcatchem.org the silyl ether,p robablyb ecause of the instability of the silyl ether formed from this silane.Finally,the dehydration of the alcohol to the alkene occurs.…”

DFT Study of the Molybdenum‐Catalyzed Deoxydehydration of Vicinal Diols

“…[8,9] Other oxomolybdenum complexes have been used for epoxidations [10] and reductions of sulfoxides and alkenes. [11] Molybdenum-catalyzed reactions that have previously been studied by computational methods include oxidations, [12] hydrosilylations, [13] and reductions using either hydrogen [11h] or silanes. [14] In line with earlier work, [15][16][17] we have carried out a computational study using density functional theory (DFT) to clarify the mechanism of the molybdenum-catalyzed DODH.…”

High Valent Oxo-molybdenum Complexes as Efficient Catalysts for C-X Bond Forming Reactions

“…Computational studies performed for this reaction have also suggested a radical mechanism (Fig. 76) [49]. This study demonstrated that after the formation of hydrogen radicals, they could add to aldehyde, forming an alkoxyl radical, which can combine with Fig.…”

“…A mechanism for the hydrosilylation of aldehydes and ketones catalyzed by MoO 2 Cl 2 (1) was proposed based on computational studies [49]. We found that the most favorable pathway to the first step, the Si-H activation, is a [2+2] addition to the Mo=O bond, forming the hydride species MoCl 2 H(O)(OSiR 3 ) H (Fig.…”

“…In the last five years, our group has demonstrated a new role of the high valent oxo-molybdenum complex MoO 2 Cl 2 (1) as an excellent catalyst for the activation of Si-H bond of silanes and also for the C-H bond formation by reduction of C=O, C=N, and C C bonds [17,[49][50][51][52].…”