New insights into hydrosilylation of unsaturated carbon-heteroatom (C═O, C═N) bonds by rhenium(V)-dioxo complexes

The theoretical design of manganese catalysts with a Si-N-Si-C-Si-C six-membered ring core-based bowl-shaped quadridentate ligand for the hydrogenation of CO/CN bonds

Herein, a new series of bowl-shaped quadridentate ligands with a Si-N-Si-C-Si-C six-membered ring core and their manganese catalysts were designed using the density functional theory (DFT) method for the hydrogenation of unsaturated CX (XN, O) bonds. The frameworks of these ligands named by LYG (LYG = P(R1)2CH2Si(CH2)(CH3)NSi(CH3)(CH2Si(CH3)CH2P(R3)2)CH2P(R2)2) have a Si-N-Si-C-Si-C six-membered ring core at the bottom of the bowl structure and each Si atom links with one phosphorus arm (-CH2PR2). The Mn catalyst Mn(CO)-LYG was constructed to catalyze the hydrogenation of CO/CN bonds. The calculated results indicate that due to the bowl-shaped structure of LYG quadridentate ligands, these Mn catalysts could be advantageous not only in the tuneup of catalytic activity and stereoselectivity by modifying three phosphorus arms but also in the homogeneous catalyst immobilization by linking with the Si-N-Si-C-Si-C six-membered ring core using different supports. This work might provide theoretical insights to design new framework transition-metal catalysts for the hydrogenation of CX bonds.

Hydrosilylation Reactions Catalyzed by Rhenium

Hydrosilylation is an important process, not only in the silicon industry to produce silicon polymers, but also in fine chemistry. In this review, the development of rhenium-based catalysts for the hydrosilylation of unsaturated bonds in carbonyl-, cyano-, nitro-, carboxylic acid derivatives and alkenes is summarized. Mechanisms of rhenium-catalyzed hydrosilylation are discussed.

Theoretical Study on the Rhodium-Catalyzed Hydrosilylation of C═C and C═O Double Bonds with Tertiary Silane

Reaction mechanisms of hydrosilylation of ketone and alkene with tertiary silane using the Wilkinson-type catalyst were theoretically investigated on the basis of density functional calculations using ωB97XD functional. Previously proposed three mechanisms, the Chalk-Harrod (CH) mechanism, the modified Chalk-Harrod (mCH) mechanism, and the outer-sphere mechanism were examined. Besides, we also found two mechanisms, the alternative CH (aCH) mechanism and the double hydride (DH) mechanism. In the aCH mechanism, a four-coordinate rhodium hydride complex formed through the elimination of R₃Si-Cl is a catalytically active species. In the DH mechanism, the active species is a six-coordinate complex with two Rh-H bonds. For the C═O double bond hydrosilylation, the rate-determining steps of the aCH and DH mechanisms are both acetone insertion into the Rh-H bond, and the order of the activation barrier is DH < aCH ≈ CH < mCH. For the C═C double bond hydrosilylation, except for the mCH pathway whose rate-determining step is the hydrosilane addition reaction, the rate-determining steps of the CH, aCH, and DH pathways are Si-C reductive elimination reactions. The order of the energy barrier is DH ≈ mCH < aCH ≈ CH. In the outer-sphere mechanism, no stable intermediate or transition state was found. Consequently, we concluded that the DH mechanism is adopted as the mechanism for the Rh-catalyzed hydrosilylation of the carbonyl group while the mCH or DH mechanism is adopted as the mechanism for alkenes under conditions where their active intermediates are formed. The present result revises a hypothesis that the hydrosilylation of the carbonyl group is in general accomplished by the mCH mechanism. The active species in the DH mechanism has one more extra Rh-H bond compared to that of the other pathways, and its interaction with a silyl group, trans-influence, and small steric effect are the origin of the highly efficient catalytic activity, which was not reported before.

Cationic rhenium(iii) complexes: synthesis, characterization, and reactivity for hydrosilylation of aldehydes

A series of novel cationic Re(iii) complexes [(DAAm)Re(CO)(NCCH₃)₂][X] [DAAm = N,N-bis(2-arylaminoethyl)methylamine; aryl = C₆F₅ (a), Mes (b)] [X = OTf (2), BAr^F₄ [BAr^F₄ = tetrakis[3,5-(trifluoromethyl)phenyl]borate] (3), BF₄ (4), PF₆ (5)], and their analogue [(DAmA)Re(CO)(Cl)₂] [DAmA = N,N-bis(2-arylamineethyl)methylamino; aryl = C₆F₅] (6) were synthesized. The catalytic efficiency for the hydrosilylation reaction of aldehydes using 4a (0.03 mol%) has been demonstrated to be significantly more active than rhenium catalysts previously reported in the literature. The data suggest that electron-withdrawing substituents at the diamido amine ligand increase the catalytic efficiency of the complexes. Excellent yields were achieved at ambient temperature under neat conditions using dimethylphenylsilane. The reaction affords TONs of up to 9200 and a TOF of up to 126 h^-1. Kinetic and mechanistic studies were performed, and the data suggest that the reaction is via a non-hydride ionic hydrosilylation mechanism.

Kinetic and Computational Studies of Rhenium Catalysis for Oxygen Atom Transfer Reactions

The rhenium(v)oxo dimer {MeReO(edt)}2 (edt = 1,2-ethanedithiolate) is an effective catalyst for the oxygen atom transfer (OAT) reaction from pyridine oxide and picoline oxide to triphenylarsine (Ph3As) as oxygen acceptor. Kinetics measurements were carried out by the initial rate method because of the monomerization reaction of the pyridine product with the {MeReO(edt)}2 catalysts. The derived rate is R = k[Re][NO] (where NO is picoline oxide or pyridine oxide) and independent of the Ph3As concentration. The rate constant at room temperature in chloroform is k(PicNO) = 268.1 ± 3.5 L mol−1 s−1 and k(PyNO) = 155.3 ± 2.3 L mol−1 s−1. The analogue rhenium(v)oxo dimer {MeReO(pdt)}2 (pdt = 1,3-propanedithiolate) does not monomerize with pyridine. However, {MeReO(edt)}2 rapidly monomerizes with pyridine. Density functional theory study of the enthalpy of the monomerization reaction shows that the {MeReO(edt)}2 reaction with pyridine is more thermodynamically favoured than {MeReO(pdt)}2 and this is attributed to the higher angle strain on the {MeReO(edt)}2 bridging sulfur. The computational study of the proposed slow step shows that enthalpy of activation (ΔH‡) of ReV oxidation to ReVII is unchanged by varying the substituent on the pyridine oxide.