Ligand insertion reaction into metal hydride

The Ligand Insertion Reaction into the Metal Hydride Bond... 

The CO insertion reaction into the metal hydride bond is in fact a member of the class of ligand insertion reactions to which much theoretical work has been devoted (28,29-35). Some years ago we analyzed the ethylene insertion into the rhodium hydride bond of a Rh(III) hexacoordinated complex (. We later focused our attention on the CO insertion reaction into the Mn-H bond ofHMn(CO)5 (37-39) and very recently we have undertaken the study of the CO2 insertion reaction into the Cr-H bond of HCr(CO)5 (C. Bo and A. Dedieu, Inorg. Chem., in press). We will concentrate here on the CO insertion reaction and compare it to the two other insertion reactions. The study of the reaction (1) was carried out at both the SCF and... 

In analogy to hydroformylation, alkenes react with SO2 and H2 to give a so-called hydrosulftnation product, sulfinic acids [116]. Cationic Pd(II) and Pt(II) complexes bearing bidentate phosphine ligands are effective catalyst precursors. A plausible mechanism for the hydrosulfination involves formation of alkyl intermediates by olefin insertion into metal hydrides, subsequent insertion of SO2, and reformation of the hydrides with the release of sulfinic acids (Scheme 7.19). However, ahphatic sulfinic acids readily undergo disproportionation to give thiosulfinic acid esters, sulfonic acids, and water at the reaction temperature. The unstable sulfinic acids can be conveniently converted into y-oxo sulfones by addition of a,-unsaturated carbonyl compounds as Michael acceptors to the reaction mixtine (Eq. 7.23) [117]. 

The most common insertion reactions involve the insertions of carbon monoxide into metal-hydrocarbyl ligands and the insertions of olefins and alkynes into metal-hydride... 

Olefins can insert into metal-alkyl bonds as well as into metal-hydride bonds provided that a vacant coordination site is available on the metal center. This reaction can thus be repeated even when the alkyl ligand lengthens, which makes the proposed mechanism a Ziegler-Natta olefin polymerization. This type of polymerization is detailed in Chap. 15.1 ... 

Thus, the isonitrile ligand easily undergoes insertion reactions, even into metal-hydride bonds. The nucleophilic additions onto the ligand carbon atom are also facile. For instance, amines readily add to yield diaminocarbene complexes ... 

In transfer hydrogenation with 2-propanol, the chloride ion in a Wilkinson-type catalyst (18) is rapidly replaced by an alkoxide (Scheme 20.9). / -Elimination then yields the reactive 16-electron metal monohydride species (20). The ketone substrate (10) substitutes one of the ligands and coordinates to the catalytic center to give complex 21 upon which an insertion into the metal hydride bond takes place. The formed metal alkoxide (22) can undergo a ligand exchange with the hydride donor present in the reaction mixture, liberating the product (15). 

All these ligands have extensive chemistry here we note only a few points that are of interest from the point of view of catalysis. The relatively easy formation of metal alkyls by two reactions—insertion of an alkene into a metal-hydrogen or an existing metal-carbon bond, and by addition of alkyl halides to unsaturated metal centers—are of special importance. The reactivity of metal alkyls, especially their kinetic instability towards conversion to metal hydrides and alkenes by the so-called /3-hydride elimination, plays a crucial role in catalytic alkene polymerization and isomerization reactions. These reactions are schematically shown in Fig. 2.5 and are discussed in greater detail in the next section. 

AUcyl ligands in niobium and tantalum complexes are susceptible to attack by electrophiles (see Electrophilic Reaction). Hydrogenation (see Hydrogenation) of niobium or tantalum M R bonds to provide the metal hydrides is an important reaction of synthetic utility. Insertion reactions of unsaturated reagents into Nb- or Ta bonds are common. The unsaturated reagents include aUcenes, aUcynes, CO, NO, RN=C=NR, CNR, and others. 

Although 187-189 were not active catalysts for polymerization process, 187 and 189 proved to be active olefin hydrosilylation catalysts, presumably 187 first reacted with a silane to form a reactive metal hydride species. They are the first examples of d° metal complexes with non-Cp ligands in the catalytic hydrosilylation of olefins. The mechanism was believed to be consistent with that of other d° metallocene-based catalysts and included two steps 1) fast olefin insertion into the metal hydride bond and 2) a slow metathesis reaction with the silane. The catalysts exhibited a high regioselective preference for terminal addition in the case of aliphatic olefins... 

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