Insertion, metal hydride

Ketenes can react in several ways with organometaUic compounds and complexes. They can add as ligands to coordinated metals forming stable ketene, ketenyl, and ketenyfldene complexes. Ketenes can be inserted into metal—hydride, metal—alkyl, metal—OR, and metal—NR2 bonds, react with metal—oxide complexes, and with coordinated Hgands. This chemistry has been reviewed (9,51). 

The Mizoroki-Heck reaction is a metal catalysed transformation that involves the reaction of a non-functionalised olefin with an aryl or alkenyl group to yield a more substituted aUcene [11,12]. The reaction mechanism is described as a sequence of oxidative addition of the catalytic active species to an aryl halide, coordination of the alkene and migratory insertion, P-hydride elimination, and final reductive elimination of the hydride, facilitated by a base, to regenerate the active species and complete the catalytic cycle (Scheme 6.5). 

Cu—C bond of the complex. Recently, activation of C02 as an r/1-C metalocarboxylate209 and photoinduced or thermal insertion of C02 into a metal-hydride bond210 have been reported. Furthermore, some metal (Zn2+, Ni2+, and Cd2+) complexes of tetraazacy-cloalkanes have been found to take up C02 easily in basic alcoholic solutions, and their structures have also been examined.211 More recently facile insertion of C02 into Rh2(/x-OH)2 to yield a carbonate complex of a rather complicated structure has been reported.212... 

From Ref. 188. Relative insertion rate of metal hydrides into TCNE. 

The fact that both the thermal and the photochemical insertion reactions yield the same products via formation of charge-transfer complexes leads to the conclusion that the reactive ion-radical pair in equation (52) is the common intermediate for both activation processes. Such a conclusion is further verified by the direct observation of anion-radical intermediates from the thermal reaction of TCNE and DDQ with various metal hydrides.188... 

Tributyltin hydride reduction of carbonyl compounds. The reduction of carbonyl compounds with metal hydrides can also proceed via an electron-transfer activation in analogy to the metal hydride insertion into TCNE.188 Such a notion is further supported by the following observations (a) the reaction rates are enhanced by light as well as heat 189 (b) the rate of the reduction depends strongly on the reduction potentials of ketones. For example, trifluoroacetophenone ( re<1 = —1.38 V versus SCE) is quantitatively reduced by Bu3SnH in propionitrile within 5 min, whereas the reduction of cyclohexanone (Erea — 2.4 V versus SCE) to cyclohexanol (under identical... 

All the above systems represent indirect routes to metal formyl species. At present, there is no proven example of the direct insertion of CO into a metal-hydride bond ... 

This type of side-on bending, which has been established by X-ray crystallographic methods for the related acyl complexes (r 5-C5H5)2Zr(COMe)Me (38) and (T>5-C5H5)2Ti(COMe)Cl (39), could overcome the thermodynamic objection, previously discussed, against the formation of a normal, linearly bonded formyl by CO insertion into a metal-hydride bond. Thermochemical data obtained from alcoholysis of zirconium tetralkyl species show that the mean bond energy of Zr—O is 50 kcal/mole greater than that of Zr—C (40). 

Hydrogenation of 1,3-dienes to terminal olefins is catalyzed by HRh(PPh3)4 and [Rh(CO)2(PPh3)2]2 in the presence of excess phosphine diene insertion into a metal- hydride bond to give a-alkenyl rather than 7r-allyl intermediates was postulated for the initial step (141). Mechanistic studies of the HRh(PPh3)4 catalyst (142) and a more reactive phosphole analog (143) HRh(DBP)4 [5-phenyl-5//-dibenzophosphole (DBP), 7] for... 

In view of the fact that early transition metal alkyls insert CO under very mild conditions (2, 5.) we chose to examine the reactions of electron-rich metal hydrides ( ) with the resultant dihapto acyl complexes. Such acyls obviously benefit from reduction of the CO bond order from three (in OO) to two. More significantly, the dihapto binding mode will significantly enhance the electrophilic character of the acyl carbon. 

Metal hydrides and acyl-like CO insertion products are two types of species likely to be present in any homogeneous or heterogeneous process for the catalytic reduction of carbon monoxide. The discovery and understanding of new types of reactivity patterns between such species are of fundamental interest. As discussed elsewhere (11,22,54-57), bis(pentamethylcyclo-pentadienyl) actinide hydrides (58) are highly active catalysts for olefin hydrogenation as well as H-H and C-H activation. 

The formation of vinylboranes and vinylboronate esters during some metal-promoted hydroboration of alkenes has led to the suggestion of an alternative mechanistic pathway. Insertion of the alkene into the metal-boron bond occurs in preference to insertion into the metal-hydride bond.44,51,52 In a competing side-reaction to reductive elimination, f3-H elimination from the resulting borylalkyl intermediate furnishes the vinylborane byproduct.52 There remains however a substantial body of evidence, both experimental53 and theoretical,54 that supports the idea that transfer of hydride to the coordinated alkene precedes transfer of the boryl fragment. 

Many of these catalysts are derived from metal complexes which, initially, do not contain metal hydride bonds, but can give rise to intermediate MH2 (al-kene) species. These species, after migratory insertion of the hydride to the coordinated alkene and subsequent hydrogenolysis of the metal alkyl species, yield the saturated alkane. At first glance there are two possibilities to reach MH2 (alkene) intermediates which are related to the order of entry of the two reaction partners in the coordination sphere of the metal (Scheme 1.2). 

One pervasive mechanistic feature of many of the hydrogenations described in other chapters of this handbook concerns the bonding of the unsaturated substrate to a metal center. As illustrated in generalized form in Eq. (1) for the hydrogenation of a ketone, a key step in the traditional mechanism of hydrogenation is migratory insertion of the bound substrate into a metal hydride bond (M-H). 

Several systems have been reported involving stoichiometric hydrogenation of ketones or aldehydes by metal hydrides in the presence of acids. An ionic hydrogenation mechanism accounts for most of these hydrogenations, though in some examples alternative mechanisms involving the insertion of a ketone into a M-H bond are also plausible. 

Carbon dioxide is known to readily insert into a metal-hydride bond to give a metal formate [57, 58] this forms the first step in insertion mechanisms of C02 hydrogenation (Scheme 17.2). Both this insertion step and the return path from the formate complex to the hydride, generating formic acid, have a number of possible variations. 

Insertion of COz into a metal-hydride bond normally requires the prior dissociation of an ancillary ligand to generate a coordinatively unsaturated complex, because C02 coordination to the metal usually precedes the formal insertion (Scheme 17.3, lower pathway). Ah initio calculations [59] support this mechanism for the insertion of C02 into the Ru-H bond of RuH2(PH3)4, a model for the catalyst RuH2(PMe3)4. However, it is theoretically possible for C02 insertion to take place without prior C02 coordination (Scheme 17.3, upper pathway) [60, 61]. The... 

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). 

Key words hydrogenation of carbon dioxide, insertion of carbon dioxide into the metal-hydride bond, reductive elimination of formic acid, C-bond metathesis... 

As briefly discussed in section 1.1, and shown in Figure 1, the accepted mechanism for the catalytic cycle of hydrogenation of C02 to formic add starts with the insertion of C02 into a metal-hydride bond. Then, there are two possible continuations. The first possibility is the reductive elimination of formic acid followed by the oxidative addition of dihydrogen molecule to the metal center. The second possible path goes through the a-bond metathesis of a metal formate complex with a dihydrogen molecule. In this section, we will review theoretical investigations on each of these elementary processes, with the exception of oxidative addition of H2 to the metal center, which has already been discussed in many reviews. 

Insertion ofC02into the Metal-Hydride and Similar Bonds... 

Insertion reactions of C02 into the metal-hydride and metal-alkyl bonds are of considerable importance, since these reactions are involved not only in the catalytic cycle of the hydrogenation of C02 into formic acid but also in the catalytic cycle of co-polymerization of C02 and epoxide. In this regard, insertions of C02 into various metal-hydride, metal-alkyl, and similar bonds have been the subject of intense experimental investigation. For instance, C02 insertions into Cu(I)-CH3, Cu(I)-OR, Cu(I)-alkyl [26-28], Ru(II)-H [29], Cr(0)-H, Mo(0)-H, W(0)-H [30], Ni(II)-H and Ni(II)-CH3 bonds [31, 32] have been so far reported. 

We recently investigated [40] the reason why C02 is inserted into the Rh(I)-H bond with a significantly lower barrier than into the Rh(III)-H bond, as shown in Table 2. As discussed above, charge-transfer from the metal-hydride moiety to the K orbital of CO2 is very important in the CO2 insertion reaction, and, at the same time, the metal-formate moiety is very much stabilized by the donation of electrons from the metal fragment. Since the Rh(I) center is more electron-rich than Rh(III), the charge-transfer from the Rh(I)-H moiety to the k orbital of C02 is favored, and the formate moiety is provided with sufficient electrons. Consequently, CO2 is more easily inserted into the Rh(I)-H bond than into the Rh(III)-H bond. 

Figure 9. Proposed intermediates for the water or alcohol assisted insertion of carbon dioxide into metal-hydride bonds.

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