Insertion into M-H bonds

A large number of catalysts or catalyst precursors for the reaction in Eq. (1) have been identified (Table 17.1). Almost all of these are complexes of Rh and Ru, although there are a few, less active, examples of fr, Pd, Ni, Fe, Ti, and Mo. The most active catalysts are RhCl(TPPTS)3, Rh(hfacac)(dcpb) and RuCl(OAc) (PMe3)4, with little difference between them after rough correction of the turnover frequencies (TOF) for pressure and temperature differences [38]. Musashi and Sakaki [39] argue that the barrier to C02 insertion into M-H bonds increases in the order Rh(I) < Ru(II) < Rh(I II) because Rh(lll)-formate bonds are too weak and Ru(II)-H bonds are too strong compared to the case of Rh(I). 

Alkynes also insert into M—H bonds. These are of interest in that the stereochemistry of insertion is often apparent in the metal vinyl complex formed. First row metals have been observed to give trans products in which case a radical mechanism may be operative134, but second and third row metals tend to give cis products as expected from a concerted insertion. 

Ans. Similarity Both prochiral and undergo enantioface selective alkene insertion into M-H bond (hydride attack). Difference The first acts as a chelating ligand all throughout the catalytic cycle, but the second only in 9.32. 

Alkyl and aryl isothiocyantes, RCNS, parallel CS2 in their capability to insert into M—H bonds and yield products having the JV-alkyl- or JV-aryl-formamide chelate ligand RN=CH=S. The iridium(III) complexes [Ir(X)2(RN=CH=S)(PPh3)2] (207 X = Cl, Br R = Me, Et, Ph) can be prepared from trans-[Ir(H)(X)2(PPh3)2] and the appropriate alkyl or aryl isothiocyanate. Two isomeric forms of (207) were deemed plausible, with (207a) the preferred stereochemistry for the alkyl products and (207b) for the aryl products.453... 

Unlike CO, RNC inserts into M—H bonds to give ij1- and r -formimidoyl compounds. 

Unsaturated ligands such as alkenes often undergo insertion into M-H bonds to give alkyls or vinyls (equation 23). This is an important step in catalytic alkene isomerization, hydrogenation, hydroformylation, hydrocyanation, and perhaps also hydrosilation, although in this last case, insertion into the M-Si bond may be preferred. This is simply the reverse of the -elimination reaction discussed above. 

Again in a brief paper with considerable impact, van Barneveld and Ponec have considered many aspects of the Fischer-Tropsch reaction. On Ni and Cu/Ni they find that to explain many known features of the reaction they must dismiss polymerization of—CH2— groups on neighbouring sites and repeated CO insertion into M—H bonds. They favour CO dissociation to C + O followed by C + xH CH c and CH c + COads —> (CH,CO) then CH,CO + wH- -H20 + CHjcCH 2- The CH ,CH 2 may add further CO, or add hydrogen to give a desorbable hydrocarbon product. The role of adsorbed CHx seems of paramount importance in their mechanism. 

Alkenes and alkynes can undergo 1,2-insertion into M-H bonds. The numbering again refers to the numbers for the carbon atoms on the double or triple bond. A schematic insertion reaction is outlined below ... 

Hydroperoxo complexes can also be obtained by a one-electron reduction of the superoxo intermediates [Ru(NH3)6]2 + is one of the most convenient reducing agents for this chemistry.15 An interesting way of preparing hydroperoxides via dioxygen activation utilizes a formal 02 insertion into M-H bond in metal hydrides examples are known for platinum, rhodium, and cobalt complexes, to name just a few.14,15... 

Other insertions into M-H bonds of paramagnetic complexes have been proposed to occur during the reactions of a few saturated hydride complexes with electron poor acetylenes, the first step being an electron transfer process. A characteristic of this mechanism is an E- rather than Z-addition stereochemistry. Examples of this phenomenon have been described for PtHjLj (L = tertiary phosphine) [139], Cp2ReH and CP2WH2 [140]. 

Mechanistic and kinetic studies of the insertion into M-H bonds and the reverse reaction... 

One final route to alkyls is the attack of a nucleophile on a metal alkene complex, but we will postpone a detailed discussion to Chapter 5. This route is more useful for the synthesis of metal vinyls from alkyne complexes vinyls are also formed from alkyne insertion into M—H bonds ... 

Insertion Reactions CO2 insertion into M—H bonds also takes place, but of the two possible regiochemistries (Eq. 12.26), only the formate, either -q -, or n -bound, has been observed in a stable product (Eq. 12.27-... 

In this chapter the elementary reaction of CO2 with a discrete M-C bond is discussed. The coupling of CO2 with unsaturated hydrocarbons, which may also imply insertion of CO2 into an M-C bond, is discussed in Chap. 5. The insertion-deinsertion of CO2 into M-R bonds (R alkyl or aryl moiety) is relevant to carboxylation-decarboxylation processes which are very important in biological systems [38]. In this case, as seen for the insertion into M-H bonds, two mechanisms are possible, as depicted in Scheme 4.11. 

Catalytic reduction of CO2 with H2 to give HCOOH involves CO2 insertion into M-H bonds. Although this is uphill thermodynamically (AG = -1-8 kcal/mol), the reaction becomes favorable under... 

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