Hydride ligands, olefin insertion

A very different neutrally charged complex for alkane activation has been reported recently and is shown in Scheme 34(A). The compound is a hydridoplatinum(II) complex bearing an anionic ligand based on the familiar nacnac-type, but with a pendant olefin moiety (97).This complex is extremely soluble in arenes and alkanes and activates C-H bonds in both types of hydrocarbons. This is indicated by deuterium incorporation from deuterated hydrocarbon into the substituents on the arene of the ligand and into the Pt hydride position (A A-d27, Scheme 34). The open site needed for hydrocarbon coordination at Pt(II) is created by olefin insertion instead of anion or solvent substitution (97). 

A 7 -dithiocarboxylate also (see Ref. 200) results when CSj reacts with the phosphine-Tt-oleiin chelating ligand in 315. The hydride ligand is transferred to the olefinic system and CS2 inserts into the metal-carbon bond. The S2C unit in 316 is formulated as a dithioallylic group which acts as a donor (207). 

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... 

Some metal hydride values have been determined (Table the values are markedly ligand dependent, as seen for example, by replacement of one CO of HCo(CO)4, a strong acid, by PPhj, which decreases the acidity of 7 pK units However, the values, which must be a measure of the polarity of the M—H bond under a certain set of conditions (those of the titration procedures), have proved of little value in predicting the chemistry of the metal hydrides, e.g., whether behavior is more typical of Co "( H), Co ( H), or Co (H). This is critical in catalysis, particularly in the direction of addition of the M—H bond across olefinic groups (i.e., in olefin insertion), which is important in hydrogenation, hydroformylation, and isomerization . This is a complex question and, as well as electronic factors, steric factors, solvent polarity, the presence of radical initiators, and even temperature changes, can be important. 

The key step in nearly all of the catalytic processes to be discussed is olefin insertion into a metal hydride [Eq. (2)] or organometallic species [Eq. (3)]. These hydrometallation and carbometallation processes also form the basis for the polymerization of alkenes. Olefin insertions generally occur with the same regiose-lectivity as hydroboration reactions [9], with the bulky metal and associated ligands residing at the least hindered site of the two carbon reactive unit. 

The hydrogen of HCo(dp)2 and HCo(PBu3)4 can be regarded as H rather than H+. Consequently, the reactivity or catalytic activity of these complexes differs. For example, the difference in iso/normal ratio in the oxo reaction was explained in terms of the difference in the ligands 53 56T Thus, when a phosphine such as PBu3 is coordinated to cobalt, the ratio of normal increases over that obtained with the corresponding carbonyl. In this case, a straight chain is formed by the olefin insertion when the acidity of the hydride is decreased. The increase of the normal aldehydes relative to iso aldehydes can be explained by the Markownikoff rule. Of course, as in many other cases, the steric effect, in addition to the electronic effect, should be considered at the same time. 

If an olefin ligand is present, oxidation of a diamagnetic hydride complex may induce olefin insertion into the M-H bond. The oxidation of complex (triphos)RhH(n-DMFU) [triphos=MeC(CH2PPh2)3 DMFU = dimethyl fumarate] consumes two electrons and aftbrds the stable olefin insertion product [(triphos)Rh CH(COOMe)CH2(COOMe) ]. This is shown to undergo a reversible one-electron reduction at a less positive potential with respect to the 2-electron oxidation of the Rh(I)-hydride complex, to afford the corresponding Rh(II) monocation, which has been spectroscopically characterized (Scheme 24)... 

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]. 

Olefin insertion is particularly facile in the case of the complexes [PtH(SnCl3)(PR3)2]. The soft ii-acceptor ligand [SnCls] stabilizes the metal-hydride bond (symbiosis of soft ligands) and hence catalyzes the insertion reaction as... 

A preliminary investigation concerning the mechanism of the Rh-catalyzed hydrogenation of acrylamide substrates with model bisphosphine ligands has been reported [67]. Acrylamide was used as the model substrate and [Rh(PH3)2] as the model catalyst (Fig. 8). The mechanism tmder investigation consists of hydrogen coordination to a [Rh(acrylamide)(PH3)2] complex based on the four possible approaches of H2, oxidative addition to form dihydrides, and olefin insertion into the Rh-H bond to form alkyl hydride complexes. Reductive elimination was not examined because it is facile in comparison to oxidative addition and olefin insertion steps. 

---