Metathesis, a-bond

A careful investigation of the reaction kinetics and detailed trapping experiments allow the conclusion that in this case a a-bond metathesis reaction mechanism applies. The polymerization reaction of PhSiH3 by CpCp Hf(SiH2Ph)Cl has been monitored by H-NMR spectroscopy. The data k(75 °C) = 1.1(1) x 10-4 M 1 s AH = 19.5(2) kcal mol" AS = -21(l)euandkH/fcD = 2.9(2) (75 °C) are in good agreement with the proposed mechanism with a metallacycle as transition state [164],... 

The discussion about the mechanism of the dehydrogenative polymerization reaction has not yet been completed. However, the reaction mechanism seems to be strongly influenced by the specific random conditions that apply for each particular system. Presumably with late transition metals a silylene mechanism is more appropriate. It may be a matter of the steric constraints of the system to shift the reaction towards a-bond metathesis. 

This catalytic reaction has been named alkane a-bond metathesis since alkyl fragments of alkane mixtures are exchanged. For example, propane is... 

Similar reactivity is observed in the cyclization of enynes in the presence of the yttrium-based catalyst 70 and a silane reductant [53,54]. The 1,6- and 1,7-enynes 90 and 91 provide -E-alkylidene-cyclopentancs 92 and -cyclohexanes 93 in very good yield (Eq. 15, Scheme 20) [55]. These transformations likely proceed by syn hydrometallation of the 7r-basic alkyne, followed by insertion of the alkene and a-bond metathesis. The reaction of 1,6-enynes tolerated... 

In a study of the methane complex [(diimine)Pt(CH3)(CH4)]+ (diimine = HN=C(H)-C(H)=NH), relevant to the diimine system experimentally investigated by Tilset et al. (28), theoretical calculations indicate preference for the oxidative addition pathway (30). When one water molecule was included in these calculations, the preference for oxidative addition increased due to the stabilization of Pt(IV) by coordinated water (30). The same preference for oxidative addition was previously calculated for the ethylenediamine (en) system [(en)Pt(CH3)(CH4)]+ (151). This model is relevant for the experimentally investigated tmeda system [(tmeda)Pt(CH3)(solv)]+ discussed above (Scheme 7, B) (27,152). For the bis-formate complex Pt(02CH)2, a a-bond metathesis was assumed and the energies of intermediates and transition states were calculated... 

Third, in analogy with the discussion presented above, oxidative addition represented by Pattern 11 may not be readily observable, and oxidative addition must also proceed mostly through more complex processes, such as that shown in Scheme 1.6 [31], More readily observable are various types of a-bond metathesis reactions of d° Cp2Zr(IV) species (Pattern 13). 

Scheme 1.68. A suggested mechanism for the formation of 11 through a-bond metathesis of zirconacyclopentadienes with H2ZrCp2.

Scheme 1.69. Various examples of a-bond metathesis reactions of three-membered zirconacycles.

Although a-bond metathesis of five-membered zirconacydes with EtMgBr [51] (Scheme 1.4) and H2ZrCp2 (Scheme 1.68) has been implicated, there are as yet very few well-established examples. The reaction of zirconacyclopentanes with alkyllithiums is interesting since it involves (i) the displacement of one of the two Cp groups, and (ii) the generation of a bimetallic species, the NMR spectroscopic data of which are consistent only with a fluxional structure as shown in Scheme 1.71 [66],... 

In many other reactions of zirconacydes catalyzed by transition metal complexes containing Cu, Ni, Pd, etc., a-bond metathesis (transmetallation) must undoubtedly be involved, but such products have not generally been identified. Partly for this reason, they are not discussed here. Readers are referred to the chapter by T. Takahashi. 

Scheme 1.71. a-Bond metathesis reaction of zirconacyclopentanes with alkyllithiums. 

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. 

Instead of having the olefin insertion reactions, the calculations indicate that M2b and M2c can only proceed uphill with the reductive elimination of HB(OH)2, leading to the formation of M3, an olefin complex which could be in principle obtained directly from the addition of olefin to the catalyst Rh (PH3)2C1. The olefin complex M3 then could undergo a-bond metathesis processes with HB(OH)2, giving two isomeric products M4 and M5 depending on the orientation of the HB(OH)2 borane. The a-bond metathesis processes are however found to be unfavorable because of the very high reaction barriers (Figure 4). 

As mentioned in the introduction, early transition metal complexes are also able to catalyze hydroboration reactions. Reported examples include mainly metallocene complexes of lanthanide, titanium and niobium metals [8, 15, 29]. Unlike the Wilkinson catalysts, these early transition metal catalysts have been reported to give exclusively anti-Markonikov products. The unique feature in giving exclusively anti-Markonikov products has been attributed to the different reaction mechanism associated with these catalysts. The hydroboration reactions catalyzed by these early transition metal complexes are believed to proceed with a o-bond metathesis mechanism (Figure 2). In contrast to the associative and dissociative mechanisms discussed for the Wilkinson catalysts in which HBR2 is oxidatively added to the metal center, the reaction mechanism associated with the early transition metal complexes involves a a-bond metathesis step between the coordinated olefin ligand and the incoming borane (Figure 2). The preference for a o-bond metathesis instead of an oxidative addition can be traced to the difficulty of further oxidation at the metal center because early transition metals have fewer d electrons. 

In this equilibrium (a a bond metathesis reaction ), the scandium-hydrogen bond in to(pentamethylcyclopentadienyl)scandium hydride is replaced by a scandium-carbon bond in phenylto(pentamethylcyclopendienyl) scandium... 

We still need to form the 08-Si9 bond, break the C7-Ti bond, and regenerate Ti(II). A a bond metathesis between the Si9-H and Ti-08 bonds can occur to give a very strong Si9-08 bond and a Ti-H bond. No change in the Ti(IV) oxidation state occurs. Reductive elimination from Ti(TV) gives the product and regenerates Ti(II). 

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