Homonuclear addition reactions

The addition of dihydrogen to triple bonds, which can be readily achieved either using ionic reagents such as LiAlH4 to give tnms double bonds or catalytically affording cis-alkenes, has found important applications in the synthesis of cyclic alkenes with interesting it-parameters. 

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The high reactivity of bent triple bonds toward both nucleophiles and electrophiles is demonstrated by the reaction of cyclooctyne with lithium and iodine, both reactions affording a homonuclear addition product [3 b] (Scheme 8-20). 

Homonuclear unbridged tm bonds exhibit a high reactivity towards insertion or addition reactions when the ligand periphery is sterically crowded. This results in a... 

Although the kinetic rate and energy partitioning are qualitatively consistent with a pure ER process, other aspects of the experiments and most of the theory (see discussion below) imply that the abstraction is more properly described as a combination of ER and HA reactions. The large a for abstraction is inconsistent with theoretical studies of a pure ER process as this requires a direct hit of the incoming H(D) with the adsorbed D(H) [380,381]. There is also no way to reconcile formation of homonuclear products with a pure ER process. In addition, similar kinetic experiments on other metals, e.g., Ni(100) [146], Pt(lll) [147,382], etc., are not even in qualitative agreement with the simple ER rate law above. In those cases, it is necessary to develop more sophisticated HA kinetic mechanisms to describe the kinetics experiments [383-385]. The key parameter of these kinetic models is the ratio of reaction to non-reactive trapping, pr/ps. For pr/ p, = 1, the HA kinetics looks very much like the simple ER case, and this is the reason H(D) + D(H)/Cu(lll) has such simple kinetics. 

Clusters based on homonuclear trimetal units are dominated by those containing transition metals from the iron triad. Clusters constructed on a Co3 framework are the only additional examples in this section. The first cluster to be reported was Co3(CO)9BNEt3 (22). This arises from the reaction of [Co(CO)4] with BBr3 in the presence of NEt3 and is clearly related to the triosmium systems of Shore et al. discussed above (Fig. 7)... 

In addition to the homonuclear reactions in Table 1, insertion of SnCU into the Ni—Fe bond of CpNi(CO)Fe(CO)2Cp occurs in 60% yield in THF solution at 40°C over 6h ... 

Dissociation of the hydrogen halides HF, HC1, DCF and HBr has been studied in shock waves. In contrast to homonuclear diatomic molecules, subsequent bimolecular reactions must be taken into account unless the ratio of reactant concentration to carrier gas concentration is kept sufficiently low. For instance, in HCl dissociation at [HCl]/[Ar] around 10 the reactions H + HCl Hg + Cl and Cl + HCl CI2 + H follow the unimolecular dissociation HCl + Ar - H + Cl + Ar. In addition the dissociation of CI2 and H2 must be considered. The interpretation of these dissociation studies may therefore be very complex. Representative results are given in Table 1.3. These fit quite reasonably into the picture found for homonuclear diatomic molecules. No direct recombination studies appear to be available. 

Fig. 5.16 Substitution, addition and elimination reactions of transition metal complexes. L = Lewis base, two electron ligand. X = one electron ligand, e.g. Cl, H, R, COR, SiR3. XY includes homonuclear species, e.g. H, Cl. ...

The enantioselective addition of dialkylzinc reagents to aldehydes is not limited to the homonuclear zinc complexes described above. Seebach has shown that Ti-TADDOL complexes can be effective catalysts for the preparation of a wide range of secondary alcohols (Equation 16) [102], The addition of Et2Zn to aldehydes is carried out with catalyst 165 in combination with 1.2 equiv Ti(Oi-Pr)4 to furnish products in 82-99% ee [17. 103). The use of excess Ti(Oi-Pr)4 is noteworthy, as it is of mechanistic significance. Catalyst turnover is only observed in this system because of the ability of the Ti(Oi-Pr)4 to serve as a reservoir for the alkoxide product. It is remarkable, however, that the presence of excess Ti(Oi-Pr)4 does not lead to any diminution in the enantioselectivity of the product formed. This represents a dramatic example of ligand-accelerated catalysis in C-C bond-forming reactions. 

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