Other Insertion Reactions

Metal complexes of heterocyclic compounds display reactivities changed greatly from those of the uncomplexed parent systems. All of the -electron system(s) of the parent heterocycle can be tied up in the complex formation, or part can be left to take part in alkenic reactions. The system may be greatly stabilized in the complex, so that reactions, on a heteroatom, for example, can be performed which the parent compound itself would not survive. Orbital energy levels may be split and symmetries changed, allowing hitherto forbidden reactions to occur. In short, a multitude of new reaction modes can be made possible by using complexes dimerization of azirines with a palladium catalyst serves as a typical example (Scheme 81). A variety of other insertion reactions, dimerizations, intramolecular cyclizations, and intermolecular addition reactions of azirines are promoted by transition metals. 

The CO insertion reaction into the metal hydride bond is in fact a member of the class of ligand insertion reactions to which much theoretical work has been devoted (28,29-35). Some years ago we analyzed the ethylene insertion into the rhodium hydride bond of a Rh(III) hexacoordinated complex (. We later focused our attention on the CO insertion reaction into the Mn-H bond ofHMn(CO)5 (37-39) and very recently we have undertaken the study of the CO2 insertion reaction into the Cr-H bond of HCr(CO)5 (C. Bo and A. Dedieu, Inorg. Chem., in press). We will concentrate here on the CO insertion reaction and compare it to the two other insertion reactions. The study of the reaction (1) was carried out at both the SCF and... 

Insertion Reactions into Metal-Metal Bonds 11.7.5. Other Insertion Reactions... 

Sulphur trioxide is well known for its ability to insert into various bonds as is evident in the formation of cyclic pyrosulphates ( carbyl sulphate ) from /i-sultones26—see Section II.A.l. The mechanism of this and other insertion reactions is uncertain26. 

By analogy with other insertion reactions, coordinately unsaturated intermediates are probably the true reactants in Eqs. (58), (59), and (60). 

Formation of metal-carbon bonds by other insertion reactions... 

Alkynes have also been used with cesium acetate both for alkynylation and in the synthesis of p)ridines and isoquinolines.The alkynylation of aromatics was promoted by the use of 8-aminoquinoline as a directing group. However, cesium acetate was used in stoichiometric amount in addition to palladium acetate (eq 5). Other insertion reactions have been performed where cesium acetate was used as an additive for nonnatural amino acid synthesis via the addition of a phenyl group during the synthetic route. Also, iodination with palladium acetate and am-ination with a rhodium complex have been successful with cesium acetate as a base. 

Experimental evidence for insertion of alkenes into the metal-nitrogen bond was reported recently in the studies of aminopalladation reactions [41,42]. Intramolecular insertion reaction was confirmed to be a yn-addition process and was monitored by NMR spectroscopy of the well-defined palladium(aryl)(amido) complexes (Scheme 11) [41, 43]. The reaction proceeded as insertion into Pd-N bond with complete chemoselectivity and the alternative route of alkene insertion into the Pd-C bond was not observed. The activation enthalpy determined for the insertion step A// = 24.8 zb 0.6 kcal/mol was comparable with the values reported for other insertion reactions, and small activation entropy = 4.6 zb 1.8 eu is consistent with intramolecular transformation [41, 43]. The final product of the reaction was formed after C-C reductive elimination, which is known to be rather fast step if at least one aryl group is involved [44, 45]. The mechanistic study of the alkene insertion into the Pd-N bond has also pointed out on possible reversible nature of such process [46]. 

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