Bidentate system

A fundamental problem in the alkylation of enamines, which is inherent in the bidentate system, is the competition between the desired carbon alkylation and attack at the nitrogen. With unactivated alkyl halides (3,267), this becomes especially serious with the enamines derived fromcycloheptan-one, cyclooctanone, cyclononanone, and enamines derived from aldehydes. Increasing amounts of carbon alkylation are found with the more reactive allyl and benzyl halides (268-273). However, with allyl halides one also observes increasing amounts of dialkylation of enamines. 

The model tridentate terimine system is terpyridine 3 and salts of its [Fe N6]2+ derivative are low spin, like those of the diimines 1 and 2, but there are important differences in the bidentate systems on the one hand and the tridentate on the other. Although the average Fe-N distance is virtually the same (-1.96 A) in all three species, in [Fe(trpy)2]2+ the Fe-Ncentrai distances are much shorter than the Fe-Ndistai and so the [Fe N6]2+ coordination unit... 

To be consistent, we shall adopt the simple orbital representations used above to describe coordinate bonding interactions. Cyclobutadiene, being a bidentate ligand system, is described by four bonding molecular orbitals. They are similar in symmetry to the general bidentate system described earlier (cf. Fig. 5) and are represented in Fig. 7. 

Another bidentate system, involving less biologically relevant S-to-P chelating ligands, with the butterfly core in Fe Fe as well as Fe°Fe° derivatives, is shown in Equation (3) ... 

Among the MgO samples screened in the asymmetric Michael addition of chalcone and dimethylmalonate using (1/ , 2/ )-( + )-l,2-diphenyl-l,2-ethylenediamine (DPED, see Figure 5.5) as a chiral auxiliary, NAP-MgO was found to be superior to NA-MgO and CM-MgO in terms of yields and ee s. In general, chiral bidentate systems eomposed of primary and secondary amines afforded better ee s, while chelation with -OH groups displayed no reaction and ee (Table 5.11). 

Scheme 10 Facile synthesis of Gabbai s first bidentate system for fluoride 86.

Early attempts using unidentate phosphines gave only low optical yield. Bidentate systems are more promising but there is still a problem of flexibility and rapid interconversion ... 

Early forays into this reaction were described by Cavell and Yates. Experimental studies demonstrated that oxidative addition of both C2 H and C2-I imidazolium ions to platinum was feasible. The C2-I substituted imidazolium ion 5 also underwent oxidative addition to [Pd(PPh3)4] however, an attempted reaction with the C2-H imidazolium met with failure (Scheme 3.4). Palladium has been shown to oxidatively add into C2 H imidazolium ions in bidentate systems resulting in palladium complexes 6 and 7 (Scheme 3.5). The oxidative addition of imidazolium ions to iridium, generating (NHC)Ir -hydrides was also reported. ... 

Under same reaction conditions but longer time (5 days), various substituted succinimides were obtained. The 2-pyridinylmethylamine bidentate system is also crucial for the success of the reaction. The carbonylation took place in a preference as the following sequence C(sp )-H bond > cyclopropyl C(sp )-H bond > methyl C(sp )-H > methylene C(sp )-H bond. 

Scheme 15.2 Other amides compared to the N//-bidentate system.

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