Trans-labilizing activity

It has been known for a few years that anation of /m -[Co(dmgH)2-(Me)(OHa)] is fast, since the methyl group has a powerful trans labilizing influence on the co-ordinated water. Recently activation parameters have been determined for replacement of this water by pyridine and by thiocyanate. Activation enthalpies are low by cobalt(m) standards 20.5 kcal mol for reaction with pyridine and 17.5 kcal mol for thiocyanate. However, these values are comparable with those for anation of the analogous, but much less reactive, complex /rans -[Co(dmgH)2(N02)-(0H2)] in fact it is the difference in activation entropies which determines the different reaction rates for these methyl and nitro-complexes. The... 

In a manner analogous to classic nitrile iinines, the additions of trifluoro-methylacetonitrile phenylimine occur regiospecifically with activated terminal alkenes but less selectively with alkynes [39], The nitnle imine reacts with both dimethyl fumarate and dimethyl maleate m moderate yields to give exclusively the trans product, presumably via epimenzation of the labile H at position 4 [40] (equation 42) The nitrile imine exhibits exo selectivities in its reactions with norbornene and norbornadiene, which are similar to those seen for the nitrile oxide [37], and even greater reactivity with enolates than that of the nitnle oxide [38, 41], Reactions of trifluoroacetomtrile phenyl imine with isocyanates, isothiocyanates, and carbodiimides are also reported [42]... 

As the trans effect theory indicates, there should be some relationship between lability of a ligand and its role as a labilizing group in another position in a complex. In an octahedral complex reacting via a dissociative mode of activation, the transition state has five strongly bound ligands. This state will be stabilized... 

The Rh and Ir complexes 85-88 (Fig. 2.14) have been tested for the intramolecular hydroamination/cyclisation of 4-pentyn-l-amine to 2-methyl-1-pyrroline (n = 1). The reactions were carried out at 60°C (1-1.5 mol%) in THF or CDCI3 The analogous rhodium systems were more active. Furthermore, the activity of 87 is higher than 85 under the same conditions, which was attributed to the hemilabihty of the P donor in the former complex, or to differences in the trans-eSects of the phosphine and NHC ligands, which may increase the lability of the coordinated CO in the pre-catalyst [75,76]. 

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