Transition metals catalysed bond activation

The importance of transition-metal-catalysed reactions lies in their ability to facilitate reactions that would not occur under normal conditions. One such reaction is nucleophilic attack on an isolated double bond. While the presence of a conjugating group promotes the attack of nucleophiles, in its absence no such reaction occurs. Coordination of an alkene to a transition metal ion such as pal-ladium(II) changes its reactivity dramatically as electron density is drawn towards the metal and away from the n orbitals of the alkene. This leads to activation towards attack by nucleophiles just as for conjugate addition and unusual chemistry follows. Unusual, that is, for the alkene the palladium centre behaves exactly as expected. 

Chemical transformations through transition metal-catalysed C-H bond activation have attracted much attention owing to the remarkable potential for atom economy and environmental sustainability. Among a range of transition-metal catalysts used, nickel in combination with various ligands plays an important role in performing the transformation. 

C-C bond formation via transition-metal-catalysed cross coupling of -activated enols has been reviewed (97 references) while less reactive than systems involving halide or triflate leaving groups, vinyl phosphates have significant advantages in terms of cost, stability, and low toxicity... 

The transition metal-catalysed amination of C-H bonds via reactive metal-imide intermediates (i.e., nitrenoids) remains a powerful taetie for C-N bond formation. In that context, the intramolecular C(ip )-H amination of biaryl azides as nitrenoid sources has been computationally explored regarding the nature of the transition metal that plays the catalytic role. Four common transition metals (Ir, Rh, Ru and Zn) have thus been considered, and while the calculations have revealed similar energy profiles regardless of the nature of the metal, catalytically active Ru speeies have nevertheless been shown to be the more efficient from a kinetic viewpoint. 

The recent developments on the metallation chemistry of oxazoles and benzoxazoles, isoxazoles and benzisoxazoles, pyrazoles and indazoles, thiazoles and benzo-thiazoles, and isothiazoles, benzo[c]isothiazoles, and benzoMisothiazoles have been reviewed. The two-decade history of catalytic carbon-carbon bond formation via direct borylation of alkane C-H bonds catalysed by transition metal complexes has been reported. The alkane functionalization via electrophilic activation has been underlined. " Recent advances of transition-metal-catalysed addition reactions of C-H bonds to polar C-X (X=N, O) multiple bonds have been highlighted and their mechanisms have been discussed. The development and applications of the transition metal-catalysed coupling reactions have been also reviewed. - ... 

In 2007, Milstein reported an approach for the transition metal catalysed intermolecular formation of amides from alcohols and amines in the absence of a hydrogen acceptor (Scheme 12.19). In contrast with conventional amide synthesis from activated carboxylic acid derivatives which produces chemical waste, this environmentally benign approach produces hydrogen gas as the only byproduct. The catalyst used for this reaction is a dearomatised Ru(PNN)pincer complex which serves as a bifunctional catalyst. The ligands, as well as the metal centre, play a role in bond making or bond breaking steps of the catalytic cycle. 

Once again, the drive for improved performance in transition metal ion-catalysed processes has continued to stimulate the synthesis of new types of organophosphine and tervalent phosphorus-ester and -amide ligands. Activity in the chemistry of heteroaromatic phosphorus ring systems and low-coordination number p -bonded systems has also remained at a high level. New mechanistic insights into the Mitsunobu reaction have been reported, and interest in synthetic applications of Staudinger/Mitsunobu procedures has continued to develop. 

---