Transition metal catalysts atom/group-transfer reactions

Fig. 10 Transition metal catalysts as initiators in atom or group transfer reactions.

Aldol reactions of silyl enolates are promoted by a catalytic amount of transition metals through transmetallation generating transition metal enolates. In 1995, Shibasaki and Sodeoka reported an enantioselective aldol reaction of enol silyl ethers to aldehydes using a Pd-BINAP complex in wet DMF. Later, this finding was extended to a catalytic enantioselective Mannich-type reaction to a-imino esters by Sodeoka s group [Eq. (13.21)]. Detailed mechanistic studies revealed that the binuclear p-hydroxo complex 34 is the active catalyst, and the reaction proceeds through a palladium enolate. The transmetallation step would be facilitated by the hydroxo ligand transfer onto the silicon atom of enol silyl ethers ... 

Metals that are capable of 2e redox changes, typically main group elements and 4d and 5d transition metals, can give heterolysis of a peroxide to form a diamagnetic oxidant that may avoid the radical pathways seen in the case of equation (14-15). O atom transfer to the substrate is possible in this way. Sharpless epoxidation provides an excellent example. In this case rBuOOH is the primary oxidant, Ti(i-OPr)4 is the catalyst precursor and a tartrate ester is the ligand that induces a high ee in the epoxy alcohol formed from an allylic alcohol. This reaction has been successfiiUy developed on an industrial scale. 

Although the ATRP is a convenient method for the synthesis of block copolymers, however it undergoes some limitations, such as the easy oxidation of the transition metal complex (Fig. 1.17a). To rise above this drawback, the reverse ATRP was developed (Fig. 1.17b) it uses the more stable Cu(II) complexes in the initiating step. However, a drawback occurs also in reverse ATRP since the transferable atom or group (X) is added to the reaction as part of the copper salt, therefore highly active catalysts should still be used in the amount comparable to the concentration of the radical initiator for this reason, complex concentration cannot be independently reduced and block copolymers cannot be formed. 

The reduction reaction occurs by the transfer of hydrogen atoms bound to the surface of the metal catalyst to the carbonyl oxygen and carbon atoms. We recall that the same types of catalysts are used for the hydrogenation of alkenes, a much faster reaction. Alkenes can be reduced at room temperature under 1 atm. pressure of hydrogen gas. Carbonyl compounds require higher temperatures and pressures as high as 100 atm. Therefore, transition metal-catalyzed reduction of carbonyl compounds that also have a carbon—carbon double bond results in reduction of both functional groups. 

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