Terminal reduction

Motherwell and Whitehead et al. reported a similar intramolecular reductive aldol reaction of aldehyde-enoate derivatives. The cyclization of 6-oxo-ester 23 was catalyzed by RhCl(PPh3)3 (1 mol%) with Et3SiH (210 mol%) as terminal reductant (Scheme 9) [17,18]. The cyclization proceeded at 50 °C for 18 h to give the aldol product 24 in 81% yield with ds-selectivity (cis trans =... 

Chiu et al. developed a catalytic reductive aldol cyclization of alkyne-diones such as 115 and 117 using [Ph3PCuH]6 (10mol%) as catalyst and polymethylhydrosiloxane PMHS (200 mol %) as terminal reductant. The... 

Electronic Structure and Spectroscopic Properties of Molybdenum and Tungsten N2, NNH, NNH2, and NNH3 Complexes with Diphosphine Co-Ligands Insights into the End-on Terminal Reduction Pathway of Di nitrogen Felix Tuczek... 

The use of organomagnesium reagents as terminal reductants in zirconocene-catalyzed diene reductive cyclization permits derivatization of the resulting bis(magnesiomethyl)cycloalkanes. However, the use of other stoichiometric reductants is likely to afford catalytic systems that exhibit complementary selectivity profiles. Molander reports the... 

Though several intermolecular catalytic reductive aldol additions are reported, corresponding reductive cyclizations have received less attention. The first reported reductive aldol cyclization involves use of a (diketonato)cobalt(ll) precatalyst in conjunction with PhSiHj as terminal reductant.48,486 The reductive cyclization is applicable to aromatic and heteroaromatic enone partners to form five- and six-membered rings. As demonstrated by the reductive cyclization of mono-enone mono-aldehyde 65a to afford aldol 65b, exceptionally high levels of ty -diastereoselectivity are observed. Interestingly, exposure of the substrate 65a to low-valent nickel in the presence of excess Et2Zn provides the isomeric homoaldol cyclization product 65c via reductive coupling to the enone /3-position (Scheme 43).47a... 

A single example of the reductive cyclization of allenic carbonyl compounds is reported, which employs a rhodium-based catalyst in conjunction with Et3SiH as terminal reductant.113 This protocol promotes hydrosilylation-cyclization to form both five- and six-membered rings with exceptional levels of yy -diastereocontrol. As revealed... 

Nitrous oxide (N2O) reductases carry out the terminal reduction step in the denitrification pathway. One has been isolated from Pseudomonas... 

While at the cathode (+ terminal), reduction of hydrogen ions takes place ... 

The presumed catalytic cycle for this coupling is the following Once formed from 23, the highly coordinatively unsaturated 14-electron palladium(O) complex 24 participates in an oxidative addition reaction with the aryl or vinyl halide to give the 16-electron palladium(II) complex 25. A copper(I)-catalyzed alkynylation of 25 then furnishes an aryl- or vinylalkynyl palladium(II) complex 27. Finally, a terminating reductive elimination step reveals the coupling prduct 9 and regenerates the active palladium(O) catalyst 24. 

ELECTRONIC STRUCTURE AND SPECTROSCOPIC PROPERTIES OF MOLYBDENUM AND TUNGSTEN N2, NNH, NNH2i AND NNH3 COMPLEXES WITH DIPHOSPHINE CO-LIGANDS INSIGHTS INTO THE END-ON TERMINAL REDUCTION PATHWAY OF DINITROGEN... 

While this methodology is very useful, it is even more desirable to use milder and more readily available terminal reductants in these chain processes. Of special importance in this respect are H2, H20, and simple alcohols as the ultimate hydrogen atom donors. We will start the discussion with the use of H2, while the use of H20 and alcohols will be treated separately at the end of this chapter. 

In order to be able to use H2 as the terminal reductant, a reagent must be available that activates H2 to generate the desired HAT reagent. Many metal complexes are capable of doing so, of course, as hydrogen activation is essential for the extremely important field of hydrogenation methodology. 

The use of hydrogen as terminal reductant has been accomplished by its activation with transition metal complexes. The resulting weak M-H bonds can be used in both radical generation and reduction through HAT. In this manner, conceptually novel radical chain reactions, such as hydrogen mediated cyclizations, or metal catalyzed processes with coupled catalytic cycles for radical generation and reduction, have been realized. The latter transformations are especially attractive for enantioselective synthesis. 

---