Carbonyl and Imine Reductions

TH of ketones by ruthenium-based catalysts represents the vast majority of protocols, following the seminal studies of the groups of Noyori and Zhang. For TH of carbonyl compounds by i-PrOH several mechanisms have been proposed. The inner-sphere TH mechanism involves four steps (i) coordination 

Quinoline-functionalized NHC complex 53 was tested with various ketones in refluxing KOH/z-PrOH. Substrates with electron-withdrawing substituents were more reactive which was taken as indicative of the hydridic nature of the reducing species. 

Finally, a Pd complex 67 was applied to TH of alkynes (Equation (13.2)). Internal substrates were selectively hydrogenated with excellent yields and over-reduction to alkane was almost negligible.  

---

Nucleophilic substitutions at the azepine nucleus are confined mainly to derivatives of hydroazepines such as lactim ethers, imidoyl chlorides and amidines, and to hydride ion reductions of carbonyl and imine groups (see Section 5.16.3.5.2). In addition some transan-nular nucleophilic displacements have been described, but such reactions are not as common with azepines as with larger ring heterocycles. 

A full report has been published on the resonance Raman spectra of M(CO)4 (di-imine) (M = Cr, Mo, or W), and their relevance to the photosubstitution reactions of the complexes has been discussed.It was observed that the complexes are only photoactive if (CO ) shows resonance enhancement of Raman intensity. This enhancement is a result of the CT transition causing delocalization of the negative charge over the cw-carbonyls and hence reduction of 7i-backbonding with the metal and weakening of the M—CO bond. 

Sodium cyanoborohydride is remarkably chemoselective. Reduction of aldehydes and ketones are, unlike those with NaBH pH-dependent, and practical reduction rates are achieved at pH 3 to 4. At pH 5—7, imines (>C=N—) are reduced more rapidly than carbonyls. This reactivity permits reductive amination of aldehydes and ketones under very mild conditions (42). 

Catalytic Reductive Coupling of Alkenes and Alkynes to Carbonyl Compounds and Imines Mediated by Hydrogen... 

Scheme 13 Enantioselective carbonyl (Z)-dienylation via reductive coupling of acetylene to aldehydes and imines mediated by hydrogen...

Ihmels H, Otto D (2005) Intercalation of Organic Dye Molecules into Double-Stranded DNA - General Principles and Recent Developments. 258 161-204 Iida H, Krische MJ (2007) Catalytic Reductive Coupling of Alkenes and Alkynes to Carbonyl Compounds and Imines Mediated by Hydrogen. 279 77-104 Imai H (2007) Self-Organized Formation of Hierarchical Structures. 270 43-72 Indelli MT, see Chiorboli C (2005) 257 63-102 Inoue Y, see Borovkov VV (2006) 265 89-146 Ishii A, Nakayama J (2005) Carbodithioic Acid Esters. 251 181-225 Ishii A, Nakayama J (2005) Carboselenothioic and Carbodiselenoic Acid Derivatives and Related Compounds. 251 227-246... 

Nitro compounds are versatile precursors for diverse functionalities. Their conversion into carbonyl compounds by the Nef reaction and into amines by reduction are the most widely used processes in organic synthesis using nitro compounds. In addition, dehydration of primary nitro compounds leads to nitrile oxides, a class of reactive 1,3-dipolar reagents. Nitro compounds are also good precursors for various nitrogen derivatives such as nitriles, oximes, hydroxylamines, and imines. These transformations of nitro compounds are well established and are used routinely in organic synthesis. 

Under the conditions of iridium-catalyzed hydrogenation, alkyne-carbonyl and alkyne-imine reductive coupling occurs in the absence of stoichiometric byproducts. For example, iridium-catalyzed hydrogenation of nonconjugated alkynes in the presence of ot-ketoesters delivers the corresponding a-hydroxy esters in... 

In the presence of excess monoalkylamine, carbonyl compounds in aqueous solution are in equilibrium with the corresponding imine. In most cases these imines cannot be isolated but they are reduced at a less negative potential than the carbonyl compound. Selective reduction of such equilibrium mixtures is a useful route to alkylamines from ketones in yields of 70-90%. The process fails with hindered ketones such as camphor and with bulky amines such as fert.-butyl amine. Overall the reaction has advantages of lower costs and simpler work-up compared to the use of cyanoborohydride reducing agents. In the electrochemical reaction, protonation of carbanion intermediates occurs from the more hindered side and where two isomeric products are fomied, the least hindered amine predominates [193]. 

---