Amination reactions reductions, carbon-nitrogen

The reductive ring opening of six-membered nitrogen-containing heterocycles was studied with A-phenyltetrahydroisoquinoline (391). Its lithiation with lithium and a catalytic amount of DTBB (4.5%) afforded the benzylic intermediate 392, which was allowed to react with electrophiles giving, after hydrolysis, functionalized amines 393 (Scheme 110) . It is noteworthy that in the reaction with carbon dioxide, instead of the corresponding lactam, amino acid 393 with X = CO2H was exclusively isolated. 

A carbon-nitrogen bond is broken during a chemical follow-up reaction in many ring closures or ring contractions [192] of heterocyclic compounds in which a nucleophilic attack on an imine followed by an elimination of the amine or ammonia takes place. This is discussed in Chapter 18, in which is also mentioned the loss of ammonia from the primarily formed dihydro compound in the reduction of certain aminoheterocyclic compounds. 

Many aldehydes (RCHO) and ketones (R2CO) are converted into amines by reductive amination reduction in the presence of ammonia. Reduction can be accomplished catalytically or by use of sodium cyanohydridoboratc, NaBHjCN. Reaction involves reduction of an intermediate compound (an imine RCH NH or R2C NH) that contains a carbon-nitrogen double bond. 

The Fixation of Carbon Dioxide and Nitrogen.—As reduced carbon compounds are convenient fuels, the possibility of achieving the reductive fixation of COa in vitro is appealing, if remote. There are very few data on photochemical reactions involving carbon dioxide, for it has no low-lying excited states and has not historically been of much interest to the photochemist. However, what appears to be the first example of photofixation of COa in a non-biological system has been briefly reported.18 Photoirradiation (with a high-pressure mercury lamp) of phenanthrene in the presence of an amine and C02 in a polar solvent (MeaSO or HCONMea) yielded 9,10-dihydrophenanthrene-9-carboxylic acid, in unspecified quantum yield. The mechanism appears to involve formation of COaT by electron transfer from the photoexcited amine, followed by attack of COaT on position 9 of phenanthrene. Similar reductive carboxylation of anthracene, pyrene, naphthalene, and biphenyl was observed. 

One of the chief values of imines is that the carbon-nitrogen double bond can be reduced to a carbon-nitrogen single bond by hydrogen in the presence of a nickel or other transition metal catalyst. By this two-step reaction, called reductive amination, a primary amine is converted to a secondary amine by way of an imine, as illustrated by the conversion of cyclohexylamine to dicyclohexylamine ... 

---