Alkenes, reductive reagent

Secondary amines can be added to certain nonactivated alkenes if palladium(II) complexes are used as catalysts The complexation lowers the electron density of the double bond, facilitating nucleophilic attack. Markovnikov orientation is observed and the addition is anti An intramolecular addition to an alkyne unit in the presence of a palladium compound, generated a tetrahydropyridine, and a related addition to an allene is known.Amines add to allenes in the presence of a catalytic amount of CuBr " or palladium compounds.Molybdenum complexes have also been used in the addition of aniline to alkenes. Reduction of nitro compounds in the presence of rhodium catalysts, in the presence of alkenes, CO and H2, leads to an amine unit adding to the alkene moiety. An intramolecular addition of an amine unit to an alkene to form a pyrrolidine was reported using a lanthanide reagent. 

Olefin inversion (c/. 7, 338). Trifluoroacetyl chloride reacts with 1,2-dialkyl epoxides in DMF stereospeciflcally by trans opening to give u/c-chlorohydrin trifluoroacetates. These products are reduced stcrcospccifically by Nal to alkenes with. ryn-elimination to give inverted alkenes. Reductions with zinc arc less selective. Inversion of olefins is also possible by addition of NCS in CFjCOOH (actual reagent is trifluoroacetyl hypochlorite) followed by reduction with Nal. 

Useful for homogeneous reduction of alkenes. As a consequence of the reagent bulk, it is understandable that the reactivity of alkene reduction is dependent on substitution the less-substituted alkenes react faster. Also, reduction occurs from the less-hindered face in a cis-stereochemistry. Many other functional groups are tolerated by conditions. 

Alkene Reduction. Sulfonylhydrazides deconpose into di-imide and sulfonic acid when heated. For this reason, they have heen utilized in the reduction of unsaturated compounds. When 2,4,6-triisopropylbenzenesulfonylhydrazide (TPSH) is used, the diimide can be generated at room temperature in the presence of base to reduce mono- or disubstituted double bonds selectively in the presence of trisuhstituted alkenes (eq 15). This reduction method tolerates sensitive groups such as esters, ketones, or organocohalt complexes and has heen applied in the preparation of polymers and in the synthesis of natural products (eq 16). This reagent was not effective in the reduction of resin-hound cyclic peptides. ... 

The biosynthesis of fatty acids is accomplished two carbons at a time by an enzyme complex called fatty acid synthetase. The biochemical reactions involved in fatty acid synthesis are described in Special Topic E WileyPLUS). Each of these biochemical reactions has a counterpart in synthetic reactions you have studied. Consider the biochemical reactions involved in adding each —CH2CH2— segment during fatty acid biosynthesis (those in Special Topic E that begin with acetyl-S-ACP and malonyl-S-ACP, and end with butyryl-S-ACP). Write laboratory synthetic reactions using reagents and conditions you have studied (not biosynthetic reactions) that would accomplish the same sequence of transformations (i.e., the condensation-decarboxylation, ketone reduction, dehydration, and alkene reduction steps). 

Figure 2.8 Sample GSK reagent guide for alkene reduction.

Many other functional groups are also reactive under conditions of catalytic hydrogenation. The reduction of nitro compounds to amines, for example, usually proceeds very rapidly. Ketones, aldehydes, and esters can all be reduced to alcohols, but in most cases these reactions are slower than alkene reductions. For most synthetic applications, the hydride transfer reagents to be discussed in Section 5.2 are used for reduction of carbonyl groups. Amides and nitriles can be reduced to amines. Hydrogenation of amides requires extreme conditions and is seldom used in synthesis, but reduction of nitriles is quite useful. Scheme 5.3 gives a summary of the approximate conditions for catalytic reduction of some common functional groups. 

Disparlure, the pheromone of the gypsy moth, can be prepared by reduction of an alkyne followed by epoxidation of the alkene. What alkyne is required What is the configuration of the alkene What reagents are required for reduction of the alkyne ... 

A catalyst, usually acid, is required to promote chemoselective and regioselective reduction under mild conditions. A variety of organosilanes can be used, but triethylsilane ia the presence of trifiuoroacetic acid is the most frequendy reported. Use of this reagent enables reduction of alkenes to alkanes. Branched alkenes are reduced more readily than unbranched ones. Selective hydrogenation of branched dienes is also possible. 

Grignard reagents that contain a /3-hydrogen—e.g. 15—can reduce a carbonyl substrate by transfer of that hydrogen as a side-reaction. The so-called Grignard reduction is likely to proceed via a six-membered cyclic transition state 16 the alkyl group of alkylmagnesium compound 15 is thereby converted into an alkene 17. 

Methods of synthesis for carboxylic acids include (1) oxidation of alkyl-benzenes, (2) oxidative cleavage of alkenes, (3) oxidation of primary alcohols or aldehydes, (4) hydrolysis of nitriles, and (5) reaction of Grignard reagents with CO2 (carboxylation). General reactions of carboxylic acids include (1) loss of the acidic proton, (2) nucleophilic acyl substitution at the carbonyl group, (3) substitution on the a carbon, and (4) reduction. 

These alkene isomers are separately available (4) by treatment of threo-S-trimethylsilyloctan-4-ol, prepared by reduction of the corresponding ketone with DIBAL in pentane at —120°C, with base or acid. The preparation of 5-trimethylsilyloctan-4-one itself illustrates three general procedures the addition of alkyl lithium reagents to vinylsilanes to generate a-lithiosilanes, the preparation of complex /5-hydroxysilanes, as diastereoisomeric mixtures, and the oxidation of such compounds to /8-ketosilanes... 

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