Reagents reducing

Note that NaBH4 reduces aldehydes (and ketones) but not esters while L1A1H4 reduces just about all carbonyl compounds. Neither reagent reduces an isolated deuble bond. 

In cases where Noyori s reagent (see p. 102f.) and other enantioselective reducing agents are not successful, (+)- or (—)-chlorodiisopinocampheylborane (Ipc BCl) may help. This reagent reduces prochiral aryl and tert-alkyl ketones with exceptionally high enantiomeric excesses (J. Chandrasekharan, 1985 H.C. Brown, 1986). The initially formed boron moiety is usually removed hy precipitation with diethanolamine. Ipc2BCl has, for example, been applied to synthesize polymer-supported chiral epoxides with 90% e.e. from Merrifield resins (T. Antonsson, 1989). 

Reaction between an absorbed solute and a reagent reduces the equilibrium partial pressure of the solute, thus increasing the rate of mass transfer. The mass-transfer coefficient hkewise is enhanced, which contributes further to increased absorption rates. Extensive theoretical analyses of these effects have been made, but rather less experimental work and design guidehnes. 

The selection of appropriate polymeric bromine reagent according to the polymeric structure (step of crosslinking, porosity, step of dilution with styrene, granulation, type of heterocyclic ring incorporated, numbers of N) is very important. From production point of view the use of polymeric reagents reduces the costs for solvents and working hours. 

Le Chatelier s principle is a compact summary of how different factors influence equilibrium. Introducing a reagent causes a reaction to proceed in the direction that consumes the reagent. Reducing the temperature removes heat from the system and causes the reaction to produce heat by proceeding in the exothermic direction. 

Microwave irradiation coupled with polyethylene glycol)-supported Burgess reagent reduced the reaction time to 2-4 min and led to improved yields. Use of harsh reagents, e. g. SOCl2, POCl3, and polyphosphoric acid, for the cydodehydration were avoided. 

Reduction of carbonyl compoundsThe reagent reduces aldehydes or ketones to alcohols in refluxing cyclohexane in 2-5 hours yields are 60-80%. The reduction probably involves hydride transfer from the carbon beta to the magnesium center. 

Several reagents reduce aldehydes preferentially to ketones in mixtures of both. Very high selectivity was found in reductions using dehydrated aluminum oxide soaked with isopropyl alcohol and especially diisopropylcarbinol [755], or silica gel and tributylstamane [756]. The best selectivity was achieved with lithium trialkoxyalumimm hydrides at —78°. In the system hexanal/ cyclohexanone the ratio of primary to secondary alcohol was 87 13 at 0° and 91.5 8.5 at —78° with lithium tris(/er/-butoxy)aluminum hydride [752], and 93.6 6.4 at 0° and 99.6 0.4 at —78° with lithium tris(3-ethyl-3-pentyl-oxy)aluminum hydride [752],... 

Lithium aluminum hydride reduced p-benzoquinone to hydroquinone (yield 70%) [576] and anthraquinone to anthrahydroquinone in 95% yield [576]. Tin reduced p-benzoquinone to hydroquinone in 88% yield [174] Procedure 35, p. 214). Stannous chloride converted tetrahydroxy-p-benzoquinone to hexa-hydroxybenzene in 70-77% yield [929], and 1,4-naphthoquinone to 1,4-di-hydroxynaphthalene in 96% yield [180]. Other reagents suitable for reduction of quinones are titanium trichloride [930], chromous chloride [187], hydrogen sulfide [248], sulfur dioxide [250] and others. Yields are usually good to excellent. Some of the reagents reduce the quinones selectively in the presence of other reducible functions. Thus hydrogen sulfide converted 2,7-dinitro-phenanthrene quinone to 9,10-dihydroxy-2,7-dinitrophenanthrene in 90% yield [248]. 

Another hydride for the preparation of aldehydes from acyl chlorides is obtained by treatment of a mixture of cuprous chloride and triphenylphos-phine, trimethyl phosphite or triisopropyl phosphite in chloroform with an ethanolic solution of sodium borohydride. Such reagents reduce acyl chlorides to aldehydes in acetone solutions at room temperature in 15-90 minutes in yields ranging from 57% to 83% [115],... 

Mechanistically, the reaction proceeds through an alkynyl chloro alkoxide which, when treated with the reducing agent, is hydroaluminated to yield the vinyl alanate, which subsequently undergoes a facile pinacol-like 1,2-rearrangement. Excess hydride reagent reduces the intermediate alkenyl ketone and the resulting 2-alkenyl carbinol 1s isolated upon aqueous workup (Scheme). Table I contains representative examples. 

Reduction of aldehydes and ketones.4 In the presence of hydrogen chloride (or a Lewis acid), the reagent reduces carbonyl compounds to alcohols in high yield and with high stereoselectivity. However, the reduction of hindered ketones requires a strong Lewis acid (A1C13). Aldehydes are reduced so much more readily than ketones that selective reductions are possible. The reagent is also useful for reduction of a,/5-cnals to allylic alcohols. 

Reduction of propargylic bromides. The reagent reduces propargylic bromides to allenes as the major products in the presence of a proton donor (water). Addition of HMPT suppresses reduction to acetylenes.2 Use of a protonic chiral reagent results in optically active allcncs. Highest inductions have been observed with ( —)-menthol and (— )-bomeol (about 20% ee).J... 

Reduction of arenesulfonic acids to arenethiols.1 The reagent reduces arenesulfononic acids to thiols in high yield. In actual practice, iodine can be used in catalytic amounts. The usual stoichiometry employed is ArS03H/I2/ P(C H,)3 = 2 1 10. 

Reduction of sulfonic odds to disulfides. This combination of reagents reduces aryl sulfonyl halides and sulfonic acids to disulfides in yields generally >90%. 

Besides heterogeneous and homogeneous catalytic hydrogenations, chemical reductions can also transform alkynes to cis alkenes. Interestingly, activated zinc in the presence of a proton donor (alcohol), although a dissolving-metal reagent, reduces disubstituted alkynes to cis alkenes 199... 

In essence, hydride ion reagents reduce chromanones to chromanols (81T2613), the reaction often being stereospecific (70JCS(C)1006). 

Selective reductions This complex borohydride is particularly useful for selective 1,2-reduction of acyclic a,/ -cnones and of conjugated cyclohexenones to allylic alcohols. However, the 1,2-selectivity is less marked with conjugated cyclopentenones. The reagent reduces unhindered cyclic ketones to the more stable (equatorial) alcohols with stereoselectivity greater than that of sodium borohydride. 

---