Stoichiometric reagent

Although an auxiliary is recovered intact after the asymmetric transformation and has the potential to be recycled, there are often problems associated with the practical implementation of this concept. 

Chiral templates can be considered a subclass of chiral auxiliaries. Unlike auxiliaries that have the potential to be recycled, the stereogenic center of a template 

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In organic synthesis, two kinds of Pd compounds, namely Pd(II) salts and Pd(0) comple.xes, are used. Pd(II) compounds are used either as stoichiometric reagents or as catalysts and Pd(0) complexes as catalysts. Pd(Il) compounds such as PdCh and Pd(OAc)2 are commercially available and widely used as... 

Olefin Isomerization. Some olefins can be isomerized in the presence of metal carbonyls. The carbonyl can act as a catalyst or a stoichiometric reagent in the reaction (137). 

Catalytic kinetic resolution can be the method of choice for the preparation of enantioenriched materials, particularly when the racemate is inexpensive and readily available and direct asymmetric routes to the optically active compounds are lacking. However, several other criteria-induding catalyst selectivity, efficiency, and cost, stoichiometric reagent cost, waste generation, volumetric throughput, ease of product isolation, scalability, and the existence of viable alternatives from the chiral pool (or classical resolution)-must be taken into consideration as well... 

Catalytic reagents (as selective as possible) are superior to stoichiometric reagents. 

Enzymes are the catalyst per excellence for reactions in water, which is their natural habitat. Moreover, the use of enzymes often circumvents the need for functional group protection and deprotection steps. For example, enzymatic hydrolysis of penicillin G to 6-APA (Fig. 2.30) proceeds in one step at ambient temperature while chemical deacylation requires three steps, a temperature of - 40 C and various stoichiometric reagents, leading to a high E factor. 

However, lithinm aluminum hydride or zinc metal and HCl (5) are required as reducing agents to reduce the thiocyanate to the thiol. These reducing agents are stoichiometric reagents and aren t environmentally acceptable at this time because of their hazardous properties and waste disposal problems on a large manufacturing scale. 

A procedure that is catalytic in Bu3SnH and uses NaBH4 as the stoichiometric reagent has been developed.201 This method has advantages in the isolation and purification of product. Entry 5 is an example of this procedure. The reaction was carried... 

Aluminum chloride, used either as a stoichiometric reagent or as a catalyst with gaseous hydrogen chloride, may be used to promote silane reductions of secondary alkyl alcohols that otherwise resist reduction by the action of weaker acids.136 For example, cyclohexanol is not reduced by organosilicon hydrides in the presence of trifluoroacetic acid in dichloromethane, presumably because of the relative instability and difficult formation of the secondary cyclohexyl carbocation. By contrast, treatment of cyclohexanol with an excess of hydrogen chloride gas in the presence of a three-to-four-fold excess of triethylsilane and 1.5 equivalents of aluminum chloride in anhydrous dichloromethane produces 70% of cyclohexane and 7% of methylcyclopentane after a reaction time of 3.5 hours at... 

Asymmetric catalytic osmylation.s Chiral cinchona bases are known to effect asymmetric dihydroxylation with 0s04 as a stoichiometric reagent (10, 291). Significant but opposite stereoselectivity is shown by esters of dihydroquinine (1) and of dihydroquinidine (2), even though these bases are diastereomers rather than enantiomers. 

Titanium catalysts have long been used in electron transfer reactions involving epoxides, mostly as stoichiometric reagents. Gansauer et al. have developed a catalytic version of these reactions using titanocenes along with zinc metal to generate the active catalyst (Scheme 60). In situ reduction of Ti(IV) with zinc metal provides Ti(III) species 231, which coordinates... 

In fact, although good enantioselectivities in ADH of dialkyl substituted olefins could be previously obtained through the use of stoichiometric reagents at low temperature [34], the catalytic ADH developed by Sharpless is by far the best method that the synthetic organic chemist has presently at his hands. l... 

Heteroatom-substituted (Fischer-type) carbene complexes are mostly used as stoichiometric reagents. For this reason only carbene complexes of reasonably cheap metals, such as chromium, molybdenum, tungsten, or iron have found broad application in organic synthesis. 

Because electrophilic carbene complexes can cyclopropanate alkenes under mild reaction conditions (Table 3.1) [438,618-620], these complexes can serve as stoichiometric reagents for the cyclopropanation of organic compounds. Thoroughly investigated carbene complexes for this purpose are neutral complexes of the type (C0)5M=CR2 (M Cr, Mo, W) and cationic iron(IV) carbene complexes. The mechanism of cyclopropanation by electrophilic carbene complexes has been discussed in Section 1.3. 

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