Reagent selection

Derivatiziag an organic compound for analysis may require only a few drops of reagent selected from silylatiag kits suppHed by laboratory supply houses. Commercial syathesis of penicillins requires silylatiag ageats purchased ia tank cars from the manufacturer (see Antibiotics, P-LACTAMS-penicillins AND others). 

In practice, these cleavage reactions are difficult to control, and usually mixtures of products form, even with stoichiometric quantities of reagents. Selectivity improves at lower temperatures, higher dilutions, and in the presence of polar solvents, eg, pyridine. This method is not used to prepare the lower alkylated—arylated organotins outside the laboratory. 

BzCN, Et3N, CH3CN, 5 min-2 h, >80% yield/ This reagent selectively acylates a primary hydroxyl group in the presence of a secondary hydroxyl... 

This reagent selectively protects a primary alcohol... 

BOC-ON=C(CN)Ph, Et3N, 25°, several hours, 72-100% yield. This reagent selectively protects primary amines in the presence of secondary amines. ... 

A,A-Diacetyl-2-trifluoromethylaniline, organic solvents, 3-24 h, rt or reflux, 54-99% yield. Acylation selectivity is a very sensitive function of steric effects this reagent selectively acylates pyrrolidine over piperidine (15 1). It is more selective than the diacetylaminoquinazolinones. ... 

CF3C02Et, Et3N. CH3OH, 25°, 15-45 h, 75-95% yield. A polymeric version of this approach has also been developed. This reagent selectively protects a primary amine in the presence of a secondary amine. ... 

Leach AR, Bradshaw J, Green DVS, Hann MM, Delany JJ III. Implementation of a system for reagent selection and library enumeration, profiling and design. / Chem Inf Comput Sci 1999 39 1161-72. 

Another type of mixed cyanocuprate has both methyl and alkenyl groups attached to copper. Interestingly, these reagents selectively transfer the alkenyl group in conjugate addition reactions.16 These reagents can be prepared from alkynes via hydrozirconation, followed by metal-metal exchange.17... 

The first step in the CSD process is solution preparation, which involves reagent selection (chemical precursors) and solvent choice.1,5-12,16 During solution preparation, other chemical modifiers may also be added to the solution to facilitate or limit chemical reactivity. Also during this stage of the process, identification of appropriate reaction conditions to promote other desired changes in precursor nature or solution characteristics is also considered. The goal for solution preparation is to develop a homogeneous solution of the necessary cation species that may later be applied to a substrate. 

For polymeric CSD processes, three classes of metal organic (metallo-organic) compounds are used most often as starting reagents metal alkoxides, metal carboxylates, and metal beta-diketonates. These species differ in their solubility and reactivity, as well as their tendency to react with one another, all of which are factors that may influence starting reagent selection. Representative structures of these classes of precursors are illustrated in Fig. 2.2.8... 

Reagent selection, solvent, and choice of solution reaction conditions serve to define the nature of the solution precursor species that are formed and that will be used for him deposition. The characteristics of these solution species serve to define him processing characteristics, including aspects such as cracking tendency,50 organic burn-out temperature,48 and for crystalline films, crystallization temperature.49,51... 

Complete control of the diastereoselectivity of the synthesis of 1,3-diols has been achieved by reagent selection in a one-pot tandem aldol-reduction sequence (see Scheme l). i Anti-selective method (a) employs titanium(IV) chloride at 5°C, followed by Ti(OPr )4, whereas method (b), using the tetrachloride with a base at -78 °C followed by lithium aluminium hydride, reverses the selectivity. A non-polar solvent is required (e.g. toluene or dichloromethane, not diethyl ether or THF), and at the lower temperature the titanium alkoxide cannot bring about the reduction of the aldol. Tertiary alkoxides also fail, indicating a similarity with the mechanism of Meerwein-Ponndorf reduction. 

All of the polymers have resonances at unusually high field (>3.0 X) suggesting that some of the protons are held in close proximity to adjacent aromatic rings. This possibility is further supported by the observation that relaxation reagents selectively collapse the lowest field peak, which is expected to be the resonance of protons least encumbered by neighboring aromatic rings and therefore more accessible to the relaxation reagent. 

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