Subject sterically hindered

The most widely used method for adding the elements of hydrogen to carbon-carbon double bonds is catalytic hydrogenation. Except for very sterically hindered alkenes, this reaction usually proceeds rapidly and cleanly. The most common catalysts are various forms of transition metals, particularly platinum, palladium, rhodium, ruthenium, and nickel. Both the metals as finely dispersed solids or adsorbed on inert supports such as carbon or alumina (heterogeneous catalysts) and certain soluble complexes of these metals (homogeneous catalysts) exhibit catalytic activity. Depending upon conditions and catalyst, other functional groups are also subject to reduction under these conditions. 

Solid-liquid phase systems with no added solvent produce esters in high yield [e.g. 2, 3] and are particularly Useful when using less reactive alkyl halides [e.g. 15], for the preparation of sterically hindered esters [16], or where other basic sites within the molecule are susceptible to alkylation, e.g. anthranilic acid is converted into the esters with minimal A-alkylation and pyridine carboxylic acids do no undergo quat-emization [17]. Excellent yields of the esters in very short reaction times (2-7 minutes) are also obtained when the two-phase system is subjected to microwave irradiation [18]. Direct reaction of the carboxylic acids with 1,2-dichloroethane under reflux yields the chloroethyl ester [19], although generally higher yields of the esters are obtained under microwave conditions [20]. 

Gibson and co-workers have introduced a well-designed latent cyclobutadiene moiety. Compound 49 reacts with a phenylethyne-cobalt complex to give PKR product 50 as a single diastereomer that resulted from the reaction at the less sterically hindered site, and 50 was subjected to the retro-Diels-Alder reaction at 205 °G under a vacuum of 6 torr to give 51 (Equation (23)). ... 

In general, simple alkyllithiums exist predominantly as either hexamers (for sterically unhindered RLi) or tetramers (for sterically hindered RLi) in hydrocarbon solvents and as tetramers in basic solvents (9-12). Polymeric organolithium compounds such as poly(styryl)lithium exist as dimers in hydrocarbon solution and are unassociated in basic solvents such as tetrahydrofuran (13-15). The state of association of poly-(dienyl)lithiums in hydrocarbon solution is a subject of current... 

It is often difficult to control the condensation reactions of silanediols so that only a single reaction occurs to give the disiloxanediol, but the sterically hindered ferrocenyl substituted diol 34 can be condensed to give 35 in good yield (equation 12). If 35 is subject to similar conditions but for 2 h at reflux temperature, then further condensation reactions... 

Nitrogen-based heterocycles can also be prepared through Ni/NHC-catalyzed cyclo addition reactions. For example, Ni/SIPr catalyzed the cycloaddition of diynes with isocyanates under the mildest conditions to date [26]. In particular, excellent yields of pyridones are obtained from diynes and isocyanates at room temperature using only 3 mol % catalyst. As shown in Eq. 8, a variety of diynes were subjected to these optimized conditions. Both aryl and alkyl isocyanates were readily converted to the respective 2-pyridone. Sterically hindered substrates appeared to have very little effect on the reaction, as excellent yields of product were obtained with bulky isocyanates and bulky diynes. 

Since epoxides are very sensitive to acid-catalyzed nucleophilic attack, whichever carbon will yield the most stable carbonium ion will be the most likely target for nucleophilic attack if an acidic group on the protein facilitates the modification reaction by hydrogen bonding to the epoxide. On the other hand, epoxides are also subject to Sfj-2 displacement reactions. In this case the carbon which will be attacked will be the least sterically hindered of the two. Unfortunately, there is no a priori way to know the mechanism of the modification reaction so that if an affinity label containing an epoxide group derivatizes a protein at least two possible products must be anticipated. 

The oxidation of substituted benzaldehydes by xanthine oxidase is sterically hindered by bulky substituents at the ortho (o) position (Table 3.5) [167], Increasing the size of the halo-substituent dramatically decreases the oxidation of the o-substituted compound, whereas that of the p-halobenzaldehyde increases due to the increased inductive effect. The positional specificity was not due to electronic effects, because the oxidation rate was also decreased with electron-donating o-substituents. Although the substrates of aldehyde oxidase have not been so rigourously examined, the enzyme does appear to be subject to similar steric considerations, as o-chloro- and o-nitrobenzaldehyde are oxidized at much lower rates than benzaldehyde itself [33]. 

Compounds of type (p -Cp)2MX2 are usually subject to a redistribution reaction such as 24.51 unless sterically hindered as in (p -C5Me5)2ThCl2 and (p -C5Me5)2UCl2. 

As mentioned previously, primary alcohols, when subjected to 1, form the corresponding carbamates, via an Sn2 mechanism, to form the corresponding olefins. For instance, 17 forms an N-carbomethoxysulfamate salt intermediate that rearranges to urethane 18 when heated. When primary sulfamate esters (19) are formed in a sterically hindered environment, the El mechanism prevails followed by rearrangement via Wagner-Meerwein, to form olefin 20.2... 

The first use of a polymer-supported Mukaiyama reagent for microwave-mediated synthesis of amides was presented in 2004 [125]. To prove its effectiveness, even in difficult coupling reactions, it was used in the microwave-accelerated synthesis of an amide from sterically hindered pivalic acid (Scheme 16.83). The mixture was subjected to microwave irradiation at 100 °C for 10 min and the desired product was obtained in 80% yield. 

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