Exclusive reactions

Because the film growth rate depends so strongly on the electric field across it (equation 1.115), separation of the anodic and cathodic sites for metals in open circuit is of little consequence, provided film growth is the exclusive reaction. Thus if one site is anodic, and an adjacent site cathodic, film thickening on the anodic site itself causes the two sites to swap roles so that the film on the former cathodic site also thickens correspondingly. Thus the anodic and cathodic sites of the stably passive metal dance over the surface. If however, permanent separation of sites can occur, as for example, where the anodic site has restricted access to the cathodic component in the electrolyte (as in crevice), then breakdown of passivity and associated corrosion can follow. 

Rhodium carboxylates have been found to be effective catalysts for intramolecular C—H insertion reactions of a-diazo ketones and esters.215 In flexible systems, five-membered rings are formed in preference to six-membered ones. Insertion into methine hydrogen is preferred to a methylene hydrogen. Intramolecular insertion can be competitive with intramolecular addition. Product ratios can to some extent be controlled by the specific rhodium catalyst that is used.216 In the example shown, insertion is the exclusive reaction with Rh2(02CC4F9)4, whereas only addition occurs with Rh2(caprolactamate)4, which indicates that the more electrophilic carbenoids favor insertion. 

Thus, iodoacetamide has the highest reactivity toward cysteine sulfhydryl residues and may be directed specifically for —SH blocking. If iodoacetamide is present in limiting quantities (relative to the number of sulfhydryl groups present) and at slightly alkaline pH, cysteine modification will be the exclusive reaction. For additional information on a-haloacetate reactivities and a protocol for blocking, see Section 4.2 (this chapter). 

As an extension towards azolic fungicides, phenacylation was next examined. Under the action of microwave irradiation, exclusive reaction in position 1 (or equivalent 2) occurred whereas mixtures of N1 N4 and N14 products were obtained by A under the same conditions [115] (Eq. (65) and Tab. 3.26). 

The intramolecular cycloaddition reactions of the nitrile oxides 357 (n = 1, 2, 3, 9), obtained in situ from the 2,5-difunctional furan hydroximoyl chlorides or nitro compounds (415) has specific features because of the 2,5-arrangement of two open chains bearing acetylenic and fulminic moieties. Only with 357 (n = 3) is the expected furanoisoxazolophane 358 formed, in acceptable yield. Compound 357 ( =9) gives a complex product mixture whereas 357 ( = 1, 2) gives rise to the exclusive reaction of the dipole with a double bond of the furan system. 

The extraordinary regioselectivity shown with the 2-methylthio group was not displayed by the analogous sulfone 238, which underwent exclusive reaction at the 4-position <2000SL905>. 

At pH <4 both 2"+ and 3,+ undergo C-H deprotonation as the exclusive reaction while in basic solution they behave as oxygen acids undergoing OH-induced OH deprotonation in a diffusion controlled process. Results indicate that in alkylaromatic radical cations, overlap between the scissile bond and the jt-system containing the unpaired electron is a fundamental requirement for the bond cleavage. The observation that for both 2 and 3, the 1,2-H atom shift occurs more rapidly than C-C ft-scission whereas the radical derived from OH de-... 

The high degree of stereoselectivity claimed is surprising. In the system used, no directive effects causing exclusive reaction of the C-4 hydroxyl group of D-glucose are apparent. Indeed, in an independent study using this... 

In most cases, O-alkylation predominates. However, with 2-propenyl halides either reaction can be made essentially the exclusive reaction by proper choice of solvent. With sodium benzenolate the more polar solvents, such as... 

CO2 insertion into TT-allylpalladium complexes exhibits the opposite regioselectivity, with, for example, exclusive reaction at the more substituted terminus of a butenylpalladium complex (equation... 

Asymmetric epoxidation of conjugated dienes and enynes catalysed by the chiral man-ganese(EI) complex 232 give monoepoxides exclusively reactions of cw-enynes give /raw.v-alkynyl epoxides as the major products with a high level of asymmetric induction362. 

The approach for the enantioselective aldol reaction based on oxazolidinones like 22 and 23 is called Evans asymmetric aldol reaction.14 Conversion of an oxazolidinone amide into the corresponding lithium or boron enolates yields the Z-stereoisomers exclusively. Reaction of the Z-enolate 24 and the carbonyl compound 6 proceeds via the cyclic transition state 25, in which the oxazolidinone carbonyl oxygen and both ring oxygens have an anti conformation because of dipole interactions. The back of the enolate is shielded by the benzyl group thus the aldehyde forms the six-membered transition state 25 by approaching from the front with the larger carbonyl substituent in pseudoequatorial position. The... 

When the alkoxyl radical and the hydrogen to be abstracted are not properly disposed for the Barton reaction, the reactions of the alkoxyl radical, for example -fragmentation, intramolecular addition to the double bond, disproportionation or a-hydrogen fission, and intermolecular hydrogen abstraction, compete with the Barton reaction or result in an exclusive reaction. Among these reactions, /l-frag-... 

Asymmetric syntheses can be carried out even more easily and elegantly than by reacting achiral substrates with enantiomerically pure chiral reagents if one allows the substrate to react with an enantiomerically pure species formed in situ from an achiral reagent and an enantiomerically pure chiral additive. The exclusive reaction of this species on the substrate implies that the reagent itself reacts substantially slower with the substrate than its adduct with the chiral additive. If high stereoselectivity is observed, it is exclusively due to the presence of the additive. The chiral additive speeds up the reaction. This is an example of ligand accelerated asymmetric catalysis. 

Section 3.4.6—which, by the way, referred to Sharpless epoxidations—you learned that catalytic asymmetric syntheses are among the most elegant asymmetric syntheses and that they can rely on a substrate reacting (exclusively) with an enantiomerically pure reagent that is formed in situ from an achiral precursor molecule and a catalytic amount of an enantiomerically pure additive. It was emphasized that an exclusive reaction of this precursor molecule/additive complex with the substrate takes place if it is much more reactive than the achiral precursor molecule without the enantiomerically pure additive. If under these circumstances additive control of stereoselectivity occurs, this principle even allows recovery of the enantiomerically pure additive after the conversion, which is more convenient than if it could only be released from the product via a chemical reaction. 

With its low yield of 40 % entry 1 is exceptional, since in this case very likely missing steric hindrance causes considerable self condensation diming enolate generation 69). This process has been reported as the exclusive reaction in attempts to deprotonate the unsubstituted ethyl cyclopropanecarboxylate 70). Decreased CH-acidity of the starting material and increased reactivity of the corresponding enolate — both caused by I-strain in the intermediate71) — should be responsible for this self condensation. It proceeds in the deprotonation phase, if not prevented by additional substituents as in almost all other cases in Table 3. 

Reactions of unsymmetrical methylene 1,3-dicarbonyl compounds with enol ethers have been investigated by Yamauchi et al. [137]. As we have mentioned earlier, the a,/ -unsaturated ketone moiety in alkylidene-/ -ketoesters reacts exclusively as the oxabutadiene. However, high regioselectivity is also observed with mixed alkyl-phenyl-1,3-diketones with exclusive reaction of the aliphatic carbonyl group, whereas in alkylidene-1,3-dicarbonyl compounds bearing an aldehyde and a keto-moiety, the a,/J-unsaturated aldehyde reacts preferentially as oxabutadiene, but not exclusively [130a]. 

Monomer complex of /-BuLi with l,4,7-trimethyl-l,4,7-triazacyclononone 9 is identified by 13C NMR and it is stable in pentane at temperatures up to 20°C and (Scheme 38) <1997T9977>. Conversely, lithiation of iV-Me was the exclusive reaction with -BuLi and r-BuLi, as indicated by the formation of TMS derivatives 185, isolated after silylation of the reaction mixture. This result evidenced the existence of uncoordinated iV-Me groups in complexes with -BuLi and r-BuLi. Dimeric structure 184 was suggested based on decreasing tendency to form monomer complexes going from /-BuLi via r-BuLi to -BuLi. 

The difference in reactivity between the aryl iodide and bromide was exploited by Sulikowski in the synthesis of a mytomycin skeleton [69]. The desired arylamine was prepared in 66% yield with exclusive reaction at the iodide, Eq.(39). 

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