Isomers, different reaction sequences

The alkyl carbonium ions which result from these reversible, relatively unselective hydride abstractions then undergo a series of 1,2- (Wagner-Meerwein) or 1,3- (protonated cyclopropane) rearrangements which eventually result in the formation of the thermodynamically most stable products. The number of different reaction sequences by which one may rationalize the formation of a given products is, of course, necessarily large. A variety of independent pathways are generally available for the interconversion of the isomers of a given species by successive alkyl shifts. 

Different reaction sequences can lead to different (octahedral) isomers (Py = pyridine, C5H5N, andX=Ns, NCS orBr) ... 

Positive-Tone Photoresists based on Dissolution Inhibition by Diazonaphthoquinones. The intrinsic limitations of bis-azide—cycHzed mbber resist systems led the semiconductor industry to shift to a class of imaging materials based on diazonaphthoquinone (DNQ) photosensitizers. Both the chemistry and the imaging mechanism of these resists (Fig. 10) differ in fundamental ways from those described thus far (23). The DNQ acts as a dissolution inhibitor for the matrix resin, a low molecular weight condensation product of formaldehyde and cresol isomers known as novolac (24). The phenoHc stmcture renders the novolac polymer weakly acidic, and readily soluble in aqueous alkaline solutions. In admixture with an appropriate DNQ the polymer s dissolution rate is sharply decreased. Photolysis causes the DNQ to undergo a multistep reaction sequence, ultimately forming a base-soluble carboxyHc acid which does not inhibit film dissolution. Immersion of a pattemwise-exposed film of the resist in an aqueous solution of hydroxide ion leads to rapid dissolution of the exposed areas and only very slow dissolution of unexposed regions. In contrast with crosslinking resists, the film solubiHty is controUed by chemical and polarity differences rather than molecular size. 

The unique antagonistic features of the (butadiene)zirconocene isomers 3a/5a have been used as a probe for the elucidation of organometaUic reaction mechanisms. In some cases it was possible to distinguish between mechanistic alternatives by simply allowing the isomeric substrates 3 and 5 to compete for a reagent. An example is as follows. Transition metal-induced C—C coupling between a conjugated diene and an olefin can occur by two basic ly different types of reaction sequence. Either a new C— C bond can be formed by olefin insertion into a metal-carbon bond of a (o--allyl)M-type intermediate (24) (95), or, alternatively, the alkene may... 

The positional isomers (265, 266 and 672) showed rather similar electronic and H-n.m.r. spectra, and the appreciable differences known for o-, m- and p-terphenyls could not be observed. The dimethoxy derivative of the para isomer (268) was prepared by the reaction sequence outlined below ... 

Time-resolved IR spectroscopy was used to identify cis and trans isomers [W(C0)4L(S)J, L = PPh3, P(0-i-Pr)3, or P(OEt)3 and S = heptane. These isomers were prepared by photolysis of [W(CO)5L] and cw-[W(CO)4L(pip)]. In heptane the cis isomer is shorter lived than the trans isomer there is no evidence for interconversion of the two. The two isomers react with CO with different rates, and the rate increases with the size of L. The term Token Ligand is coined to indicate that there is a specific interaction between a solvent molecule and what would otherwise be a vacant coordination site. Reactions of ds-[W(CO)4L(pip)] with L, L and V = phosphine or phosphite, proceed by reversible pip dissociation, forming square-pyramidal [W(CO)4L] in which L is equatorially coordinated. The general reaction sequence is given in Scheme 6. Dissociative loss of chlorobenzene from ci5-[W(C0)4 P(0-/-Pr)3 (PhCl)] displays activation parameters of A/Zj = 13.0 kcal mol and ASf = -f 5.6 cal K" mol". Rate constants for thermal reactions of ci5-[W(CO)4(L)(PhCl)] with P(0-f-Pr)3, given in Table 10.5, are predominantly... 

In some cases a choice of multicomponent or linear protocol for the treatment of pyruvic acids, aminoazole, and aldehydes allows obtaining different heterocycles. For instance, MCR involving 5-aminopyrazoles or sequence pathway via preliminary synthesis of arylidenpyruvic acids led to positional isomers 36 and 37, respectively (Scheme 15) [4, 61, 68]. It is interesting to note that the same strategy applied to 3-amino-l,2,4-triazole or to amino-W-aryl-lH-pyrazole-4-carboxamide reactions gave no effect and the final compound for both the protocols were the same [52, 61, 62]. 

The total synthesis of carbazomycin D (263) was completed using the quinone imine cyclization route as described for the total synthesis of carbazomycin A (261) (see Scheme 5.86). Electrophilic substitution of the arylamine 780a by reaction with the complex salt 779 provided the iron complex 800. Using different grades of manganese dioxide, the oxidative cyclization of complex 800 was achieved in a two-step sequence to afford the tricarbonyliron complexes 801 (38%) and 802 (4%). By a subsequent proton-catalyzed isomerization, the 8-methoxy isomer 802 could be quantitatively transformed to the 6-methoxy isomer 801 due to the regio-directing effect of the 2-methoxy substituent of the intermediate cyclohexadienyl cation. Demetalation of complex 801 with trimethylamine N-oxide, followed by O-methylation of the intermediate 3-hydroxycarbazole derivative, provided carbazomycin D (263) (five steps and 23% overall yield based on 779) (611) (Scheme 5.91). 

Below the temperatures for the NTC regime, the peroxy radical (ROO) may be involved in a chain-branching sequence of reactions that is responsible for the positive temperature dependence. The oxidation rate varies significantly between different hydrocarbons or hydrocarbon isomers, depending on their structure. The first step is an internal isomerization,... 

Many enzymes exist within a cell as two or more isoenzymes, enzymes that catalyze the same chemical reaction and have similar substrate specificities. They are not isomers but are distinctly different proteins which are usually encoded by different genes.22 23 An example is provided by aspartate aminotransferase (Fig. 2-6) which occurs in eukaryotes as a pair of cytosolic and mitochondrial isoenzymes with different amino acid sequences and different isoelectric points. Although these isoenzymes share less than 50% sequence identity, their internal structures are nearly identical.24-27 The two isoenzymes, which also share structural homology with that of E. coli,28 may have evolved separately in the cytosol and mitochondria, respectively, from an ancient common precursor. Tire differences between them are concentrated on the external surface and may be important to as yet unknown interactions with other protein molecules. 

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