Group isomerization

FIGURE 6.10 The side chain of histidine is readily acylated (A) by activated residues. The imidazolide produced is an activated species similar to the intermediate generated by reaction (B) of a carboxylic acid with coupling reagent carbonyldiimidazole. (Staab, 1956). Imida-zolides acylate amino and hydroxyl groups. Isomerization of histidyl during activation results from abstraction (C) of the a-proton by the 7t-nitrogen. 

It should be noted that there are some limitations to the use of the tert-butyldimethylsilyl group (and, presumably, other trialkylsilyl groups) for the selective protection of hydroxyl groups. Isomerization between 0-2 and 0-3 occurs with 2 - and 3 -0-(tert-butyldimethyl-silyl) derivatives of nucleosides when they are in prolonged contact with silica gel,309 or when stored in aqueous pyridine, aqueous triethyl-... 

The ers-forms of arylthiocarbazone derivatives (RNHN=C(SCH3)-N=NR) with ortho groups isomerize readily on exposure to diffuse daylight.61... 

The products of oxidative addition of acyl chlorides and alkyl halides to various tertiary phosphine complexes of rhodium(I) and iridium(I) are discussed. Features of interest include (1) an equilibrium between a five-coordinate acetylrhodium(III) cation and its six-coordinate methyl(carbonyl) isomer which is established at an intermediate rate on the NMR time scale at room temperature, and (2) a solvent-dependent secondary- to normal-alkyl-group isomerization in octahedral al-kyliridium(III) complexes. The chemistry of monomeric, tertiary phosphine-stabilized hydroxoplatinum(II) complexes is reviewed, with emphasis on their conversion into hydrido -alkyl or -aryl complexes. Evidence for an electronic cis-PtP bond-weakening influence is presented. 

The driving force for rearrangement is relief of steric strain between the isopropyl group and one of its adjacent methyl groups. Isomerization is acid-catalyzed. Protonation of the ring generates the necessary carbocation intermediate and rearomatization occurs by loss of a proton. 

Disconnection 3 shows that any alcohol may be disconnected at a bond next to the OH group. Isomeric alcohols 18 and 20 can both be made from acetone using perhaps a Grignard reagent 19 in the first case and available BuLi in the second. 

Synthesis starting from amidrazones (Scheme 59, c -d") is the most versatile preparation of the 1,2,4-triazole ring. Application to specific cases is governed by three considerations. One is the formation of the required amidrazone from a hydrazine with or without substituents and amide derivatives RC(=NH)X or RC(=NCOR )X where X = halide, OH, OAlk, SAlk, NH2, etc. With two reactive hydrazinic NH groups, isomeric amidrazones can be formed but the relative basicities usually predict the correct product. Unsymmetrical hydrazines YZNNH2 also form amidrazones but these cannot be cyclized unless Y or Z is reactive. 

Functional group isomerism, e.g. in alcohols and ethers, aldehydes and ketones. Isomerism in aromatic compounds. 

FUNCTIONAL GROUP ISOMERISM (A SUB-GROUP OF STRUCTURAL ISOMERISM)... 

A new type of isomerism, called functional group isomerism, is where two or more compounds can have different functional groups present in their molecules but still retain the same overall molecular formula, e.g. methoxymethane and ethanol (Figure 6.4.10). 

Tamao, K., Kiso, Y., Sumitani, K., Kumada, M. Aikyi group isomerization in the cross-coupiing reaction of secondary alkyl Grignard reagents with organic haiides in the presence of nickei-phosphine compiexes as cataiysts. J. Am. Chem. Soc. 1972, 94, 9268-9269. 

Cross-coupling reactions between the isopropyl Grignard reagent and alkenyl bromides or iodides are also catalyzed by Fe ° or Cu complexes, no alkyl group isomerization being observed (equations 33... 

The reaction of the r-butyl Grignard reagent with chlorobenzene in the presence of NiCl2(DMPE) results in the formation of isobutylbenzene in low yields through tertiary to primary group isomerization no reaction whatsoever is observed with NiCl2(DPPP) (equation 100). 

Only a limited number of vinyl sulfones, e.g. phenyl ( )-2-phenylvinyl sulfone (14), undergo codimerization with MCR Homocyclodimerization of MCP is the most efficient side reaetion. Interestingly, yields and product distributions are solvent dependent. No reaction takes place with catalytic amounts of bis(t -cycloocta-l,5-diene)nickel(0)/triphenylphosphane. In this case the vinyl sulfones are strongly coordinated to the catalyst metal, thus preventing interaction with MCP. When the sulfones bear alkyl-substituted vinyl groups, isomerization to yield allyl sulfones usually proceeds faster than cycloaddition, at least in the case of palladium(O) catalysis. 

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