Isomers regiospecific

Perfluoroalkylation can be accomplished via direct reaction of peifluoroalkyl halides and copper with aromatic substrates [232, 233, 234, 235, 236] Thus, perfluoroalkyl iodides or bromides react with functionalized benzenes m DMSO m the presence of copper bronze to give the corresponding perfluoroalkylated products directly in moderate to good yields [233] (equation 157) Mixtures of ortho, meta, and para isomers are obtained [232, 233], The use of acetic anhydride as solvent gives similar results [234, 235], Similarly, the direct reaction of perfluoroalkyl iodides and pyrroles with copper metal regiospecifically gives the 2-perfluoroalkylpyrroles [236] (equation 158). 

Azidofurazans and -furoxans undergo dipolar cycloaddition reactions with unsaturated compounds, in some cases regiospecifically. Thus, reaction of 3-amino-4-azidofurazan with l-morpholinyl-2-nitroethene (toluene, reflux, 70 hours) gives 4-nitro-l,2,3-triazole 204 in 87% yield (99MI1, 000KGS406). Cycloaddition of the same azide to alkynes was accomplished by formation of a mixture of position isomers 205 and 206. Regiospecific addition was observed only in singular cases... 

Free-radical addition of Se-phenyl areneselenosulfonates to acetylenes is also a facile process, occurring regiospecifically and stereoselectively to afford the E-isomer of a -(phenylseleno) vinyl sulfone (26) in high yield88. 

Formal isomerization of the double bond of testosterone to the 1-position and methylation at the 2-position provides yet another anabolic/androgenic agent. Mannich condensation of the fully saturated androstane derivative 93 with formaldehyde and di-methylamine gives aminoketone 94. A/B-trans steroids normally enolize preferentially toward the 2-position, explaining the regiospecificity of this reaction. Catalytic reduction at elevated temperature affords the 2a-methyl isomer 95. It is not at all unlikely that the reaction proceeds via the 2-methylene intermediate. The observed stereochemistry is no doubt attributable to the fact that the product represents the more stable equatorial isomer. The initial product would be expected to be the p-isomer but this would experience a severe 1,3-diaxial non-bonded interaction and epimerize via the enol. Bromination of the ketone proceeds largely at the tertiary carbon adjacent to the carbonyl (96). Dehydrohalogenation... 

A remaining class of dimerizations is represented by the reactions shown in Eqs. 36—39. These are singlet state reactions, and they are all characterized by stereospecificity and regiospecificity. Production of the trans isomer from the steroidal dienone in Eq. 39 is anomalous and poses a... 

The synthesis of analogous iminoacyl complexes by isonitrile insertion into linear alkyl-zirconocene chlorides is also known. In an overall regiospecific hydrocyanation of alkenes, iminoacyls 21 derived from tBuNC or Me3SiCN (as the Me3SiNC isomer) may be treated with I2 to rapidly generate an imidoyl iodide and subsequently the nitrile 22 (Scheme 3.6) [22], Less hindered iminoacyl complexes (e. g. R = Bu, Cy) may be hydrolyzed to afford aldehydes 23 [23]. 

In the case of dioximes bearing different substituents (R Ri) the furoxan isomer obtained can depend on the configuration of the starting dioxime. The classic example is the oxidation of p-methoxybenzil dioxime with ferricyanide (Scheme 6.5) this reaction affords the 4-phenyl furoxan isomer 18 in a regiospecific manner if the dioxime amphi-form 19 is used, the 3-phenyl isomer 20 is produced if the amphi-... 

Table 8.17 shows the scope of the reaction of acetylcobalt tetracarbonyl with polyenes. The reactions are regiospecific with the acetyl group adding to the terminal unsaturated carbon atom of the ir-electron system to produce the E-a,p-unsaturated ketones [9]. In the reaction with fulvenes [10], only the 1-acetyl and 1,4-diacetyl derivatives are formed, with no evidence of the 2-isomer. This is an indication of the relative stabilities of the cyclic it-allyl complexes, compared with the exocyclic complex. It has been postulated that, in the reactions of conjugated systems, the initial o-allyl adduct proceeds to the products via the it-allyl complex (cf Scheme 8.1), whereas in the case of unconjugated tt-systems, the initial o-adduct is more stable and tends to undergo a further carbonylation reaction. 

The concept of "selectivity" must be clearly distinguished from the term "specificity" [4][5], Specific, applied to a reaction, means that two (or more) isomeric starting materials give -under the same reaction conditions- different reaction products which are also isomers. Depending upon the isomers we may be considering, we may refer to "regiospecificity" (structural isomers) or to "stereospecificity" (either diastereospecificity or enantiospecificity). For instance, the formation of we5o-2,3-dibromobutane by addition of bromine to ( )-2-butene, in contrast with the formation of the d,l - 2,3-dibromobutene from the (Z)-2-butene, is a case of diastereospecificity. 

In fact, the formation of the rranj-isomer in the photoaddition of 2-cyclohexenone to isobutylene and the regiospecificity observed were unexpected on the basis of some earlier studies. However, isomerisation of 2-cvclohexenone to a distorted rran -isomer could account for the observed results [4]. 

Depending upon the choice of substrates, the hydrosilylation of alkenes with (TMS)3SiH can also be highly stereoselective. The reaction of (TMS)3SiH with methylmaleic anhydride, proceeded regiospecifically to the less substituted side, but also diastereoselectively to afford the thermodynamically less stable cis isomer (Reaction 5.6) [25]. Stereoselectivity increased by decreasing the reaction temperature, indicating the difference in enthalpy of activation for syn vs anti attack. 

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