Addition regiospecific

Binucleophiles give regiospecific addition to trifluoromethylacetylene with and without subsequent cyclization. Initial attack by the softer nucleophile occurs preferentially at the carbon p to the trifluoromethyl group [6] (equation 7). 

A regiospecific addition takes place when diazomethane reacts with an acetylenic ester [17] (equation 26)... 

Bis(lrifluoromethyl)oximes also are highly reactive dipolarophiles, as evidenced by the facile regiospecific addition of diazomethane [32] (equation 28)... 

Reactions of trifluoroacetonitnle oxide with conjugated dienes also lead to regiospecific additions [36] (equation 36) Its addition to the strain-activated double... 

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... 

Jones and coworkers200 found that a variety of sulphenic acids may be generated by thermolysis of the readily available /J-cyanosulphoxides (equation 81) and observed their highly regiospecific addition also to non-conjugated alkynes (Table 12). As expected for a pericyclic mechanism, the reaction afforded the product of a stereospecific cis-addition. However, the regioselectivity of the addition suggests that the partial carbon-sulphur bond in the transition state 148 is polarized in such a way that the carbon atom has some cationic character (equation 82). 

Prior to 1985, much had been learned about the chemistry of the silicon-carbon double bond through the study of the reactions of silenes with a wide variety of reactants. Thus it was known that all silenes studied reacted readily with alcohols (particularly methanol) by regiospecific addition across the ends of the Si=C bond in which the MeO group became attached to silicon and the alcoholic H to carbon, as in Eq. (22). 

Silylstannanes undergo regiospecific addition to alkenes and alkynes with scission of the Sn—Si bond, as illustrated in reactions 15 and 16272. 

Irradiation of 8 with trimethylamine 11 yields the regiospecific adducts 13, 17 and 2027. The reaction of 8 with triethylamine 10 in either hexane or acetonitrile solution (>290 nm) results in the formation of a single adduct 14 (24% yield) accompanied by comparable amounts of 17 (15%) and also 20 (78%)28 (equation 3). Regiospecific addition of triethylamine to a-methylstyrene 9 is also observed to give 15 (32%), 18 (11%) and 21 (8%)26 (equation 3). 

Depending on the linear, branched or cyclic structure of the unsaturated compound, a variety of dialkyl sulfides has been obtained in the reaction with H2S (equations 16-18). The regiochemistry depended markedly on the structure of the diene. For a mechanistic purpose, some experiments have been carried out using deuterium sulfide, D2S. The results have been interpreted in terms similar to those of Nordlander and coworkers9 (vide infra). The thiylation of 1,3-dienes was assumed to start with a regiospecific addition of a proton or a deuteron to one of the two double bonds to form two isomeric ion pairs as in equation 14 which, in the poorly dissociating solvent, collapse into products with equal probability. 

Another method of achieving regiospecific addition to terminal... 

Cycloaddition of p-methoxyphenyl azide to alkynic dipolarophiles at room temperature gives triazoles (697) and (698) (Equation (54)). A regiospecific addition is only observed in the case of Z = CH(OMe)2 <89H(29)967>. Phenyl azide and substituted benzyl azides undergo 1,3-dipolar cycloadditions with DM AD, phenylacetylene, and ethyl propiolate to afford 1-phenyl- and 1-benzyl-... 

The latter transformation requires the use of a small amount of an acid or its ammonium salt. By using [Cp2TiMe2] as the catalyst, primary anilines as well as steri-cally hindered tert-alkyl- and sec-alkylamines can be reacted.596 Hydroamination with sterically less hindered amines are very slow. This was explained by a mechanism in which equlibrium between the catalytically active [L1L2Ti=NR] imido complex and ist dimer for sterically hindered amines favors a fast reaction. Lantha-nade metallocenes catalyze the regiospecific addition of primary amines to alkenes, dienes, and alkynes.598 The rates, however, are several orders of magnitude lower than those of the corresponding intramolecular additions. 

Aliphatic and alicyclic thiones are less well investigated, principally because of their inherent instability. Adamantanethione, however, has been studied in detail and undergoes [ 2 + 2] cycloaddition to electron-deficient and electron-rich alkenes from both T, and S2 excited states. Addition from the latter is more efficient than that from the triplet and is again stereospecific but not regiospecific. Addition of adamantanethione (412) to ethyl vinyl... 

Regiospecific addition took place affording good yields of thietanes 143503 and, with alkynes, diene systems were formed504. Fischer has also described the reactions of thioketones with (CO)sW=NPh which undergo metathesis with these substrates to yield N-phenyl imines 144505 as shown in equation 154. 

With alkynic 1,3-dipolarophiles the dibenzo derivatives (103) react as azomethine imines giving the adducts (104). Regiospecific addition is observed when monosubstituted alkynes are used. In some reactions the cycloadduct (104) is accompanied by a Michael adduct (e.g. 103 R2 = C(C02Me)=CHC02Me). Reaction of compound (103 R1 = Me, R2 = H) with two equivalents of DMAD gives a product which appears to be the 2 1 cycloadduct (105 R1 = Me) (73TL597, 74H(2)27). 

Theoretical analysis of this [4% + 27r]-cycloaddition reaction by consideration of frontier-orbital interactions between the electron-rich olefin (highest occupied molecular orbital, HOMO) and the electron-poor 5-nitropyrimidine (LUMO) has shown that the FMO perturbation theory correctly predicts an exclusive regiospecific addition of the enamine to N-l and C-4 of the pyrimidine ring (86JOC4070). 

The photocycloaddition of thiocarbonyl-containing compounds to alkenes provides an easy route to thietanes. 2,4,6-Tri-(t-butyl)thiobenzaldehyde (249) undergoes regiospecific addition to substituted allenes (250) to give the vinyl-thietanes (251). The formation of the ketones (252) by irradiation of the aryl... 

According to a detailed mechanistic studythe reaction involves reversible, regiospecific addition of NaHFe2(CO)s to the C =C double bond of the enone, affording the corresponding binuclear iron enolate. Cleavage of the latter to the mononuclear iron enolate represents the rate-determining step, protonolysis of which provides the saturated ketone. 

The mechanism for the reaction involves a regiospecific addition of bromine radical to the double bond. 

Electrophilic Addition.— The olefinic bond in an ergost-22-ene undergoes stereospecific and regiospecific addition reactions, controlled by its dissymmetric environment. The preferred conformation, revealed by A-ray analysis, is represented by (77). Frontal hindrance by the steroid nucleus, and the 24-methyl... 

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