Disubstituted alkenes, regioselectivity

Distortionless enhancement by polarization transfer (DEPT), 725-6 Disubstituted alkenes, regioselectivity, 842-4 o-Ditoluidine, glucose determination, 632, 634 o,o -Dityrosine, low-density lipoprotein, 610 DMD see Dimethyldioxirane DMDO see Dimethyldioxirane DNA... 

As regards the regioselectivity of monosubstituted, cis and trans disubstituted alkenes, bromination most frequently exhibits anti-Markovnikov behaviour. Only in the case of propene and 1-butene, i.e. when the double bond bears only one linear substituent, bromination in methanol is predominantly but not completely Markovnikov. Steric effects obviously play an important role in determining the... 

Ruasse et al, 1978) is totally regioselective and shows X-dependent chemoselectivity. This is partly in agreement with the kinetic data, which indicate no primary carbocation but rather a competition between the benzylic carbocation and the bromonium ion, depending on X. According to the data of Table 6, bridged intermediates would lead to more dibromide than open ions do. From these results and from those on gem-, cis- or frans-disubstituted alkenes, empirical rules have been inferred for chemoselectivity (i) more solvent-incorporated product is formed from open than from bridged ions (ii) methanol competes with bromide ions more efficiently than acetic acid. 

The reaction is applicable to acyclic and cyclic enol ethers and to various (3-dicarbonyl compounds, but fails with silyl enol ethers and simple 1,2-disubstituted alkenes. When applicable, this route to furans is useful because the yields and regioselectivity are consistently satisfactory. The paper includes a preparation of the reagent by reaction of Mn(NO,)3 with Ac20 at 100° to give Mn,0(0Ac)7 H0Ac in 60% yield. 

While monosubstituted alkenes usually react with high regioselectivity, it is not true for disubstituted alkenes. Formation of mixtures of type 163 and 164 (equation 105) has been observed in most cases when unsymmetrical alkenes bearing two different substituents possess similar stereoelectronic properties. In general, regioselectivity is controlled by a combination of HOMO-LUMO interactions, steric effects and hydrogen bonding between suitable substituents in both alkene and nitrone molecules . ... 

Regioselectivity with cis and trans disubstituted alkenes. The large-group non-bonded effect... 

Reactions of nitrile oxides with 1,1-disubstituted alkenes afford products in which the oxygen atom of the nitrile oxide gets attached to the most crowded carbon atom of the dipolarophile. This high regioselectivity does not seem to depend on the type of substituent present on the alkene (142-152). Some of the results cannot be satisfactorily interpreted on the basis of FMO theory (149,151). Both steric and electrostatic effects often counteract each other and contribute to the regioselectivity actually observed. With trisubstituted alkenes, the orientation of cycloaddition is apparently dominated by this phenomenon. The preference is for the more substituted carbon atom to end up at the 5-position of the heterocyclic product (153,154). However, cases of opposite regiodifferentiation are also found, in particular with donor-substituted alkenes (42,155-157) (Scheme 6.21). 

Disubstituted alkenes undergo iodohydroxylation regioselectively when the allylic position bears a hydroxyl group [110]. The attack on the distal carbon of the iodonium intermediates is favored on the polarity alternation basis. 

Terminal alkenes react almost instantaneously in rhodium-catalyzed hydroboration, whereas disubstituted alkenes require the use of excess reagent and increased amounts of catalyst (2-5%)362 Other alkenes are unreactive. High regioselectivities can be achieved 363,364... 

In the preparative application of [2 + 2]-photocycloadditions of cyclic enones to (substituted) alkenes, two factors concerning product formation are of decisive relevance, namely the regioselectivity and the (overall) rate of conversion. Regarding the regioselectivity in the addition to mono- and 1,1-disubstituted alkenes, Corey had shown that the preferred addition mode of cyclohex-2-enone to isobutene or 1,1-dimethoxyethylene was the one leading to—both cis- and trans-fused—bicyclo[4.2.0]octan-2-ones with the substituents on C(7) [8]. In contrast, in the reaction with acrylonitrile, the alternate orientation was observed to occur preferentially. Similar results were also reported by Cantrell for the photocycloaddition of 3-methyl-cyclohex-2-enone to differently substituted alkenes [14]. No significant differences in the overall rates of product formation for the different alkenes were observed in these studies. In order to explain these observed... 

Thus, with electron-rich or electron-poor monosubstituted alkenes, the reaction with tri-fluoroacetonitrile oxide is more regioselective than with benzonitrile oxides. and the oxygen of the nitrile oxide adds to the substituted carbon. With other dipolarophiles, cyeloaddition is less regioselective. Competitive reactions include an electrophilic addition on the nitrile oxide, in particular with 1,1-disubstituted alkenes. and a further cyeloaddition which can occur when the dipolarophile is used in excess. ... 

As before, the reaction is best with monosubstituted alkenes regioselectivity is not high with 1,2-disubstituted alkenes, and the yields are less impressive (eqnation 12). ... 

The reaction rate of the addition decreased on going from phenyl-substituted alkenes to mono-, di-, and trisubstituted alkenes. A preference for E double bonds over Z double bonds was exhibited by the monoimido complex 1. Complete regioselectivity was observed with mono-and trisubstituted alkenes and gmi-disubstituted alkenes, since the C-N bond is formed exclusively at the least substituted alkene carbon, for example of 1-methyl- and 1-phenylcyclo-hexene. However, for 1,2-disubstituted alkenes the regioselectivity depended on the substrate70,71. Unexpectedly, benzylic amination was predominant in the reaction mixtures derived from 1-phenylpropenes and 1,2-dihydronaphthalene (Tabic 5). 

Despite the obvious advantages the catalytic methods suffer from some limitations. A considerably lesser degree of regioselectivity was observed in the oxyamination reactions of terminal alkenes and asymmetrically disubstituted alkenes, such as 1-phenyl-1-propenes, with respect to the comparable results from stoichiometric reactions (cf. Tabic 5 and 6). Furthermore, reduced reactivity was observed, In fact, neither method (A or B) was successful with tetramethylethylene, cholesteryl acetate, diethyl ( )-2-butenedioate, 2-cyclohexenone, 1-acetoxycyclohexene or 1-phenylcyclohexene73. 

When an alcohol has two or three different P carbons, dehydration is regioselective and follows the Zaitsev rule. The more substituted alkene is the major product when a mixture of constitutional isomers is possible. For example, elimination of H and OH from 2-methyl-2-butanol yields two constitutional isomers the trisubstituted alkene A as major product and the disubstituted alkene B as minor product. 

---