Ethene, 1,2-disubstituted

Table 1.3 Regioselectivity of Diels-Alder reactions of disubstituted 1,3-butadienes with monosubstituted ethenes (RCH=CH2)...

Another situation is observed when salts or transition metal complexes are added to an alcohol (primary or secondary) or alkylamine subjected to oxidation in this case, a prolonged retardation of the initiated oxidation occurs, owing to repeated chain termination. This was discovered for the first time in the study of cyclohexanol oxidation in the presence of copper salt [49]. Copper and manganese ions also exert an inhibiting effect on the initiated oxidation of 1,2-cyclohexadiene [12], aliphatic amines [19], and 1,2-disubstituted ethenes [13]. This is accounted for, first, by the dual redox nature of the peroxyl radicals H02, >C(0H)02 and >C(NHR)02 , and, second, for the ability of ions and complexes of transition metals to accept and release an electron when they are in an higher- and lower-valence state. 

These effects can occur when the active site at which a measurable phenomenon occurs is in close proximity to the substituent. Among the many systems exhibiting direct steric effects are ortho-substituted benzenes, 1, cis-substituted ethylenes, 2, and the ortho- (1,2-, 2,1- and 2,3-) and peri- (1,8-) substituted naphthalenes, 3, 4, 5 and 6, respectively. Other examples are d.v-1,2-disubstiUited cyclopropanes, c/ s-2,3-disubstituted norbornanes and ci.s-2,3-disubstituted [2.2.2]-bicyclooctanes, 7, 8 and 9, respectively. Some systems generally do not show steric effects. Vicinally substituted systems such as disubstituted methanes, 10, and 1,1-disubstituted ethenes, 11, are examples, 2,3-Disubstituted heteroarenes with five-membered rings such as thiophenes and selenophenes... 

The reaction of 2,5-disubstituted 3-furyl bromides with butyl-lithium and octafluorocyclopentene in dry tetrahydrofuran (THF) at —78°C gave bis(furyl)ethenes 82 (06JMC4690). Photochrome 83 (05JOC10323, 06EJ03105) (29-46% yield) and a series of its derivatives 84 (55-65%) containing different substituents at position 6 of the ben-zofuran ring were synthesized from 3-bromo-2-methyl-1-benzofuran (08JPP(A)146). 

N,N -Disubstituted Bipyridyls and Dipyridyl Ethenes 2.1 Variation of N-Substituents in 4,4 -Bipyridyl... 

Carbazole reacts with cyclohexene in a reaction catalyzed by aluminium chloride to give a low yield of 3-cyclohexylcarbazole and a main product of the 3,6-disubstituted material. The formation of 1-ethylcarbazole in high yield by treatment of carbazole and ethene at 300°C with aluminium-mercuric chloride and aniline is believed to proceed via 138. 1-Isopropylcarba-zole was similarly prepared using propene. ... 

Olefinic carbons of 1,2-disubstituted ethenes also reflect the stereochemical orientation of the substituents. In most cases chemical shifts in the Z-configurated ethenes are slighter smaller than in the C-ethenes, however, the difference is not large enough for a secure structural assignment unless both isomers are available for comparison. 

Configurations of ethene compounds can be determined easily and securely by measuring coupling constants. The simplest case is the differentiation of diastereomeric 1,2-disubstituted ethenes, because vicinal H, H coupling constants are always larger in the E- than in the Z-isomer, and the ranges do not overlap. 

For the cleavage of alkenes from a support by metathesis, several strategies can be envisaged. In most of the examples reported to date, ring-closing metathesis of resin-bound dienes has been used to release either a cycloalkene or an acyclic alkene into solution (Figure 3.38, Table 3.44). Further metathesis of the products in solution occurs only to a small extent when the initially released products are internal alkenes, because these normally react more slowly with the catalytically active carbene complex than terminal alkenes. If, however, terminal alkenes are to be prepared, selfmetathesis of the product (to yield ethene and a symmetrically disubstituted ethene) is likely to become a serious side reaction. This side reaction can be suppressed by conducting the metathesis reaction in the presence of ethene [782,783]. 

Di- and polysubstituted pyrrolizines are uncommon. The only pyrrolizine isolated from the intramolecular aldol condensation of 2-acetyl-l-(butan-3-on-l-yl)pyrrole was the 6,7-disubstituted compound 223.122 A small amount of the 5,6-disubstituted pyrrolizine (145d) was obtained when the 2-formylpyr-ryl anion was condensed with l,2-di(phenylsulfonyl)ethene.90 By contrast with the production of the single isomer (223), the homologous diketone 224... 

As a comparison of the enthalpies of formation of isomeric aldehydes and ketones shows, the disubstituted carbonyl compounds (ketones) are more stable than the monosubstituted carbonyl compounds (aldehydes), analogous to the stability order for the corresponding 1,1-disubstituted ethenes and the 1-n-alkenes. For the three aldehyde/methyl ketone isomeric pairs (nc = 3-5), initially it seems that the enthalpies of isomerization are fairly constant. However, if the interpolated value for pentanal is used, the trend is clearly that of more negative enthalpy of isomerization with increasing c(g) —31.7, —33.9, —34.5 kJmol-1. The non-constant enthalpies are expected because the slopes, ag, in Table 4 are quite different for aldehydes and ketones. The greater contribution to the isomerization enthalpy differences comes from the methyl ketone which is more stabilized than the aldehyde by an additional —CH2— group. The enthalpies of isomerization of the corresponding alkenes are much less exothermic (—6.1 0.8 kJmol-1) than those of the carbonyl compounds. 

Solutions ofLiOCH and NaCsCH in liquid NH3 are mostly prepared by the so-called titration method, which involves controlled addition of pieces of metal to boiling ammonia while introducing acetylene. The blue colour of the dissolved metal serves as an indicator. This method is also applicable in the case of homologues, but is less attractive for economical reasons (one third of the acetylene is reduced to ethene). Derivatization of solutions of HteCM obtained from MNH2 or M and acetylene, in most cases gives only minor amounts of disubstituted acetylenes RC=CR, so it is inferred that di-metallated acetylene C C- occurs at best in only very small concentrations in liquid ammonia. 

The primary product (153) is photochemically converted into a derivative of benz[fl]anthracene-7,12-dione (154). A variety of heteroatom-containing polycyclic aromatic compounds was synthesized by using a 1,1-disubstituted ethene in which one of the substituents is a phenyl group and the other a 2-furyl, 2-benzo[ ]furyl, 2-thienyl, 3-... 

Steric hindrance is tolerated in the organic halide component of the reaction. The reaction of 2,5-diisopropyliodobenzene with methyl acrylate proceeds in 79% yield at 125 °C, for example. The aUcene component, conversely, is sensitive to the number and size of the substituents on the alkene. Useful yields of product are only obtained for ethene and mono- and disubstituted aUcenes. If the alkene is... 

Alkenes. The increased electronegativity of the sp carbon and the modest anisotropy of the carbon-carbon double bond result in a high-frequency (low-field) position for protons on alkene carbons. The range is quite large (8 4.5-7.7), as the exact resonance position depends on the nature of the substituents on the double bond. The value for ethene is 8 5.28. 1,1-Disubstituted hydrocarbon alkenes (vinylidenes), including exomethylene groups on rings (=CH2), resonate at a somewhat lower frequency cf. isobutylene [(CH3)2C=CH2,... 

Examples of tridentate phosphines include the synthesis of tris(diphenylphosphino)ethene (101) in 51% yield by the base-catalyzed addition of Ph2PH to the 1,2-disubstituted phosphine Ph2PCCPPh2 (Equation (27)).217 This protocol has also been used in the preparation of various polyphosphines using (Ph2P)2C=CH2 and an appropriate P—11 source, again with KOBu1 as base.218,219... 

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