Alkenes normal

The reaction of alkenylcarbene complexes and electron-poor alkenes normally leads to mixtures of the expected [2S+1C] vinylcyclopropane derivatives (see... 

The addition to alkenes normally leads to unstable adducts that lose carbon dioxide under the reaction conditions. The intramolecular cycloaddition of the sydnone (30) takes place at room temperature, however (Equation (5)) and the cycloadduct (31) has been characterized <86HCA927>. The unstable species formed by the loss of carbon dioxide are also azomethine ylides. It is therefore possible for a second 1,3-dipolar addition to take place, as illustrated in Scheme 6 for the reaction of 3-phenylsydnone with Al-phenylmaleimide <86TL317,92JA8414>. This 2 1 addition has been used as the basis of a synthesis of polyimides. Imides of the type (32) were used as the dipolarophiles and their reaction with 3-phenylsydnone gave linear polymers <87MM726>. 

Alkenes strained by twist or r-bond torsion, such as E-cyclooctene, exhibit much lower barriers due to relief of strain in the TS for the oxygen transfer step. While the epoxidation of symmetrically substituted alkenes normally involve a symmetrical approach to the TT-bond, the TSs for epoxidation of E-cyclooctene and E-l-methylcyclooctene exhibit highly asymmetric transition structures. The AAE = 3.3 kcalmol" for E- versus Z-cyclooctene is clearly a reflection of the relative SE of these two medium ring alkenes (16.4 vs 4.2 kcalmol ) ". The classical activation barrier (AE ) for the highly strained bicyclo[3.3.1]non-l-ene is also quite low (Table 10, Figure 26). In these twist-strain alkenes, the approach of the peracid deviates markedly from the idealized spiro approach suggesting fliat this part of the potential energy surface is quite soft. 

Irradiation of anthracenes with alkenes normally results in a 1,4-cycloaddition across the 9,10-positions of the central ring (3.SS), although other types of adducts have also been reported. A reaction... 

The C—C=C angle in alkenes normally is about 122°, which is 10° larger than the normal C—C—C angle in cycloalkanes. This means that we would expect about 20° more angle strain in small-ring cycloalkenes than in the cycloalkanes with the same numbers of carbons in the ring. Comparison of the data for cycloalkenes in Table 12-5 and for cycloalkanes in Table 12-3 reveals that this expectation is realized for cyclopropene, but is less conspicuous for cyclobutene and cyclopentene. The reason for this is not clear, but may be connected in part with the C-H bond strengths (see Section 12-4B). 

Optically active metal complexes have been recognized as excellent catalysts for the enantioselective cyclopropanation of carbenes with alkenes. Normally, diazo compounds react under metal catalysts in the dark to afford carbenoid complexes as key intermediates. Katsuki et al. have reported the ds-selective and enantioselective cyclopropanation of styrene with a-diazoacetate in the presence of optically active (R,R)-(NO + )(salen)ruthenium complex 80, supported under illumination (440 nm light or an incandescent bulb) [59]. The irradiation causes dissociation of the apical ligand ON + in 80, and thus avoids the splitting of nitrogen from the a-diazoacetate. 

Another potential dark source of in the atmosphere, more particularly in the boundary layer, is from the reactions between ozone and alkenes. The ozonolysis of alkenes can lead to the direct production of the OH radical at varying yields (between 7 and 100%) depending on the structure of the alkene, normally accompanied by the co-production of an (organic) peroxy radical. As compared to both the reactions of OH and NO3 with alkenes the initial rate of the reaction of ozone with an alkene is relatively slow, this can be olfset under regimes where there are high concentrations of alkenes and/or ozone. For example, under typical rural conditions the atmospheric lifetimes for the reaction of ethene with OH, O3 and NO3 are 20 h, 9.7 days and 5.2 months, respectively in contrast, for the same reactants with 2-methyl-2-butene the atmospheric lifetimes are 2.0 h, 0.9 h and 0.09 h. 

Figure 5. Plot of the average 1-alkene/normal alkane ratios vs. carbon number in NOSR cores 25 (o) and 15/16 (%) and the Geokinetics core (a).

The intramolecular cycloaddition of ketenes with alkenes is a versatile synthetic procedure for the preparation of bicyclo[3.2.0]heptan-6-ones and bicyclo[3.3.1]heptan-6-ones19a-b. Three-atom tethers offer the best compromise between product strain, which is prohibitive with two-atom tethers, and entropy of activation, which decreases the rate of reaction with longer tethers. Cycloadditions with four-atom tethers arc quite rarc19a,b. As in intcrmolccular cycloadditions, syn addition to the alkene normally occurs. 

Bromination of simple alkenes normally proceeds via a bromonium ion and is stereospecifically anti. Exceptions occur when the bromonium ion is in equilibrium with a corresponding carbocation. 

Hydroboration of mono- and disubstituted alkenes with borane gives rise typically to a trialkylborane product. However, trisubstituted alkenes normally give a dialkylborane and tetrasubstituted alkenes form only the monoalkylboranes (5.4). The extent of hydroboration may also be controlled by the stoichiometry of alkene and borane. This has been exploited in the preparation of a number of mono-and dialkylboranes that are less reactive and more selective than borane itself. Important in this respect are the so-called disiamylborane 1 (name derived from... 

Epoxidation of alkenes normally occurs with approach of the peroxy-acid from the less-hindered side of the double bond. However, where there is a polar substituent, particularly in the allylic position, this may influence the direction of attack by the peroxy-acid. Thus, whereas 2-cyclohexenyl acetate gives a mixture consisting predominantly of the trans-epoxide (as expected with attack from the less-hindered side of the double bond), the free alcohol gives almost exclusively the cw-epoxide under the same conditions (5.42). The stereoselectivity and the faster rate of reaction with the hydroxy compound result from hydrogen bonding of the reactants. 

Electrophilic addition of HCl or HBr to an alkene normally occurs with Mar-kovnikov regiochemistry, resulting in an alkyl halide with the halogen located at the more substituted carbon of the starting alkene. Since the presence of peroxides (ROOR) initiates a radical mechanism, HBr can also be added to an alkene with anti-Markovnikov orientation. Anti addition of bromine or chlorine to an alkene gives a trans-dibromo or dichloro product. 

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