Olefin solvent effects

In similarly substituted olefins Kf is strongly influenced by steric effects, as shown by the comparison of tetraisobutylethylene with adamantylideneadamantane and (i,/-D3-trishomocubylidene-D3-trishomocubane. In particular, the comparison between cyclohexene and the two tetrasubstituted cage olefins indicates that Kf increases at least by a factor of 103 on passing from a 1,2 disubstituted to a tetrasubstituted olefin. This dependence is likely to be similar in other solvents, because solvent effects on Kf are modest. 

In these solvents at sufficiently low Br2 concentration (< 10-3 m) the kinetics are first order both in the olefin and in Br2 and the main solvent effect consists of an electrophilic solvation of the departing Br ion. A nucleophilic assistance by hydroxylic solvents has also been recognized recently (ref. 26) (Scheme 10). So far, return during the olefin bromination in methanol had been admitted only for alkylideneadamantanes, and was ascribed to steric inhibition to nucleophilic attack at carbons of the bromonium ion (ref. 26). 

Kinetic data can be discussed in terms of bromine bridging in ionic intermediates if the transition states of the ionization step are late. It appears that this is the case in the bromination of a wide variety of olefins, and in particular of alkenes, stilbenes and styrenes. Large p- and m-values for kinetic substituent and solvent effects (p. 253) consistent with high degrees of charge development at the transition states, are found for the reaction of these compounds. It can therefore be concluded that their transition states closely resemble the ionic intermediates. 

Table 13 Values of p and m for ring-substituent and solvent effects in the bromination of aromatic olefins trans-Ar—C(R)=CHR in methanol at 25°C.

On the other hand, transition-state positions in bromination can be evaluated from solvent effects and their Winstein-Grunwald m-coefficients, since these latter are related mainly to the magnitude of the charge in the activated complexes (p. 274). The p- and m-values for most olefins included either in selectivity relationship A (44) or in B (45) are compared in Table 17. The m-value varies significantly with the reactivity as does p. Since m-variations arise from transition-state shifts, p-variations necessarily come, at least in part, from the same effect. 

Table 21 Solvent effects in bromination of conjugated olefins transition-state shifts and nucleophilic solvent assistance.

The second series of data on protic solvent effects in bromination that are related to transition states comprises the m-values of solvent-reactivity correlations. First, it is important to underline that 7-parameters, the solvent ionizing powers, established from solvolytic displacements, work fairly well in this electrophilic addition. This is expected since bromination, like SN1 reactions, leads to a cation-anion pair by heterolytic dissociation of the bromine-olefin CTC, a process similar to the ionization of halogenated or ether derivatives (Scheme 14). 

In contrast, 1,2-H shift to olefin 106 is the dominant reaction of carbene 104, and this process is slow enough to be measured by LFP r = 300 ns in cyclohexane and 560 ns in pentane at 25°C.117 There is a polar solvent effect the lifetime decreases to 52 ns in acetonitrile. However, at least in the case of cyclohexane, the lifetime is solvent limited, with a KIE of 1.5 on the lifetime in cyclohexane- (460 ns). Carbene 104 is much longer-lived than dimethylcarbene (r 21 ns in pentane) or methylcarbene (<1 ns).22,89... 

Using a protocol for tandem carbonylation and cycloisomerization, Mandai et al.83 were able to synthesize cyclopentene and cyclohexene derivatives in high yield, including fused and 5/>/>0-bicycles (Scheme 25). The cyclohexene Alder-ene products were not isolable methanol addition across the exocyclic double bond (in MeOH/ toluene solvent) and olefin migration (in BuOH/toluene solvent) were observed. The mechanism of methanol addition under the mild reaction conditions is unknown. In contrast to many of the other Pd conditions developed for the Alder-ene reaction, Mandai found phosphine ligands essential additionally, bidentate ligands were more effective than triphenylphosphine. 

A special problem can be the passivation of the electrode surface by insulating layers, for example, formation of oxides on metals at a too high anodic potential or precipitation of polymers in aprotic solvents from olefinic or aromatic compounds by anodic oxidation. As a result, the effective surface and the activity of the... 

T he epoxidation of olefins using organic hydroperoxides has been studied in detail in this laboratory for a number of years. This general reaction has also recently been reported by other workers (6,7). We now report on the effects of five reaction variables and propose a mechanism for this reaction. The variables are catalyst, solvent, temperature, olefin structure, and hydroperoxide structure. Besides these variables, the effect of oxygen and carbon monoxide, the stereochemistry, and the kinetics were studied. This work allows us to postulate a possible mechanism for the reaction. 

Mechanism. The following types of evidence ere pertinent in selecting on acceptable mechanism for olefin epoxidation by means of peroxy acids (1) the nature of the peroxy acid and the electronic effect of eubBtituents on its reactivity (2) the electronic effect of substituents on the reactivity of the olefin component (3) stereochemical factors affecting the reactivity of the olefin (4) the possibility of acid dialysis (5) solvent effects and (6) neighboring group effects. 

Also known are 2 + 2 cycloadditions proceeding by way of a bipolar ion, path (b) of Scheme l.28 These reactions occur in situations such as that depicted in Equation 12.14, where the intermediate zwitterion (10) is strongly stabilized. Tetracyanoethylene adds by this mechanism to /Mnethoxyphenyl-,29 alkoxyl-,30 and cyclopropyl-31 substituted olefins. The additions show large solvent effects.32 Partial loss of stereochemistry occurs as in the biradical cases, but it is much less pronounced. 

The characteristic features of the reaction between dehydrohydamtoins and dienes are regiospecificity of products formation (in nearly all cases), the apparent absence of solvent effect, and the rate enhancement caused by electron-donating groups on the diene. These same phenomena are prominent in the concerted [4 + 2] cycloaddition of olefins to dienes, and the two reactions appear to have the same mechanism. 

Foote, C.S. and R.W. Denny. 1971. Chemistry of singlet oxygen. XIII. Solvent effects on the reaction with olefins. ]. Am. Chem. Soc. 93 5168-5172. 

Ohashi et al. [128] found that the yields of ortho photoaddition of acrylonitrile and methacrylonitrile to benzene and that of acrylonitrile to toluene are considerable increased when zinc(II) chloride is present in the solution. This was ascribed to increased electron affinity of (meth)acrylonitrile by complex formation with ZnCl2 and it confirmed the occurrence of charge transfer during ortho photocycloaddition. This was further explored by investigating solvent effects on ortho additions of acceptor olefins and donor arenes [136,139], Irradiation of anisole and acrylonitrile in acetonitrile at 254 nm yielded a mixture of stereoisomers of l-methoxy-8-cyanobicyclo[4.2.0]octa-2,4-diene as a major product. A similar reaction occurred in ethyl acetate. However, irradiation of a mixture of anisole and acrylonitrile in methanol under similar conditions gave the substitution products 4-methoxy-a-methylbenzeneacetonitrile (49%) and 2-methoxy-a-methylbenzeneacetonitrile (10%) solely (Scheme 43). 

As in the case of olefin or diene homopolymerization by RLi, copolymerization is particularly sensitive to solvent effects. Initial-charge (all monomers added together) copolymerization of butadiene and styrene tends to result in a tapered block copolymer (a block of butadiene with increasing levels of styrene, followed by a block of styrene) in hydrocarbon solvents and a random copolymer (a uniform distribution of butadiene and styrene) in polar media. 

Another noticeable characteristic of captodative olefins is the influence of the reaction medium. The stabilizing effect of solvent on the persistency of a captodatively radical has been reported experimentally for the bond homolysis of bis(3,5,5-trimethyl-2-oxomorpholin-3-yl) [111], but was not found for the 2,3-diphenyl-2,3-dimethoxysuccinonitrile homolysis [112]. Theoretically the solvent-assisted stabilization las been predicted for the captodative substituted nitriles in solvent with large dielectric constants [113-114], Table 16 illustrates the solvent effect on the spontaneous thermal polymerizations [115]. The polymer yields are... 

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