Alkenes selectivity factors

Kinetic resolution.l33 Since enantiomers react with chiral compounds at different rates, it is sometimes possible to effect a partial separation by stopping the reaction before completion. This method is very similar to the asymmetric syntheses discussed on p. 102. An important application of this method is the resolution of racemic alkenes by treatment with optically active diisopinocampheylborane,134 since alkenes do not easily lend themselves to conversion to diastereomers if no other functional groups are present. Another example is the resolution of allylic alcohols such as 45 with one enantiomer of a chiral epoxidizing agent (see 5-36).135 In the case of 45 the discrimination was extreme. One enantiomer was converted to the epoxide and the other was not, the rate ratio (hence the selectivity factor)... 

It is clear that all three types of selectivity are relevant to catalytic hydrogenation reactions and from a consideration of the reaction scheme for alkyne hydrogenation (Fig. 4), it can be deduced that all three factors may be operative simultaneously. Clearly, the selectivity for the formation of the alkene relative to alkane will depend upon a number of factors. If both the alkene and the alkane are formed during one residence of the parent molecule on the surface, the selectivity will depend upon the relative values of k, and k2 (Type II selectivity) and upon the ratio kjk4 (Type II selectivity). Since both of these depend upon the specific properties of the catalyst, they have been termed the mechanistic selectivity factor [38], Once the alkene is produced, the system contains another potential adsorbate and Type I selectivity must be taken into account. It... 

When the barrier to hydrometallation is solely a consequence of the strength of the metal-alkene interaction, the implication is unfavorable for selectivity. Suppose the kinetic scheme of equation (8) operates then the rate for formation of (3) from a given alkene will be related to the product of the constants k and K for that alkene. However, factors which tend to make K larger (more stable alkene complex) will make k smaller and thus tend to cancel out, resulting in little kinetic differentiation between substrates. 

There are many problems still to be solved for this quite simple reaction. For the reaction to be reversible and fast the alkyls must be unstable, but the criteria for stability of different types of alkyls are not fully established. It must be noted that the alkyl group occupies only one coordination site, whereas in the transition state and in the hydridoalkene complex, two sites must be available, so that clearly a prime factor is coordinative unsaturation which allows decomposition of the alkyl by elimination of alkene. The factors affecting the direction of addition, or the selectivity in elimination from secondary alkyls to give cis- or inm -isomers, are similarly not well understood. For the first, it appears that the greater the polarity of the M—H bond in the direction Ma —H[Pg.787]

The lithium-free experiments of Table 11 tell a consistent story, one that has changed little compared to the overview presented in 1970 (1). Typical nonstabilized ylides of the Ph3P=CHR family are > 90% selective for the formation of (Z)-alkenes at room temperature. The (Z)-alkene selectivity increases to > 95% as the reaction temperature is lowered (Table 11, entries 10-18) and is highest for tertiary aldehydes (entries 78-87). Relatively few -branched ylides have been studied, but the more recent entries appear to follow the usual pattern of (Z)-alkene selectivity (entries 88,95,96). Somewhat lower Z E selectivities appear in one study of the cyclopropylcarbinyl member of the -branched ylide family (entries 90-92) (73), but these experiments involve benzaldehyde as the substrate and employ the relatively inefficient base sodium hydride for ylide generation. These factors may increase the risk of catalyzed equilibration of intermediates. A more recent study reports a typical Z E ratio of 9 1 in the case of a 2,3-diphenyl-cyclopropylcarbinyl ylide (salt-free conditions 2,3-diphenylcyclopropane-carboxaldehyde substrate) (77). 

Nevertheless, the use of relative reactivities to characterize carbenic philicity is restrictive the apparent philicity is related to the alkenes selected for the relative reactivity measurements. What if the set of alkenes were expanded by the addition of an even more electron-deficient alkene Such a test was applied in 1987 [65], using a-chloroacrylonitrile, 26, which is more 7t-electron deficient than acrylonitrile, 27. We found that PhCF or PhCCl added 15 or 13 times, respectively, more rapidly to 26 than to 27. In preferring the more electron-deficient olefin, the carbenes exhibited nucleophilic character. However, because they also behave as electrophiles toward other alkenes (Table 4), they must in reality be ambiphiles. In fact, we now realize that all carbenes have the potential for nucleophilic reactions with olefins the crucial factor is whether the carbene s filled a orbital (HOMO)/alkene vacant Ji orbital (LUMO) interaction is stronger than the carbene s vacant p orbital (LUMO)/aIkene filled k orbital (HOMO) interaction in the transition state of the addition reaction. [63]... 

Stabilized ylides are those that possess an R substituent (Figure 1.1) that is anion-stabilizing/electron-withdrawing (e.g. CO2CH3, CN). Such ylides tend to be less reactive than other ylides and usually only react with aldehydes to give the E-alkene. This -selectivity has been attributed to the fact that stabilized ylides react with aldehydes under thermodynamic control. Consequently, the less crowded and hence favored trans-oxaphosphetane gives rise to the observed -alkene. Other factors that influence the E/Z ratio of alkenes in reactions of a stabilized ylide with an aldehyde are listed in Table 1.1. 

Under PKR conditions, two enantiomeric substrates are simultaneously converted into two structurally and configurationally different chiral products by reaction with chiral reagents or catalysts. It has been shown that to achieve the same selectivity, the selectivity factor s can be significantly lower for PKR than for a traditional kinetic resolution. As yet, there has been only one report of an asymmetric HWE reaction under PKR conditions [88], in which one equivalent of racemic aldehyde 143 was converted into alkene products 144 and 145 by reaction with half an equivalent each of two chiral phosphonates 28 and 20 bearing different chiral auxiliaries (Scheme 7.24). These alkene products, 144 and 145, were readily separable as a result of the difference in polarity between the two auxiliaries. It was clearly shown that the diastereoselectivities of the alkene products were dramatically improved compared to those obtained in the respective individual kinetic resolutions, especially in the case of alkene 145. 

The N+ relationship, as discussed above, is a systematization of experimental facts. The equation of Scheme 7-4 has been applied to nearly 800 rate constants of over 30 electrophiles with about 80 anionic, neutral, and even cationic nucleophiles covering a range of measured rate constants between 10-8 and 109s 1 (Ritchie, 1978). Only about a dozen rate constants deviated from the predicted values by more than a factor of 10, and about fifty by factors in the range 5-10. It is therefore, very likely that this correlation is not purely accidental. Other workers have shown it to be valid for other systems, e.g., for ferrocenyl-stabilized cations (Bunton et al., 1980), for coordinated cyclic 7r-hydrocarbons (Alovosus and Sweigart, 1985), and for selectivities of diarylcarbenes towards alkenes (Mayr, 1990 Mayr et al., 1990). On the other hand, McClelland et al. (1986) found that the N+ relationship is not applicable to additions of less stable triphenylmethyl cations. 

Kochi (1956a, 1956b) and Dickerman et al. (1958, 1959) studied the kinetics of the Meerwein reaction of arenediazonium salts with acrylonitrile, styrene, and other alkenes, based on initial studies on the Sandmeyer reaction. The reactions were found to be first-order in diazonium ion and in cuprous ion. The relative rates of the addition to four alkenes (acrylonitrile, styrene, methyl acrylate, and methyl methacrylate) vary by a factor of only 1.55 (Dickerman et al., 1959). This result indicates that the aryl radical has a low selectivity. The kinetic data are consistent with the mechanism of Schemes 10-52 to 10-56, 10-58 and 10-59. This mechanism was strongly corroborated by Galli s work on the Sandmeyer reaction more than twenty years later (1981-89). 

The catalytic hydroformylation of alkenes has been extensively studied. The selective formation of linear versus branched aldehydes is of capital relevance, and this selectivity is influenced by many factors such as the configuration of the ligands in the metallic catalysts, i.e., its bite angle, flexibility, and electronic properties [152,153]. A series of phosphinous amide ligands have been developed for influencing the direction of approach of the substrate to the active catalyst and, therefore, on the selectivity of the reaction. The use of Rh(I) catalysts bearing the ligands in Scheme 34, that is the phosphinous amides 37 (R ... 

The reactivity of different alkenes toward mercuration spans a considerable range and is governed by a combination of steric and electronic factors.24 Terminal double bonds are more reactive than internal ones. Disubstituted terminal alkenes, however, are more reactive than monosubstituted cases, as would be expected for electrophilic attack. (See Part A, Table 5.6 for comparative rate data.) The differences in relative reactivities are large enough that selectivity can be achieved with certain dienes. 

In order to rationalize the catalyst-dependent selectivity of cyclopropanation reaction with respect to the alkene, the ability of a transition metal for olefin coordination has been considered to be a key factor (see Sect. 2.2.1 and 2.2.2). It was proposed that palladium and certain copper catalysts promote cyclopropanation through intramolecular carbene transfer from a metal carbene to an alkene molecule coordinated to the same metal atom25,64. The preferential cyclopropanation of terminal olefins and the less hindered double bond in dienes spoke in favor of metal-olefin coordination. Furthermore, stable and metastable metal-carbene-olefin complexes are known, some of which undergo intramolecular cyclopropane formation, e.g. 426 - 427 415). 

Xiang, Y., Larsen, S.C. and Grassian, V.H. (1999). Photooxidation of 1-alkenes in zeolites a study of the factors that influence product selectivity and formation. J. Am. Chem. Soc. 121, 5063-5072... 

It is observed that insertion into a zirconacyclopentene 163, which is not a-substituted on either the alkyl and alkenyl side of the zirconium, shows only a 2.2 1 selectivity in favor of the alkyl side. Further steric hindrance of approach to the alkyl side by the use of a terminally substituted trans-alkene in the co-cyclization to form 164 leads to complete selectivity in favor of insertion into the alkenyl side. However, insertion into the zirconacycle 165 derived from a cyclic alkene surprisingly gives complete selectivity in favor of insertion into the alkyl side. In the proposed mechanism of insertion, attack of a carbenoid on the zirconium atom to form an ate complex must occur in the same plane as the C—Zr—C atoms (lateral attack 171 Fig. 3.3) [87,88]. It is not surprising that an a-alkenyl substituent, which lies precisely in that plane, has such a pronounced effect. The difference between 164 and 165 may also have a steric basis (Fig. 3.3). The alkyl substituent in 164 lies in the lateral attack plane (as illustrated by 172), whereas in 165 it lies well out of the plane (as illustrated by 173). However, the difference between 165 and 163 cannot be attributed to steric factors 165 is more hindered on the alkyl side. A similar pattern is observed for insertion into zirconacyclopentanes 167 and 168, where insertion into the more hindered side is observed for the former. In the zirconacycles 169 and 170, where the extra substituent is (3 to the zirconium, insertion is remarkably selective in favor of the somewhat more hindered side. 

The investigation of factors affecting facial selectivity in the hydroboration of steroidal -alkenes revealed the facial (a vs /3) stereoselectivities of hydroboration of androst-5-enes (69) and B-norandrost-5-enes (70) do not parallel the difference between the calculated force-field energies for a- and jS-cyclobutane models (71)-(74). This finding appears to suggest that the facial selectivity is not determined by the four-centre transition state but by the relative ease of formation of the initial tt-complex. ... 

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