Relative reactivity of alkenes

Table 6.3. Relative Reactivity of Alkenes toward Halogenation...

The many successful applications of nitrile oxide cycloadditions in synthesis are intimately linked with theory, both the simple FMO variety as well as the more sophisticated ab initio treatment, where the work of Sustmann and subsequently of Houk and his group has been seminal. We, the practitioners, have thus been supplied with a consistent view on the nature of 1,3-dipoles, their reactivity toward dipolarophiles, and the origin and interpretation of stereoselectivity of cycloaddition chemistry. It is of course desirable that our understanding of the relative reactivities of alkenes as well as of many 1,3-dipoles would be also improved, thereby leading to simple, extended recipes for the chemist practicing synthetics. We hope that this account will stimulate further advances in this field of cycloaddition chemistry and promote further uses of nitrile oxides in organic synthesis. 

The catalyzed reaction of acetylenic esters and alkenes can lead to ene products and/or cis [2 + 2]cycloaddition. The relative reactivity of alkenes established by reactions with dienes is 1,1-disubstituted > trisubstituted > monosubstituted and 1,2-disubstituted. Ene reactions predominate with alkenes containing two substi tuents on one carbon.1... 

The acylperoxy radical was found to epoxidize olefins much faster than peracids also formed under reaction conditions. The result ruled out the role of the latter.267 The addition of RCO3 was observed to occur 105 faster than that of ROO. The relative reactivity of alkenes suggests a strongly electrophilic radical forming the polar transition state 30 ... 

The relative reactivity of alkenes toward reduction by diimide depends on the degree of substitution. Increasing alkyl substitution results in decreasing reactivity, and strained alkenes exhibit higher reactivity than nonstrained compounds 187... 

Table 5 Relative Reactivities of Alkenes Toward Various Electrophiles... 

Hydroboration. When these two reagents are mixed in THF, hydrogen is evolved and a solution of a violet titanium-boron complex forms. This complex catalyzes the hydroboration of alkenes with LiBH4 to form lithium alkylboro-hydrides, which are converted to alcohols by NaOCHj and H2O2. The relative reactivity of alkenes is terminal > cyclic > internal. The reaction involves anti-Markownikoff addition. 

Good yields of 1 1 adducts have also been obtained from Lewis acid catalyzed reactions of methyl chloropropiolate or DMAD with alkenes. With methyl chloropropiolate, cyclobutenes are obtained exclusively from mono- and 1,2-di-substituted alkenes and mixtures of ene adducts and cyclobutenes are obtained with 1,1-di-, tri- and tetra-substituted alkenes. With DMAD, ene adducts are obtained exclusively from 1,1-di-, tri- and tetra-substituted alkenes and mixtures of ene adducts and cyclobutenes are obtained hrom 1,2-disubstituted alkenes. EtAlCb is a more effective catalyst than AlCb since it is a proton scavenger as well as a Lewis acid. With AlCb, lactone byproducts are also formed. - The relative reactivity of alkenes in these Lewis acid catalyzed reactions was found to be 1,1-di- > tri- > tetra- mono- > 1,2-di-substituted. With reactive ene components such as methylenecyclohexane, ethyl-idenecyclohexane and -pinene, good yields of ene adducts can be obtained with DMAD by thermal reaction at 110-130 "C for 0.25-6 d. ... 

Two-laser two-photon results revealed photoisomerization of the cation E,E-11 to its stereoisomer Z,E-11, which undergoes thermal reversion with a lifetime of 3.5 ps at room temperature. Absolute rate constants for reaction of styrene, 4-methylstyrene, 4-methoxystyrene and /i-methyl-4-methoxystyrene radical cation with a series of alkanes, dienes and enol ethers are measured by Laser flash photolysis [208]. The addition reactions are sensitive to steric and electronic effects on both the radical cation and the alkene or diene. Reactivity of radical cations follows the general trend of 4-H > 4-CH3 > 4-CH3O > 4-CH30-jff-CH3, while the effect of alkyl substitution on the relative reactivity of alkenes toward styrene radical cations may be summarized as 1,2-dialkyl < 2-alkyl < trialkyl < 2,2-dialkyl < tetraalkyl. 

The relative reactivity of alkenes toward various carbenes and carbenoids has been compiled Recently, a selectivity index, mcxv which can be estimated from the dual... 

In the following sections we shall see that the mechanism is also consistent with (c) the orientation of addition (d) the relative reactivities of alkenes, and (e) the occurrence of rearrangements. 

The openess of the strained 9-BBN molecule makes possible hydroboration of even hindered alkenes not readily attacked by SiajBH. This indicates a lower degree of steric control in the reaction with 9-BBN, but 9-BBN is more sensitive to electronic effects, e.g., the difference in reactivity between p-trifluoromethylstyrene and p-methoxystyrene is 1.5 for SiajBH and 67 for 9-BBN The relative reactivities of alkenes toward 9-BBN vary over 10 (see Table 3). A high preference for hydroboration of terminal double bonds and a small difference in the reactivity of cis-trans pairs is evident. Unlike other hydro-borating agents, 9-BBN reacts faster with terminal double bonds than with internal triple bonds e.g. ... 

As part of their study of the relative reactivity of alkenes and alkynes, Modena and coworkers determined the reactivity ratio toward carbocations. These cations were generated by interaction of either diphenylmethyl chloride or 1-phenylethyl chloride with zinc chloride. Absolute rate constants were not determined. In general, the reactivities of alkynes and alkenes are of the same order of magnitude (see also Section II. A). 

TABLE 9.1 Relative Reactivity of Alkenes in Bromine Addition ... 

Relative reactivities of alkenes for hydrogenation by Wilkinson s catalyst. 

The relative reactivities of alkenes are a key component in the development of efficient cross-metathesis reactions and minimizing homodimerization. Powerful empirical models of olefin reactivity have been experimentally developed [27], but relatively few computational studies have been directedtoward understanding the relative reactivity of substituted olefins. This is in part because the number of pathways available increases dramatically for the cross-metathesis of substituted olefins, especially when secondary metathesis processes are considered. However, several computational studies have explained the origins of the low reactivities of certain alkenes, and these are discussed here. 

Scheme 10.7 Relative reactivity of alkenes toward (CO)5W=CPh2.

Alkenoic acids given in Scheme 20 are also cyclized to give good yields of lactones.f t The relative reactivity of alkenes generally follows the order disubstituted > trisubstituted > monosubstituted, probably due to a fine balance between the electron density and steric bulkiness. exo-Methylene-y-butyrolactones can similarly be synthesized from the corresponding alkenyl carboxylic acids. ... 

Further, when double and triple bonds are both present in the same substrate (vide infra), their relative reactivity appears to depend upon what is being added. Thus, generalizations regarding the relative reactivities of alkenes and alkynes can be made only within narrow Omits. 

Table 4.13 Effect of halogen substitution on the relative reactivity of alkenes in hydroboration with (9-BBN), [1] ...

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