Relative reactivity cycloadditions

An interesting probe of reactivity was presented by Burton in his study of cycloadditions of l,2-disubstituted-3,3,4,4-tetrafluorocyclobutenes and 1,2-disub-stituted-3,3,4,4,5,5-hexaf1uorocyclopentenes with butadiene, 2-methylbutadiene, and 2,3-dimethylbutadiene [86], On the basis of the extent of their conversions to adducts, the relative reactivities of the cyclobutenes and of the cyclopentenes are as shown in equation 74. A typical reaction is shown in equation 75. 

Diels-Alder cycloadditions are sensitive to steric effects of two major types in the diene. Bulky substituents on the termini of the diene hinder approach of the two components to each other and decrease the rate of reaction. This effect can be seen in the relative reactivity of 1-substituted butadienes toward maleic anhydride.19... 

Some trends in relative reactivity for intramolecular ketene cycloadditions have been examined by internal competitions.172 For example, 12 gives exclusively 13, pointing to a preference for five-membered rings over six-membered ones. 

If one examines the minimal sequences of reaction steps for [2+2] cycloadditions, Eqs. 12—18, 32—35, one concludes that stereochemistry of addition, and perhaps relative reactivities might be calculable at several points. Oriented complexes could control regiospecificity, or the transition state leading to a biradical could be the important stage. Relative rates of product formation would be derived from relative perturbation stabilization energies for different configurations of the two reactants. 

Cycloaddition with nitrile oxides occur with compounds of practically any type with a C=C bond alkenes and cycloalkenes, their functional derivatives, dienes and trienes with isolated, conjugated or cumulated double bonds, some aromatic compounds, unsaturated and aromatic heterocycles, and fullerenes. The content of this subsection is classified according to the mentioned types of dipolarophiles. Problems of relative reactivities of dienophiles and dipoles, regio- and stereoselectivity of nitrile oxide cycloadditions were considered in detail by Jaeger and... 

Although allene itself is reluctant to react with ordinary 1,3-dienes, it underwent successful [4+2]-cycloadditions with relatively reactive cyclopentadienes to afford 5-methylenebicyclo[2.2.1]hept-2-ene derivatives [145]. 

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... 

Edelson and Turro 79> have examined the kinetics of the cycloaddition of 2,2-dimethylcyclopropanone with furan and with cyclopentadiene at 0 °C, as well as the relative reactivities of a number of alkyl-... 

To the extent that the enolate resulting from conjugate addition at the (3-carbon can be stabilized, the rate of this reaction pathway is enhanced. For example, (3-Michael additions are observed for MVK, acrolein, and acetylenic electrophiles even without the presence of a Lewis acid. Furthermore, MVK reacts with the 2,5-dimethylpyrrole complex (22) to form a considerable amount of (3-alkylation product, whereas only cycloaddition is observed for methyl acrylate. The use of a Lewis acid or protic solvent further enhances the reactivity at the (3-position relative to cycloaddition. While methyl acrylate forms a cycloadduct with the 2,5-dimethylpyrrole complex (22) in the absence of external Lewis acids, the addition of TBSOTf to the reaction mixture results in exclusive conjugate addition (Tables 3 and 4). 

Thus, the results of this comparative study clearly indicated that (1) the 7t-facial selectivity of the Diels-Alder reactions of vinyl sulfoxides must be explained by assuming a steric approach control of the diene on the less hindered face of the dienophile (that supporting the lone pair at sulfur), taking into account the relative reactivity of the retainers around the C-S bond, in addition to their populations (2) both the endo and the 7i-facial selectivities of the cycloadditions of vinylsulfox-ides with acyclic dienes are substantially higher than with cyclic ones. 

Tlie decomposition of diazo compounds can be run under very difierem conditions. Their photochemical decomposition has been largely investigated in the gas phase and in solution in the presence of olefinic and saturated hydrocarbons as substrates the fundamental concepts concerning the relative reactivity and selectivity of singlet-triplet states of carbenes are essentially the result of such studies and they have formed the basis for the determination of the correla tion between the spin state of the carbene and its selectivity and reactivity in imerliun and cycloaddition reactions. 

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