Strained olefins reactivity

Reactive unsaturated nitroso compounds such as 1112 can also be readily prepared from a-halooximes such as 1111 on treatment with powdered Na2C03 in diethyl ether to give, in the presence of enoltrimethylsilyl ether 1113 or strained olefins such as norbomene and other dienophiles, hetero-Diels-Alder adducts such as 1114 and 1115 in moderate yields [150-155] (Scheme 7.47). 

Luef, W., Strained Olefins Structure and Reactivity ofNonplanar Carbon-Carbon Double Bonds, 20, 231. 

The higher reactivity of ring-strained olefins has been exploited by several workers in 1,3-dipolar cycloaddition reactions of milnchnones. Thus, Kato and co-workers (112) reported that miinchnone 38 reacts with 1,2,3-triphenyl-1//-phosphirene (202) to give l-methyl-2,3,4,5-tetraphenylpyrrole (203) (45% yield). Control experiments demonstrated that phosphirene 202 does not decompose to diphenylacetylene appreciably under the reaction conditions. Moreover, the reaction of diphenylacetylene and miinchnone 38 afforded only a 21% yield of pyrrole 203. 

In contrast to 52 (Scheme 12), diastereomer 55 (Scheme 13), because of its more exposed and highly reactive strained olefin, undergoes rapid polymerization in the presence of 4a. The less reactive Ru complex 56 [23] can however be used under an atmosphere of ethylene to effect a tandem ROM/CM to generate 57. The resulting triene can be induced to undergo Mo-catalyzed ARCM (5 mol % 4a) to afford optically pure 58, the AROM/RCM product that would be obtained from 55. 

Early work on strained double bonds has been reviewed.7-9 Double bonds in strained bicylic systems and medium-sized cycloalkenes are particularly reactive and add azides quantitatively in an exothermic reaction43-45 81,82 that could be useful in derivatization83 and quantitative analysis.84 The reaction of organic azides with strained, olefinic bonds in cyclic systems, first recorded by Alder,43-45 has been the subject of numerous theoretical and... 

The fluoride ion-induced reductive / -elimination makes it possible to generate highly strained olefins [Eq. (31)]. /J-Silyl A3-iodane 40 generates five-mem-bered cumulene with remarkable reactivity at room temperature and affords Diels-Alder adduct 40a (7 % yield) by the reaction with benzene [63]. 

Strained Olefins Structure and Reactivity of Nonplanar Carbon-Carbon Double... 

All of the preceding syntheses exploited the reactions of o-benzoquinones with reactive (strained) olefins. A different approach led to the successful synthesis 62)... 

Neunhoeffer and Wiley (78HC226) discovered that 1,2,4-triazine served as a reactive, electron-deficient diene in inverse electron demand Diels-Alder reactions with electron-rich or strained olefins. Cycloaddition occurs exclusively across C-3/C-6 of the triazine nucleus and there is a strong preference for the nucleophilic carbon of the dienophile to be attached to C-3... 

Another type of strained olefins which add azides are the mediumsized ircnj-cycloolefins. In this series increase in the ring size relieves the strain and thus lowers the reactivity . 

It was thus early recognized that structural deformations of double bonds are related to instability and enhanced reactivity. It is this relationship that induced extensive studies of strained carbon compounds and distorted olefins. Moreover, it is evident that even today, the inherent chemical information potential of strained olefins has only been partially explored (4-6). 

Whereas transition metal complexes of alkenes and their chemistry have been well explored, comparatively little is known about the structure and reactivity of n complexes obtained from strained olefins. The stability of transition metal complexes of alkenes in general is preferably discussed in terms of the Dewar-Chatt-Duncanson model (171). A mutual er-type donor-acceptor interaction accounts for the bonding overlap of the bonding 71-MO of the olefin with vacant orbitals of the metal together with interaction of filled d orbitals with the 7r -MO of the double bond (back bonding) leads to a partial transfer of. electron density in both directions (172). The major contribution to the stabilizing interaction is due to back-bonding. 

Sterically demanding substituents, which give rise to distortions, may also reduce reactivity. Examples are given by the strained olefins tri-tert-butyl-ethene (66) and tetraneopentylethene (82) the former reacts only slowly and the latter not at all with bromine (103,239). Tetra-rert-butylcyclobutadiene (87) and betweenanenes 141, 144, and 146 also belong to this class of highly strained compounds where reactions are prevented by steric hindrance (95,240). 

For the hydronium-ion-catalyzed hydration of bicyclo[4.2.1]non-l-ene (177) and bicyclo[4.2.1]non-l(8)-ene (176), appreciable solvent isotope effects have been observed. Since these correspond to those found for reactions of unstrained olefins, it was concluded that the hydration proceeds as with unstrained alkenes by a two-step mechanism protonation of the double bond is followed by addition of the nucleophile (152b). The strained olefin with its distorted double bond is higher in energy and more reactive than an unstrained alkene. Hence, the transition state for protonation of Bredt-olefins is expected to be an early one (95). 

Little work has been carried out to probe the reactivity of the double bond of strained olefins by hydroboration. In an early investigation it was found that hydroboration of bicyclo[3.3. l]non-l -ene (69) occurs with low regioselectivity but with reasonable stereospecificity (279 280 281 30 64 6) (252) (Scheme 13). However, it remains to be seen whether 281 is a primary product of the reaction with borane. 

The intra- and intermolecular addition of carbon centered radicals to carbon-carbon double bonds is currently of great synthetic interest and much work has been done for control of regioselectivity (256,257). Comparatively little work has been done to investigate the reactivities of strained olefins. The rates of gas-phase additions of HO" and N03" radicals to the cyclic olefins with ring size 5-7, norbornene (287) and bicyclo[2.2.2]octene (288), showed little variation (krel < 1.8 and krel < 5, respectively) and were insensitive to the ring strain energies (258). 

In summary, the reactivity of strained olefins is different from that of simple alkenes. The wealth of transformations initiated by strained double bonds has recently been reviewed in great detail (276). In this chapter some structural and strain-related features of reactivity observed for olefins with nonplanar double bonds have been presented. It is apparent that in polar reactions as well as cycloadditions of these molecules the release of strain may contribute to enhanced rates, but it is certainly not the controlling factor. In view of the operation of microscopic reversibility, it may be questioned whether the buildup of strain in reverse reactions—eliminations and cycloreversions—is the controlling factor. 

STRAINED OLEFINS STRUCTURE AND REACTIVITY OF NONPLANAR CARBON-CARBON DOUBLE BONDS 231... 

Hydroalumination followed by protonolysis is less convenient for stereoselective cis hydrogenation of the double bond. Simple, vicinally disubstituted double bonds sluggishly undergo hydroalumination and the carbon-aluminum bond undergoes inversion in hydrocarbon solvents at moderate temperatures. Strained olefins are more reactive and can react stereoselec-tively under carefully controlled conditions126. 

The stability of the a-dithiocarbonyl compounds follows closely their potential utility for 4ir participation in Diels-Alder reactions with typical olefinic and acetylenic dienophiles [a-dithioamide > a-dithionoester > a-dithionothioester (dimer equilibrium at 25°C) > a-dithione > a-dithioalde-hyde]. The a-dithiocarbonyl compounds are electron-deficient and consequently react rapidly with electron-rich and strained olefins in inverse electron demand (LUMOd,ene controlled) Diels-Alder reactions and more slowly with unactivated or electron-deficient dienophiles in apparent normal (HOMODiels-Alder reactions (Scheme 8-IX).36 Subtle differences in the reactivity and observed course of reactions of the various a-dithiocarbonyl compounds have been detailed.36... 

In many instances, the entropic assistance provided in the intramolecular Diels-Alder reaction is sufficient to promote azadiene participation in Diels-Alder reactions.12 The incorporation of the azadiene system, or dienophile, into a reactive or sensitive system, e.g., heterocumulene or strained olefin, allows a number of specialized azadiene Diels-Alder reactions. Many such examples may represent stepwise, polar [4 + 2] cycloaddition reactions. 

For preliminary studies on the scope of the [4 + 2] cycloaddition reactions of 1,2,4-triazines with electron-rich dienophiles including enol ethers, enamines, ketene acetals, ynamines and strained or reactive olefins, see Dittmar, W., Sauer, J., and Steigel, A. (1969). Tetrahedron Lett., 5171 Reim, H., Steigel, A., and Sauer, J. (1975). Tetrahedron Lett., 2901. 

V-Sulfinylaniline and related Af-arylsulfinylimines participate as 2ir components of [4 + 2],6 [2 + 2], and [3 + 2] cycloadditions in addition to their observed 47t participation in Diels-Alder reactions with strained and reactive olefins [Eq. (41).]103... 

All electron-rich dienophiles including 0,0-ketene acetals, O.S-ketene acetals, 5,5-ketene thioacetals, 0,A/-ketene acetals, MS-ketene acetals, N,N-ketene aminals, enol ethers, enamines, and reactive or strained olefins cycloadd exclusively across C-3/C-6 of the 1,2,4-triazine nucleus [Eq. (26)].90-92,94... 

In 1972, Mock considered double-bond reactivity and its relationship to torsional strain, by which he understood the strain imposed on a double bond in medium-ring fra 5-cycloalkenes or by steric compression of large cis substituents [28]. He argued that the loss of 7t overlap due to a torsion about the double bond can be partially compensated by rehybridization in these two situations, leading, respectively, to syn and anti pyramidalization of the double bond consequently, such bonds will favor different modes of addition (cis and trans). The proposition was supported by examples of X-ray structures of strained olefins, STO-3G energy calculations for the twisted and pyramidalized ethylene geometries, and by analysis of the out-of-plane vibrational frequencies of ethylene. Mock concluded that small ground-state distortions may produce sizable effects in the transition states. 

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