Allyl resonance

C 1 IS more reactive because the intermediate formed by electrophilic attack there IS a relatively stable carbocation A benzene type pattern of bonds is retained m one nng and the positive charge is delocalized by allylic resonance... 

To involve allylic resonance in stabilizing the arenium ion formed during attack at C 2 the benzenoid character of the other ring is sacrificed... 

The allylic resonance may give rise to formation of a mixture of isomeric allylic bromides, e.g. 6 and 8 from but-l-ene. The product ratio depends on the relative stability of the two possible allylic radical species 5 and 7 ... 

The C—C bonds are close to being ordinary single bonds (bcc — 1-12, Rcc = 1.45 A), showing that little vestige of allylic resonance remains (as the geometry in Fig. 4.86(c) makes apparent), and the p3 species (4.111a) is therefore better characterized as a metallabicyclobutane. For the p1 complex (4.11 lb), the single ctauc bond (Fig. 4.87(c)) has the approximate composition... 

Even the allyl anion can be seen as an example of resonance-enhanced coordination. As shown in Section 4.9.2, r -CsHs- complexation is accompanied by a shift toward the localized H2C —CH=CH2 resonance structure that places maximum anionic character at the metal-coordinated carbon atom. In effect, the carbanionic lone pair nc is shared between intramolecular nc 7icc (allylic resonance) and intermolecular nc—>-n M (metal coordination) delocalizations, and the former can be diminished to promote the latter, if greater overall stabilization of the metal-ligand complex is achieved thereby. 

The allyl-resonance stabilized E- and Z-pent-l,3-dienyl-2-cations (22 and 23) are the smallest member of vinyl cations observed as persistent species in superacid solution 49 These are difficult to generate experimentally50 but structures with only five heavy atoms are suitable candidates for coupled cluster model calculations. A challenging task of quantum chemistry was to assign the 13C NMR spectrum of the mixture of isomers (Fig. 3), which exhibits pairs of signals of 22 and 23 which differ only by a few ppm, to the chemical shifts for the specific carbon atoms of the E- and Z-isomers, respectively. 

The checkers found that the 1H NMR spectrum of (2-bromoallyl)-diisopropoxyborane is concentration dependent. When the NMR spectrum was measured at a concentration of 20 pL of product in ca. 0.5 mL of CDCI3, (2-bromo-allyl)diisopropoxyborane was observed along with a substantial amount of a second material that had 1H resonances for the vinylic and allylic protons that were very similar in chemical shift to the vinylic and allylic resonances of the desired product. However, when the 1H NMR spectrum of a much more concentrated solution (ca. 250... 

Thus the rotation of the allylic system out of the plane (required before ring closure can occur) results in the reduction of the available allylic resonance energy by about 5 kcal mole". This particular steric requirement does not apply to the isomerizations to the dienes, and these have much the same values as for vinylcyclopropane itself, viz. ... 

The simplest explanation of these results is that the reaction involves the cleavage of the cyclopropane ring to give a biradical, which is doubly stabilized by allylic resonance. Rotation of the biradical allows ring... 

The difference in conjugation between neutral molecules and their ion-radicals can also be traced for keto-enol tautomerism. As a rule, enols are usually less stable than ketones. Under the equilibrium conditions, enols exist only at a very low concentration. However, the situation becomes different in the corresponding cation-radicals, where gas-phase experiments have shown that enol cation-radicals are usually more stable than their keto tautomers. This is because enol cation-radicals profit from allylic resonance stabilization that is not available to ketones (Bednarek et al. 2001, references therein). 

The carbon—carbon double bond is the distinguishing feature of the butylenes and as such, controls their chemistry. This bond is formed by sp orbitals (a sigma bond and a weaker pi bond). The two carbon atoms plus the four atoms in the alpha positions therefore lie in a plane. The pi bond which lies over the plane of the atoms acts as a source of electrons in addition reactions at the double bond. The carbon—carbon bond, acting as a substitute, affects the reactivity of the carbon atoms at the alpha positions through the formation of the allylic resonance structure. This structure can stabilize both positive and... 

The shielding of the Ca and Cy resonances in 349 and 350 indicates allyl resonance delocalization of the positive charge. The unusual high field resonance of the signals for the sp2-hybridized carbons C/ and Cp in accord with those for other vinyl cations, are due to the sp-hybridization of the geminal Ca carbon. The methyl groups at Cy are non-equivalent because of the partial double bond character of the Ca—Cp bond. 

The close resemblance of the para carbon shift of the silyl-substituted cations 372-375 (168-170 ppm) to that of the mesitylallenyl cation 385 (165.9 ppm), which in addition to aryl conjugation is stabilized by /i-allyl resonance, shows that hyperconjugative interaction of a jS-C—Si cr-bond with the vacant 2p(jr) orbital on C+ in 372-375 is about as efficient as /l-jr-conjugation in 385. 

For two isomeric halides to yield the same carbocation on ionization, they must have the same carbon skeleton. They may have their leaving group at a different location, but the carbocations must become equivalent by allylic resonance. 

In an ESR study of 1,1,3,3-difluoroallyl radicals, Krusic and coworkers were able to demonstrate that the barrier to rotation of such apparently planar radicals is substantially reduced [18]. Although allyl itself has a rotational barrier of 15 kcal/mol [19, 20], 1,1,3,3-tetrafluoroallyl, 1, had a barrier of but 7.2 kcal/mol. The observed 19F hfs constants (42.6 and 39.7 G) were consistent with 1 being a planar system. It is likely that the lowering of the rotational barrier of 1 derives from a destabilizing interaction between the fluorine lone pairs and the doubly-occupied allyl tt-MO which diminishes the net allylic resonance energy, as well as from stabilization of the transition state due to pyramidalization. 

Quantum chemical studies of cyclizations of enediynes and enyneallenes have been reviewed.180 The intermediates are computationally tractable as a result of the unrestricted broken spin symmetry (UBS) approach using GGA functionals for the description of open-shell biradicals. The intermediacy of biradicals in Cope-type rearrangements, to which the Bergman and Myers-Saito reactions belong, are shown to be predictable using a very simple rule biradicals are likely to be intermediates if they are stabilized either by allyl resonance or by aromaticity. 

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