Assistance, intramolecular participation

Figure 7-2 indicates that this intramolecular assistance takes place with those compounds having substituents to the right of X = OMe = —0.78), for in this portion of the a scale compounds 8 solvolyze more rapidly than do 7. At the X = OMe member, however, the two series have essentially identical reactivities, and this behavior continues at more negative cr. It. therefore, appears that intramolecular participation by the double bond occurs when it is needed when the X substituent is sufficiently electron-donating to stabilize the cation, intramolecular assistance is not needed, so it does not occur, and the saturated and unsaturated series show the same reactivity. 

More recently, Usui et al. studied the stereochemical aspects of this rearrange-ment. (5)-2-Phenylpropionic acid was stereoselectively obtained by the AgBF4-catalyzed rearrangement of (S)-2-chloropropiophenone dimethyl acetal with a complete chirality transfer, suggesting that the rearrangement proceeds with the backside phenyl group participation at the C-Cl bond, that is, with an Ag+ -aided, phenyl-assisted intramolecular SN2 mechanism (Scheme 3.52). 

Neber rearrangement, anamalous 23,212 31, 376 Neighboring group participation (s. a. Assistance, intramolecular)... 

Aryl thioethers s. Thioethers, ar. Arylthiomethylation 26,844 Aryl tridiloromethyl carbinols -, reactions with nucleophiles, review 22, 629 suppl. 26 Arynes s. Benzyne..., Hetarynes Ascaiite 11,963 suppl. 28 Asscher-Yofsi reaction 21,642 Assistance, intramolecular s. Neighboring group participation... 

Series 7 gives a linear plot with p = —5.27, consistent with the S l mechanism. Series 8, however, shows a discontinuity, which Gassmann and Fentiman interpreted as a change in mechanism. Compounds in series 8 are capable of intramolecular assistance (neighboring group participation) by electron donation from the double bond to stabilize the cation, as in 9. 

The driving force behind this remarkable reaction in which the sulphone participates intramolecularly in a 5-exo-tet ring closure process96 is that the geometry of the intermediate bromonium ion is nicely set up for the attack by the sulphone oxygen. Probably, the process is assisted by the ready cleavage of the sulphur to the t-butyl bond, with the concomitant formation of t-butyl bromide. 

In contrast to the intramolecular process, simple allenes do not participate in the intermolecular [5 + 2]-reaction with VCP 18. However, when a second functionality is incorporated into the allene, an efficient and facile cycloaddition occurs, presumably assisted by a directing effect of the secondary functional group. The [5 + 2]-reaction also works with styrenyl- and cyano-substituted allenes as directing groups (Scheme 12). As would be expected from... 

Intramolecular nucleophilic displacements, such as those in lactone formation, have faster reaction rates than intermolecular S,42 reactions because the latter require two species to collide. The neighboring participant is said to furnish anchimeric assistance. 

The reported analyses of the products suggested that the COOCH2CH3 substituent provided anchimeric assistance for the reaction paths 2 and 3. The mechanism was explained in terms of an intramolecular solvation of the bromide ion through an intimate ion-pair intermediate which decomposes in two different directions (equation 74, where X = Br and CH3CH2 replaces CH3). The formation of bromobutyric acid and ethylene (path 3) indicated a normal six-membered transition state for their formation as found in primary ethyl esters pyrolyses in the gas phase. Bromobutyric acid, which is known to be unstable at room temperature, rapidly produced butyrolactone. The consecutive reaction of path 4, under the experimental conditions, was ascribed to a similar mechanism where an intimate ion-pair intermediate is formed through COOH participation (equation 76). 

The entropic assistance provided in the intramolecular Diels-Alder reaction often is sufficient to promote heterodiene 4ir participation in poorly matched [4 + 2] cycloaddition reactions, including those with unactivated and electron-deficient dienophiles. - " ... 

One of the most effective approaches to implementing the Diels-Alder participation of 1-oxa-1,3-butadienes is through the use of an intramolecular [4 + 2] cycloaddition reaction.A select set of thermal and Lewis acid-catalyzed intramolecular cycloaddition reactions of unactivated and electron-rich alkenes with a,P-unsaturated aldehydes and ketones has been detailed. Two examples of the poorly matched intramolecular Diels-Alder reaction of an a,P-unsaturated aldehyde (4 ir component) with an a, 3-unsaturated amide (2ir component) have proven successful (190-160 °C) and may be attributed to the entropic assistance provided by the intramolecular reaction. These observations have been applied in... 

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