Addition reactions hyperconjugative effects

Finally, Holm and 0gaard Madsen (1992) determined the secondary /3-deuterium KIEs for the addition of /3-deuterated Grignard reagents to unsaturated ketones. Hie authors attributed the small, <5%, KIEs they found in these radical reactions to opposing steric and hyperconjugative effects. 

Additional acylation studies were also reported (24), (26). In the first case it is claimed that acylation of thiophene is achieved by means of HC104 and acetic anhydride affording a 65 % yield of 2-acetylthiophene. In the second paper Levine and coworkers reported that while 2,5-dimethylthiophene could be readily acetylated, 2,5-dichlorothiophene acetylated sluggishly. This is, however, readily explained, since the presence of chlorine atoms on the thiophene ring decreased its reactivity in electrophilic substitution reactions. In the case of methyl substitution, however, the 3 and 4 positions of the ring are activated toward electrophilic substitution by the inductive and hyperconjugative effects. Thus 2,5-dimethylthiophene was successfully acylated by the boron fluoride etherate method in high yield with three aliphatic anhydrides. 

An alkyne is less reactive than an alkene. A vinyl cation is less able to accommodate a positive charge, as the hyperconjugation is less effective in stabilizing the positive charge on a vinyl cation than on an alkyl cation. The vinyl cation is more stable with positive charge on the more substituted carbon. Electrophilic addition reactions allow the conversion of alkenes and alkynes into a variety of other functional groups. 

In general, the effects of alkyl and alkoxyl (e.g. CH3—O) groups are well known and understood. For example, ortho and para alkyls (at positions 2,4,6) stabilize the phenoxyl radical by inductive and hyperconjugative effects and, in addition, ortho groups provide steric hindrance to minimize undesirable wasting reactions such as pro-oxidation (equation 21). In addition, the conjugative effect of a heteroatom, for example at the para-position, provides stabilization through resonance (Scheme 4). 

Here the reaction is assisted by conjugative and hyperconjugative effects, and is retarded by the inductive effect which withdraws electrons from the reaction site. A similar effect is found in the addition of bromine to olefins (RCH=CH2) in acetic acid at 25°, viz-... 

Simple linear equations could also be developed for the other three systems of Figure 4, PA of aldehydes and ketones(4e), and their hydride ion affinities, both of the neutral (4f) and protonated forms (4g), However, in addition to effective polarizability and electronegativity, hyperconjugation had also to be used as a parameter, as p-orbitals carrying a partial positive charge are involved in the reactions 4e to 4g (26),... 

More often, however, fluorocarbanions are intermediates in reactions of fluoroalkenes,35 e.g. by addition of nucleophiles, especially fluoride. Deprotonation of hydrofluorocarbons also yields the anion. Whereas a-fluorination destabilizes the anion, /J-fluorination is essentially stabilizing. The latter effect may be assisted by so-called fluorine-nonbond resonance (negative anionic hyperconjugation) which has been discussed in diverse ways supported by some,3 but criticized by others.2 31... 

The other approach, with great potential, involves cyclisation reactions of polyfluoro compounds, with in situ aromatisation via loss of one or more fluorines. The reaction mode involves nucleophilic addition to gm-difluoro-methylidene intermediates. This moiety is highly susceptible to nucleophilic addition to the fluorine-bearing carbon, even in the absence of other activation. A transient intermediate anion is thought to be stabilised by hyperconjugation and inductive effects from the two P-fluorines, this being a much more favourable situation than an anion a to the fluorines. ... 

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