Addition reactions electronic effects

Interestingly, [IrCl(cod)]2 did not catalyze the reaction. The [Ir(OMe)(cod)]2]/dtbpy catalytic system in nonpolar solvents such as hexane was found to be very effective for the synthesis of arylboronates, using stoichiometric amounts of arene and pin2fi2 at room temperature (Table 1). The borylation t) ically occurs at the para or meta position with respect to the functional groups on the arene. The ortho C-H positions are less active due to the steric hindrance. In addition, the electronic effect plays a minor role, where the electron-poor carbon seems to be more active. This allows the reaction to occur regiospecifically (Table 1). For example, the borylation of 1,3-disubstituted arenes selectively occurs at the common meta position, while the borylation of heterocycles such as benzo[l)]thiophene, benzo[fc]furan, or indole occurs selectively at the 2-position (Table 1). 

The BDE theory does not explain all observed experimental results. Addition reactions are not adequately handled at all, mosdy owing to steric and electronic effects in the transition state. Thus it is important to consider both the reactivities of the radical and the intended coreactant or environment in any attempt to predict the course of a radical reaction (18). AppHcation of frontier molecular orbital theory may be more appropriate to explain certain reactions (19). 

Methacryhc acid and its ester derivatives are Ctfjy -unsaturated carbonyl compounds and exhibit the reactivity typical of this class of compounds, ie, Michael and Michael-type conjugate addition reactions and a variety of cycloaddition and related reactions. Although less reactive than the corresponding acrylates as the result of the electron-donating effect and the steric hindrance of the a-methyl group, methacrylates readily undergo a wide variety of reactions and are valuable intermediates in many synthetic procedures. 

For many species the effective atomic number (FAN) or 18- electron rule is helpful. Low spin transition-metal complexes having the FAN of the next noble gas (Table 5), which have 18 valence electrons, are usually inert, and normally react by dissociation. Fach normal donor is considered to contribute two electrons the remainder are metal valence electrons. Sixteen-electron complexes are often inert, if these are low spin and square-planar, but can undergo associative substitution and oxidative-addition reactions. 

These effects can be attributed mainly to the inductive nature of the chlorine atoms, which reduces the electron density at position 4 and increases polarization of the 3,4-double bond. The dual reactivity of the chloropteridines has been further confirmed by the preparation of new adducts and substitution products. The addition reaction competes successfully, in a preparative sense, with the substitution reaction, if the latter is slowed down by a low temperature and a non-polar solvent. Compounds (12) and (13) react with dry ammonia in benzene at 5 °C to yield the 3,4-adducts (IS), which were shown by IR spectroscopy to contain little or none of the corresponding substitution product. The adducts decompose slowly in air and almost instantaneously in water or ethanol to give the original chloropteridine and ammonia. Certain other amines behave similarly, forming adducts which can be stored for a few days at -20 °C. Treatment of (12) and (13) in acetone with hydrogen sulfide or toluene-a-thiol gives adducts of the same type. 

Formation of five-membered ring systems (1,2-addition) can compete with formation of the seven-membered heterocycles (1,4-addition). If the first step of the reaction sequence, namely the nucleophilic attack of the terminal heteroatoin of the diene, is hindered by steric or electronic effects, the five-membered ring product is formed exclusively. 

The ortho effect may consist of several components. The normal electronic effect may receive contributions from inductive and resonance factors, just as with tneta and para substituents. There may also be a proximity or field electronic effect that operates directly between the substituent and the reaction site. In addition there may exist a true steric effect, as a result of the space-filling nature of the substituent (itself ultimately an electronic effect). Finally it is possible that non-covalent interactions, such as hydrogen bonding or charge transfer, may take place. The role of the solvent in both the initial state and the transition state may be different in the presence of ortho substitution. Many attempts have been made to separate these several effects. For example. Farthing and Nam defined an ortho substituent constant in the usual way by = log (K/K ) for the ionization of benzoic acids, postulating that includes both electronic and steric components. They assumed that the electronic portion of the ortho effect is identical to the para effect, writing CTe = o-p, and that the steric component is equal to the difference between the total effect and the electronic effect, or cts = cr — cte- They then used a multiple LFER to correlate data for orrAo-substituted reactants. 

In the case of the reaction between 2-diazopropane and diphenyldiacetylene, the reverse (as compared with other diynes) orientation of addition of the first molecule of the diazo compound with a predominant formation of 4-phenylethynylpyrazole is observed. Therefore, it is noteworthy that whereas the regioselectivity of the addition of diazoalkanes to alkenes is well studied audits products have, as a rule, the structure been predicted with respect to electron effects, the problem of orientation... 

In addition to its effect on stability, delocalization of the unpaired electron in the allyl radical has other chemical consequences. Because the unpaired electron is delocalized over both ends of the nr orbital system, reaction with Br2 can occur at either end. As a result, allylic bromination of an unsymmetrical alkene often leads to a mixture of products. For example, bromination of 1-octene gives a mixture of 3-bromo-l-octene and l-bromo-2-octene. The two products are not formed in equal amounts, however, because the intermediate allylic radical is... 

One further comparison aromatic aldehydes, such as benzaldehyde, are less reactive in nucleophilic addition reactions than aliphatic aldehydes because the electron-donating resonance effect of the aromatic ring makes the carbonyl group less electrophilic. Comparing electrostatic potential maps of formaldehyde and benzaldehyde, for example, shows that the carbonyl carbon atom is less positive (less blue) in the aromatic aldehyde. 

Since equatorial attack is roughly antiperiplanar to two C-C bonds of the cyclic ketone, an extended hypothesis of antiperiplanar attack was proposed39. Since the incipient bond is intrinsically electron deficient, the attack of a nucleophile occurs anti to the best electron-donor bond, with the electron-donor order C—S > C —H > C —C > C—N > C—O. The transition state-stabilizing donor- acceptor interactions are assumed to be more important for the stereochemical outcome of nucleophilic addition reactions than the torsional and steric effects suggested by Felkin. 

Reaction 31 appears to be little affected by substituent electronic effects or by steric effects of either sulfonyl chloride or styrenes. Treatment of /5-chlorosulfones with triethylamine in benzene affords the corresponding a, /5-unsaturated sulfones in excellent yield. The copper-catalyzed addition of sulfonyl iodides to simple and cyclic alkenes has also been exploited76. 

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