Steric ortho effect

The ethylation of toluene by diethylhalonium ions gives ethyltoluenes with ortho para isomer ratios between 0.60 and 0.96. The ortho para isomer ratios obtained for the alkylation of toluene in conventional Friedel-Crafts ethylations range from 1.17 to 1.84 (average 1.60). Such differences are considered to be due to the steric ortho effect caused by diethylhalonium ions, and are in accordance with the most probable displacement reaction on the bulky diethylhalonium ions by the aromatic substrate. This can be envisioned to proceed through an SN2-type transition state involving no free alkyl cations [Eq. (5.109)]. 

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. 

Fujita and Nishioka have attempted to place ortho effects on the same numerical scale as meta and para effects. They assume that the normal ortho electronic effect can be represented by the standard substituent constant appropriate to the reaction (cr, cr", cr, cr°), that the steric effect is given by E , and that the proximity effect is measured by the Swain-Lupton Then a multiple LFER is written... 

Fig. 13 The steric assistance mechanism for the ortho-effect. Energy profile for the para-isomer is given in dotted lines whereas data for ort/zo-isomer are shown in solid lines. Calculations were performed at the BLYP/6-31G (in bold) and B3LYP/6-31G levels. P stands for products, R stands for reactants. Adapted from reference49.

Contreras and Pardey reported on the activity of ds-[Rh(CO)2(amine)2]PF6 complexes.141 They found that the turnover frequencies of the Rh/amine catalysts were strongly dependent on the nature of the amine ligand, as pointed out earlier by Laine and Wilson.77 They suggested that there is either a promoting effect related to the basicity of the ligand, or a steric hindrance effect of two ortho methyls. Results are displayed in Table 32. Conditions T = 100 °C Pco =1.9 atm acetonitrile/H20 = 8 2 v/v 10 x 10-3 mol Rh complex. 

A large steric interaction between the bridging methylene and nitro group of the o-complex has been suggested to account for decreased a-reactivity the fused-ortho effect , though analysis of the sterics in bicycloannelated derivatives show this is unlikely (Section 2.1). For discussion of the fused-ort/io effect , see ref. 34a,b. 

In aromatic compounds carbon-13 shifts are largely determined by mesomeric (resonance) and inductive effects. Field effects arising from through-space polarization of the n system by the electric field of a substituent, and the influences of steric (y) effects on the ortho carbon nuclei should also be considered. Substituted carbon (C-l) shifts are further influenced by the anisotropy effect of triple bonds (alkynyl and cyano groups) and by heavy atom shielding. 

The ortho effect is not entirely steric in origin. See M. Char ton, Prog. Pkys. Org. Chem.y 8, 235 (1971). 

Partial Rate Factors of a Methyl Group. Comparison of the f0Me and /mMe values of toluene with those of 4-hydroxy-3-methoxytoluene does not reveal significant differences. The second /0Me value that was determined for position 2 in the latter compound is lower than that for position 6 owing to steric inhibition effects caused by two ortho substituents. Consequently, the additivity principle appears valid for the 1,3,4-trisubstitution pattern. 

A review of experimental work prompted the suggestion of the importance of dipolar interactions (Hammond and Hawthorne, 1956). de la Mare and Kidd (1959), observing a parallelism in the parajmeta and ortho/meta ratios, predicted the ortho effect to be primarily electronic in origin. Norman and Radda (1961) explored the general significance of this idea. They studied the orthojpara ratios for the substitution of a series of monosubstituted benzenes by two reagents with the same electrophilic properties but different steric requirements. The reactions, nitration by N02+ and chlorination by CI+, fulfill the requirements. The results are summarized in Table 3. 

Brown, H.C., Barbaras, G.K. (1947) Dissociation of the compounds of trimethylboron with pyridine and the picolines evidence for the steric nature of the ortho effect. J. Am. Chem. Soc. 69, 1137-1144. 

A recent advance on the method outlined above (Eq. (3), schemes 1-5), is based on the discovery of the ortho effect - when the palladacyde contains an ortho substituent, as in 8, arylation occurs entirely at the aryl and not at the norbornyl site [3], This is probably because of the steric effect of the ortho substituent which renders attack at the aryl site much more favorable by preferentially weakening the palladium-carbon bond of the precursor of 15 (Scheme 6). 

Kinetic effects are observed in replacement of chloride ion in trans-(Pt(PEt3)2(o-tolyl)Cl) by various pyridines (Scheme 32). The reactivity of 2-methylpyridine is much lower ( 100 times) than that of other 3-, 4-, or 5-methyl- or dimethylpyridines. This steric ortho-retarding effect is much greater than that found for other substrates of Pt(II), Pd(II), and Au(III) (74JCS(D)927). 

While generally agreeing with the contention that the ortho effect is steric in ori-gin, Platz and coworkers noted that the barrier for 21d 23d is 1 kcal mol" larger... 

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