Ortho-tert-butyl substituents

Scheme 8 Ligand C-H activation at the ortho-tert-butyl substituents to afford palladacycles 1.27c and 1.28c [53]...

Scheme 11.34 Effect of the bulky ortho-tert-butyl substituent on the reaction of singlet nitrene cyclization...

A similar reaction sequence of triisopropylphenylphosphole or mesitylphosphole (17b and 17a, respectively) with phosphorus tribromide afforded the corresponding 2-substituted products. The reaction of dibromophosphine 37 with nucleophiles followed by oxidation or hydrolysis gave phosphonic or //-phosphinic derivatives (39 or 41, respectively) (Scheme 9) [48, 49], The regioselectivity is obviously the consequence of the presence or the lack of the steric hindrance with ortho tert-butyl groups, only position 3 is available, while with the smaller triisopropyl substituent, position 2 may be the appropriate reaction site. 

Figure 5.33 presents Friedel-Crafts acylations, taking benzoylations of toluene (top line) and para-tert-butyl toluene (Figure 5.33, bottom) as an example. The methyl group of toluene preferentially directs the benzoyl residue into the para-position. The ortho-benzoylated toluene occurs only as a by-product. In para-tert-butyl toluene both the methyl- and the tert-butyl substituent direct the electrophile towards the ortho-position, since both para-positions are occupied and could at best react with de-ferf-butylation, i.e., in a—sterically hindered — ipso-substitution (cf. Figure 5.5). Indeed, we see reaction ortho to the methyl group and not ortho to the ferf-butyl group. This selectivity can be ascribed to minimized steric interactions in the preferred sigma complex intermediate.

All alkyl groups not just methyl are activating substituents and ortho para direc tors This IS because any alkyl group be it methyl ethyl isopropyl tert butyl or any other stabilizes a carbocation site to which it is directly attached When R = alkyl... 

The reaction of arylphospholes with phosphorus tribromide was extended to di-te/f-butyl-methylphenylphosphole (17c) leading after further steps, to a mixture of 3- and 2-substituted products (43 and 45, respectively) (Scheme 10) [40], The di-tert-butyl-tolyl substituent clearly occupies an intermediate position between the triisopropylphenyl and tri-terf-butylphenyl ones regarding the steric hindrance caused by the ortho alkyl substituents. 

The main considerations in protecting the hydroxyl of tyrosine are the stability to acid of the protector and the protector s tendency to alkylate the ortho-position of the ring when it is removed (Figure 6.7 see Section 3.7). The standard protector for Fmoc chemistry is tert-butyl, but the substituent is sensitive to acid. Preferable... 

The temperature and intensity of the p peak scarcely change on going from H, F, Cl, Br to I in Illers data. However, the data of Boyer and Turley [10] for three isomers of poly(chlorostyrene) show that the p relaxation intensity decreases on going from para to ortho substituents. Gao and Harmon [6] analysed the methyl and tert-butyl para sustituent effect on the P relaxation in PS. Small modifications of the peak position were observed and the P-activation energy was the same as obtained for PS... 

Some azobenzenes that are locked against rotation by bulky substituents in all four ortho positions may show fluorescence when frozen rigidly at 77 K 2,2 ,4,4 ,6,6 -hexaisopropyl 2,2 -difflethyl-4,4 6,6 -tetra-tert-butyl azobenzene belong to this series. Azobenzenophanes 7 to 13 do not emit, even at 77 K this is the expectation for card-packed dimers. 

There are, however, azobenzenes that have wavelength-independent isomerization quantum yields and thus obey Kasha s rule. The structure of these molecules inhibits rotation. Rau and Liiddecke investigated azobenzeno-phane 9 and Rau the azobenzene capped crown ether 14, and these researchers found identical E,E —> E,Z, and E —> Z quantum yields respectively, regardless of which state was populated. The photoisomerization of azobenzenophanes and 13 could not be evaluated in the same way because the photoisomerization is intensity-dependent. A series of azobenzenes substituted in all ortho positions to the azo group has equal quantum yields for n —> n and k —> k excitation if the substituents are ethyl, isopropyl, tert.butyl, or phenyl. This provides clues for the elucidation of the isomerization mechanism (Section 1.6). 

Substituted and Heat Reactive. The third class, substituted and heat-reactive resins, are made by using para-substituted phenols where the substituent is a four-carbon or higher group such as tert-butyl, tert-octyl, and phenyl. Small amounts of ortho-substituted phenols and unsubstituted phenols are sometimes coreacted but, in general, the functionality is 2, and only linear molecules are formed. They are brittle solids that do not form films. The substituent makes the resins less polar and hence they are soluble in ketones, esters, and aromatic hydrocarbons, with limited solubility in alcohols and aliphatic hydrocarbons. The phenolic resins based on longer chain aliphatic phenols are more compatible with drying oils, alkyds, and rubbers. 

Selectivity is more complicated with a methyl or chloro substituent. Again, meta substitution is always significant, but ortho substitution can account for 50-70% of the mixture in some cases [2]. More reactive anions (1,3-dithianyl) and less substituted carbanions (e.g., tert-butyl lithioacetate) tend to favor ortho substitution. Representative examples are shown in Table 3. Entries 2-4 show that variation of reaction temperatures from -100 °C to 0 °C has no significant effect in that highly selective system. The added activating effect of the Cl substituent allows addition of the pinacolone enolate anion (entry 11), whereas no addition to the anisole nor toluene ligand is observed with the same anion. 

A similar transformation to that shown in Scheme 3, but enantioselective, was carried out with a,p-unsaturated tert-butyl ester 29 and aryUithium reagent 8, providing the 1,4-addition product 30 in up to 88% ee (Scheme 9) [27]. The ortho (relative to the imsaturated ester portion) substituent is important for improving the enantioselectivity. A survey of various Ugands showed that 18 was more effective than 19, or chiral amino ether ligands. 

So, the only question left is how the substituents are arranged. Two tert-butyl groups could be arranged ortho, meta or para to each other, but only the para arrangement is possible for A because only when the two groups are para are all the protons of the aromatic ring identical (check for yourself). 

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