Tert group

A complication in the preparation of compounds having sec- and tert-alkyl groups in the 2- and in the 4-positions is their vulnerability to thermal dealkylation with acidic catalysts, particularly with 1% oleum in the range 160-200°C, and, as has been mentioned, to aluminium phenoxide when this is used at high temperature (ref.12). Indeed above 250°C all sec- and tert- groups are eliminated from alkylated phenols. The role of thermal dealkylation has been mentioned for the preparation of nonylphenol from dinonylphenol. In the case of 5-methyl-2,4-di-tert-butylphenol and 4-methyl-2,6-di-tert-butylphenol resulting from the tert-butylation of 3-methyl and 4-methylphenol mixtures, the former... 

Kinetic data are available for the nitration of a series of p-alkylphenyl trimethylammonium ions over a range of acidities in sulphuric acid. - The following table shows how p-methyl and p-tert-h xty augment the reactivity of the position ortho to them. Comparison with table 9.1 shows how very much more powerfully both the methyl and the tert-butyl group assist substitution into these strongly deactivated cations than they do at the o-positions in toluene and ferf-butylbenzene. Analysis of these results, and comparison with those for chlorination and bromination, shows that even in these highly deactivated cations, as in the nitration of alkylbenzenes ( 9.1.1), the alkyl groups still release electrons in the inductive order. In view of the comparisons just... 

The alternative method of elimination of chlorine and a sulfinate group is particularly useful for the preparation of tert.-butylbutatriene, since the corresponding dichloride cannot be obtained in good yields. The required sulfinate... 

If a bromomethyl- or vinyl-substituted cyclopropane carbon atom bears a hydroxy group, the homoallyiic rearrangement leads preferentially to cyclobutanone derivatives (J. Sa-laun, 1974). Addition of amines to cydopropanone (N. J. Turro, 1966) yields S-lactams after successive treatment with tert-butyl hypochlorite and silver(I) salts (H.H. Wasserman, 1975). For intramolecular cyclopropane formation see section 1.16. 

The first practical method for asymmetric epoxidation of primary and secondary allylic alcohols was developed by K.B. Sharpless in 1980 (T. Katsuki, 1980 K.B. Sharpless, 1983 A, B, 1986 see also D. Hoppe, 1982). Tartaric esters, e.g., DET and DIPT" ( = diethyl and diisopropyl ( + )- or (— )-tartrates), are applied as chiral auxiliaries, titanium tetrakis(2-pro-panolate) as a catalyst and tert-butyl hydroperoxide (= TBHP, Bu OOH) as the oxidant. If the reaction mixture is kept absolutely dry, catalytic amounts of the dialkyl tartrate-titanium(IV) complex are suflicient, which largely facilitates work-up procedures (Y. Gao, 1987). Depending on the tartrate enantiomer used, either one of the 2,3-epoxy alcohols may be obtained with high enantioselectivity. The titanium probably binds to the diol grouping of one tartrate molecule and to the hydroxy groups of the bulky hydroperoxide and of the allylic alcohol... 

We shall describe a specific synthetic example for each protective group given above. Regiosdective proteaion is generally only possible if there are hydroxyl groups of different sterical hindrance (prim < sec < tert equatorial < axial). Acetylation has usually been effected with acetic anhydride. The acetylation of less reactive hydroxyl groups is catalyzed by DMAP (see p.l44f.). Acetates are stable toward oxidation with chromium trioxide in pyridine and have been used, for example, for protection of steroids (H.J.E. Loewenthal, 1959), carbohydrates (M.L. Wolfrom, 1963 J.M. Williams, 1967), and nucleosides (A.M. Micbelson, 1963). The most common deacetylation procedures are ammonolysis with NH in CH OH and methanolysis with KjCO, or sodium methoxide. 

The protecting group Y of the amine is generally an alkoxycarbonyl derivative since their nucleophilicity is low. Benzyloxy- or tert-butoxycarbonyl derivatives usually do not undergo azlactone formation. 

As expected, the formation of a carbonyl group is not possible with tert-allylic alcohols. Although the aromatic ring bears electron-donating groups, the 2,2-disubstituted chromene 119 was formed smoothly with the tert-allylic alcohol 118[100]. 

Those derived from isobutane are the 2 methylpropyl (isobutyl) group and the 1 1 dimethylethyl (tert butyl) group Isobutyl is a primary alkyl group because its poten tial point of attachment is to a primary carbon tert Butyl is a tertiary alkyl group because Its potential point of attachment is to a tertiary carbon... 

Dimethylethyl group (common name tert butyl)... 

Highly branched groups such as tert butyl are commonly described as bulky... 

A tert butyl group is so large that tert butylcyclohexane exists almost entirely in the conformation m which the tert butyl group is equatorial The amount of axial tert butylcyclohexane present is too small to measure... 

Less than 0 01 % (Serious 1 3 diaxial repulsions involving tert butyl group)... 

If a disubstituted cyclohexane has two different substituents then the most stable conformation is the chair that has the larger substituent m an equatorial orientation This IS most apparent when one of the substituents is a bulky group such as tert butyl Thus the most stable conformation of cis 1 tert butyl 2 methylcyclohexane has an equatorial tert butyl group and an axial methyl group... 

Add methyl group to axial position at C 3 so that It IS trans to tert butyl group... 

The lUPAC rules permit cer tain common alkyl group names to be used These include n propyl isopropyl n butyl sec butyl isobutyl tert butyl and neopentyl (Section 2 13)... 

The properties of tert butyl cation can be understood by focusing on its structure which IS shown m Figure 4 9 With only six valence electrons which are distributed among three coplanar ct bonds the positively charged carbon is sp hybridized The unhybridized 2p orbital that remains on the positively charged carbon contains no elec Irons Its axis is perpendicular to the plane of the bonds connecting that carbon to the three methyl groups... 

FIGURE 4 9 tert Butyl cation (a) The positively charged carbon is sp hybridized Each methyl group IS attached to the positively charged carbon by a cr bond and these three bonds he in the same plane (b) The sp hybridized car bon has an empty 2p orbital the axis of which is perpen dicular to the plane of the carbon atoms... 

We noted m Section 2 13 that the common names of certain frequently encoun tered alkyl groups such as isopropyl and tert butyl are acceptable m the lUPAC sys tern Three alkenyl groups—vinyl, allyl, and isopropenyl—are treated the same way... 

The difference m stability between stereoisomeric alkenes is even more pronounced with larger alkyl groups on the double bond A particularly striking example compares as and trans 22 5 5 tetramethyl 3 hexene m which the heat of combustion of the cis stereoisomer is 44 kJ/mol (10 5 kcal/mol) higher than that of the trans The cis isomer IS destabilized by the large van der Waals strain between the bulky tert butyl groups on the same side of the double bond... 

FIGURE 5 11 Confer mations of as and trans 4 tert butylcyclohexyl bromide and their relationship to the preference for an anti copla nar arrangement of proton and leaving group... 

Use curved arrows to show the bonding changes in the reaction of CIS 4 tert butylcyclohexyl bromide with potassium tert butoxide Be sure your drawing correctly represents the spatial relationship between the leaving group and the proton that is lost... 

Together these two products contain all eight carbons of the starting alkene The two carbonyl carbons correspond to those that were doubly bonded m the original alkene One of the doubly bonded carbons therefore bears two methyl substituents the other bears a hydrogen and a tert butyl group The alkene is identified as 2 4 4 trimethyl 2 pentene (CH3)2C=CHC(CH3)3 as shown m Figure 6 15... 

Both compounds react by an S l mechanism and their relative rates reflect their acti vation energies for carbocation formation Because the allylic chloride is more reactive we reason that it ionizes more rapidly because it forms a more stable carbocation Struc turally the two carbocations differ m that the allylic carbocation has a vinyl substituent on Its positively charged carbon m place of one of the methyl groups of tert butyl cation... 

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... 

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