De-t-butylation

De-t-butylation of phenols t-Butyl groups are often used for protection of various positions of arenes, and are commonly removed by a reaction involving a f-butyl acceptor plus a Lewis acid. Surprisingly, A1C13 alone can effect de-f-buty-lation of phenols at room temperature in nitromethane or methylene chloride. Removal of an ort/io-f-butyl group occurs more readily than that of a para-group. 

De-f-butylation during electrophilic substitution is a fairly common phenomenon in thiophene chemistry. The A1C13 catalyzed acylation of 2-methyl-5-(-butylthiophene with acetyl chloride gives 3,5-diacetyl-2-methylthiophene as the main product (69BSF991). Treatment of the acid (324) with PPA in toluene results in ring closure and de-t-butylation to form (325) (73BSF343). 

Compound (79) was easily de-t-butylated by Lewis acid-catalyzed thermolysis to yield (80) and an isomeric betaine (81) (Equation (5)) <93JOC6620>. 

For example /-butyl phenyl ether with aluminium chloride forms para-t-butyl phenol155. Often the de-alkylated phenol is also formed in considerable quantity. The reaction formally resembles the Fries and Claisen rearrangements. Like the Fries rearrangement the question of inter- or intramolecularity has not been settled, although may experiments based on cross-over studies156, the use of optically active ethers157 and comparison with product distribution from Friedel-Crafts alkylation of phenols158 have been carried out with this purpose in view. 

The pH of the reaction stream was adjusted to neutral and the dark stream was washed with methyl-t-butyl ether to remove neutral organic impurities. The major impurity observed in the MTBE wash is phenyl oxazole formed by de-iodination of the starting iodoxazole. Phase separation of this wash is a difficult one due to the dark color of the reaction stream. 

When a copper complex of 83 is used as the catalyst, the reaction can also proceed with excellent selectivity (98% de and >98% ee for the enJo-isomer).30 Note that 83 is similar to 81, but in this compound the two phenyl groups are replaced by two t-butyl groups. 

De Jong et al. (1976c, 1977) have studied the rate of decomplexation of t-BuNHjPFj (AM) complexes of crown ethers. The exchange broadening of the t-butyl nmr signal was used to determine the rate of cation exchange between a kinetically stable (X.AM) and a kinetically unstable (Y.AM)... 

Abb. 1. Ausschnitt aus dem NMR-Spektrum des Pivaloxoaium-Salzes 55b 36b bei verschiedenen Temperaturen nur t-Butyl-Signale (60 MHz. CD3NO2)... 

Die Starke Nachbargruppenwirkung des Dreiringes im Cyclopropylme-thylkation kommt in starken Konjugationseffekten in den UV- und NMR-Spektren zum Ausdruck 44, 45). So zeigt das Dimethyl-cyclo-propylmethyl-Kation (22) ein Maximum bei 289 mjr mit s = 10,800 40) (t-Butyl-Kation Xmax = 300 mp, e < 500 46)). 

Deprotection is effected in the case of methyl or ethyl esters by hydrolysis, usually under basic conditions (Section 9.6.17, p. 1266). t-Butyl esters are de-protected by mild acidic conditions, for example, formic acid at room temperature,159 toluene-p-sulphonic acid in benzene solution, or hydrogen bromide in acetic acid at 10°C.160... 

N1 - LUTEINIZING HORMONE-RELEASING HORMONE, (D-SERINE-t-BUTYL(sup 6),DES-GLYCINE-NH2(sup 10))-, ETHYLAMIDE... 

More practical from the point of view of starting material availability is the use of the N-t-butyl-A-methyl amides 178, whose f-butyl group may be removed with acid.163 De-J-butylation of amides can be capricious however, though it seems much more successful with acyclic than cyclic amides. 

Thus the partial molar enthalpy of Bu4N+Br increases sharply when t-butyl alcohol is added to aqueous solutions until x2 - 0 1 and then decreases slowly (Mohanty et al., 1971). Similar complex patterns emerge in the enthalpies of these salts when amide cosolvents are added, e.g. formamide (de Visser and Somsen, 1974a,b). Striking changes are observed in the partial molar heat capacities of salts in TA mixtures when x2 is varied (Avedikian et al., 1975). Thus for Am4N+Br in t-butyl alcohol + water mixtures (Mohanty et al., 1972), the partial molar heat capacity increases as x2 increases to a maximum near x2 — 0-04, drops sharply to a minimum and then... 

SYNS tetc.BUTYLESTER KYSELINY PEROXY-BENZOOVE (CZECH) terc.BUTYLPER-BENZOAN (CZECH) t-BUTYL PERBENZOATE t-BUTYL PEROXY BENZOATE OESPEROX 10 NOVOX PERBENZOATE de BUTYLE TERTIAIRE (FRENCH) TRIGONOX C... 

The reaction of this silyl ketene acetal with Di-t-butyl Azodicar-boxylate in the presence of TiC affords the adduct in 45-70% yield with ca. 90% de. The subsequent treatment of the adduct with Trifluoroacetic Acid and Lithium Hydroxide affords (R)-a-hydrazo acids (eq 5). ... 

Conjugate Additions. The reaction of this acrylate derivative with lithium t-butyl hydroperoxide (generated from anhydrous t-Butyl Hydroperoxide and n-Butyllithium) in THF leads to the corresponding epoxide (eq 5) in 95% yield with a de of 40%. ... 

The data in Table 25 and Figs. 5, 6 and 7 suggest that Ingold s (1957) calculations (de la Mare et al., 1955) of rate data for Finkelstein reactions of chloride ion with alkyl bromides, except t-butyl bromide (cf. Winstein et al., 1959), do roughly reflect changes in energy differences between each alkyl bromide and its transition state. These changes appear to be more or less independent of solvent and of the source of chloride ion. 

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