De-r-butylation

Although there are a number of reports in the literature of this process, (see ref. 1) only one of these relates to a kinetic study of the reaction168. The ease with which the reaction takes place depends upon the stability of the leaving carbonium ion. Consequently, de-r-butylation is mcst frequently observed and in a kinetic study of the sulphonation of /-butylbenzene in aqueous sulphuric acid (see p. 72) this side reaction was sufficiently prominent for rates to be easily measurable (Table 225)168. Comparison of the rates with those in Table 42 shows that de-... 

Trans acylations have been encountered in the acylation of 2-acylthiophenes. Thus the action of benzoyl chloride on 2-acetylthiophene in presence of excess A1C13 has led to a complex mixture, from which 2-benzoyl-, 2,4-dibenzoyl- and 2,5-dibenzoyl-thiophene were isolated (73ZOR1959). Other examples of ipso acylation including acyl-debromination and acyl-de-r-butylation have been recorded. Unexpected products have been reported in some intramolecular cyclizations. Thus cyclization of the acid (103) leads to (104) and not (105) (63AHCHH). Acid-catalyzed cyclization of ylidene-malononitriles has been reported (75JOC1840). Yields are in the range 30-60% (Scheme 19). 

De-f-butylation during electrophilic substitution is a fairly common phenomenon in thiophene chemistry. The A1C13 catalyzed acylation of 2-methyl-5-f-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-r-butylation to form (325) (73BSF343). 

Trans-r-butylation and de-r-butylation reactions are facile processes, and form the basis of many applications of the r-butyl group as a positional protective group. This aspect was well reviewed by Tashiro, and is also discussed in Section I.8.3.2. Tashiro and coworkers have more recently applied the trans-r-butylation and de-r-butylation reactions to prepare metacyclophanes (39) and related compounds. 

Olah et al showed Nafion-H to be a very efficient de-r-butylation agent for aromatic compounds, and Rosevear and Wilshire have applied the trans-r-butylation reaction to the synthesis of a variety of ultraviolet absorbers of the general structure (40), as shown in Scheme 14. 

Methyl-5-phenyl-3,6-dihydro-27/-l,3,4-thiadiazin-2-imine (164) is available in excellent yield by de-r-butylation of the 2-(t-butylamino)-derivative (163) with 48% HBr <90EGP280760>. Deallyl-ation is achieved in a similar manner. Curiously, in both instances, ring contraction to a pyrazole (Section 6.17.5.2) does not appear to occur. 

Chiral aminals 1,4-dihydropyridine-3-carboxaldehydes.1 The chiral aminal 2 prepared from pyridine-3-carboxylaldehyde and (S,S)-12 reacts with organocopper reagents in the presence of methyl chloroformate to give almost exclusively products of 1,4-addition, as expected from reactions of the free aldehyde.3 No products of 1,2-addition are formed, but 1,6-adducts are minor products in some cases. The 1,4-adducts are formed in 82-93% de (R-configuration). Addition of butyl and ethyl groups is best effected with lithium cuprates, but addition of methyl, vinyl, or aryl groups is best effected with organomagnesium cuprates. Under these conditions,... 

A sample of r-butyl lithium was prepared with isotopically pure Li and H-de coup led 13C spectra were acquired over a range of temperatures. The traces show how the appearance of the quaternary carbon resonance depends on temperature. Use these spectra to propose a structure for the compound and to determine the nature of the fluxional process which is occurring. 

The condensation of the dilithio derivative of (R)-(+)-3-(p-tolylsulfinyl)propionic acid with protected glycoaldehy-des (O-r-butyl and 0-benzyl) gives 5-alkoxy-4-hydroxy-3-(p-tolylsulfinyl)pentanoic acids, which spontaneously cyclize to the corresponding 3-sulfinyl-4-alkoxymethyl butanolides (eq 4). Pure diastereomers can be separated by flash chromatography and are obtained in comparable amounts. The corresponding optically pure butenolides are obtained by pyrolytic elimination of the sulfoxides and then transformed into natural (-i-)-(/ )-umbelactone (eq5). 

De-O-methylatUin. Aryl methyl ethers are cleaved with L-Selectride in refluxing THF. By this method removal of the protecting group from methyl carbamates is possible in the presence of r-butyl carbamates. ... 

The behaviour of the most sterically hindered 2,6-di-rerr-butylphenol (25) and its derivatives was very dependent upon the reaction conditions. In the presence of a base favouring electron-transfer (such as BTMG), oxidation reactions took place with triphenylbismuth dichloride or triphenylbismuth carbonate leading to the diphenoquinone (26). Para-phenylation with formation of (27) was observed for the first time in the reaction of the potassium phenolate of 2,6-di-re rr-butylphenol with triphenylbismuth dichloride and in the reaction of the phenol (25) with tetraphenylbismuthonium tosylate in the presence of BTMG. Even ortho- and para-phenylation with concomitant de-rerr-butylation occurred in the reaction of the potassium salt of 2,4,6-tri-re r/-butylphenol with triphenylbismuth dichloride. 2... 

An X-ray structure of the corresponding de-terr-butyl compound 90c (R > = showed this compound to also exist in a cone conformation, indicating that the p-substituent has little effect on the solid state conformation of calix[4]arenes. 

Silver oxide Nitrobenzenes from 2,5-cyclohexadienone oximes with preferential C-de-r rr-butylation Ag O <-... 

Zircanocene complexes with silyl- or r-butyl-substituted 1,3-butadienes undergo two syn-S 2 reactions with various aldehydes to yield cw-[3]cumulenic diols with a high de value. ... 

Zur Darstellmig des 5-Isopropyl [69]- und des 5-sec. Butylazulens [70] cyclisierten F. Sorm und J. Hlavni Cka (132) y-(2-Carboxymethyl-cyclopentyl)-buttersaure zum Bicyclo-(0,3,5 )-decanon-( 5), setzten letz-teres mit Isopropyl-lithium, bzw. sek. Butyl-lithium zum entsprechenden 5-Alkyl-bicyclo-(0,3,5)-decanol-(5) um, das sie zu den genannten Azulenen [69], [70] (R Isopropyl, sek.-Butyl) dehydratisierten und dchy-drierten... 

H 1. R—MgBr/CuJ/THF/Elher Zugabe des Eduktes bei --10° dann 20 25s A- 6h 2. HCl/HjO R 5-Amino-4-butyl-l-benzo- thiophen 5-Amino-4-benzyl-l-benzo- thiophen 60 63 2... 

Gomes de Azevedo, R. et al.. Thermophysical and thermodynamic properties of l-butyl-3-methylimidazolium tetrafluoroborate and l-butyl-3-methylimid-azolium hexafluorophosphate over an extended pressure range, ]. Chem. Eng. Data, 50, 997, 2005. 

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