Rerr-butyl group

Together, these two products contain all eight carbons of the starting alkene. The two carbonyl carbons conespond to those that were doubly bonded in the original alkene. One of the doubly bonded carbons therefore bears two methyl substituents the other bears a hydrogen and a rerr-butyl group. The alkene is identified as 2,4,4-trimethyl-2-pentene, (CH3)2C=CHC(CH3)3, as shown in Figure 6.15. 

When two positions aie compaiably activated by alkyl groups, substitution usually occurs at the less hindered site. Nitration of p-terr-butyltoluene takes place at positions ortho to the methyl group in preference to those ortho to the larger rerr-butyl group. This is an example of a steric effect. 

The rerr-butyl group is cleaved as the conesponding carbocation. Loss of a proton from terr-butyl cation converts it to 2-methylpropene. Because of the ease with which a tert-butyl group is cleaved as a car bocation, other acidic reagents, such as trilluoroacetic acid, may also be used. 

The Diels-Alder reactions of the methyl or ethyl ester of benzenesulfonylindole-2-acrylic acid with several l-alkoxycarbonyl-l,2-dihydropyridines are reported and only a single stereoisomer was obtained, as in the case of l-methoxy(ethoxy)-carbonyl-1,2-dihydropyridines. However, when the Diels-Alder reaction of 17 was carried out with 8g[R = (CHsjsC], a mixture of two stereoisomers 18gand25were obtained in a 1 1 ratio (65% total yield). The bulky rerr-butyl group creates sufficient steric interference with the indole ring to cause the loss of stereochemistry ... 

Less than 0.01 % (Serious 1,3-diaxial repulsions involving rerr-butyl group)... 

The steric bulk of the rerr-butyl group significantly diminishes the rate of silyla-tion with /m-butyldimethylsilyl chloride (TBSQ, mp 86-89 C bp 125 C) so convenient rates are best achieved by the addition of basic activators such as imidazole or DMAP and by using dipolar aprotic solvents such as DMF. Primary alcohols react much faster than secondary alcohols [Scheme 4.60] but tertiary alcohols are inert. 

Substitution of hydrogen by methyl results in a slight rate increase, probably as a result of an electronic effect, while a 1-ferf-butyl substituent produces a significant rate decrease. Apparently, any steric retardation to approach of the dienophile by a methyl substituent is insignificant compared to its electronic effect. With the larger rerr-butyl group, the steric effect is dominant. 

More stable because the large rerr-butyl group is equatorial (less potential energy)... 

NMR spectroscopy is ideal for detecting charged fluorinated intermediates and has been applied to the study of increasingly stable carbocation and carbanion species. Olah [164, 165] has generated stable fluorocarbocations m SbFj/SOjClF at low temperatures The relatively long-lived perfluoro-rerr-butyl anion has been prepared as both the cesium and tris(dimethylamino)sulfonium (TAS) salts by several groups [166, 167, 168], Chemical shifts of fluonnated carbocations and carbanions are listed m Table 23. 

All alkyl groups, not just methyl, are activating substituents and ortho, paia directors. This is because any alkyl group, be it methyl, ethyl, isopropyl, rerr-butyl, or any other, stabilizes a caibocation site to which it is directly attached. When R = alkyl. 

Mohanty et al. were the first to introduce pendent r-butyl groups in die polymer backbones. The resulting material was quite soluble in aprotic dipolar solvents.83 The PEEK precursors were prepared under a mild reaction condition at 170°C. The polymer precursor can be converted to PEEK in die presence of Lewis acid catalyst A1C13 via a retro Friedel-Crafts alkylation. Approximately 50% of die rerr-butyl substitutes were removed due to die insolubility of the product in die solvent used. Later, Risse et al. showed diat complete cleavage of f< rf-butyl substitutes could be achieved using a strong Lewis acid CF3SO3H as both die catalyst and the reaction medium (Scheme 6.15).84... 

Experimental results [1361] and theoretical treatment [28] indicate that the cyclo-propanation of alkenes by electrophilic carbene complexes is a concerted process. Z-Olefins normally lead to the formation of the corresponding c7. -cyclopropanes, and -olefins yield fran -cyclopropanes. The relative configuration of the carbene-bound substituent and the substituents of the alkene in the final cyclopropane seems to be mainly determined by the steric bulk of these groups. In cyclopropanations of terminal alkenes with ethyl diazoacetate low diastereoselectivities are often observed [1024,1351]. These can be improved by increasing the steric demand of the substituents at the carbene or at the alkene [1033,1362]. High diastereoselectivities can, e.g., often be achieved with terf-butyl, neopentyl or 2,6-di(rerr-butyl)phenyl diazoacetate [1362] as carbene complex precursors (Figure 4.19). 

In contrast, Tsuji s group coupled 2-methylfuran with ethyl acrylate to afford adduct 6 via a Pd-catalyzed reaction using rerr-butyl peroxybenzoate to reoxidize Pd(0) to Pd(II) [16]. The palladation of 2-methylfuran took place at the electron-rich C(5) in a fashion akin to electrophilic aromatic substitution. The perbenzoate acted as a hydrogen acceptor. 

The replacement of a tert-butyl group by SiMe3 going from di-ferf-butyl ketone to rerr-butanoyl trimethylsilane and from di-terf-butyl thioketone to ferf-thiobutanoyl trimethylsilane lg increased Xmax and deshielded the quaternary carbon. Furthermore, an inversion in polarization of the C=S bond and a decrease of the HOMO-LUMO energy differences were observed in the ferf-thiobutanoyl trimethylsilane with respect to di-rm-butyl thioketone. 

Preparation of Enantiopure a-Substituted p-Amino Acids. In preliminary studies, racemic 2-rerr-hutyl-perhydropyrimidinone, rac-4, was alkylated with high di-astereoselectivity via its corresponding enolate (eq 5). The high stereoselectivity encountered in the reaction of rac-4-Li with various electrophiles was ascribed to steric hindrance generated by the axial disposition of the tert-butyl group at C(2), which directs approach to the electrophile from the enolate face opposite to this group. 

To test these alternative hypotheses, a tremendous amount of work has been done, by Brown and by others. For example, camphene hydrochloride is known to undergo ethanolysis 6000 times as fast as rer/-butyl chloride, and this had been attributed to anchimeric assistance with formation of a bridged ion. Brown pointed out that the wrong standard for comparison had been chosen. He showed that a number of substituted (3°) cyclopentyl chlorides (examine the structure of camphene hydrochloride closely) also react much faster than rerr-butyl chloride. He attributed these fast reactions—including that of camphene hydrochloride—to relief of steric strain. On ionization, chloride ion is lost and the methyl group on the ap hybridized carbon moves into the plane of the ring four non-bonded interactions thus disappear, two for chlorine and two for methyl. For certain systems at least, it became clear that one need not invoke a nonclassical ion to account for the facts. 

O-rm-Butyl trichloroacetimidate. prepared in 70% yield by reacting potassium rm-butoxide with trichloroacetonitrile. reacts with carboxylic acids and alcohols in the presence of a catalytic amount of boron trifluoride etherate at room temperature in cyclohexane-dichloromethane [Scheme 6.3S]. 7 The method also converts alcohols to ferr-butyl ethers (see section 4.3.2). A very similar reaction that allows rerr-butylation under essentially neutral conditions on a large scale involves reaction of a carboxylic acid with 3-4 equivalents of N,V-di-isopropyl-0-/er/-butylisourea [Scheme 6.36]. The reaction proceeds via a tertiary carbocation ion intermediate and since capture of the cation is inefficient, excess isourea is required. The presence of alcohols is tolerated but not thiols or unhindered amines. The reaction conditions are compatible with a range of acid sensitive groups such as N-trityl derivatives and cyclopentylidene acetals. ... 

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