Butyl substituent

The strong preference for a /-butyl group to occupy the equatorial position has made it a useful group for the study of conformationally biased systems. The presence of a /-butyl substituent will ensure that the equilibrium lies heavily to the side having the /-butyl group equatorial but does not stop the process of conformational inversion. It should be emphasized that conformationally biased is not synonymous with conformationally... 

There are three derivatives of butadiene having one t-butyl substituent and nine di-t-butyl derivatives. Predict the preferred conformation for each of these 12 compounds. 

Neopentyl (2,2-dimethylpropyl) systems are resistant to nucleo diilic substitution reactions. They are primary and do not form caibocation intermediates, but the /-butyl substituent efiTectively hinders back-side attack. The rate of reaction of neopent>i bromide with iodide ion is 470 times slower than that of n-butyl bromide. Usually, tiie ner rentyl system reacts with rearrangement to the /-pentyl system, aldiough use of good nucleophiles in polar aprotic solvents permits direct displacement to occur. Entry 2 shows that such a reaction with azide ion as the nucleophile proceeds with complete inversion of configuration. The primary beiuyl system in entry 3 exhibits high, but not complete, inversiotL This is attributed to racemization of the reactant by ionization and internal return. 

Entry 3 has only alkyl substituents and yet has a significant lifetime in the absence of oxygen. The tris(/-butyl)methyl radical has an even longer lifetime, with a half-life of about 20 min at 25°C. The steric hindrance provided by the /-butyl substituents greatly retards the rates of dimerization and disproportionation of these radicals. They remain highly reactive toward oxygen, however. The term persistent radicals is used to describe these species, because their extended lifetimes have more to do with kinetic factors than with inherent stability." Entry 5 is a sterically hindered perfluorinated radical and is even more long-lived than similar alkyl radicals. 

These strategies guide the retrosynthetic conversion of 272 to 278 and the further conversion of 278 via 279 to 282. The r-butyl substituent actuates the clearability of the stereocenters in 279. Further retrosynthetic simplification as dictated by basic FG-, stereochemical and topological strategies then leads from 280 to 281 and to 282, a previously described substance. The successful synthesis followed closely the above outlined retrosynthetic scheme. An enantioselective process was devised for the synthesis of 281 from 282 (see Section 10.12).67, 83... 

Several derivatives of l-(2-alkenoyl)-3-isopropyl-2-imidazolidinones having ethyl, isopropyl, and t-butyl / -substituents can be successfully applied to give satisfactory enantioselectivities (Scheme 7.46). 

The rearrangement with ring contraction probably is the most important synthetic application of the Favorskii reaction it is for example used in the synthesis of steroids. Yields can vary from good to moderate. As solvents diethyl ether or alcohols are often used. With acyclic a-halo ketones bearing voluminous substituents in a -position, yields can be low a tcrt-butyl substituent will prevent the rearrangement. 

Detecting chirality centers in a complex molecule takes practice because it s not always immediately apparent that four different groups are bonded to a given carbon. The differences don t necessarily appear right next to the chirality center. For example, 5-bromodecane is a chiral molecule because four different groups are bonded to C5, the chirality center (marked with an asterisk). A butyl substituent is similar to a pentyl substituent but it isn t identical. The difference isn t apparent until four carbon atoms away from the chirality center, but there s still a difference. 

During the course of the conversion of intermediate 18 into intermediate 16, the imposing ferf-butyl substituent at C-8 guides the formation of the adjacent stereocenter at C-9 and it is now called upon to guide, or at least influence in a favorable way, the stereo-... 

In the case of an aromatic 7t-nucleophile, a dramatic reversal of diastercoscleetivity is found on going from a phenyl to a cyclohexyl or fert-butyl substituent, which is attributed to a balanee of. syn-axial and A(1,3) interactions, while a boat-like transition state may also play a role149. 

In the original work1 the neopentyl substituent is incorrectly shown as a /-butyl substituent. 

N.m.r. spectroscopy has played an important part in determining the stereochemistry of the 1,3-dioxaphosphorinanes (52). The presence of the saturated six-membered ring means that there are usually conformational effects to be unravelled before configurational assignments can be made. The chair conformation is generally dominant. Phosphorus substituents which exhibit shielding effects show that in many P " phosphorinanes this substituent occupies an axial position and Sis( has been used to establish the equatorial conformation of a t-butyl substituent at C(5). Even in P" derivatives the isomer possessing the bulkiest P-substituent in an axial... 

Under comparable reaction conditions, no C—H bond activation is observed for adducts of 6-Phbipy and 6-Rbipy. Nevertheless, [Au(N,N,C)Cl] derivatives can be obtained with 6-Phbipy [18] and with 6-tBubipy (tBu = CMe3) [20]. The former is obtained by a transmetallation reaction of the arylmercury(II) derivative with [AuClJ, while activation of a C(sp )—H bond of the tert-butyl substituent is accomplished by reaction of the Au(N)Cl3 adduct 3 (N = 6-tBubipy) with AgBp4 in the presence of excess ligand (Scheme 2.2). 

The prediction and interpretation of alkylation stereochemistry requires consideration of conformational effects in the enolate. The decalone enolate 3 was found to have a strong preference for alkylation to give the cis ring junction, with alkylation occurring cis to the f-butyl substituent.58... 

The enantioselectivity is thought to result from both steric blocking by the f-butyl substituent on the oxazoline ring and an attractive van der Waals interaction of an aryl ring and the oxazoline ring, as shown in Figure 5.5. 

Substitution of hydrogen by methyl results in a slight rate increase as a result of the electron-releasing effect of the methyl group. A r-butyl substituent produces a large rate decrease because the steric effect is dominant. 

The presence of a /-butyl substituent on both C(2) and C(3), however, prevents attainment of the s-cis conformation, and D-A reactions of 2,3-di-(/-butyl)-1,3-butadiene have not been observed.21... 

---