R-Butyl substituent

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... 

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 ratio fa /p(Me)//0 /P(r-Bu) is 15,700 for the alkylbenzenes but only 69 for the 2-alkylthiophenes. Likewise the fa /m(Me)//0 /m(t-Bu) ratios are 44,700 (alkylbenzenes), 76.4 (2-alkylthiophenes), and 17.6 (3-alkylthi-ophenes). These results are free from any bond-order effects (which would equally affect the activation by both methyl and r-butyl substituents). They indicate that the 2-t-butyl group produces more hindrance than the 3-/-butyl group, possibly through buttressing between it and the sulfur d orbitals. This view is reinforced by results for electrophilic substitution of benzo[b]thiophene (Chapter 8, Section 2.A.b.i). 

Fig. 3. Structure of anion 2 (thermal ellipsoids at 30 % probability level) together with characteristic ranges for bond lengths and angles. For a better insight into the structure, the r-butyl substituents at the nitrogen atoms have been omitted. S Si NMR parameters and further structural details are given in Tables 1 or 2, respectively.

Sulfur reacts with hexamethylsilacyclopropane in tetrahydrofuran (THF) to give 2,3-dithia-l-silacyclopentane in 38% yield <79JOM(164)305>. Exposure of rranj-1,1-di-r-butyl-2,3-dimethyl-silacyclopropane (47) to elemental sulfur at room temperature for 3-6 h gave trans-1,1 -di-i-butyl-1,2-silathietane (48) and rra s-3,3-di-r-butyl-4,5-dimethyl-l,2-dithia-3-silacyclopentane (49) in 60% and 19% yield respectively (Equation (10)). Identical results were obtained with the m-silacyclo-propane, suggesting a stepwise radical mechanism <900M2205>. The presence of r-butyl substituents on silicon was a key factor in this reaction, since sulfur insertion cannot be stopped at a single atom when there are small substituents on silicon <79JOM(164)305>. 

Neopentyl (2,2-dimethylpropyl) systems are resistant to nucleophilic substitution reactions. They are primary and do not form carbocation intermediates moreover the r-butyl substituent hinders back-side displacement. The rate of reaction of neopentyl bromide with iodide ion is 470 times less than that of n-butyl bromide. Under solvolysis conditions the neopentyl system usually reacts with rearrangement to the... 

Relatively small substituents at C(2) and C(3) of the diene exert little steric influence on the rate of D-A addition. 2,3-Dimethylbutadiene reacts with maleic anhydride about ten times faster than butadiene because of the electron-releasing effect of the methyl groups. 2-f-Butyl-l,3-butadiene is 27 times more reactive than butadiene. The r-butyl substituent favors the s-cis conformation because of the steric repulsions in the s-trans conformation. 

The orientational behaviour of amine oxide pyrolyses has been adequately summarised by Cope and TrumbulE . As for the acetate decompositions, orientation in the simple alkyl systems is controlled primarily by statistical factors but departure from this influence is noted with the bulky r-butyl substituent and the acid strengthening beta phenyl substituents Eclipsing effects are greater in the planar five-membered transition states than in the puckered six systems and this is borne out by the greater preference for trans-olefin formation from amine oxides than esters and xanthates (152, cf. 149). 

The importance of coplanarity of the eliminating fragments is emphasised by the large difference in the Arrhenius activation energies for elimination from cw-4-r-butylcyclohexanol (E (cis) = 21 kcal.mole" ) and rra/iJ-4-r-butylcyclo-hexanol (Epitrans) = 38 kcal.mole ) . In the trans isomer, elimination must occur from a diequatorial syn-clinal conformation, or a diaxial conformation with the bulky r-butyl substituent in an unfavourable axial conformation, or an initial epimerisation to the cis isomer, or via a boat conformation, all possibilities which involve much higher energy intermediates than the diaxial elimination from the cis isomer. The products of elimination from the four stereoisomers of 1-decalol are also consistent with anti stereospecificity ... 

Entry 3 in Scheme 12.1 has only alkyl substituents and yet has a significant lifetimes in the absence of oxygen. The tris(r-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 r-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, since their extended lifetimes have more to do with kinetic factors than with inherent stability." Entry 5 is a sterically hindered perfiuorinated radical, which is even more stable than similar alkyl radicals. 

Finally, the di-tert-butyl or DBP derivatives are all found to be monomeric, adopting 3-5 coordination environments. These derivatives are the few that can be isolated witiiout solvents however, tiiey can be easily solvated by simple dissolution in a Lewis basic solvent. The monomeric nature is not solely due to the 2,6-r-butyl substituents, since the mesityloxide (OMes) species have been found to form both monomeric and dinuclear complexes. Figure 6 shows... 

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