Fcrt-Butyl group

A fcrt-butyl group is so large that fcrt-butylcyclohexane exists almost entirely in the conformation in which the r< rt-butyl group is equatorial. The amount of axial tert-butylcyclohexane present is too small to measure. 

The difference in stability between stereoisomeric alkenes is even more pronounced with larger alkyl groups on the double bond. A particularly striking example compares cis- and tra/ s-2,2,5,5-tetramethyl-3-hexene, in which the heat of combustion of the cis stereoisomer is 44 kJ/mol (10.5 kcal/mol) higher than that of the trans. The cis isomer is destabilized by the large van der Waals strain between the bulky fcrt-butyl groups on the same side of the double bond. 

Oxidation of 142c with lead tetraacetate gave 9//-9-oxodibenzo[c,c][l,2]-thiazine 5,5-dioxide (143). The redox potentials of some 9-substituted dibenzo[c,e][l,2]thiazine 5,5-dioxides (142) have been determined. Kaufmann and Zobel first prepared 2,3-dihydro-3-oxonaphtho-[l,8-d,e][l,2]thiazine 1,1-dioxide (146). More recently, this was prepared by Lombardino by an abbreviated process involving lithiation of 144 and treatment with carbon dioxide (Eq. 35). Cyclization of 145 with poly-phosphoric acid simultaneously removed the fcrt-butyl group to produce compound 146 directly. 

In each of these problems, a fcrt-butyl group is the larger substituent and will be equatorial in the most stable conformation. Draw a chair conformation of cyclohexane, add an equatorial tcrf-butyl group, and then add the remaining snbstiment so as to give the required cis or trans relationship to the teri-bntyl gronp. 

The transalkylation of fm-alkylbenzenes follows a different route, since they have no abstractable benzylic hydrogen. They were shown to transalkylate by a dealkylation-transalkylation mechanism with the involvement of free ferf-alkyl cations. The exceptional ability of fcrt-alkyl groups to undergo transalkylation led to the extensive utilization of these groups, especially the fm-butyl group, as positional protective groups in organic synthesis pioneered by Tashiro.223... 

If a disubstituted cyclohexane has two different substituents, then the most stable conformation is the chair that has the larger substituent in an equatorial orientation. This is most apparent when one of the substituents is a bulky group such as tert-butyl. Thus, the most stable conformation of cis-1 -fcrt-butyl-2-methylcyclohexane has an equatorial ferf-butyl group and an axial methyl group. 

The use of unsubstituted or 4-methyl phenols resulted in the formation of cluster compounds [58]. However, 2,6-di(fcrt-butyl) substituted aryloxide ligands allowed the isolation of mononuclear 3-coordinate homoleptie complexes of the lanthanide elements, the coordination mode of which was first demonstrated with the N(SiMe3)2 ligand [59], The 2,6-substitution pattern is very effective because the alkyl groups are directed towards the metal center and impose a steric coordination number onto the metal which is comparable to the Cp ligand (Cp 2.49 OC6H3rBu2-2,6 2.41) [60],... 

The ferf-butyl cations liberated from compounds Ar—tert- Bu, upon ipso reaction of a proton, may also react again with the aromatic compound from which they stemmed. If this course is taken, the te/7-butyl groups are ultimately bound to the aromatic nucleus with a regioselectivity that is dictated by thermodynamic control. Figure 5.4 shows how in this way 1,2,4-tri-ferf-butylbenzene is smoothly isomerized to give 1,3,5-tri-fcrt-buty 1-benzene. 

Synthesis of amide functionalised NHC ligands is facile and follows a standard protocol of unsymmetrically substituted imidazolium salts (see Chapter 1). In the present case, Arnold et al. reacted iV-fcrt-butyl-imidazole with iV-tcrt-butyl-aminoethyl bromide hydro-bromide [34]. Subsequent reaction with potassium hydride yields the amino functionalised carbene, probably in the form of a hydrogen bond stabilised zwitterion (see Figure 4.28). Stepwise reactiou of the amino functionalised imidazolium salt hydrobromide first strips off the hydrobromide and then deprotonates the imidazolium ring (not the secondary amino group) to form a lithium chelate complex. 

This is a typical nucleophilic acyl substitution reaction, with the amine of the amino acid as the nucleophile and fcrt-butyl carbonate as the leaving group. The terf-butyl carbonate then loses CO2 and gives fc/t-butoxide, which is protonated. 

For example, the fcrt-butoxycarbonyl protecting group, abbreviated as Boc, is formed by reacting the amino acid with di-tert-butyl dicarbonate in a nucleophilic acyl substitution reaction. 

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