F-Butyl group

Rotation hindered by bulky substituents (f-butyl groups)... 

Two families of complexes having either exocyclic chirality borne by the nitrogen atoms (79,81,82) or endocycUc chirality (f-butyl groups) relayed by benzyUc groups on the nitrogen atoms (83, 84) were tested in the optimized conditions (Fig. 15). 

According to molecular mechanics (MM) calculations, the minimum energy conformation of the enolate is a twist-boat (because the chair leads to an axial orientation of the f-butyl group). The enolate is convex in shape with the second ring shielding the bottom face of the enolate, so alkylation occurs from the top. 

A study of the lithium enolate of pinacolone with several a-phenyl aldehydes gave results generally consistent with the Felkin model. Steric, rather than electronic, effects determine the conformational equilibria.77 If the alkyl group is branched, it occupies the large position. Thus, the f-butyl group occupies the large position, not the phenyl. 

Several enolates of 4,4-dimethyl-3-(trimethylsiloxy)-2-pentanone have been investigated.106 The lithium enolate reacts through a chelated TS with high 2,2 -anti stereoselectivity, based on the steric differentiation by the f-butyl group. 

Entries 10 to 12 illustrate the use of organocopper conjugate addition in the synthesis of relatively complex molecules. The installation of a f-butyl group adjacent to a quaternary carbon in Entry 10 requires somewhat forcing conditions, but proceeds Transition Metals in ooc yie - I 1 Entry 11, the addition is to a vinylogous ester, illustrating the ability... 

In general, an increase in steric demand of the substituents on silicon atoms appears to lead to the formation of more stable disilenes. For example, 3, which has two f-butyl groups, is more stable than 1 while 4, which has two 1-adamantyl groups, is more stable than 3.9 Disilene 4... 

These results suggest that the transition state which leads for example to the compound (38) is not symmetrical and completely productlike. It may well be that bond formation at the position para to the f-butyl group is more advanced in the transition state, as represented schematically in (40). 

Double hydrophilic star-block (PEO-fo-PAA)3 copolymers were prepared by a combination of anionic and ATRP of EO and fBuA [150]. Three-arm PEO stars, with terminal - OH groups were prepared by anionic polymerization, using l,l,l-tris(hydroxymethyl)ethane, activated with DPMK as a trifunctional initiator. The hydroxyl functions were subsequently transformed to three bromo-ester groups, which were utilized to initiate the polymerization of f-butyl acrylate by ATRP in the presence of CuBr/PMDETA. Subsequent hydrolysis of the f-butyl groups yielded the desired products (Scheme 74). 

The high selectivity can be explained by the model shown in Scheme 25. Because of the steric bulk of the f-butyl groups of bisoxazoline ligand 96, reface cyclization (transition states 97 and 98) should be favored over si-face... 

The Mannich reaction has been used to synthesize numerous heterocyclic nitramine explosives. Adolph and Cichra prepared a number of A-heterocycles containing ferf-butyl A-blocking groups. The nitrolysis of these f-butyl groups provides the corresponding A-nitro derivatives in excellent yields (Section 5.6.2.2). Some of the nitramine products from these reactions are powerful, energetic explosives with attractive properties. 

Pentanitropiperidine (91) is prepared from the condensation of 2,2-dinitro-l,3-propanediol (89) with formaldehyde and f-butylamine under slightly acidic conditions, followed by nitrolysis of the f-butyl group of the resulting piperidine (90) with mixed acid or absolute nitric acid. ... 

More practical from the point of view of starting material availability is the use of the A-f-butyl-A-methyl amides 30, whose f-butyl group may be removed with acid (Scheme 17) . De-f-butylation of amides can be capricious however, though it seems much more successful with acyclic than cyclic amides. 

Secondary and tertiary sulphonamides are among the most powerful ort/io-directing groups known " . There is no danger of attack at S, and A,Af-dimethylsulphonamides 112 may be lithiated with n-BuLi alone °. Secondary A-f-butyl sulphonamides 113 are particularly useful because the f-butyl group is readily removed in polyphospho-ric acid. Carbonation of 114 and cyclization in acid give the saccharin analogues 115 (Scheme 49) °. 

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