Ferf-butyl groups

The limitations of the system with regard to substrates and oxidants was attributed to the strong electron-withdrawing character of the perfluorinated chains and the lower steric hindrance in the position adjacent to phenols, in marked contrast to the ferf-butyl groups present in Jacobsen s catalyst, hi view of this, a second generation of fluorinated salen ligands le and If was... 

In the Fmoc protection approach, the acid-labile ferf-butyl groups are often used for side-chain protection. The base-labile Fmoc groups can be easily removed during a synthesis using piperidine (Fig. 4). The final global deprotection together with cleavage from the polymeric support is achieved with TFA. 

Highly branched poly(methacryhc acid) was synthesized by SCVCP of tert-butyl methacrylate with the inimer 12 via GTP, followed by hydrolysis [28]. Acid-catalyzed hydrolysis of the ferf-butyl groups and neutralization with NaOH produced a water-soluble, highly branched poly(sodium methacrylate). 

Lomas and Dubois (113b) also reported that by substitution of 1-adamantyl group(s) for one or two of the ferf-butyl group(s) of di-rm-butyl-o-tolylcarbinol (74), the barrier to rotation was-considerably raised, AG at 200°C being 33.9 and 39.1 kcal/mol, respectively, for the mono-1-adamantyl and di-1-adamantyl compounds in dodecane. Being rigid, the 1-adamantyl group causes more steric interference in the transition state for rotation than does the rm-butyl. 

Adolph and Cichra prepared a number of cyclic nitramines from the nitrolysis of tert-butyl protected Mannich products (Table 5.5). Nitrolysis of the fert-butyl groups was achieved with mixed acid, pure nitric acid or a mixture of nitric acid in acetic anhydride depending on the substrate. Pure nitric acid was found to affect the nitrolysis of both the ferr-butyl groups of (97), (Table 5.5, Entry 4) whereas the use of mixed acid led to the isolation of the product where only one of the ferf-butyl groups had undergone nitrolysis. Some of the cyclic nitramine products... 

Marchand and co-workers reported a synthetic route to TNAZ (18) involving a novel electrophilic addition of NO+ NO2 across the highly strained C(3)-N bond of 3-(bromomethyl)-l-azabicyclo[1.1.0]butane (21), the latter prepared as a nonisolatable intermediate from the reaction of the bromide salt of tris(bromomethyl)methylamine (20) with aqueous sodium hydroxide under reduced pressure. The product of this reaction, A-nitroso-3-bromomethyl-3-nitroazetidine (22), is formed in 10% yield but is also accompanied by A-nitroso-3-bromomethyl-3-hydroxyazetidine as a by-product. Isolation of (22) from this mixture, followed by treatment with a solution of nitric acid in trifluoroacetic anhydride, leads to nitrolysis of the ferf-butyl group and yields (23). Treatment of (23) with sodium bicarbonate and sodium iodide in DMSO leads to hydrolysis of the bromomethyl group and the formation of (24). The synthesis of TNAZ (18) is completed by deformylation of (24), followed by oxidative nitration, both processes achieved in one pot with an alkaline solution of sodium nitrite, potassium ferricyanide and sodium persulfate. This route to TNAZ gives a low overall yield and is not suitable for large scale manufacture. 

Now, as the substituent gets bigger, the proportion of axial conformer will diminish even further. With a substituent as big as a ferf-butyl group, the equilibrium... 

To take this general principle to its extreme, we noted above that ferf-butyl groups are sufficiently large that they never occupy an axial position. It is possible to make di-ferf-butylcyclohexanes where conformational mobility would predict that one of these groups would have to be axial, namely cis-1,2-, trans-1,3- or d -1,4-derivatives. As a result, in these cases, we do not see an axial ferf-butyl, but... 

Using this information in conjunction with a study into the preferred conformations of iminium ions generated from catalysts 12 and 21, Houk suggests that the additional steric bulk of the ferf-butyl group causes the benzyl arm of the catalyst to shield better the Si face of the C=C double bond - a requirement for high ees in an open transition state. For both the Diels-Alder and pyrrole/indole alkylation... 

Benzamides 565 without any substituent at the para position reacted with lithium and a catalytic amount of naphthalene under Barbier-type reaction conditions (in the presence of a carbonyl compound) in THF at —78 °C to give, after hydrolysis, the corresponding dearomatized products 566 (Scheme 151). When 567 was used as starting material with a 4-ferf-butyl group in p-position, and using 3-pentanone as electrophile and under the same reaction conditions, the fraw -product 568 was the only one isolated . 

H-l,2,3-Benzodithiazol-6-ones 141 were prepared from p-benzoquinone-4-oximes, S2CI2, N-ethyldi/sopropylamine and NCS (1998T223 Scheme 72). Some ring chlorination occurred and 2,6-substituents were retained in the products except for the ferf-butyl group, which was replaced by chlorine. 1,4-Naphthoquinone 4-oxime and 1,2-naphthoquinone 2-oxime similarly gave dithiazole derivatives 142 and 143 (1998T223). 

Other dithienylethenes have been synthesized and investigated for their photo-chromic properties by replacing the ferf-butyl group by bis(p-methylphenyl) amine, thus raising the glass transition temperatures [293, 294]. 

The anti selectivity increases as the disubstituted side of the double bond becomes more crowded (Scheme 8). This is illustrated with the trisubstituted alkenes 16, 17 and 13. Alkene 16 shows the normal cis effect selectivity where only 10% of the anti ene adduct is formed. However, as the size of the cis alkyl substituent increases from methyl in 16, to isopropyl in 17 and ferf-butyl in 13, the anti selectivity increases from 10% to 42% and to >97%, respectively. The same trend is also noted in substrate 19. A substantial deviation from cis effect selectivity is observed by replacing one methyl group in 18 with a ferf-butyl group in 19. The totally unreactive methylene hydrogens in 18 (cis effect) become reactive in 19, producing the exo ene adduct in 38% yield. 

Another type of adducts [8, Eq. (3)] was formed by the reaction of di(fert-butyl)aluminum chloride with dilithium bis(trimethylsilyl)hydrazide in low yields below 30% [19]. The structure of 8 consists of a distorted heterocubane with four vertices occupied by nitrogen atoms, two of which are connected by an intact N—N bond across one face of the cube. The cation positions are occupied by two aluminum and two lithium atoms, of which the last ones bridge the N—bond. Part of the hydrazide molecules was cleaved, and the aluminum atoms are bonded to one ferf-butyl group only. On the basis of the NMR spectroscopic characterization many unknown by-products were formed in the course of that reaction, and no information is available concerning the reaction mechanism. Compound 8 may be described as an adduct of dilithium bis(trimethylsilyl)hydrazide to a dimeric iminoalane containing a four-membered AI2N2 heterocycle. Further... 

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