F-butoxy

A/-Trimethoxybora2ines are available from reaction of dichloroboranes and 0-methyl-X,X-his(trimethylsilyl)hydroxylamine (eq. 31). The B-trichloro-bora2iQes undergo substitution reactions at the B atoms to give B-tri(/ f -butoxy)- or B-tri(/ f2 -but5i)-A/-trimethoxybora2iaes (101)... 

Another reagent which effects chlorination by a radical mechanism is f-butyl hypochlorite. The hydrogen-abstracting species in the chain mechanism is the f-butoxy radical. 

Many radicals undergo fragmentation or rearrangement in competition with reaction with monomer. For example, f-butoxy radicals undergo p-scission to form methyl radicals and acetone (Scheme 3.6). 

The radicals formed by imimolecular rearrangement or fragmentation of the primary radicals arc often termed secondary radicals. Often the absolute rate constants for secondary radical formation are known or can be accurately determined. These reactions may then be used as radical clocks",R2° lo calibrate the absolute rate constants for the bimolecular reactions of the primary radicals (e.g. addition to monomers - see 3.4). However, care must be taken since the rate constants of some clock reactions (e.g. f-butoxy [3-scission21) are medium dependent (see 3.4.2.1.1). 

For the case of MMA polymerization with a source of f-butoxy radicals (DBPOX) as initiator and toluene as solvent, most initiation may be by way of solvent-derived radicals"1"" (Scheme 3.9). Thus, a high proportion of chains (>70% for 10% w/v monomers at 60 °C22) will be initiated by benzyl rather than 1-butoxy radicals. Other entities with abstractable hydrogens may also be incorporated as polymer end groups. The significance of these processes increases with the degree of conversion and with the (solvent or impurity) monomer ratio. 

The reaction medium may also modify the reactivity of the primary, or other radicals without directly reacting with them. For example, when f-butoxy reacts... 

The low conversion initiator efficiency of di-r-butyl pcroxyoxalatc (0.93-0.97)1-1 is substantially higher than for other peroxyeslers [/-butyl peroxypivalale, 0.63 /-butyl peroxyacetate, 0.53 (60 °C, isopropylbenzene)195]. The dependence of cage recombination on the nature of the reaction medium has been the subject of a number of studies. 12I,1<>0 20CI The yield of DTBP (the main cage product) depends not only on viscosity but also on the precise nature of the solvent. The effect of solvent is to reduce the yield in the order aliphatic>aromatie>protic. It has been proposed199 that this is a consequence of the solvent dependence of p-scission of the f-butoxy radical which increases in the same series (Section 3.4.2.1.1). 

The pathways whereby oxygen-centered radicals interact with monomers show marked dependence on the structure of the radical (Table 3.8). For example, with MMA the proportion of tail addition varies from 66% for f-butoxy to 99% for isopropoxycarbonyloxy radical. The reactions of oxygen-centered radicals are discussed in detail in the following sections. 

As seen earlier in this section, the energy released upon formation of the benzophenone ketyl radical is about 104 kcal/mole. This value is almost identical to that released upon formation of f-butyl alcohol from the f-butoxy radical,... 

The f-butoxy radical can be produced from the radical decomposition of f-butylhypochlorite using azobisisobutyronitrile (AIBN) as catalyst(33) ... 

The f-butoxy radicals thus formed then abstract hydrogen from hydrocarbons to generate hydrocarbon radicals and f-butyl alcohol. The hydrocarbon radicals in turn generate more r-butoxy radicals by attacking the t-butyl-hypochlorite ... 

When f-butoxy methoxy benzyl acetate groups are attached to the same dendrimers, which are similar in shape to but less dependent on the solvent than the phenylalanine moieties, a roughly constant optical rotation per end group is obtained for all generations [2]. In this case the contributions to optical activity of the end groups seem to be additive and insensitive to differences in packing. 

Fig. 15. Optical activities of poly(propylene imine) dendrimers, functionalized at the periphery with protected phenylalanine or f-butoxy methoxy benzyl acetate groups, depend on the number of end groups [2]...

For the halogen-metal exchange reaction of bulkier halopyrimidines, steric hindrance retards the nucleophilic attack at the azomethine bond. As a consequence, halogen-metal exchange of 5-bromo-2,4-di-r-butoxypyrimidine (43) with n-BuLi could be carried out at -75 °C [20]. The resulting lithiated pyrimidine was then treated with n-butylborate followed by basic hydrolysis and acidification to provide 2,4-di-f-butoxy-5-pyrimidineboronic acid (44). 5-Bromopyrimidine 43 was prepared from 5-bromouracil in two steps consisting of a dehydroxy-halogenation with phosphorus oxychloride and an SnAt displacement with sodium r-butoxide. 

Reaction Between Organometallic Reagents and Peroxides f-Butoxy-de-metallation... 

This mechanism, involving a free radical R , is compatible with the allylic rearrangements found.250 The finding that t-butyl peresters labeled with ibO in the carbonyl oxygen gave ester with 50% of the label in each oxygen251 is in accord with a combination of R with the intermediate 19, in which the copper is ionically bound, so that the oxygens are essentially equivalent. Other evidence is that f-butoxy radicals have been trapped with dienes.252 Much less is known about the mechanisms of the reactions with metal acetates.253... 

When methyl radicals are generated from diacetyl peroxide they give substitution products with monoalkylfurans, but 2,5-dimethylfuran is hardly attacked (Scheme 41) (73TL637). f-Butoxy radicals generated from t- butyl hydroperoxide and Fe2+ in methanol react with furan to give addition products (64G67), but when generated photochemically from t- butyl peroxide they convert 2-methylfuran into the furfuryl radical, identified by ESR spectros-... 

Tn addition to arylthiazoles, heteroarylthiazoles also have been synthesized using halothiazoles and heteroarylboronic acids. Suzuki coupling of 2-bromothiazole and 5-indolylboronic led to 5-substituted indole 32 [24]. The Suzuki coupling of 2,4-dibromothiazole with 2,4-di-f-butoxy-5-pyrimidineboronic acid (33) resulted in selective helreoarylation at the 2-position to give pyrimidylthiazole 34 although the yield was low [25-27]. 

Large ring heterocyclic radicals are not particularly well known as a class. Their behavior often resembles that of their alicyclic counterparts, except for transannular reactions, such as the intramolecular cyclization of 1-azacyclononan-l-yl (Scheme 1) (72CJC1167). As is the case with alicyclic ethers, oxepane in the reaction with f-butoxy radical suffers abstraction of a hydrogen atom from the 2-position in the first reaction step (Scheme 2) (76TL439). 

A wide choice of peptide synthesizers is currently available, ranging from manual to fully automated. They are all based on solid-phase peptide synthesis methodologies in which either f-butoxy carbonyl (t-boc) (11), or 9-fluor-enylmethoxycarbonyl (Fmoc) (12) is the major protecting group during synthesis. A detailed description of peptide synthesis is clearly beyond the scope of this chapter, and further information on practical and theoretical approaches to this chemistry may be found elsewhere (13-15). However, a brief outline of solid-phase synthesis may prove useful. 

Reduction ofpyrimidine-2(lH)-ones.1 The pyrimidinone 2 is reduced by metal hydrides such as lithium tri-f-butoxy aluminum hydride to a mixture of the 3,6- and 3,4-dihydro derivatives in the ratio 9 1. In contrast, Meerwein-Ponndorf-Verley reduction with 1 in isopropanol results only in the 3,4-dihydro derivative (3a) but the reaction is slow and stops after two days to provide only a 25% yield. However, introduction of a halo substituent at C5 results in enhanced yields of the 3,4-dihydro derivatives, with the highest yields obtained with the 5-chloro derivative. 

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