R-butoxy

IV-Oxidation of 3,6-dialkoxypyridazines (OMe, OEt, OPr", OBu") gives monoxides, while 3,6-di-r-butoxy- and 3,6-dibenzyloxy-pyridazine cannot be IV-oxidized, but give 6-hydroxy-pyridazin-3(2/f)-one and 6-benzyloxypyridazin-3(2//)-one as hydrolysis products. Methyl-thiopyridazines give both 5-oxidation and IV-oxidation products with various oxidizing agents in some instances. 

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) (72CJCH67). As is the case with alicyclic ethers, oxepane in the reaction with r-butoxy radical suffers abstraction of a hydrogen atom from the 2-position in the first reaction step (Scheme 2) (76TL439). 

For R = r-butoxy, the rate data are given for several temperatures in decane. 

The selectivity and product composition is different from that obtained for direct chlorination. The selectivity of the r-butoxy radical is intermediate between that of chlorine and bromine atoms. The selectivity is also solvent- and temperature-dependent. 

However, consideration of polar factors in the traditional sense does not provide a ready explanation for the regiospecificity shown by the r butoxy radicals (which arc electrophilic, Tabic 1.3) in their reactions with the tluoro-olcfins (Tabic 1.2).22,23 Apparent ambiphilicity has been reported21 for other not very electrophilic radicals in their reactions with olefins and has been attributed to the polarizability of die radical. 

Alkyl radicals, when considered in relation to heteroatom-centered radicals (e.g. r-butoxy, benzoyloxy), show a high degree of chcmo- and rcgiospecificity in their reactions. A discussion of the factors influencing the rate and rcgiospecificity of addition appears in Section 2.3. Significant amounts of head addition arc observed only when addition to the tail-position is sterically inhibited as it is in a,p-disubstituted monomers. For example, with p-alkylacrylates, cyclohexyl... 

Pioneering work by Wallingj94 established that the specificity shown by t-butoxy radical is solvent dependent. Work21 22396 on the reactions of /-butoxy radicals with a series of a-mcthylvinyl monomers has shown that polar and aromatic solvents favor abstraction over addition, and [3-scission over either addition or abstraction. Recently, Weber and Fischer418 and Tsentalovich at a/.410 reported absolute rate constants for [3-scission of r-butoxy radicals in various solvents. These studies indicate that p-scission is strongly solvent dependent while abstraction is relatively insensitive to solvent. 

Chalfont ei a m were the first to apply the spin trapping technique in the study of radical polymerization. They studied radicals produced during S polymerization initiated by r-butoxy radicals with 108 as the radical trap. Since... 

Watanabe et al,25-5 52s applied AMS dimer (116) as a radical trap to examine the reactions of oxygen-centered radicals (e.g. r-butoxy, cumyloxy, benzoyloxy). AMS dimer (116) is an addition fragmentation chain transfer agent (see 6.2.3.4) and reacts as shown in Scheme 3,96. The reaction products are macromonomers and may potentially react further. The reactivity of oxygen centered radicals towards 116 appears to be similar to that of S.2 1 Cumyl radicals are formed as a byproduct of trapping and are said to decay mainly by combination and disproportionation. 

The reaction of matrix-isolated triazidophenylsilane with r-BuOH led to the formation of /V-(di-f-butoxysilyl)aniline and amino(di-r-butoxy)phe-nylsilane, the trapping products of the nitrile and isonitrile.6... 

The r-butoxy radical, therefore, should serve as an excellent model to compare with the benzophenone triplet since the energetics are essentially the same. 

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 ... 

It was recognized (32) that not all species 4 (R = sec-alkyl) undergo disproportionation readily, since the reagent formed from camphor and LAH was more stereoselective than LAH in the reduction of 5. This was attributed to the bulky isobomyloxy complex formed in the reaction of camphor with LAH, which is analogous to the tri-r-butoxy complex. Disproportionation to the tetraalkoxy-aluminum species in these cases is disfavored by steric hindrance. 

LiAlH(OBu )3 does not undergo disproportionation to the tetraalkoxy species. Reduction of the aromatic ketones studied involved either monomeric LAH or both this species and the monoalkoxy species, depending on the steric hindrance of the substrate. In a similar study of the reduction of camphor in THF (39), the kinetic results were also consistent with disproportionation of r-butoxy species (eqs. [6] and [7]), active reducing species being LAH and LiAl(OBu )H3. In the reduction of camphor with a series of reagents prepared by the reaction of LAH... 

The migration of the 0-r-butoxy group with concomitant generation of the 0-t-butoxy isocyanate 562 was observed in the Lossen rearrangement of 561 induced by base deprotonation " (equation 250). 

Reduction of a-alkoxy-fl-keto esters.1 r-Butoxy esters of a-alkoxy-/3-keto acids are reduced by NaBH4 in 2-propanol selectively to erythro-a-alkoxy-fl-hydmxy curboxylates. The report suggests that the selectivity results from reduction of an intermediate five-membcred chelate involving Na+ and, in fact, no stereoselectivity is observed if a crown ether is present. 

The short-lived [MH2(Cp)2] and [TaH4(dmpe)2] have been obtained from the Mv hydrides using photogenerated r-butoxy radicals, and were characterized by low temperature ESR.575 On the other hand, thermally stable, well-defined dinuclear or mononuclear MIV hydrides have been prepared by oxidative addition of H2 to dinuclear Mm or mononuclear Mn halide phosphine adducts, respectively. They constitute attractive entries to lower oxidation state compounds, and will be reviewed in Sections 34.4.3.l.i and 34.6.1.2.i. 

Comparative studies indicate that abstraction of hydrogen atoms from the methyl group of 1-methylpyrrole is easier than from the methyl group of toluene. Subsequent dimerization and polymerization of the 1-pyrrolylmethyl radicals, initiated by the addition of r-butoxy radicals, occurs and low yields (0.5%) of l,2-bis(l-pyrrolyl)ethane and l-methyl-3-(l-pyrrolylmethyl)pyrrole have been isolated from the tars (B-77MI30500). The radical dimerization of pyrrole initiated with r-butoxy radicals produces 2-(2-pyrrolyl)-A1-pyrroline (B-77MI30500). 

Another electrophilic reagent which has been used to introduce an oxygen function is alkyl peroxybenzoate. Thus 5-methoxy-2-thienyllithium on reaction with r-butyl peroxy-benzoate gives 2-r-butoxy-5 -methoxythiophene in 72% yield (790R(26)1>. 

Simple 5-pyrimidineboronic acid is not trivial to make because the requisite hthiopyrimidine would add to the azomethine bond. The tendency towards these side reactions is less severe in case of 2,4-di-tert-butoxy-5-bromopyrimidine (20) and the halogen-metal exchange can be conducted at -75 °C [29]. The nucleophilic attack towards the azomethine bond is retarded due to the steric hindrance. Therefore, the halogen-metal exchange of 2,4-di-r-butoxy-5-bromopyrimidine (20) followed by quenching with n-butylborate, basic hydrolysis and... 

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-it-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-r-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. 

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