Radical alkoxycarbonyl

Ryu and coworkers subsequently reported photochemically initiated radical alkoxycarbonylation reactions of alkyl iodides 177 using 5 mol% of Pd(PPh3)4 as the catalyst (Fig. 42) [210]. The reaction proceeds by light-stimulated reductive generation of an alkyl radical by the palladium(O) catalyst 179A. This radical has three options to react further. Direct addition to CO forms the acyl radical 177A. 

Synthetic approaches that have been developed or extended in the review period and have the potential for general use include radical alkoxycarbonylation to give 4-alkoxycarbonylpyridazines (Section 6.01.5.6) preparation of 4(5)-aminopyridazines by direct amination with ammonia or amide ions followed by oxidation (Section 6.01.5.4.3) C—C bond formation by palladium catalyzed coupling reactions of pyridazine O-triflates (Section 6.01.7.12.2) and halopyridazines (Section... 

Ester groups in R —CO—OR compounds are named (1) by the prefix alkoxycarbonyl- or aryloxycarbonyl- for —CO—OR when the radical R contains a substituent with priority for citation as principal group or (2) by the prefix acyloxy- for R —CO—O— when the radical R contains a substituent with priority for citation as principal group. Examples are... 

Pyridinium bis(alkoxycarbonyl)methylides radical anions ESR, 2, 144 radical cations ESR, 2, 144... 

As data for the rates of spin-trapping reactions are accumulated, so it becomes possible to use the competition experiment in reverse , i.e. to determine rates of rearrangement, fragmentation, atom transfer, etc. which can compete with spin trapping. An attempt to estimate rates of decarbonylation of acyl radicals depended on this approach (Perkins and Roberts, 1973). Although the results obtained were intuitively reasonable, they depended on the assumption that the rate of scavenging of acyl radicals by MNP would be no different from that measured for the butoxycarbonyl radical. This still awaits experimental verification. Another application, reported recently, was to the rates of rearrangement (23) of a series of (o-(alkoxycarbonyl)-alkyl radicals... 

The absolute rate constants for a variety of cyclizafions have been measured. In particular, the rates of decarbonylafion of a variety of alkoxycarbonyl radicals have been obtained by LFP studies on PTOC oxalates." From these data, rate constants for the reduction of alkoxycarbonyl radicals with BusSnH and their 5-exo cyclizafions were determined. Whereas cyclizations were slightly faster than the analogous alkyl radical 5-exo cyclizations, their reactions with BusSnH were 10 times slower, indicating that cyclization processes should be synthetically useful. The rate constants for the cyclization of a number of variously substituted a-amide radicals have been determined together with their relative reactivities towards reduction using BusSnH (Scheme 16). Cyclizations of secondary-based radicals were found to be similar to the corresponding alkyl-substituted radicals. In addition, the rate constants were subject to minor electronic... 

The tandem -endo/5-exo cyclization of the (alkoxycarbonyl)methyl radical (32) has indicated that 8-endo cyclization is favoured over 5-exo cyclization in this system (Scheme 24). Ab initio studies indicated that this was due to the initial radical favouring a (Z)-(32) over an ( )-(32) conformation. 

It is clear from a study of thermal and radical-induced decompositions of N-alkoxycarbonyldihydropyridines that radical processes are of minor importance, and that pyridine formation is probably a consequence of 1,2-elimination of formate (Scheme 6). It has also been concluded that the rate of 1,4-elimination of formate from iV-alkoxycarbonyl-l,4-dihydropyridines at higher temperatures is too rapid to be explained by a homolytic process. 

Radicals similar to carbamoyl are the alkoxycarbonyl radicals, ROCO. Also, these radicals were successfully used to carboxylate protonated heteroaromatic bases with good yields and selectivity. " ... 

The 2,3-double bond of compound 145 can be dibrominated with bromine <1982AP761>. The reaction is an analog of the dibromination of 126 described in Section 8.06.5.3. The 2-bromination of 152 to give 153 (Equation 6) was claimed to be a radical reaction, but is more likely to be an electrophilic attack, as a base catalyst was used and the reaction needed the presence of a 2-alkoxycarbonyl group to proceed <1984H(22)2789>. In a similar way, the a-carbons of dihydrooxazin-2-ones and dihydrooxazin-3-ones can also be deprotonated and then reacted with electrophiles these reactions are described in Section 8.06.6.5. 

The elusive radical cation of pyridine (140) has been obtained by irradiation of pyridine in CFCb at 4 K (79MI20403) and g values and hyperfine coupling constants have been measured for the parent molecule and deuterated derivatives. This species is of cr-type, the odd electron spending most of its time in the N sp2 lone pair orbital. Radical cations and anions of pyridinium bis(alkoxycarbonyl)methylides have been produced in the former case (78CC817) as a cyclopropenone complex, and in the latter by reduction of pyridinium bis(methoxycarbonyl)methylide with sodium (79JMR(35)l7l). The coupling constants in the ESR spectrum of both the radical cation and the anion agree to some extent with simple Huckel MO calculations. 

Attack by alkoxycarbonyl radicals, which are isoelectronic with but less nucleophilic than carbamoyl radicals, has been less well studied than acylation and amidation. An example is provided by the reaction of quinoline with ethyl pyruvate, hydrogen peroxide and an iron(II) salt (Scheme 213) (73TL645). 

The oxidation of indoles and pyrroles by Fe(III) ions is less predictable than other chemical oxidations. 2-Methyl- and 3-methyl-indoles, respectively, yield (187) and (188), and whilst pyrroles may form pyrrole black , the rate of oxidation of pyrrole and of 1-methylpyrrole appears to be relatively slow. C-Alkyl and electron-donating substituents enhance the formation of oligimers, e.g. (189) -> (190) and (191) -> (192), and although electron-withdrawing substituents reduce the susceptibility of the pyrrole ring to oxidation, acyl- and alkoxycarbonyl-pyrroles of the type (193) are readily oxidized to the thermochromic dimer (194), which is in equilibrium with the dimer (195) via the monomeric pyrrolyl radical (72BCJ3584). 

Carboxylic acids are the most general, versatile and useful source of carbon-centered radicals successfully used for selective alkylation and acylation of protonated heteroarenes. Alkyl, acyl, carbamoyl, and alkoxycarbonyl radicals have been obtained by oxidative decarboxylation of the corresponding acids with peroxydisulfate as an oxidant and Ag(I) as catalyst. 

Stobadine ((-)-cis-2,8 dimethyl-2,3,4,4a,5,9b-hexahydro-l-H-pyrido (4,3b) indole) is a model drug with pyridoindole structure with cardioprotective and antioxidant properties [109]. Based on stobadine, (-)-ris-2,8-dimethyl-2,3,4,4a,5,9b-hexahydro-lH-pyrido[4,3-b]indole (Fig. 4), a well-known antioxidant, free-radical scavenger and neuroprotectant, stobadine derivatives with improved pharmacodynamic and toxicity profiles were developed [110]. A stobadine molecule was modified mostly by electron donating substitution on the benzene ring and by alkoxycarbonyl substitution at N-2 position. Significant antioxidant activity was observed in the new compounds. A link... 

A similar reaction has been adopted for the preparation of amides from alkenes by the photoaddition of "CONH2 radicals generated from formamide by benzophe-none [7], or via a TBADT [25] photocatalyzed hydrogen abstraction. Solar light has been used in the first case. Furthermore, the introduction of an ester function has been accomplished by generating an alkoxycarbonyl radical by photolysis of [bis(alkoxyoxalyloxy)iodo]benzene at 0-5 °C and ensuing addition to vinyl sulfones in the presence of 1,4-cyclohexadiene [9]. 

The rate constants of the alkoxycarbonyl radicals in Table 9 have been estimated to provide an idea of how stable a radical of this type can be and yet react with a substrate before decarboxylation. Alkoxycarbonyl radicals enter reactions of the type shown in eqns... 

Estimated Rate Constants for the Decarboxylation of Acyloxy and Alkoxycarbonyl Radicals at 20°C... 

Several examples of ring-opening polymerization proceeding via free-radical mechanism have been reported, e.g. the free-radical polymerization of vinylcyclo-propane 7 and its derivatives having alkoxycarbonyl substituents 98,9) ... 

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