Reaction Initiated by C-O Bond Formation

A rhodium(III)/copper(II)-mediated process was reported to provide tetra-substituted enol esters in a trans-selective fashion. Overall, the reaction consists of a heteroaryl acyloxylation of alkynes. The process was initiated by a rhodium(III)-catalyzed C-2-selective activation of electron-rich het-eroarenes, such as benzo[I>]furan, and furan. Upon addition across an alkyne, a transmetalation to copper(II) enabled reductive C—O bond formation (14AGE14575). 

A further variation of these functionalizations of cyanoarenes is the NOCAS process [14, 15]. As shown in Scheme 14.2, path g, this involves the addition of a nucleophile (which is often the solvent) to the donor radical cation. The thus-formed neutral radical adds to the acceptor radical anion, while rearomatization by the loss of an anion leads again to an overall ipso-substitution. AUenes could be used as the donors in these reactions, as shown recently by Arnold [50]. Accordingly, the irradiation of TCB in the presence of tetramethylaUene (15) in a 3 1 MeCN/MeOH mixture afforded 1 1 1 arene-allene-methanol adduct 16 in 48% yield (Scheme 14.9, central part). Interestingly, the addition of methanol took place exclusively at the central allene carbon, while aromatic substitution occurred through the terminal carbons. co-Alkenols, in which an O-nucleophile and an easily oxidized moiety are both present, could also be used. In the latter case, the initial ET was followed by a cyclization, yielding aryl-substituted tetra-hydrofurans or tetrahydropyrans as the final products via a tandem Ar—C, C—O bond formation [51]. 

Conjugated dienes coordinated to a transition metal can readily be transformed into a rr-allylmetal complex by functionalization at the 4-position.f This makes dienes useful substrates for catalytic transformations since the 7r-allyl complex formed can undergo further reaction. t A number of Pd-catalyzed reactions of conjugated dienes are known that proceed via rr-allylpalladium intermediates and lead to useful 1,4- or 1,2-funtional-ization of the diene. There are two types of reactions of this kind (i) Pd(0)-catalyzed reactions that involve initial oxidative addition and (ii) Pd(II)-catalyzed reactions that involve electrophilic activation of the diene by the metal followed by nucleophilic attack. This section deals with C— N and C—O bond formation via these two types of reactions. This topic has previously been reviewed in connection with Pd-catalyzed additions to conjugated dienes. [5] [n] -jijg present review will focus mainly on the work published since 1997 but will also briefly discuss previous work. 

An efficient route to quinazolinones was reported by Willis et al. They described the palladium-catalyzed coupling of o-bromobenzoate esters such as 50 with monosubsti-tuted ureas (Scheme 24.25) [100]. The reactions proceeded via initial intermolecular C—N bond formation followed by intramolecular base-promoted cyclization to yield the 3-alkylated quinazolinedione products in good yields. Complete regiocontrol was observed, which originates from the initial aryl C—Nbond formation occurring at the least hindered N-atom of the urea nucleophile. 

Three different mechanisms of perester homolytic decay are known [3,4] splitting of the weakest O—O bond with the formation of alkoxyl and acyloxyl radicals, concerted fragmentation with simultaneous splitting of O—O and C—C(O) bonds [3,4], and some ortho-substituted benzoyl peresters are decomposed by the mechanism of decomposition with anchimeric assistance [3,4]. The rate constants of perester decomposition and values of e = k l2kd are collected in the Handbook of Radical Initiators [4]. The yield of cage reaction products increases with increasing viscosity of the solvent. 

The third intra-pair reaction to be discussed involves bond formation between radical anion and cation without intervening transfer both singlet and triplet radical ion pairs can couple. For example, the bifunctional radical cation 24 generates two chloranil adducts, most likely via zwitterions (e.g., 74 and 75 ), initiated by forming a C O bond. The CIDNP results indicate that 74 and 75 are formed from a singlet radical ion pair. Adduct 75 is a minor product, as the major spin density of 24 + is located in the allyl function which, therefore, is expected to be the principal site of coupling. 

Both N-N and N-C bond fission occurs on irradiation of the hydrazone derivatives (191). The photodegradation of the phenylhydrazone and the hydrazone of benzil have also been described. a-Ketoiminyl radicals are formed on irradiation of oximino ketones at low temperature. A study of the photochemical decomposition of sulfamic esters and their use as initiators of cross-linking of a melamine resin have been described. The bispyridinyl radical (192) is formed by one electron reduction of the corresponding pyridinium salts. The irradiation of this biradical at 77 K results in C-N bond fission with the formation of benzene-1,3-diyl. The predominant products from the irradiation (X,> 340 nm) of (193) in methanol were identified as A -hydroxy-2-pyridone and (194) from the fission of the C-O bond. Other products were 2-pyridone, (195) and (196) that arise from O-N bond fission. The reaction is to some extent substituent dependent and a detailed analysis of the reaction systems has identified an intramolecular exciplex as the key intermediate in the C-O bond heterolysis. 

Enantioselective bicvclization can be initiated by the Lewis acid catalyzed ring opening of a chiral acetal40. The C-O bond next to the axial methyl group breaks and the olefin attacks from the opposite face. A chairlikc transition state leads to the optically active /ranx-hydrindane system. Formation of the anti-trans-bicycle 3 is favored over reaction to the syn-/w/tx-produc13. 

JV-Oxide rearrangements in heteroaromatic compounds are frequently induced by either photochemical or acid anhydride initiated processes, and usually involve formation of a C—O bond at the carbon a to the original N-oxide. 60 One of the few -rearrangements is observed by treating 1,3-dimethylpyrido[2,3-r/Jpyrimidine-2,4(l //,3//)-dione 8-oxide (1 see Section 7.2.2.1.1.5.1.) with acetic acid/acetic anhydride at 90 CC, which gives 6-acetoxy-l,3-dimethylpyrido[2,3-t/]pyrimidine-2,4(l//,3//)-dione (2). When the reaction is run at reflux temperature in trifluoroacctic anhydride/trifluoroacetic acid, a 46% yield each of the a- and -product, 3 and 4, respectively, is obtained.310... 

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