MARTÍN SANDRA ELIZABETH
Capítulos de libros
Título:
Recent Advances on the Photoinduced Radical Nucleophilic Substitution Reactions
Autor/es:
MARÍA E. BUDÉN; SANDRA E. MARTÍN; ROBERTO A. ROSSI
Libro:
Handbook of Organic Photochemistry and Photobiology
Editorial:
CRC Press
Referencias:
Lugar: London; Año: 2012; p. 347 - 366
Resumen:
Nucleophilic substitution is feasible through processes that involve electron transfer
(ET) steps. In these reactions a compound bearing an adequate leaving group is substituted at
the ipso position by a nucleophile. Several reviews have been published on the subject in
relation to the mechanism and to the synthetic applications of the process which has become one of the common methodologies in modern synthesis. In this review we will describe the
most recent advances in photostimulated SRN1 reactions. With this aim in mind, we expect to
cover recent SRN1 substitutions, with an emphasis on the scope of the process in terms of
synthetic capability and target applications. The critical review illustrates the mechanistic
considerations required for planning synthetic applications and a wide range of synthetic
protocols.
The SRN1 reaction, which stands for Unimolecular Radical Nucleophilic Substitution,
is a chain process that involves radicals and radical anions as intermediates. Several
nucleophiles can be used for SRN1 reactions such as carbanions and heteroatom anions
resulting in the formation of new C-C or C-heteroatom bonds in good yields. An exception to
this is the reaction of aromatic alkoxides with aromatic substrates. In these cases C-O bond
formation is not observed; instead C-C bond formation is achieved instead. For example, the
unsubstituted phenoxide ions have been reported to react with ArX to afford the ortho- (ca.
40%) and para- (ca. 20%) C-C coupled products.1 On the other hand, alcoxide ions afford the
O-alkylation products with aliphatic substrates with EWG at the alpha carbon.
There is a theoretical study (DFT/B3LYP level) on the reaction of aromatic alcoxide
ions with phenyl and 4-nitrobenzyl radicals with the aim of interpreting the factors that
control the regiochemical outcome of these reactions. While FMO predicts the experimental
regioselectivity of phenyl radicals with good accuracy, it fails in the nitrobenzyl case.
A similar behavior is observed with nitrogen nucleophiles (anions from aromatic
amines, pyrrole, diazoles and triazoles) in reactions with aromatic substrates, no C-N is
observed, but C-C bond is formed. On the other hand, with nitrogen nucleophiles N-alkylation
products are formed with aliphatic substrates with EWG at the alpha carbon.
Anions derived from tin, phosphorus, arsenic, antimony, sulfur, selenium and
tellurium react through the heteroatom to form a new C-heteroatom bond by this mechanism.
(ET) steps. In these reactions a compound bearing an adequate leaving group is substituted at
the ipso position by a nucleophile. Several reviews have been published on the subject in
relation to the mechanism and to the synthetic applications of the process which has become one of the common methodologies in modern synthesis. In this review we will describe the
most recent advances in photostimulated SRN1 reactions. With this aim in mind, we expect to
cover recent SRN1 substitutions, with an emphasis on the scope of the process in terms of
synthetic capability and target applications. The critical review illustrates the mechanistic
considerations required for planning synthetic applications and a wide range of synthetic
protocols.
The SRN1 reaction, which stands for Unimolecular Radical Nucleophilic Substitution,
is a chain process that involves radicals and radical anions as intermediates. Several
nucleophiles can be used for SRN1 reactions such as carbanions and heteroatom anions
resulting in the formation of new C-C or C-heteroatom bonds in good yields. An exception to
this is the reaction of aromatic alkoxides with aromatic substrates. In these cases C-O bond
formation is not observed; instead C-C bond formation is achieved instead. For example, the
unsubstituted phenoxide ions have been reported to react with ArX to afford the ortho- (ca.
40%) and para- (ca. 20%) C-C coupled products.1 On the other hand, alcoxide ions afford the
O-alkylation products with aliphatic substrates with EWG at the alpha carbon.
There is a theoretical study (DFT/B3LYP level) on the reaction of aromatic alcoxide
ions with phenyl and 4-nitrobenzyl radicals with the aim of interpreting the factors that
control the regiochemical outcome of these reactions. While FMO predicts the experimental
regioselectivity of phenyl radicals with good accuracy, it fails in the nitrobenzyl case.
A similar behavior is observed with nitrogen nucleophiles (anions from aromatic
amines, pyrrole, diazoles and triazoles) in reactions with aromatic substrates, no C-N is
observed, but C-C bond is formed. On the other hand, with nitrogen nucleophiles N-alkylation
products are formed with aliphatic substrates with EWG at the alpha carbon.
Anions derived from tin, phosphorus, arsenic, antimony, sulfur, selenium and
tellurium react through the heteroatom to form a new C-heteroatom bond by this mechanism.
