6
D. W. Stephan
The description of FLP reductions of silyl-enol ethers by Erker was extended
to a range of enone and ynone substrates using tBuCH C(Ph)B(C 6 F 5 ) 2 [32],
CpFe(C 5 H 4 PMes 2 )/B(C 6 F 5 ) 3 [33], MesB(C 6 F 5 ) 2 [34] and a series of alkenyl boranes
with DABCO (Scheme 1.3) [35]. More recently, B(C 6 F 5 ) 3 has been used in a onepot Lewis acid-mediated generation of aza-Morita−Baylis−Hillman adducts and
subsequent FLP hydrogenation [36].
In 2010, we extended FLP hydrogenations to substituted nitrogen-heterocycles,
reducing substituted quinolines, phenanthroline, acridine as well as several indole
derivatives using B(C 6 F 5 ) 3 under H 2 [37]. Subsequently, the scope was extended to
a series of pyridines, quinolines, and other N-heterocycles [38–43].
In 2012, a collaboration with Paradies [44] showed that the FLP
(C 6 F 5 )Ph 2 P/B(C 6 F 5 ) 3 effectively reduced 1,1-disubstituted olefins in the presence of
H 2 (Scheme 1.3). Similarly, dissociation of the adduct (Et 2 O)B(C 6 F 5 ) 3 to the corresponding FLP was also capable of mediating the hydrogenation of 1,1-disubstituted
olefins [45]. Similarly, B(C 6 F 5 ) 3 and Ph 2 P(C 6 F 5 ) were shown to catalyze the FLP
hydrogenation of polyaromatic compounds in 2012 [46]. For example, several
anthracenes and tetracene derivatives were converted to the dihydro-analogs although
102 atm of H 2 and 80 °C were required for 10–48 h (Scheme 1.3).
Subsequently in 2012, electron-poor alkenes of the form RCH C(C(O)Et) 2 were
reduced by Alcarazo and coworkers using DABCO and B(C 6 F 5 ) 3 as the FLP catalyst
(Scheme 1.3) [22]. A 2014 study described the reduction of a series of electronrich nitro-olefins and acrylates as well as silylated fulvenes [47] using (THF)B(2,6C 6 F 2 H 3 ) 3 and lutidine or collidine under 4 bar H 2 at room temperature. In more recent
studies, vinylferrocene was reduced using the cyclopentane-based intramolecular
FLP C 5 H 8 (PMes 2 )B(C 6 F 5 ) 2 [48] while a series of terminal, di- and tri-substituted
olefins were reduced using 20 mol% of HB(C 6 F 5 ) 2 as a catalyst by Wang and Li [49].
The latter system required 6 bar H 2 and 140 °C, as it proceeds via initial hydroboration
followed by hydrogenolysis which regenerates the borane and provides the reduced
product.
Also in 2012, an examination of the activation of H 2 by the FLP derived from
alkyl-aryl amines and B(C 6 F 5 ) 3 revealed the formation of expected ammonium hydridoborate at room temperature while heating to 110 °C afforded reduction of the
corresponding cyclohexylammonium salts [50]. Indeed, a series of aromatic reductions were performed, although these reactions terminate by sequestration of the
borane as the hydridoborate counterion. Interestingly, such reductions remain a challenge for transition metal-based catalysts. In a related 2015 study, Du and coworkers
[51] showed that in contrast to the above aniline systems, reductions of tetrahydronaphthylamines could be catalyzed using 10 mol% of B(C 6 F 5 ) 3 under H 2 at 60
°C.
In 2013, Repo and coworkers [52] developed the intramolecular B/N FLP,
C 6 H 4 NMe 2 (BC 6 F 5 )H and exploited it to catalytically convert alkynes to cis-olefins
(Scheme 1.3). These authors demonstrated that the reduction is initiated by hydroboration of the alkyne, followed by H 2 cleavage and intramolecular protodeborylation
liberates the alkene. These authors latter showed that C 6 H 4 (NMe 2 )BCl 2 [53] was also
an effective catalyst for the production of cis-alkenes. In a related sense, Du et al.
D. W. Stephan
The description of FLP reductions of silyl-enol ethers by Erker was extended
to a range of enone and ynone substrates using tBuCH C(Ph)B(C 6 F 5 ) 2 [32],
CpFe(C 5 H 4 PMes 2 )/B(C 6 F 5 ) 3 [33], MesB(C 6 F 5 ) 2 [34] and a series of alkenyl boranes
with DABCO (Scheme 1.3) [35]. More recently, B(C 6 F 5 ) 3 has been used in a onepot Lewis acid-mediated generation of aza-Morita−Baylis−Hillman adducts and
subsequent FLP hydrogenation [36].
In 2010, we extended FLP hydrogenations to substituted nitrogen-heterocycles,
reducing substituted quinolines, phenanthroline, acridine as well as several indole
derivatives using B(C 6 F 5 ) 3 under H 2 [37]. Subsequently, the scope was extended to
a series of pyridines, quinolines, and other N-heterocycles [38–43].
In 2012, a collaboration with Paradies [44] showed that the FLP
(C 6 F 5 )Ph 2 P/B(C 6 F 5 ) 3 effectively reduced 1,1-disubstituted olefins in the presence of
H 2 (Scheme 1.3). Similarly, dissociation of the adduct (Et 2 O)B(C 6 F 5 ) 3 to the corresponding FLP was also capable of mediating the hydrogenation of 1,1-disubstituted
olefins [45]. Similarly, B(C 6 F 5 ) 3 and Ph 2 P(C 6 F 5 ) were shown to catalyze the FLP
hydrogenation of polyaromatic compounds in 2012 [46]. For example, several
anthracenes and tetracene derivatives were converted to the dihydro-analogs although
102 atm of H 2 and 80 °C were required for 10–48 h (Scheme 1.3).
Subsequently in 2012, electron-poor alkenes of the form RCH C(C(O)Et) 2 were
reduced by Alcarazo and coworkers using DABCO and B(C 6 F 5 ) 3 as the FLP catalyst
(Scheme 1.3) [22]. A 2014 study described the reduction of a series of electronrich nitro-olefins and acrylates as well as silylated fulvenes [47] using (THF)B(2,6C 6 F 2 H 3 ) 3 and lutidine or collidine under 4 bar H 2 at room temperature. In more recent
studies, vinylferrocene was reduced using the cyclopentane-based intramolecular
FLP C 5 H 8 (PMes 2 )B(C 6 F 5 ) 2 [48] while a series of terminal, di- and tri-substituted
olefins were reduced using 20 mol% of HB(C 6 F 5 ) 2 as a catalyst by Wang and Li [49].
The latter system required 6 bar H 2 and 140 °C, as it proceeds via initial hydroboration
followed by hydrogenolysis which regenerates the borane and provides the reduced
product.
Also in 2012, an examination of the activation of H 2 by the FLP derived from
alkyl-aryl amines and B(C 6 F 5 ) 3 revealed the formation of expected ammonium hydridoborate at room temperature while heating to 110 °C afforded reduction of the
corresponding cyclohexylammonium salts [50]. Indeed, a series of aromatic reductions were performed, although these reactions terminate by sequestration of the
borane as the hydridoborate counterion. Interestingly, such reductions remain a challenge for transition metal-based catalysts. In a related 2015 study, Du and coworkers
[51] showed that in contrast to the above aniline systems, reductions of tetrahydronaphthylamines could be catalyzed using 10 mol% of B(C 6 F 5 ) 3 under H 2 at 60
°C.
In 2013, Repo and coworkers [52] developed the intramolecular B/N FLP,
C 6 H 4 NMe 2 (BC 6 F 5 )H and exploited it to catalytically convert alkynes to cis-olefins
(Scheme 1.3). These authors demonstrated that the reduction is initiated by hydroboration of the alkyne, followed by H 2 cleavage and intramolecular protodeborylation
liberates the alkene. These authors latter showed that C 6 H 4 (NMe 2 )BCl 2 [53] was also
an effective catalyst for the production of cis-alkenes. In a related sense, Du et al.