1 Frustrated Lewis Pair Catalysis: An Introduction
15
Scheme 1.9 Examples of
FLP-mediated
hydroaminations
NH
R
X
N
R
X
10 mol%
B(C6F5)3
R'
R'
O
N
N
EtO 2 C
CO 2 Et
+
+
5mol%
B(C 6 F 5 ) 3
10 mol%
Brønsted acid
O HN
N
CO 2 Et
CO 2 Et
O
N
Boc
Ph
+
5mol%
B(C 6 F 5 ) 3
10 mol%
Brønsted acid
O
NH
Boc
Ph
1.3.4 Amination
One of the first papers on FLP chemistry described the addition of phosphine and
borane to olefins [4]. This work proved seminal in the development of a catalytic
strategy to hydroamination. While stoichiometric addition of sterically encumbered
amines and B(C 6 F 5 ) 3 to alkynes afforded the zwitterionic addition product of the
form [R’ 2 NHC(R)=C(H)B(C 6 F 5 ) 3 ], use of a catalytic amount of borane with slow
addition of the terminal alkyne gave the corresponding enamine product was prepared
(Scheme 1.9) [120]. This metal-free route to hydroamination also proved to be
amenable to tandem hydrogenation catalysis as subsequent addition of H 2 effected
the conversion of the enamines to the corresponding amines in a single pot. The
hydroamination of alkynes was subsequently extended to intramolecular systems
affording a catalytic route to a variety of heterocyclic derivatives [121].
Wasa and coworkers [122] have developed enantioselective α-aminations of
ketones with dialkyl-azodicarboxylates catalyzed by FLPs derived from a Lewis
acid and a Brønsted base (Scheme 1.9). These reactions afford an elegant route to
α-aminocarbonyl compounds in high enantiomeric purity. In a conceptually related
study, Wasa’s group also described the use of this same FLP to direct the Mannichtype reactions of a broad range of carbonyl compounds and aldimines with imines
to afford both α- and β-amino esters (Scheme 1.9) [123].
1.3.5 Hydroarylation
Electrophilic phosphonium cations have been exploited as Lewis acid catalysts
to affect the hydroarylation of olefins by substituted anilines, bis-arylamines,
phenol, furan, thiophene, pyrrole, and indole derivatives under mild conditions
(Scheme 1.10) [124]. In a similar vein, the electrophilic dicationic [(PhO)P(2-(N-
15
Scheme 1.9 Examples of
FLP-mediated
hydroaminations
NH
R
X
N
R
X
10 mol%
B(C6F5)3
R'
R'
O
N
N
EtO 2 C
CO 2 Et
+
+
5mol%
B(C 6 F 5 ) 3
10 mol%
Brønsted acid
O HN
N
CO 2 Et
CO 2 Et
O
N
Boc
Ph
+
5mol%
B(C 6 F 5 ) 3
10 mol%
Brønsted acid
O
NH
Boc
Ph
1.3.4 Amination
One of the first papers on FLP chemistry described the addition of phosphine and
borane to olefins [4]. This work proved seminal in the development of a catalytic
strategy to hydroamination. While stoichiometric addition of sterically encumbered
amines and B(C 6 F 5 ) 3 to alkynes afforded the zwitterionic addition product of the
form [R’ 2 NHC(R)=C(H)B(C 6 F 5 ) 3 ], use of a catalytic amount of borane with slow
addition of the terminal alkyne gave the corresponding enamine product was prepared
(Scheme 1.9) [120]. This metal-free route to hydroamination also proved to be
amenable to tandem hydrogenation catalysis as subsequent addition of H 2 effected
the conversion of the enamines to the corresponding amines in a single pot. The
hydroamination of alkynes was subsequently extended to intramolecular systems
affording a catalytic route to a variety of heterocyclic derivatives [121].
Wasa and coworkers [122] have developed enantioselective α-aminations of
ketones with dialkyl-azodicarboxylates catalyzed by FLPs derived from a Lewis
acid and a Brønsted base (Scheme 1.9). These reactions afford an elegant route to
α-aminocarbonyl compounds in high enantiomeric purity. In a conceptually related
study, Wasa’s group also described the use of this same FLP to direct the Mannichtype reactions of a broad range of carbonyl compounds and aldimines with imines
to afford both α- and β-amino esters (Scheme 1.9) [123].
1.3.5 Hydroarylation
Electrophilic phosphonium cations have been exploited as Lewis acid catalysts
to affect the hydroarylation of olefins by substituted anilines, bis-arylamines,
phenol, furan, thiophene, pyrrole, and indole derivatives under mild conditions
(Scheme 1.10) [124]. In a similar vein, the electrophilic dicationic [(PhO)P(2-(N-