electrophilic Au atom, which would facilitate the final protodeauration step of the
catalytic cycle [109].
Attempts to utilise a chiral ligand scaffold to enhance the enantioselectivity of
bimetallic Zn complexes have also been explored [112]. The efficiency of the
bimetallic complex 72, which contains a (S,S)-1,2-diphenylethane scaffold bridging
the two alkyl zinc centres, as a catalyst for the intramolecular hydroamination of
aminoalkenes was investigated (Scheme 27). Unfortunately, very poor enantioselectivity (<10% ee) was obtained for this reaction. Unrestricted rotation about the
C–C bond of the ethane scaffold could lead to a large separation between the two Zn
atoms, decreasing the potential for both metals to interact with the substrate. The
vigorous reaction conditions (120
C) would also decrease discrimination between
similar diastereomeric intermediates.
In an effort to increase the potential for enantioinduction in the hydroamination
of amino alkenes, bimetallic complexes of Zn (73) and Mg (74) have been prepared
NH
5 mol% cat.
10 mol% AgOTf
Toluene (25
o
C) or
Dioxane (80
o
C)
O
PhHN
N
Ph
Ph
NHPh
O
Ph
Ph
Monometallic catalyst:
71
15 h at 80
o
C= 92 % conv.
NH
O
PhHN
Ph
Ph
AuP
AuP
*
N
Ph Ph
PhHN
O
AuP
*
H
AuP
Favourable substrate alignment
enhances N-C bond formation
Increased acidity
promotes protodeauration
Bimetallic activation proposed:
N
Ph
Ph
NHPh
O
Au-Au bonding enhances
enantiodescrimination
P
P
Ar 2
Ar 2
Au
Au
Cl
Cl
With Au-Au interaction:
70-(S) (Ar= 3,5-Me2C6H3)
3 h at 25
o
C= 96 % conv.; 42 % ee
PAr 2 AuCl
PAr 2 AuCl
PtBu 2 AuCl
Without Au-Au interaction:
66-(S) (Ar= 3,5-MeC6H3)
3 h at 25
o
C= 95 % conv.; 17 % ee
>>
a)
b)
Scheme 26 (a) Enhanced stereoselectivity for the hydroamination reaction using the chiral
bimetallic Au(I) complex (70-(S)) containing an Au–Au interaction compared to the noninteracting bi- and monometallic Au complexes (66 and 71), and (b) proposed bimetallic Au(I)
activation of the substrate through favorable alignment
132
M.J. Page et al.
catalytic cycle [109].
Attempts to utilise a chiral ligand scaffold to enhance the enantioselectivity of
bimetallic Zn complexes have also been explored [112]. The efficiency of the
bimetallic complex 72, which contains a (S,S)-1,2-diphenylethane scaffold bridging
the two alkyl zinc centres, as a catalyst for the intramolecular hydroamination of
aminoalkenes was investigated (Scheme 27). Unfortunately, very poor enantioselectivity (<10% ee) was obtained for this reaction. Unrestricted rotation about the
C–C bond of the ethane scaffold could lead to a large separation between the two Zn
atoms, decreasing the potential for both metals to interact with the substrate. The
vigorous reaction conditions (120
C) would also decrease discrimination between
similar diastereomeric intermediates.
In an effort to increase the potential for enantioinduction in the hydroamination
of amino alkenes, bimetallic complexes of Zn (73) and Mg (74) have been prepared
NH
5 mol% cat.
10 mol% AgOTf
Toluene (25
o
C) or
Dioxane (80
o
C)
O
PhHN
N
Ph
Ph
NHPh
O
Ph
Ph
Monometallic catalyst:
71
15 h at 80
o
C= 92 % conv.
NH
O
PhHN
Ph
Ph
AuP
AuP
*
N
Ph Ph
PhHN
O
AuP
*
H
AuP
Favourable substrate alignment
enhances N-C bond formation
Increased acidity
promotes protodeauration
Bimetallic activation proposed:
N
Ph
Ph
NHPh
O
Au-Au bonding enhances
enantiodescrimination
P
P
Ar 2
Ar 2
Au
Au
Cl
Cl
With Au-Au interaction:
70-(S) (Ar= 3,5-Me2C6H3)
3 h at 25
o
C= 96 % conv.; 42 % ee
PAr 2 AuCl
PAr 2 AuCl
PtBu 2 AuCl
Without Au-Au interaction:
66-(S) (Ar= 3,5-MeC6H3)
3 h at 25
o
C= 95 % conv.; 17 % ee
>>
a)
b)
Scheme 26 (a) Enhanced stereoselectivity for the hydroamination reaction using the chiral
bimetallic Au(I) complex (70-(S)) containing an Au–Au interaction compared to the noninteracting bi- and monometallic Au complexes (66 and 71), and (b) proposed bimetallic Au(I)
activation of the substrate through favorable alignment
132
M.J. Page et al.