abnormal NHC ligand showed very low activity for the dehydrogenation of COA
with TBE (8:1) at 200
C (TONs ¼ 4 after 10 h) due to its low stability and low
solubility in the reaction media [50].
The Chianese group synthesized more rigid bis(NHC) iridium complexes, 12a–
12f, which were shown to be active pre-catalysts for the acceptorless dehydrogenation of alkanes (Fig. 8) [51, 52]. For the acceptorless dehydrogenation of COA,
the catalytic systems generated in situ from 12c, 12d, and 12e with NaO
t
Bu gave
TONs of 103, 84, and 35, respectively, after 12 h at reflux of COA (bp ¼ 150
C).
The catalytic activity of 12c appears to exhibit no inhibition in the presence of COE
or N 2 . By using the higher-boiling cyclodecane, TONs up to 102 were achieved
with the pre-catalyst 12c after 22 h. Acceptorless dehydrogenation of linear alkanes
was also carried out. TONs up to 97 were obtained when using the pre-catalyst with
n-undecane, which is comparable to the results obtained from the most active
iridium PCP catalysts. Under similar conditions, pre-catalysts 12c and 12d showed
lower reactivity (TON ¼ 50).
Other non-phosphine-based catalysts active for alkane dehydrogenation were
developed by the Jensen group [53]. The complex (
tBu4 AsOCOAs)IrHCl, 13
(Fig. 9), combined with NaO
t
Bu catalyzed the transfer dehydrogenation of COA
with TBE (1:1) giving TONs of up to 930 after 24 h with initial rates of 600 TO h
À1
at 200
C. The leveling off in catalytic activity was explained by inhibition by the
COE product, as well as thermal decomposition of the catalyst over time.
3 Applications to the Synthesis of Aromatics
As the global demand for chemicals grows, so does that for aromatics, which
constitute a significant fraction of the major building blocks of the chemical
industry. As petroleum is displaced by natural gas, and as decreased gasoline
refining (in favor of diesel [54]) limits production of aromatic by-products, the
Fig. 6 (PCyP)Ir(H)
(Cl) complex reported by
Wendt [48]
Fig. 7 CCC iridium
complexes reported by
Braunstein [49, 50]
Transfer Dehydrogenations of Alkanes and Related Reactions Using Iridium. . .
195
with TBE (8:1) at 200
C (TONs ¼ 4 after 10 h) due to its low stability and low
solubility in the reaction media [50].
The Chianese group synthesized more rigid bis(NHC) iridium complexes, 12a–
12f, which were shown to be active pre-catalysts for the acceptorless dehydrogenation of alkanes (Fig. 8) [51, 52]. For the acceptorless dehydrogenation of COA,
the catalytic systems generated in situ from 12c, 12d, and 12e with NaO
t
Bu gave
TONs of 103, 84, and 35, respectively, after 12 h at reflux of COA (bp ¼ 150
C).
The catalytic activity of 12c appears to exhibit no inhibition in the presence of COE
or N 2 . By using the higher-boiling cyclodecane, TONs up to 102 were achieved
with the pre-catalyst 12c after 22 h. Acceptorless dehydrogenation of linear alkanes
was also carried out. TONs up to 97 were obtained when using the pre-catalyst with
n-undecane, which is comparable to the results obtained from the most active
iridium PCP catalysts. Under similar conditions, pre-catalysts 12c and 12d showed
lower reactivity (TON ¼ 50).
Other non-phosphine-based catalysts active for alkane dehydrogenation were
developed by the Jensen group [53]. The complex (
tBu4 AsOCOAs)IrHCl, 13
(Fig. 9), combined with NaO
t
Bu catalyzed the transfer dehydrogenation of COA
with TBE (1:1) giving TONs of up to 930 after 24 h with initial rates of 600 TO h
À1
at 200
C. The leveling off in catalytic activity was explained by inhibition by the
COE product, as well as thermal decomposition of the catalyst over time.
3 Applications to the Synthesis of Aromatics
As the global demand for chemicals grows, so does that for aromatics, which
constitute a significant fraction of the major building blocks of the chemical
industry. As petroleum is displaced by natural gas, and as decreased gasoline
refining (in favor of diesel [54]) limits production of aromatic by-products, the
Fig. 6 (PCyP)Ir(H)
(Cl) complex reported by
Wendt [48]
Fig. 7 CCC iridium
complexes reported by
Braunstein [49, 50]
Transfer Dehydrogenations of Alkanes and Related Reactions Using Iridium. . .
195