and the carbene carbon. The buried volume (% VBur
)
8,9 of
Phosphines and NHC ligands play a crucial role in Au-
catalyzed reactions where very different reactivities as well
as selectivities were often observed depending on ligands.16
Thus, we decided to evaluate new ADC-gold complexes
5a and 5b in representative Au-catalyzed reactions to gain
insights into their properties. First, Au(I)-catalyzed intramo-
lecular hydroamination was investigated (Table 2).17 The
5a (44.3%) is larger than that of similarly substituted
NHC-Au complex 5f (39.3%), possibly owing to the larger
N-Ccarbene-N angle of 5a (115.0(2)°) than that of 5f
(107.8(2)°) (Figure 3). Importantly, the bulkier adamantyl
Table 2. Au(I)-Catalyzed Hydroamination of Alkenyl Ureaa
entry
catalyst
% VBur time (h) % convnb
1
2
3
4
5
6
7
tBu2Ph2ADC-AuCl (5a)
Adm2Ph2ADC-AuCl (5b) 45.7
Pyrr2ADC-AuCl (5c)
Et2tBuHADC-AuCl (5d)
tBu2H2ADC-AuCl (5e)
ItBu-AuCl (5f)
44.3
24
24
24
24
24
24
16
82
87c
<2
<2
3
3
>98
Figure 3. % VBur of ADC and NHC ligands in Au complexes
29.5
36.0
39.2
39.3
45.6
(calculated using X-ray crystallographic data).
IPr-AuCl (5g)
substituent further increases % VBur of 5b to 45.7%, which
constitutes one of the highest among the reported ADC
ligands4 and is comparable to that of bulky NHCs such as
IPr (45.6%).
a 0.033 M concd [LAuCl] and AgOTf in dioxane were stirred for 30
min before addition of 7. b Determined by 1H NMR. c >98% conversion
observed after 48 h.
Electron-donating capacities of the new ADCs were
estimated by density functional theory (DFT) calculations
(Table 1). The energies of the carbene lone pair orbital (Eσ)
reaction proceeded smoothly at room temperature when
catalyzed by ADC-Au catalyst 5a (entry 1). The bulkier
adamantyl catalyst 5b gave better conversion after 24 h (entry
2 vs entry 1), whereas less bulky ADC catalysts 5c and 5d
Table 1. Energies of Carbene Lone Pair Orbitals, Eσ [eV]
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were computed, after an initial geometry optimization, using
the B3LYP,10 TPSSTPSS,11 M06,12 and M06L13 functional
and the 6-31G** basis set14 on carbenes 6a-d and 6f,g.
These calculations were run using GAUSSIAN 09.15 The
ADCs (6a-d) have higher Eσ values than the NHCs (6f,g),
suggesting that they are better σ-donors. The same trends
were observed across the carbenes for all four functionals.
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