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ACS Catalysis
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catalysts (Table 1, entries 7 and 8). In these latter cases, the fluoride
anion is indeed not naked, and hence less reactive.
(Table 1, entries 1 and 11), and the latter was hence selected as a
catalyst in the mechanism depicted in Scheme 2. The role of the
fluoride anion (or alkoxides) as a precatalyst (mechanism A) or as
a catalyst (mechanism B) is also explored, thereafter.
1
2
3
4
5
6
7
8
Disilanes can also be activated by non-fluorinated bases such as
alkoxides.24 In fact, both KOMe and KOtBu catalyzed the reaction,
leading to full conversion of N2O within 1 h and 2 h, respectively
(Table 1, entries 9 and 10). At this stage, KOMe and KOtBu could
act as precatalysts for the generation of a silanolate species.21a,c
This hypothesis was validated by the successful use of KOSiMe3
as a catalyst (Table 1, entry 11).
Table 2. Screening of disilanes for the reduction of N2O[a]
CsF (10 mol%)
N2O + R3SiSiR3
N2 + R3SiOSiR3
DMSO-d6, 20 °C
Entry
R3SiSiR3
Yield[b]
Final yield[b]
N2/N2O[c]
Interestingly, no reaction was detected in either THF or MeCN,
even when 18-crown-6 was added to help solubilize CsF. In
contrast, when a DMSO/THF 1:1 mixture was used as solvent, the
reaction proceeded but required 48 h to reach full conversion,
showing a detrimental effect of THF (Table S2). This observation
was ascribed to a lower solubility of both the fluorinated base and
N2O in less polar solvents.
after 1 h
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
1
2
3[d]
(Me3Si)2 (1)
(PhMe2Si)2 (3)
(Ph2MeSi)2 (4)
55%
63%
68%
77% (4 h)
93% (3 h)
93% (2 h)
99:1
100:0
100:0
[a] Conditions: N2O (1 bar, ca. 2 mL), R3SiSiR3 (0.12 mmol), catalyst
(12 µmol, 10 mol%), DMSO-d6 (0.5 mL). [b] Yields measured by GC-
analysis (disilane 1) or by NMR-analysis (compounds 3 and 4). Internal
standard: 1,3,5-trimethoxybenzene. [c] Corrected GC-ratios. Residual
amount of N2 from the atmosphere <10%. [d] To overcome the poor
solubility of the starting material in DMSO, the reaction mixture was stirred
2 h at 20 °C under argon prior to adding N2O. See SI for more details (p.
S11).
Table 1. Catalyst screening for the reduction of N2O with disilane
1.[a]
cat
(10 mol%)
N2O + Me3SiSiMe3
N2 + Me3SiOSiMe3
DMSO-d6, 20 °C
1
2
The silanolate Me3SiO– (I) can react either with N2O or with
(Me3Si)2 (1). Whereas the activation barrier of the reaction with
N2O was high in energy (ΔG‡= +44.4 kcal.mol-1, Figure S10), the
nucleophilic addition of silanolate I to disilane 1 appeared more
favorable (ΔG‡= +18.5 kcal.mol-1). After release of the siloxane
(Me3Si)2O (2), it generates a highly nucleophilic Me3Si– anion
(III), in an overall exergonic sequence (ΔG= –4.7 kcal.mol-1)
(Figure 1c). This result is supported by the observation in NMR of
the instantaneous formation of Me3SiOSiMe3 (2) when KOSiMe3
and disilane 1 are mixed (Figure S7).
Entry
Cat.
Time
Conversion
Yield in
N2/N2O
[c]
of 1 (%)[b]
2 (%)[b]
1
2
CsF
-
4 h
3 days
24 h
24 h
24 h
1 h
92
0
77
0
99:1
5:95
3
LiF
12
8
0
10:90
5:95
4
NaF
0
5
KF
2
0
7:93
6
TMAF
KHF2
TBAT
KOMe
KOtBu
KOSiMe3
100
35
28
98
92
86
76
2
97:3
Me3SiOSiMe3
X
Me3SiSiMe3
2
1
7
8[d]
24 h
24 h
1 h
25:75[e]
30:70
100:0[e]
100:0[e]
90:10[e]
B
X
as catalyst ?
11
85
62
80
Me3SiX
Me3SiX
9
(Me3Si)2
Me3SiOSiMe3
1
2
10
11
2 h
4 h
A
SiMe3
Me3SiO
[a] Conditions: N2O (1 bar, ≈2 mL), (Me3Si)2 (0.12 mmol), catalyst
(12 µmol, 10 mol%), DMSO-d6 (0.5 mL). [b] Conversions and yields
measured by GC-MS analysis. Internal standard: 1,3,5-
trimethoxybenzene. The difference between conversion and yield is
ascribed to the volatility of the silylated compounds and to the
production of Me3SiF, Me3SiOMe or Me3SiOtBu in the course of the
reaction. [c] Corrected GC-ratios. Residual amount of N2 from the
atmosphere <10% [d] Scrambling of R3Si residues: Ph3SiF and
Ph3SiOSiMe3 are detectable in GC-MS. [e] H2 is also detected in GC
(<2%). See SI for more details (p. S7).
X
as precatalyst
I
III
bond
NO
cleavage ?
selectivity ?
N2O
N2
[Me3SiN2O]
X = F, OR
IV
Scheme 2. Two plausible mechanisms: with the fluoride anion or
the alkoxides as precatalysts (A) or as catalysts (B).
In contrast to silanolate I, silyl anion III efficiently reacts with
N2O via a nucleophilic attack. Considering that the two degenerate
LUMOs of N2O are mostly developed on the two nitrogen atoms
(Figure 1a), the nucleophilic attack can occur either on the central
or the terminal nitrogen centers. Computational results show a
significant difference in the activation barrier energy of both
pathways: the nucleophilic attack of Me3Si– (III) on the central
nitrogen is kinetically favored by 10.8 kcal.mol-1 (TS3 and TS’3)
and, under the applied conditions (20 °C), the attack on the terminal
N atom is hardly accessible (ΔG‡=+26.2 kcal.mol-1). As such, the
less stable Me3Si(N2O)– intermediate IV is the major product. Little
is known about the ambivalent reactivity of N2O as an electrophile.
According to DFT calculations, metal-hydride complexes attack
preferentially at the terminal nitrogen.25 Likewise, non-metallic
systems, such as N-heterocyclic carbenes reported by Severin et
al.26 or frustrated Lewis pairs,27 have been experimentally shown
to react via the terminal nitrogen of N2O. The selectivity observed
To test the influence of the stereoelectronic properties of the
silane reductant, the deoxygenation of N2O was carried out with
(PhMe2Si)2 (3) and (Ph2MeSi)2 (4) (Table 2, entries 2 and 3).
Interestingly, the more Lewis acidic the disilane was, the faster the
reaction proceeded, as reflected in the yields in the corresponding
siloxanes, after 1 h, of 55%, 63% and 68%, for disilanes 1, 3 and 4,
respectively (Table 2). In contrast, neither (Bpin)2 nor PhMe2Si–
Bpin proved successful under these conditions.
Unfortunately, no intermediate could be either isolated or
observed by NMR in the course of the reaction or through
stoichiometric reactions. Hence DFT calculations were used (for
computational details see SI, part V), to track how the N–O bond is
cleaved under mild and metal-free conditions to promote the
reduction of the kinetically stable N2O molecule. According to the
experimental results, CsF and KOSiMe3 show a similar activity
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