Reductive Deamination with Hydrosilanes Catalyzed by B(C6F5)3

Article abstract of DOI:10.1002/anie.202004651

The strong boron Lewis acid tris(pentafluorophenyl)borane B(C₆F₅)₃ is known to catalyze the dehydrogenative coupling of certain amines and hydrosilanes at elevated temperatures. At higher temperature, the dehydrogenation pathway competes with cleavage of the C?N bond and defunctionalization is obtained. This can be turned into a useful methodology for the transition-metal-free reductive deamination of a broad range of amines as well as heterocumulenes such as an isocyanate and an isothiocyanate.

Full text of DOI:10.1002/anie.202004651

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Accepted Article
Title: Reductive Deamination with Hydrosilanes Catalyzed by
B(C6F5)3
Authors: Huaquan Fang and Martin Oestreich
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10.1002/anie.202004651
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COMMUNICATION
Reductive Deamination with Hydrosilanes Catalyzed by B(C6F5)3
Huaquan Fang and Martin Oestreich*
Abstract:
The
strong
boron
Lewis
acid
tris(pentafluorophenyl)borane B(C6F5)3 is known to catalyze the
dehydrogenative coupling of certain amines and hydrosilanes at
elevated temperatures. At higher temperature, the dehydrogenation
pathway competes with cleavage of the C–N bond and
defunctionalization is obtained. This can be turned into a useful
methodology for the transition-metal-free reductive deamination of a
broad range of amines as well as heterocumulenes such as an
isocyanate and an isothiocyanate.
Selective defunctionalization of synthetic intermediates is a
key strategy in organic synthesis.[1] Reductive alcohol
deoxygenation is an important example of it[2] and can be
achieved by the reaction of alcohols or derivatives thereof and
hydrosilanes in the presence of catalytic amounts of B(C6F5)3
(Scheme 1, top left).[3] The ionic mechanism involves the
thermodynamically favorable formation of Si–O bonds and
[HB(C6F5)3]–,[4] and these reactions are ultimately borohydride
reductions. Employing various boron Lewis acid/hydrosilane
combinations, Gagné showcased the impressive chemo- and
regioselectivity of this methodology when applied to complex
molecules.[5] Morandi[6] and our laboratory[7] also contributed to
this field. We asked ourselves whether reductive deamination of
1°, 2°, and 3° amines would be possible in the same manner
Scheme 1. B(C6F5)3-catalyzed reductive defunctionalization with hydrosilanes.
R1, R2, and R = alkyl or aryl; n = 1–3. HMPA = hexamethylphosphoric triamide.
We began our investigation with optimizing the reductive
deamination of 1-(naphthalen-2-yl)ethan-1-amine (1aa → 2a;
Table 1). The reaction of 1aa with 4.0 equiv of PhSiH3 in the
presence of 20 mol% of B(C6F5)3 in benzene at 120 °C furnished
β-ethylnaphthalene (2a) in 48% yield after 24 h (entry 1). Among
several solvents screened, 1,2-C6H4F2 was the best (entries 2–
8). The high reaction temperature of 120 °C was essential, and
there was hardly any conversion at 80 °C (entry 9). Lower
catalyst loadings were not detrimental, and 5.0 mol% of B(C6F5)3
was sufficient to reach quantitative yield (entries 10–12). For this,
the reaction time of 24 h was needed (see the Supporting
Information for a yield/time analysis). Less PhSiH3 resulted in a
lower yield (entry 13). Using MePhSiH2 or Me2PhSiH instead of
PhSiH3 as reductants also gave excellent yields (entries 14 and
15). Other tertiary hydrosilanes were however too unreactive
under these reaction conditions (see the Supporting Information
for details). In the case of Me2PhSiH the byproduct
bis(dimethyl(phenyl)silyl)amine was detected in 65% yield,
suggesting the intermediacy of the aforementioned disilazane.
(Scheme 1, top right). Based on a 45-year-old report on the
[8]
reductive cleavage of disulfonimides with NaBH4
,
we
anticipated that in-situ generation of disilazanes by
dehydrogenative Si–N coupling[9] followed by another reaction
with
a B(C6F5)3/hydrosilane pair would also result in the
displacement of the amino group (Scheme 1, bottom). Exisiting
protocols for reductive deamination either rely on preceding
conversion of the amino group into a better leaving group
followed by hydride substitution[8,10] or on transition-metal-
catalyzed hydrogenolysis.[11] We disclose here an alternative to
these methods, thereby expanding the toolbox of B(C6F5)3
chemistry.[12]
Table 1. Selected examples of the optimization of the B(C6F5)3-catalyzed
reductive deamination.[a]
B(C6F5)3 (catalytic)
NH2
Me
H
R4–nSiHn (excess)
Me
solvent
for 24 h
1aa
2a
Entry
B(C6F5)3
[mol%]
Hydrosilane
[equiv]
Solvent
T
[°C]
Yield
[%][b]
[*]
Dr. H. Fang, Prof. Dr. M. Oestreich
Institut für Chemie, Technische Universität Berlin
Strasse des 17. Juni 115, 10623 Berlin (Germany)
E-mail: martin.oestreich@tu-berlin.de
1
2
3
20
20
20
PhSiH3 (4.0)
PhSiH3 (4.0)
PhSiH3 (4.0)
benzene
toluene
120
120
120
48
47
Homepage: http://www.organometallics.tu-berlin.de
Supporting information for this article is given via a link at the end of
the document.
1,4-xylene
52
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10.1002/anie.202004651
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COMMUNICATION
B(C6F5)3 (5.0 mol%)
PhSiH3 (4.0 equiv)
NRR'
H
4
20
20
20
20
20
20
10
5.0
2.5
5.0
20
20
PhSiH3 (4.0)
PhSiH3 (4.0)
PhSiH3 (4.0)
PhSiH3 (4.0)
PhSiH3 (4.0)
PhSiH3 (4.0)
PhSiH3 (4.0)
PhSiH3 (4.0)
PhSiH3 (4.0)
PhSiH3 (2.0)
MePhSiH2 (4.0)
Me2PhSiH (4.0)
C6H5CF3
120
120
120
120
120
80
57
R2
R2
R1
R1
1,2-C6H4F2
120 °C for 24 h
R3
R3
5
C6H5F
58
1
2
primary (R = R' = H) secondary (R = Alkyl, R' = H) tertiary (R = R' = Alkyl)
6
C6H5Cl
59
primary amines
7
1,2-C6H4Cl2
1,2-C6H4F2
1,2-C6H4F2
1,2-C6H4F2
1,2-C6H4F2
1,2-C6H4F2
1,2-C6H4F2
1,2-C6H4F2
1,2-C6H4F2
91
NH2
Me
NH2
Me
NH2
X
8
98
n
9
1
2d (X = H): 87%[b]
1aa
99% (94%)[a]
1ba 2b ( ):
99% (91%)
2a ( ):
1pa
1qa
2p (n = 1):
1da
1ea
1fa
99%
2e (X = 4-Ph): 99% (96%)
2f (X = 2-Me): 93%
2g (X = 3-Me): 83%
2h (X = 4-Me): 96%
2i (X = 4-F): 82%[c]
2j (X = 4-Cl): 43%
2k (X = 4-Br): 62%[b]
2l (X = 4-I): 62%[d]
2h (X = 4-CO2Me): 62%[c]
2n (X = 4-OMe): 94%[d,e]
2o (X = 4-CF3): 3%[c]
2q (n = 2):
99%
10
11
12
13
14
15
120
120
120
120
120
120
98
1ga
1ha
1ia
99 (92)
89
NH2
Me
NH2
Ph
O
1ja
1ca
1sa
1ua
2c:
63%
1ra
2r:
58
1ka
1la
79% (74%)[c]
NH2
NH2
iPr
99
1ma
1na
1oa
tBu
96
2s:
1ta
2t:
[a] All reactions were performed on a 0.10-0.20 mmol scale in a sealed tube.
[b] Yields determined by NMR spectroscopy using mesitylene as an internal
standard; isolated yield in parenthesis.
67%[d]
48%[d,f]
NH2
NH2
NH2
F
NH2
Me
Ph
Me
Ph
Ph
Me
The optimized setup was then applied to mainly benzylic
amine derivatives (Scheme 2). Aside from model compound 1aa
with a β-naphthyl group, an α-naphthyl and a benzofuran-5-yl
substituent as in 1ba and 1ca at the reactive site also worked.
The functional-group tolerance was assessed with 1-
phenylethan-1-amine derivatives 1da–oa decorated with various
electron-donating or -withdrawing substituents on the aryl group
(gray box). Yields were generally moderate to good, and all halo
groups were compatible (1ia–la). Substrate 1ma bearing a
methyl ester underwent exhaustive defunctionalization to yield
2h rather than 2m with the carboxy group fully reduced to a
methyl substituent. Similarly, the deamination of 1na containing
a methyl ether resulted in demethylation[3b] but when employing
Me2PhSiH instead of PhSiH3 the methoxy group remained intact,
affording 2n in 94% yield along with 4-methoxystyrene in 6%
yield (β-elimination). In turn, trifluoromethyl-substituted substrate
1oa was too unreactive, and 2o and 2h were obtained in 3%
and 5% yields, respectively. The opposed reactivities of 1na (X
= 4-OMe) and 1oa (X = 4-CF3) already indicate that this
Me
2u:
1va
2v:
1wa
2w:
1xa
2x:
99% (89%)
99% (90%)[c]
62%[d]
81%[d]
secondary amines
NH2
NH2
NHMe
NHBn
Me
Me
1ya
2y:
1za
2z:
83% (78%)[c]
41% (26%)[c]
1db
2d:
1dc
2d:
97%[d,g]
95%[d,g]
tertiary amines
NMe2
Me
NMe2
Me
NMe2
Me
NEt2
Ph
N
Ph
Me
1a'd
56%[h,i]
1dd
2d:
1ed
2e:
2a':
1ue
2u:
98%[h]
75%[d,h]
81%[h]
Scheme 2. Scope I: B(C6F5)3-catalyzed reductive deamination of 1°, 2°, and
3° amines. [a] 5.0 mmol scale. [b] B(C6F5)3 (20 mol%) was used. [c] B(C6F5)3
(10 mol%) was used. [d] B(C6F5)3 (30 mol%) was used. [e] Me2PhSiH (4.0
equiv) was used. [f] 19% of (3-methylbutan-2-yl)benzene was formed. [g]
PhSiH3 (3.0 equiv) was used. [h] PhSiH3 (2.0 equiv) was used. [i] 22% of 1-
methyl-3-ethylindoline was formed.
reductive deamination may involve the intermediacy of
a
benzylic carbocation. This was further substantiated by another
observation. Benzylic amine 1ta with an α-tert-butyl group gave
The reductive deamination was successfully extended to
secondary and tertiary benzylic amines (Scheme 2, lower). The
yields were generally in range of those obtained for the
corresponding primary amines. The deamination of 1a’d
afforded the desired indole derivative 2a’ in 56% yield but the
corresponding indoline was also formed in 22% yield. Two drug
molecules were subjected to the reductive deamination (Scheme
3): The antidepressant Sertraline (1b’b) was degraded to tetralin
2b’ in 87% yield and the antihistamine Meclizine (1c’f) furnished
1-benzyl-4-chlorobenzene (2c’) in 99% yield.
the expected hydrocarbon 2t in 48% yield; however,
a
rearranged product generated by a [1,2]-migration of one of the
methyl groups of the tert-butyl residue to the assumed benzylic
carbocation did form in 19% yield. Other α-alkyl- and α-aryl-
substituted benzylic amines 1ra, 1sa, and 1ua reacted in good
and high yields, respectively. Primary amines 1va–ya with a fully
substituted α-carbon atom (benzylic or aliphatic) participated
equally well; these transformations are likely to pass through
tertiary carbenium ions as intermediates. Conversely, a simple
primary amine such as 1za furnished α-methylnaphthalene (2z)
in mediocre yield.
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10.1002/anie.202004651
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COMMUNICATION
NHMe
H
-substitution: competition experiments
NH2
+
NH2
Me
NH2
B(C6F5)3 (30 mol%)
PhSiH3 (3.0 equiv)
Me
Me
+
1,2-C6H4F2
120 °C for 24 h
1d'a
1da
1wa
Cl
Cl
B(C6F5)3 (30 mol%)
1,2-C6H4F2
Cl
Cl
1b'b (Sertraline)
2b': 87% (85%)
PhSiH3 (x equiv) 120 °C for 24 h
H
H
H
Me
Me
Me
+
+
Me
H
B(C6F5)3 (30 mol%)
N
PhSiH3 (2.0 equiv)
x
2d':
none
none
2d:
2w:
1,2-C6H4F2
120 °C for 24 h
N
3.0
6.0
traces
90%
42%
68%
Cl
2c': 99% (97%)
Cl
1° versus 2° versus 3°: reactivity assessment
1c'f (Meclizine)
B(C6F5)3 (30 mol%)
NH2–nMen
n
0
1
2
120 °C 80 °C
H
PhSiH3 (4–n equiv)
73%
97%
75%
5%
Scheme 3. Scope II: B(C6F5)3-catalyzed reductive deamination for the
degradation of drug molecules.
Me
Me
94%
12%
1,2-C6H4F2
for 24 h
1da (n = 0)
2d
1db (n = 1)
1dd (n = 2)
To assess the relative reactivity of the different types of
amines, we conducted several control experiments in the
presence of 30 mol% of B(C6F5)3 (Scheme 4). We reacted 1.0
equiv of an equimolar mixture of benzylamines with different
degrees of substitution at the α-carbon atom with 3.0 and 6.0
equiv of PhSiH3 (top). With 3.0 equiv, cumene (2w) formed
exclusively in 42% yield; increasing the amount of the
trihydrosilane to 6.0 equiv, 2w was formed next to ethylbenzene
(2d) in yields of 68% and 90%, respectively. The formation of
toluene (2d’) was not detected. These results show that a
primary amine with a tertiary alkyl group is more reactive than
those with secondary and primary alkyl groups. This trend is in
accordance with the stability of their corresponding benzylic
carbocations.[13] Reactions were routinely run at 120 °C (cf.
Table 1) where there was little dependence of conversion on the
number of substituents at the nitrogen atoms in benzylic amine
derivatives 1d (bottom). However, when performing the same
set of reactions at 80 °C, the reductive deamination was
reasonably chemoselective in favor of the secondary amine 1db.
Hence, 1db is more reactive than the tertiary amine 1dd and the
parent compound 1da in the B(C6F5)3-catalyzed reductive
deamination.
Scheme 4. Reactivity study.
To gain further insight into the reaction mechanism, several
stoichiometric experiments were performed (Scheme 5).[14] As
part of the reaction optimization we had already found that 24 h
is necessary to achieve full conversion of the primary amine 1aa.
Mixing this amine and B(C6F5)3 in equimolar ratio led to
quantitative formation of the Lewis adduct 3aa within 10 min at
room temperature (top left). The subsequent reaction of 3aa with
o
PhSiH3 (4.0 equiv) at 80 C generated a new Lewis pair 4aa in
90% yield after 8 h; no further reaction of 3aa and PhSiH3
occurred at room temperature (top right). 4aa is formally
composed of amine 1aa and Piers’ borane HB(C6F5)2 and can
also be synthesized in quantitative yield within a few minutes at
room temperature by the reaction of 1aa and 1.0 equiv of
HB(C6F5)2 (not shown). Apparently, B(C6F5)3 and PhSiH3
underwent the known exhange of substituents,[15] and the
formation of C6F5(Ph)SiH2 (5) was confirmed spectroscopically;
the 86% yield of 5 was estimated by 19F NMR spectroscopy (see
the Supporting Information for details). The deaminated product
2a did only form in 2% yield, and we showed that 20 mol% of
Piers’ borane cannot promote the deamination of 1aa at 120 °C
with 4.0 equiv PhSiH3 as the reductant (not shown). The
degradation of the prevalent Lewis pair 1aaꞏB(C6F5)3 by excess
trihydrosilane to catalytically inactive 1aaꞏHB(C6F5)2 therefore
causes catalyst deactivation and in part explains the long
reaction times at relatively high catalyst loadings of 5.0 mol% or
more. Premixing 1aa and 4.0 equiv of PhSiH3 followed by the
addition of 1.0 equiv of B(C6F5)3 again generated Lewis pair 3aa
in 86% yield along with its FLP-type adduct with PhSiH3 in
approximately 10% yield (bottom). Silylammonium borohydrides
such as 6aa are kinetically stable and do not rapidly release
dihydrogen[9a,16] which is in agreement with the high reaction
temperature. Heating this reaction mixture at 80 °C for 8 h brings
about the same outcome as the reverse order of addition. We
actually believe that the dehydrogenative Si–N coupling of the
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10.1002/anie.202004651
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COMMUNICATION
bissilylammonium borohydride intermediate is in competition
with its dissociation into the corresponding benzylic carbocation
and disilazane (Scheme 6).[17] The carbocation is then captured
by the borohydride to afford the defunctionalized product.[18]
However, the hydride source cannot be designated with
certainty because trihydrosilane can also deliver one of its
hydrides as verified in a control experiment. The reductive
deamination can be initiated with the trityl cation; for example,
20 mol% of Ph3C+[B(C6F5)4]– converted amine 1aa into
hydrocarbon 2a in 42% yield under otherwise identical reaction
conditions (4.0 equiv of PhSiH3 in 1,2-C6H4F2 at 120 °C for 24 h;
not shown).
protocol (Scheme 7, left). An imine such as 10 does also react
(Scheme 7, right). Application of this methodology in organic
synthesis is currently ongoing in our laboratory.
stoichiometric experiment with stepwise addition: B(C6F5)3 first
H
NH2
H
B(C6F5)3
B(C6F5)3 (1.0 equiv)
Scheme 7. Scope III: B(C6F5)3-catalyzed reductive C–N bond cleavage in
heterocumulenes and in an imine. [a] B(C6F5)3 (20 mol%) was used.
N
1,2-C6H4Cl2
RT for 10 min
Ar
Me
1aa
Ar
Me
3aa: quant.
H
H
C6F5
H
Acknowledgements
PhSiH3 (4.0 equiv)
H
B(C6F5)2
N
+
+
Si
H
1,2-C6H4Cl2
80 °C for 8 h
(no reaction at RT)
Ph
Ar
Me
H
Ar
Me
4aa: 90%
H.F. gratefully acknowledges the Alexander von Humboldt
Foundation for a postdoctoral fellowship (2018–2020). M.O. is
indebted to the Einstein Foundation (Berlin) for an endowed
professorship.
5: 86%
2a: 2%
stoichiometric experiments with stepwise addition: PhSiH3 first
PhSiH3 (x equiv)
then
H
N
H
N
NH2
H
B(C6F5)3
H
SiPhH2
[HB(C6F5)3]–
H
B(C6F5)3 (1.0 equiv)
+
+
1,2-C6H4Cl2
RT for 10 min
x
Ar
Me
1aa
Ar
Me
Keywords: amines • boron • defunctionalization • reduction •
Ar
Me
3aa:
86%
52%
32%
Ar
Me
silanes
6aa:
10%
30%
32%
2a:
2%
4.0
10
20
12%
17%
[1]
[2]
[3]
a) S. W. McCombie in Comprehensive Organic Synthesis, Vol. 8 (Eds.:
B. M. Trost, I. Fleming), Pergamon, Oxford, 1991, pp. 811–833; b) A.
Modak, D. Maiti, Org. Biomol. Chem. 2016, 14, 21–35.
H
C6F5
H
N
H
for x = 4.0
H
B(C6F5)2
For reviews, see: a) P. J. Deuss, K. Barta, Coord. Chem. Rev. 2016,
306, 510–532; b) J. M. Herrmann, B. Konig, Eur. J. Org. Chem. 2013,
7017–7027; c) W. Hartwig, Tetrahedron 1983, 39, 2609–2645.
a) V. Gevorgyan, J.-X. Liu, M. Rubin, S. Benson, Y. Yamamoto,
Tetrahedron Lett. 1999, 40, 8919–8922; b) V. Gevorgyan, M. Rubin, S.
Benson, J.-X. Liu, Y. Yamamoto, J. Org. Chem. 2000, 65, 6179–6186;
c) R. D. Nimmagadda, C. McRae, Tetrahedron Lett. 2006, 47, 5755–
5758; d) W. Yang, L. Gao, J. Lu, Z. Song, Chem. Commun. 2018, 54,
4834–4837.
+
+
Si
H
1,2-C6H4Cl2
80 °C for 8 h
Ph
Ar
Me
H
Ar
Me
4aa: 90%
5: 79%
2a: 8%
Scheme 5. Stoichiometric experiments with stepwise addition of the reactants
(Ar = β-naphthyl).
[4]
a) D. J. Parks, J. M. Blackwell, W. E. Piers, J. Org. Chem. 2000, 65,
3090–3098; b) S. Rendler, M. Oestreich, Angew. Chem. Int. Ed. 2008,
47, 5997–6000; Angew. Chem. 2008, 120, 6086–6089; c) K. Sakata, H.
Fujimoto, J. Org. Chem. 2013, 78, 12505–12512; d) A. Y. Houghton, J.
Hurmalainen, A. Mansikkamäki, W. E. Piers, H. M. Tuononen, Nature
Chem. 2014, 6, 983–988; e) T. Fallon, M. Oestreich, Angew. Chem. Int.
Ed. 2015, 54, 12488–12491; Angew. Chem. 2015, 127, 12666–12670.
a) L. L. Adduci, T. A. Bender, J. A. Dabrowski, M. R. Gagné, Nat. Chem.
2015, 7, 576–581; b) T. A. Bender, J. A. Dabrowski, M. R Gagné, ACS
Catal. 2016, 6, 8399‒8403; c) T. A. Bender, P. R. Payne, M. R. Gagné,
Nat. Chem. 2018, 10, 85–90; d) Y. Seo, M. R. Gagné, ACS Catal. 2018,
8, 81–85.
[5]
[6]
Scheme 6. Mechanistic proposal for disilazane as leaving group
(monosilazane omitted for clarity and trisilazane not considered for steric
reasons): competing dehydrogenation (right) and dissociation pathways (left).
a) N. Drosos, B. Morandi, Angew. Chem. Int. Ed. 2015, 54, 8814–8818;
Angew. Chem. 2015, 127, 8938–8942; b) N. Drosos, G.-J. Cheng, E.
Ozkal, B. Cacherat, W. Thiel, B. Morandi, Angew. Chem. Int. Ed. 2017,
56, 13377–13381; Angew. Chem. 2017, 129, 13562–13566; c) G.-J.
Cheng, N. Drosos, B. Morandi, W. Thiel, ACS Catal. 2018, 8, 1697–
1702.
In summary, we have developed a B(C6F5)3-catalyzed,
efficient reductive deamination of 1°, 2°, and 3° mainly benzylic
amines with hydrosilanes as the stoichiometric reducing agent.
The method extends to other C–N bonds, and an isocyanate,
isothiocyanate, and thionyl imide such as 7–9 were shown to
undergo the defunctionalization using the same standard
[7]
a) I. Chatterjee, D. Porwal, M. Oestreich, Angew. Chem. Int. Ed. 2017,
56, 3389–3391; Angew. Chem. 2017, 129, 3438–3441; see also: b) S.
C. Richter, M. Oestreich, Chem. Eur. J. 2019, 25, 8508–8512.
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10.1002/anie.202004651
Angewandte Chemie International Edition
COMMUNICATION
[8]
[9]
R. O. Hutchins, F. Cistone, B. Goldsmith, P. Heuman, J. Org. Chem.
1975, 40, 2018–2020.
a) J. Hermeke, M. Mewald, M. Oestreich, J. Am. Chem. Soc. 2013, 135,
17537–17546; see also: b) L. Greb, S. Tamke, J. Paradies, Chem.
Commun. 2014, 50, 2318–2320 (aniline derivatives and nitrogen
heterocycles).
[10] Selected examples of transformations of the amino group into a better
leaving group followed by hydride substitution, see: a) A. Nickon, A.
Sinz, J. Am. Chem. Soc. 1960, 82, 753–754; b) C. L. Bumgardner, K. J.
Martin, J. P. Freeman, J. Am. Chem. Soc. 1963, 85, 97–99; c) A.
Nickon, A. S. Hill, J. Am. Chem. Soc. 1964, 86, 1152–1158; d) G.
Doldouras, J. Kollonitsch, J. Am. Chem. Soc. 1978, 100, 341–342; e) A.
R. Katritzky, K. Horvath, B. Plau, J. Chem. Soc., Perkin Trans. 1 1980,
2554–2560; f) C. L. Bumgardner, V. R. Desai, J. Fluorine Chem. 1987,
36, 307–312; for a short summary, see: g) T. Wirth, C. Rüchardt,
Chimia 1988, 42, 230–231.
[11] Selected examples of transition-metal-catalyzed hydrogenolysis, see:
a) W. F. Maier, P. Grubmüller, I. Thies, P. M. Stein, M. A. McKervey, P.
von Ragué Schleyer, Angew. Chem. Int. Ed. 1979, 18, 939–940;
Angew. Chem. 1979, 91, 1004–1005; b) M. J. Guttieri, W. F. Maier, J.
Org. Chem. 1984, 49, 2875–2880; c) S. Lemaire-Audoire, M. Savignac,
J. P. Genêt, J.-M. Bernard, Tetrahedron Lett. 1995, 36, 1267–1270; d)
M. Kanai, M. Yasumoto, Y. Kuriyama, K. Inomiya, Y. Katsuhara, K.
Higashiyama, A. Ishii, Org. Lett. 2003, 5, 1007–1010; e) T. C. Nugent,
D. E. Negru, M. El-Shazly, D. Hu, A. Sadiq, A. Bibi, M. N. Umar, Adv.
Synth. Catal. 2011, 353, 2085–2092; f) A. J. Yap, B. Chan, A. K. L.
Yuen, A. J. Ward, A. F. Masters, T. Maschmeyer, ChemCatChem 2011,
3, 1496–1502.
[12] a) M. Oestreich, J. Hermeke, J. Mohr, Chem. Soc. Rev. 2015, 44,
2202–2220; see also: b) D. Weber, M. R. Gagné in Organosilicon
Chemistry: Novel Approaches and Reactions (Eds.: T. Hiyama, M.
Oestreich), Wiley-VCH, Weinheim, 2019, pp. 33–85.
[13] This order of reactivity is generally opposite to that of the
deoxygenation where tertiary alcohols do not react[3b] or do not fully
convert[3c,3d] into the hydrocarbon. However, trityl alcohol underwent
smooth deoxygenation,[3b] likely following an SN1 mechanism. The
formation of stabilized carbenium ions as intermediates is crucial in the
deamination as disilazanes are poorer leaving groups than disiloxanes.
Hence, less substituted oxonium ions can also participate in SN2
displacements with the borohydride while related ammonium ions do
not, even at 120 °C.
[14] 1,2-C6D4Cl2 was used instead of 1,2-C6D4F2 as we did not have access
to this deuterated solvent.
[15] D. J. Parks, W. E. Piers, G. P. A. Yap, Organometallics 1998, 17,
5492–5503.
[16] V. Sumerin, F. Schulz, M. Nieger, M. Leskelä, T. Repo, B. Rieger,
Angew. Chem. Int. Ed. 2008, 47, 6001–6003; Angew. Chem. 2008, 120,
6090–6092.
[17] Si–N coupling is observed with substrates less likely to form carbenium
ions.[13] For example, the corresponding bissilylamine was found as the
major product in the reaction of 1-cyclohexylethan-1-amine and PhSiH3
(cf. Ref. [9a]).
[18] I. Chatterjee, Z.-W. Qu, S. Grimme, M. Oestreich, Angew. Chem. Int.
Ed. 2015, 54, 12158–12162; Angew. Chem. 2015, 127, 12326–12330.
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10.1002/anie.202004651
Angewandte Chemie International Edition
COMMUNICATION
Suggestion for the Entry for the Table of Contents
COMMUNICATION
H. Fang, M. Oestreich*
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Reductive Deamination with
Hydrosilanes Catalyzed by B(C6F5)3
I mean defunct. Combinations of the
boron Lewis acid B(C6F5)3 and
hydrosilanes enable the reductive
cleavage of C–N bonds in various,
mainly
benzylic
amines
and
heterocumulenes. By this, these
functionalized substrates can be
converted into the corresponding
hydrocarbons in moderate to good
yields.
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