Autocatalytic Carbonyl Arylation through In Situ Release of Aryl Nucleophiles from N-Aryl-N′-Silyldiazenes

Article abstract of DOI:10.1002/anie.201916004

A method for the catalytic generation of functionalized aryl alkali metals is reported. These highly reactive intermediates are liberated from silyl-protected aryl-substituted diazenes by the action of Lewis basic alkali metal silanolates, resulting in desilylation and loss of N₂. Catalytic quantities of these Lewis bases initiate the transfer of the aryl nucleophile from the diazene to carbonyl and carboxyl compounds with superb functional-group tolerance. The aryl alkali metal can be decorated with electrophilic substituents such as methoxycarbonyl or cyano as well as halogen groups. The synthesis of a previously unknown cyclophane-like [4]arene macrocycle from a 1,3-bisdiazene combined with a 1,4-dialdehyde underlines the potential of the approach.

Full text of DOI:10.1002/anie.201916004

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Accepted Article
Title: Autocatalytic Carbonyl Arylation through In-Situ Release of Aryl
Nucleophiles from N-Aryl-N’-Silyldiazenes
Authors: Clément Chauvier, Lucie Finck, Elisabeth Irran, and Martin
Oestreich
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201916004
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10.1002/anie.201916004
Angewandte Chemie International Edition
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Autocatalytic Carbonyl Arylation through In-Situ Release of Aryl
Nucleophiles from N-Aryl-N’-Silyldiazenes
Clément Chauvier+, Lucie Finck+, Elisabeth Irran, and Martin Oestreich*
Dedicated to Professor Rolf Huisgen on the occasion of his 100th birthday
and to Professor Reinhard Brückner on the occasion of his 65th birthday
Abstract: A method for the catalytic generation of functionalized aryl
alkali metals is reported. These highly reactive intermediates are
liberated from silyl-protected aryl-substituted diazenes by the action
of alkali metal silanolates, resulting in desilylation and loss of N2.
Catalytic quantities of those Lewis bases initiate the transfer of the
aryl nucleophile from the diazene to carbonyl and carboxyl
compounds with superb functional-group tolerance. The aryl alkaline
metal can be decorated with electrophilic substituents such as
methoxycarbonyl or cyano as well as halogen groups. The synthesis
of an unknown cyclophane-like [4]arene macrocycle from a 1,3-
bisdiazene combined with a 1,4-dialdehyde underlines the potential
of the approach.
Eq. 3), yet with no demonstration of its synthetic value and using
NaOMe as an overstoichiometric activator.[10] This seminal
contribution has been largely overlooked and, hence, did not
witness any further development. The present work shows how
this approach can be turned into a catalytic process with
excellent functional-group tolerance in both the diazene and the
carbonyl compound, including transformations of a difunctional
building block (Scheme 1, Eq. 4).
Synthetic chemistry without aryl nucleophiles based on lithium,
magnesium (Grignard), and zinc is not imaginable anymore.[1]
Usual methods of their preparation include reductive metalation
and halogen–metal exchange of aryl halides, and the resulting
polar organometallic reagents can be converted into one another
by transmetalation. These procedures are not always
chemoselective, and the high reactivity of those nucleophiles is
often detrimental to their functional-group tolerance. Especially
Knochel and co-workers provided viable solutions to these
problems, thereby turning polyfunctionalized zinc and Grignard
reagents into everyday chemicals.[2–4]
An alternative to these reactive compounds are easy-to-
handle and storable, less polarized aryl pronucleophiles based
on silicon, mainly their trimethylsilyl derivatives.[5] Aside from the
fact that these are typically accessed from one of the
aforementioned reagents, their fluoride- or alkoxide-promoted
activation for aryl transfer to aldehydes is only applicable to
electron-deficient aryl groups attached to the silicon atom;[6]
even parent Ph–SiMe3 does not react[7,6e] (Scheme 1, Eq. 1).
That gap was closed with the more electrophilic Ph–Si(OMe3)
and Bu4N+F– (TBAF) as the Lewis-basic activator (Scheme 1, Eq.
2).[8] To overcome this limitation, we considered the related
activation of N-aryl-N’-silyldiazenes (Ar–N=N–SiR3) that can be
readily synthesized in two steps from aryl hydrazines with no
need for aryl halides.[9] We envisioned that Lewis base activation
of Ar–N=N–SiR3 could unleash a reactive aryl nucleophile
equivalent by desilylation and denitrogenation. This conceptual
framework was formulated by Bottaro forty years ago (Scheme 1,
Scheme 1. Silicon-based aryl pronucleophiles in transition-metal-free 1,2-
addition to aldehydes (alcohols after hydrolysis). EWG = electron-withdrawing
group; EDG = electron-donating group; FG = functional group; X = aryl
substituent; R = alkyl or aryl group. tBu-P4 = 3-tert-butylimino-1,1,1,5,5,5-
hexakis(dimethylamino)-3-{[tris(dimethylamino)phosphoranylidene]amino}-
1λ5,3λ5,5λ5-1,4-triphosphazadiene
(Schwesinger
base);
TBAF
=
tetrabutylammonium fluoride.
We began our investigation with testing various initiators in
the reaction of 4-tolyl-substituted diazene 1a and benzaldehyde
(2a) in THF (Table 1). Lithium salts such as the alkoxide tBuOLi
[pKa (H2O) ≈ 16.5] and the less basic silanolate Me3SiOLi[11] [pKa
(H2O) ≈ 12.7] both initiated the reaction at room temperature,
affording the silyl ether 3aa in high yields within one hour
(entries 1 and 2). With the same initiator loading of 10 mol%,
improved reaction kinetics were achieved with the sodium and
potassium salt of trimethylsilanol, respectively;[11] full conversion
was reached in less than five minutes accompanied by vigorous
evolution of N2 (entries 3 and 4). The same outcome was
obtained with 5.0 mol% Me3SiONa but yields dwindled with 1.0
[*]
Dr. C. Chauvier,[+] L. Finck,[+] E. Irran, 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
Homepage: http://www.organometallics.tu-berlin.de
[+]
These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201916004
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COMMUNICATION
mol% Me3SiONa or Me3SiOK even at prolonged reactions times
(entries 5–7). These results emphasize the influence of the alkali
metal cation in this reaction. Polar co-solvents such as N-
methylpyrrolidone (NMP) accelerated the already fast reactions.
For completion, we included fluoride sources such as CsF and
anhydrous TMAF into our screening. CsF did promote the aryl
transfer yet at low reaction rate (entry 8) while essentially no
conversion was seen with the poorly soluble ammonium fluoride
(entry 9).
yield. The reaction of sterically more hindered 1c required the
addition of 18-crown-6, and the “more naked” silanolate and
alkoxide intermediate enabled the formation of the silyl ether 3ca
in 68% yield. We also prepared a wide range of Me3Si-
substituted diazenes with functionalized aryl groups (1e–o;
Table 2, entries 4–14). Without exception, these reacted in good
yields under the standard setup. The successful reaction of
electron-rich 1f to silyl ether 3fa closes an important gap (cf.
Scheme 1, top). Further notable examples include the aryl
transfers from diazenes 1g, 1h, and 1n containing sensitive
functional groups (CO2Me in 3ga, CN in 3ha, and NO2 in 3na).
Even the transfer of aryl nucleophiles containing a bromo or iodo
groups as in 1k and 1l (competing halogen–metal exchange) or
a fluorine substituent in the ortho-position as in 1m (competing
β-elimination/aryne formation) proceeded in high yields. These
examples highlight the chemoselectivity of the method and its
orthogonality with classical carbonyl arylations. The productive
combination of these diazenes and a broad range of aromatic,
heteroaromatic, and cinnamic aldehydes 2b–i, is further
evidence of this (Figure 1). Enolizable aldehydes were not
compatible but α-branched 2-ethylbutyraldehyde (2j) yielded the
silyl ether 3aj in 70% yield along with the silyl enol ether in 30%
yield. Other aliphatic aldehydes 2k and 2l reacted with high
chemoselectivity to furnish 3ak and 3fl in good yields (Figure 1).
Table 1: Selected examples of the optimization.
Entry
Initiator
mol%
10
10
10
10
5
Time
1 h
Yield [%][a]
91
1
2
3
4
5
6
7
8
9
tBuOLi
Me3SiOLi
Me3SiONa
Me3SiOK
Me3SiONa
Me3SiONa
Me3SiOK
CsF
1 h
> 95
98 (70)[b]
90
< 5 min
< 5 min
< 5 min
20 h
> 95
6
Table 2: Scope I: Variation of the silyl and aryl groups of the diazene.
1
1
20 h
58
10
10
20 h
60
TMAF
20 h
trace
Entry
Diazene
SiR3
FG
Silyl
ether
Yield
[%][a]
[a] Determined by calibrated GLC analysis with tetracosane as an internal
standard. [b] Isolated yield on 0.40 mmol scale after flash
chromatography on silica gel in parentheses. TMAF
tetramethylammonium fluoride.
a
1[a]
2[b]
3
1b
1c
1d
1e
1f
SiEt3
4-Me
4-Me
4-Me
H
3ba
3ca
3da
3ea
3fa
76
68
86
72
82
80
72
99
81
87
83
65
67
73
=
SitBuMe2
SiMe2Ph
SiMe3
SiMe3
SiMe3
SiMe3
SiMe3
SiMe3
SiMe3
SiMe3
SiMe3
SiMe3
SiMe3
4
5
4-OMe
4-CO2Me
4-CN
4-F
It is important to note that hydrazine 4a had never been
detected by GLC analysis in those experiments. This would
emerge from the addition of the aryl nucleophile across the N=N
double bond of the diazene followed by silylation.[12] This
suggests that the aldehyde substrate outcompetes the diazene
as the electrophile. The silylated arene 5a, which formally arises
from the silylation of the corresponding aryl anion, did usually
form in trace amounts, likely because of the slight excess of the
diazene reagent 1a employed. In turn, compounds 4a and 5a
were the major products of the alkoxide-initiated degradation of
1a in the absence of the aldehyde substrate (see the Supporting
Information for details).
6[a]
7
1g
1h
1i
3ga
3ha
3ia
8
9
1j
4-Cl
3ja
10
11
12
13[a]
14
1k
1l
4-Br
3ka
3la
4-I
1m
1n
1o
2-F
3ma
3na
3oa
3-NO2
2,5-Me2, 4-F
To demonstrate the scope of the new method, we continued
with 10 mol% Me3SiONa in THF at room temperature as the
standard procedure. Diazenes with silyl groups other than Me3Si
were examined (1b–d; Table 2, entries 1–3). It was only the
Me2PhSi-substituted derivative 1d that behaved similarly to 1a,
affording the silyl ether 3da in 86% yield. Conversely, 1b with a
Et3Si group and 1c with a tBuMe2Si group exhibited either low or
no conversion of benzaldehyde. However, little re-optimization
showed that Me3SiOK instead of Me3SiONa promotes the aryl
transfer from 1b to benzaldehyde, and 3ba was isolated in 76%
[a] Me3SiOK instead of Me3SiONa. [b] Me3SiOK/18-crown-6 (1.0:1.2 molar
ratio) instead of Me3SiONa.
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Aromatic aldehydes
Aliphatic aldehydes
Ketones
Me3SiOK
(10 mol%)
Me3SiO
OSiMe3
OSiMe3
O
OMe
N
SiMe3
Me3SiO R'
Ar
+
Ar
N
THF
RT
– N2
R'
R
R
MeO
O
1
6a d
7
(1.2 equiv)
X
– (0.40 mmol)
MeO
OMe
3aj: 70%[b–d]
3fb: 89%
SiMe3
3ec: 83%
O
OSiMe3
OSiMe3
OMe
Me3SiO
Me3SiO
Me3SiO
F
Br
3ae: 57%[e]
3od: 60%
OSiMe3
3ak: 90%
FG
X
Cl
OSiMe3
OSiMe3
7ac: 65%[a] (E/Z > 99:1)
7aa (FG = Me, X = H): 69%
7ja (FG = Cl, X = H): 49%
7ka (FG = Br, X = H): 79%
7ib (FG = F, X = F): 76%
7jd: 54%
I
Br
I
CO2Me MeO
3lf: 65%
3lg: 56%
3fl: 76%
Esters
Me3SiOK
Heteroaromatic aldehyde
Cinnamic aldehyde
O
(20 mol%)
N
SiMe3
Me3SiO Ar
Ar Ar'
+
Ar
N
OSiMe3
O
OSiMe3
THF
RT
– N2
MeO
Ar'
1 (3.0 equiv)
8a,b (0.40 mmol)
9
Cl
– MeOSiMe3
3jh: 73%
3ai: 73% (E/Z > 99:1)
OMe
F
Figure 1. Scope II: Various diazene/aldehyde combinations.[a] [a] Unless
noted otherwise, the reactions were performed on a 0.40 mmol scale with 10
mol% Me3SiONa in THF at room temperature. [b] Me3SiOK instead of
Me3SiONa. [c] Formed along with the corresponding silyl enol ether in 30%
yield. [d] Yield determined by NMR spectroscopy using CH2Br2 as an internal
standard. [e] The reaction was performed on a 1.8 mmol scale with 5.0 mol%
Me3SiONa in THF at room temperature.
Me3SiO
Me3SiO
Me3SiO
MeO
F
9ea: 73%
9fb: 67%
9ia: 68%
Scheme 2. Scope III: Ketones and esters as electrophiles. [a] Formed along
with the corresponding 1,4-adduct in 6% yield.
Less electrophilic ketones were also competent substrates but,
as is the case of enolizable acetophenone, deprotonation was
the predominant pathway to afford the corresponding silyl enol
ether in 60% yield (not shown, see the Supporting Information).
Conversely, 6a–d reacted in the planned way with Me3SiOK as
the initiator (Scheme 2, top). No reaction or low conversion were
observed with Me3SiOLi and Me3SiONa, presumably because of
the low reactivity of intermediate tertiary alkoxide and, hence, its
inability to maintain catalytic turnover. Unlike the 1,2-selective
aryl transfer to trans-cinnamaldehyde (2i → 3ai, Figure 1), the
reaction of the aryl nucleophile with trans-chalcone (6c) led to
the formation of both the 1,2- (7ac, 65%) and the 1,4-adduct
(6%). Moreover, modification of the reaction setup with a higher
loading of Me3SiOK (20 mol%) and slow addition of the diazene
(3 equiv) to a solution of a methyl benzoates 8 and THF even
allowed for a two-fold aryl transfer to give the tertiary silyl ethers
9 in reasonable yields (Scheme 2, bottom). To the best of our
knowledge, this is an unprecedented catalytic arylation of
unactivated carboxylic acid derivatives with non-stabilized
carbanion equivalents.[13] We note here that the occurrence of
this nucleophilic addition is also diagnostic of the in-situ
formation of highly reactive aryl anions.[14]
Our next plan was to explore the possibility as to whether the
diazene platform would also enable reactions of aryl
dinucleophiles.[15] For this, we synthesized the 1,3-bismetalated
benzene equivalent 1p from 1,3-diaminobenzene in 26% yield
over three steps (see the Supporting Information for details and
crystallographic characterization[16]). The bisdiazene 1p is a
storable, crystalline deep blue solid with decent thermal stability
(up to 130 °C). The reaction of 1p and benzaldehyde (2a, 1.4
equiv) in the presence of 20 mol% Me3SiOK afforded diol 10pa
in 65% yield after deprotection with TBAF (Scheme 3, left).
O
O
O
2a (1.4 equiv)
Me3Si
N
N
SiMe3
2m (0.80 equiv)
N
N
a) Me3SiOK b) TBAF
(20 mol%) (excess)
a) Me3SiOK b) Me3SiI
1p (X-ray)
(20 mol%)
THF
H2O
CH3CN
THF
RT
THF
RT
RT
0 °C to RT
OH
OH
H
H
Ph
Ph
10pa: 65% (d.r. = 50:50)
11pm: 10%
molecular structure of 11pm
Scheme 3. Scope IV: Equivalent of an aryl bisnucleophile in the reaction with
an aldehyde (left) or a dialdehyde (right); molecular structure of a cyclophane-
like [4]arene macrocycle (middle, thermal ellipsoids are shown at the 50%
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10.1002/anie.201916004
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COMMUNICATION
probability level and all hydrogen atoms except the two pointing toward the
center are omitted for clarity).
trapped in situ with functionalized carbonyl and carboxyl
compounds. Conceptually, these arylation reactions are similar
to a Barbier-like setup[24] where the polar organometallics are
generated in situ, thereby avoiding their delicate preparation and
handling. However, our method makes use of Me3SiOM (with M
= Li, Na, and K)[11] as initiatiors whereas Barbier reactions
typically rely on overstoichiometric amounts of reducing metals.
Aside from the excellent functional-group tolerance, the new
protocol differs from established methods in that it is halide-free,
starting from aryl hydrazines rather than aryl halides.
The fact that 10pa had been employed as a precursor of
porphinoid macrocycles[17] inspired us to make use of building
block 1p in the practical assembly of otherwise difficult to
prepare macrocycles. The idea was to combine 1,3-difunctional
1p and terephthalaldehyde (2m) with its 1,4-substitution pattern,
hoping that the alternate 1,3/1,4 motifs of the rings would result
in cyclization rather than polymerization to poly(diarylcarbinols).
The reaction of 1p and 0.80 equiv 2m initiated with 20 mol%
Me3SiOK led to a complex product mixture of poly- and
Acknowledgements
oligomeric material, from which
a tetrameric macrocyclic
compound could be identified by HRMS analysis.[18]
Defunctionalization of the crude residue, that is removal of the
silyl ethers by a reported procedure,[19] considerably simplified
the analysis and allowed for the isolation of the unknown
cyclophane-like [4]arene macrocycle 11pm in 10% yield over
two steps (Scheme 3, right).[20] This seemingly low yield
compares well with others from difficult macrocyclizations
involving aromatic precursors lacking preorganization, especially
in relatively high concentration (0.1 M).[21] The molecular
structure of 11pm was confirmed by X-ray diffraction[16] (Scheme
3, middle) and shows preference for a chair-like conformation in
the solid state whereas a boat-like conformation was computed
to be more stable by approximately 0.5 kcal/mol (see the
Supporting Information for details). Compound 11pm is the
simplest, unfunctionalized member of an emerging class of
C.C. gratefully acknowledges the Alexander von Humboldt
Foundation for a postdoctoral fellowship (2018–2019). M.O. is
indebted to the Einstein Foundation Berlin for an endowed
professorship.
Conflict of interest
The authors declare no conflict of interest.
Keywords: autocatalysis • chemoselectivity • Lewis bases •
nucleophilic addition • silicon
[1]
[2]
Organometallics in Synthesis: Third Manual (Ed.: M. Schlosser), Wiley,
Hoboken, 2013.
hybrid macrocycles[22]
derivatives.[23]
currently comprising
just two
a) A. D. Benischke, M. Ellwart, M. R. Becker, P. Knochel, Synthesis
2016, 48, 1101–1107; b) G. Dagousset, C. François, T. Leόn, R. Blanc,
E. Sansiaume-Dagousset, P. Knochel, Synthesis 2014, 46, 3133–3171;
c) P. Knochel, W. Dohle, N. Gommermann, F. F. Kneisel, F. Kopp, T.
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Although an in-depth mechanistic analysis is still pending,
we propose the autocatalytic cycle outlined in Scheme 4. After
initiation with either of the three trimethylsilanolate alkali salts
the aryl alkali metal will form as a result of desilylation and loss
of N2. The in-situ-formed aryl nucleophile then adds to the
carbonyl compound, forming an alkali metal alkoxide that will in
turn engage in the desired degradation of the diazene reagent.
This step then propagates the catalytic cycle.
[3]
[4]
W. E. Parham, C. K. Bradsher, Acc. Chem. Res. 1982, 15, 300–305.
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[5]
[6]
N
SiR3
Ar
N
Me3SiO– M+
– N2
O
– Me3SiOSiR3
Initiation
M
N
N
R'
R
Ar
e
ion
Addit
releas
N2
1,2-
ic
n
alyt
ocat
Aut
Pro
e
cycl
M+ –
O R'
N
M
[7]
[8]
[9]
Ar
N
Ar
R
atio
pag
R3SiO R'
Ar
N
SiR3
Ar
N
R
[10] J. C. Bottaro, J. Chem. Soc., Chem. Commun. 1978, 990.
[11] For a review of trimethylsilanolate alkali salts, see: K. Bürglová, J.
Haláč, Synthesis 2018, 50, 1199–1208.
Scheme 4. Proposed autocatalytic cycle. M = alkali metal; R and R’ = alkyl or
[12] A. R. Katritzky, J. Wu, S. V. Verin, Synthesis 1995, 651–653.
[13] For the addition of stabilized heteroaryl anions to lactones, see: A. Ricci,
M. Fiorenza, M. A. Grifagni, G. Bartolini, Tetrahedron Lett. 1982, 23,
5079–5082.
aryl; Ar = aryl.
To summarize, we showed that N-aryl-N’-silyldiazenes
constitute a versatile platform from which various highly reactive
and, at the same time, functionalized aryl nucleophiles can be
released at ambient temperature. The reaction of a related
bisdiazene illustrates the potential of the method to formally
generate dinucleophiles. These reactive intermediates can be
[14] J. A. Murphy, S.-z. Zhou, D. W. Thomson, F. Schoenebeck, M. Mahesh,
S. R. Park, T. Tuttle, L. E. A. Berlouis, Angew. Chem. Int. Ed. 2007, 46,
5178–5183; Angew. Chem. 2007, 119, 5270−5275.
[15] a) M. Fossatelli, R. den Besten, H. D. Verkruijsse, L. Brandsma, Recl.
Trav. Chim. Pays-Bas 1994, 113, 527–528; b) F. Bickelhaupt, Angew.
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10.1002/anie.201916004
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COMMUNICATION
Chem. Int. Ed. Engl. 1987, 26, 990–1005; Angew. Chem. 1987, 99,
1020–1035.
[16] CCDC 1971069 (1p) and CCDC 1971057 (11pm) contain the
supplementary crystallographic data for this paper. These data can be
obained free of charge from The Cambridge Crystallographic Data
Centre.
[17] M. Stępień, L. Latos-Grażyński, Chem. Eur. J. 2001, 7, 5113–5117.
[18] Larger macrocycles such as the [6]arene derivative were also observed
by HRMS but could not be isolated nor characterized after
defunctionalization.
[19] E. J. Stoner, D. A. Cothron, M. K. Balmer, B. A. Roden, Tetrahedron
1995, 41, 11043–11062.
[20] For the synthesis of parent [1,1,1,1]metacyclophane, see: a) J. E.
McMurry, J.C. Phelan, Tetrahedron Lett. 1991, 32, 5655–5658; for the
synthesis of parent [1,1,1,1]paracyclophane, see: b) Y. Miyahara, T.
Inazu, T. Yoshino, Tetrahedron Lett.1983, 24, 5277–5280.
[21] Examples are provided in the following review: V. Martí-Centelles, M. D.
Pandey, M. I. Burguete, S. V. Luis, Chem. Rev. 2015, 115, 8736–8834.
[22] An example of an application of a hybrid macrocycle: D. A. Fowler, A. S.
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[24] P. Barbier, Compt. Rend. 1899, 128, 110.
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10.1002/anie.201916004
Angewandte Chemie International Edition
COMMUNICATION
Suggestion for the Entry for the Table of Contents
COMMUNICATION
Under cover of nitrogen. Silyl-
protected aryl-substituted
diazenes act as masked aryl
nucleophiles. By initiation with
Me3SiOM (M = Li, Na, and K)
aryl transfer to carbonyl and
carboxyl groups occurs at
ambient temperature. This mild
nucleophile generation is
C. Chauvier, L. Finck, E. Irran, M.
Oestreich*
Page No. – Page No.
Autocatalytic Carbonyl
Arylation through In-Situ
Release of Aryl Nucleophiles
from N-Aryl-N’-Silyldiazenes
compatible with electron-
withdrawing and -donating
functional groups (FGs).
This article is protected by copyright. All rights reserved.