Titanium(III)-Catalyzed Reductive Decyanation of Geminal Dinitriles by a Non-Free-Radical Mechanism

Article abstract of DOI:10.1002/anie.201908372

A titanium-catalyzed mono-decyanation of geminal dinitriles is reported. The reaction proceeds under mild conditions, tolerates numerous functional groups, and can be applied to quaternary malononitriles. A corresponding desulfonylation is demonstrated as well. Mechanistic experiments support a catalyst-controlled cleavage without the formation of free radicals, which is in sharp contrast to traditional stoichiometric radical decyanations. The involvement of two Ti^III species in the C?C cleavage is proposed, and the beneficial role of added ZnCl₂ and 2,4,6-collidine hydrochloride is investigated.

Full text of DOI:10.1002/anie.201908372

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Accepted Article
Title: Titanium(III) Catalyzed Reductive Decyanation of Geminal
Dinitriles by a Non-Free Radical Mechanism
Authors: Jens Weweler, Sara L. Younas, and Jan Streuff
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201908372
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http://dx.doi.org/10.1002/ange.201908372

10.1002/anie.201908372
Angewandte Chemie International Edition
COMMUNICATION
Titanium(III) Catalyzed Reductive Decyanation of Geminal
Dinitriles by a Non-Free Radical Mechanism
Jens Weweler,[a] Sara L. Younas[a] and Jan Streuff*[a]
Bu3SnH, AIBN
or NHC-BH3, DTBP
or SmI2/HMPA
Abstract:
A titanium catalyzed mono-decyanation of geminal
dinitriles is reported. The reaction proceeds under mild conditions,
tolerates numerous functional groups, and can be applied to
Previously:
stoichiometric
quaternary malononitriles.
A corresponding desulfonylation is
demonstrated as well. Mechanistic experiments support a catalyst-
controlled cleavage without the formation of free radicals, which is in
sharp contrast to traditional stoichiometric radical decyanations. The
involvement of two TiIII species in the C-C cleavage is proposed and
the beneficial role of added ZnCl2 and Coll•HCl is investigated.
CN
R'
NC CN
R'
R
R
Cp2TiCl2
reductant
• mild conditions
• no free radicals
• FG compatibility
This work:
single-electron
transfer catalysis
The controlled cleavage of carbon-carbon bonds is a highly
topical research area and a challenge to modern transition metal
catalysis.[1–3] One particular type is the cleavage of C-CN bonds,
which can be used either as an entry point for subsequent bond
Scheme 1. Free radical decyanations and the envisioned titanium(III) catalysis.
inhibition of the catalyst was concluded. Further experimentation
revealed that this inhibition could be prevented and the catalyst
activity even improved if zinc chloride was added to the
decyanations, giving 82% yield for 2a and 74% yield for 2b after
only 24 h. This scenario was supported by preliminary DFT
calculations, which confirmed the product inhibition and the
liberation of the inhibited catalyst by the addition of ZnCl2.[16]
Only traces of the decyanation product were observed without
the titanium catalyst.
constructing events,[4] or for
a
reductive, selective
decyanation.[5,6] In this context, the reductive decyanation of
geminal dinitriles provides direct access to functionalized
alkylnitriles from easy-to-prepare malononitrile precursors,
making it a powerful alternative to conventional nitrile a-
functionalizations.[7] However, only
a
limited number of
stoichiometric examples have been reported for this
transformation to date. These comprise traditional free radical
defunctionalizations with tin hydride, tris-trimethylsilylsilane, and
NHC-borane reagents as hydrogen radical donors,[8] or strong
reducing conditions with stoichiometric amounts of SmI2/HMPA
and other so-called "super electron donors" (Scheme 1).[9,10]
With the goal to close this methodological gap, we herein report
a broadly applicable catalytic reductive decyanation in presence
Cp2TiCl2 (10 mol%)
Zn (2 equiv)
Coll•HCl (1.0 equiv)
TMSCl (3.0 equiv)
CN
CN
H
Ph
Ph
CN
THF, 35 ºC
1a
2a
no changes: 80% (48 h)
with 0.5 equiv 2b: 23% (48 h)
with 1.0 equiv. ZnCl2: 82% (24 h)
a
titanium(III) single-electron-transfer catalyst.[11,12] The
added
here
of
CN
CN
H
reaction is not to be confused with free radical nitrile
translocation reactions.[13]
as above
Ph
Ph
CN
1b
2b
The catalytic decyanation was first investigated using 2-
benzylmalononitrile (1a) as substrate, having both nitriles in a
homobenzylic position (Scheme 2). An initial optimization study
showed that nitrile 2a could be obtained in a good yield of 80%
after 48 h from a reaction with titanocene dichloride (10 mol%),
zinc as reducing agent, and 2,4,6-collidine hydrochloride
(Coll•HCl) and chlorotrimethylsilane (TMSCl) as additives in THF
at 35 ºC. Without either additive, the yield of 2a was inferior. The
reaction was highly chemoselective (spot-to-spot) and worked
also for 2-phenylmalononitrile (1b), albeit with a significantly
lower yield (30%) in benzyl cyanide (2b). Interestingly, it was
found that adding 2b to the decyanation of 1a also greatly
diminished the reaction outcome to 23% yield. Based on
previous reports on titanium(III)-nitrile complexes and our
experience in titanium(III) catalysis involving nitriles,[14,15] product
no changes: 30% (48 h)
with 1.0 equiv. ZnCl2: 74% (24 h)
Scheme 2. Initial optimization studies.
Several malononitriles were then decyanated accordingly on
a 0.5 mmol scale with a simple filtration as a sufficient workup
procedure (Scheme 3). The reaction showed an unusually broad
substrate scope for a C-CN cleavage method. For example, the
decyanation at a homobenzylic position proceeded smoothly in
presence of bromo (2c), ester (2d), acetoxy (2e), nitrile (2f),
thioether (2g), ether and free alcohol functions (2h). Arylated
malononitriles 1i and 1j, containing trifluoromethyl and methoxy
groups, also underwent the decyanation to the corresponding
benzyl nitriles 2i and 2j in 44% and 70% yield, respectively.
Likewise, an ortho-tolylmalononitrile was mono-decyanated in
71% yield (2k). Symmetric and unsymmetric quaternary
malononitriles could be employed as well, which led to the
nitriles 2l–2n (54–88%). Here, the increased steric hindrance led
to a slower reaction, which was compensated by a prolonged
reaction time of 48 h. Other structurally diverse substrates
containing cyclohexyl and indole groups smoothly underwent the
[a]
J. Weweler, S. L. Younas, Dr. J. Streuff
Institut für Organische Chemie
Albert-Ludwigs-Universität Freiburg
Albertstr. 21, 79104 Freiburg im Breisgau, Germany
E-mail: jan.streuff@ocbc.uni-freiburg.de
Supporting information for this article is given via a link at the end of
the document.
1
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10.1002/anie.201908372
Angewandte Chemie International Edition
COMMUNICATION
Cp2TiCl2 (10 mol%)
Zn, ZnCl2, Coll•HCl
TMSCl
A series of experiments was then carried out to elucidate the
decyanation mechanism. Geminal dinitriles with a tethered pent-
4-en-1-yl group were previously reported to readily undergo a 5-
exo-trig cyclization after a homolytic C-CN cleavage under free
radical conditions.[8b,c] The titanium(III) catalysis, however, led to
an exclusive decyanation of compound 4 to nitrile 5 without the
generation of the cyclization product 6 (Scheme 5a). Further
proof of a non-free radical mechanism was unambiguously
obtained by the decyanation of radical clock substrate 7 that
only led to the desired nitrile 8 without any formation of ring-
opening products. The decyanation of 1a was then carried out
with 94%-deuterated Coll•DCl to confirm that the newly
introduced hydrogen atom was transferred via a proton transfer
from the collidinium salt. A 68% deuterium incorporation was
achieved, while a reaction with Coll•HCl run in THF-d8 as solvent
resulted in no deuterium transfer. Next, the order in catalyst was
determined from two experiments run with 5.0 and 12.5 mol%
Cp2TiCl2 on a 4 mmol scale that were followed by NMR analysis
of taken samples (Figure 1). A visual kinetic analysis then
NC CN
NC
R
H
R
R'
R'
2a–r
THF, 35 ºC, 24 h
1a–r
homobenzylic:
X
MeO2C
AcO
HO
H
H
H
CN
CN
CN
2a, X = H, 82%
2d, 89%
2e, 56%
2a, 9 mmol scale: 70%
2c, X = Br, 82%
NC
MeS
H
H
H
H
CN
CN MeO
CN
CN
2f, 81%
benzylic:
2g, 60%
2h, 41%
H
H
Me
H
CN
CN
CN
F3C
MeO
H
2b, 74%
2i, 44%
2j, 70%
2k, 71%
quaternary:
revealed a second order in catalyst by time normalization.[17]
A
NC
H
plot yield vs. t [cat]2.0 led to an excellent overlay of the two
NC
H
NC
curves, which confirmed a second order in the catalyst.
Me
We also investigated the direct effect of added ZnCl2 on the
titanium(III) catalyst by cyclic voltammetry (Figure 2). The
voltammograms of Zn-reduced Cp2TiCl2 were recorded in the
presence of Coll•HCl, ZnCl2, or Coll•HCl and ZnCl2 (each in a
ten-fold excess to simulate the conditions of the catalysis). With
added Coll•HCl, the ion pair [CollH]+ [Cp2TiCl2]– (E = –1.25 V)
2l, 88% (48 h)
2m, 66% (48 h)
2n, 54% (48 h)
diverse:
H
H
CN
Ph
H
H
CN
Ph
N
CN
CN
H
2o, 80%
2p, 76%
2q, 72%
2r, 42% (48 h)
a)
Cp2TiCl2 (10 mol%)
Zn, ZnCl2, Coll•HCl
TMSCl
NC CN
NC H
NC
Ph
Scheme 3. Scope of the catalytic C-CN cleavage reaction.
Ph
Ph
+
THF, 35 ºC, 24 h
4
5, 70%
6
decyanation to the corresponding nitriles 2o and 2p.
Malononitriles containing a styryl moiety or a b-vinyl group were
decyanated to give 2q and 2r in 72% and 42% (48 h) yield,
respectively. The catalytic reductive decyanation reaction of 1a
was also demonstrated on a 9 mmol (1.4 g) scale, resulting in a
slightly lower yield (70%).
not observed
Cp2TiCl2 (10 mol%)
Zn, ZnCl2, Coll•HCl
TMSCl
CN
CN
CN
recovered
7 (41%)
Ph
Ph
+
7
8, 45%
THF, 35 ºC, 48 h
no ring-opening observed
b)
Cp2TiCl2 (10 mol%)
Zn, ZnCl2, TMSCl
Coll•DCl (1.0 equiv)
We also tested whether he reaction could be extended
towards the removal of a different functional group and it was
(68% D)
D
CN
CN
Ph
Ph
CN
found that a-cyanosulfone
desulfonylation to 2a in 56% yield (Scheme 4). Since thiophenol
was observed as byproduct, the amounts of zinc and
hydrochloride were increased to compensate for the additional
sulfone reduction and thiolate protonation. Unreacted
3 did indeed undergo a clean
THF, 60 ºC, 24 h
1a
2a-d1, 77%
a
Scheme 5. a) Negative radical clock experiments. b) Deuteration experiment
confirming the introduction of the new hydrogen via proton transfer.
3
accounted for the mass balance and no background reaction
took place. The further elaboration of this catalytic
desulfonylation will be reported separately.
Cp2TiCl2 (10 mol%)
Zn (3.0 equiv)
SO2Ph
CN
H
Coll•HCl (2.0 equiv)
Ph
Ph
CN
ZnCl2, TMSCl
THF, 60 ºC
3
2a, 56%
Scheme 4. Titanium(III) catalyzed desulfonylation.
Figure 1. a) Reaction kinetics of experiments with 5 and 12.5 mol% catalyst.
b) Variable time normalization analysis revealing a catalyst order of 2.0.
2
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10.1002/anie.201908372
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COMMUNICATION
N
N
ZnCl2
III
Cp Ti Cl
2
2
R
R'
Zn, THF
A
catalyst
reduction
coordination
Cl
IV
Cp Ti
Cl
N
Cl
III
TiCp2
III
2
2
Cp2Ti
Cl
E
CN
N
R
R'
+
R
R'
B
TMS–CN
Cl
N
IV
Cp2Ti
double-SET
C-C cleavage
product
liberation
Cl
IV
TiCp2
TMSCl
Coll•HCl
+
N
C
R
R'
C
D
Figure 2. Cyclic voltammograms of Zn-Cp2TiCl2 with added Coll•HCl, added
Scheme 6. Proposed simplified catalytic cycle.
ZnCl2, and added ZnCl2 and Coll•HCl, all at a sweep rate of 0.1 V s–1
.
In conclusion,
a titanium(III) catalyzed decyanation of
and the dimer [(Cp2TiCl)2] (E = –0.81 V) were observed.[18–20]
Usually, Zn-reduced solutions of Cp2TiCl2 also show the
monomer [Cp2TiCl] (E = –0.75 V),[20] which appeared to be
absent under these conditions. With ZnCl2 added, however, the
cation [Cp2Ti]+ (E = –0.43 V)[20] became the only observable
species.[21] This was in agreement with the earlier proposal that
geminal dinitriles has been developed that represents the first
example of such a decyanation reaction via single-electron-
transfer catalysis. The reaction proceeds under mild conditions,
can be applied to a broad substrate scope, and it shows an
excellent chemoselectivity. It has been demonstrated that the
cleavage does not proceed via a free radical mechanism but via
a unique catalyst-controlled C-CN scission involving two titanium
species, which renders it complementary to previous
decyanation protocols. Further applications towards other C-C
and C-heteroatom bond cleavage reactions are ongoing and will
be reported in due course.
[Cp2TiCl] and ZnCl2 form
a closely bound ion pair in
solution.[22,23] Based on previous studies on cationic titanium(III)
species, we concluded that this ion pair was [Cp2Ti]+ [ZnCl3]–
[12g,15]
.
If Coll•HCl and ZnCl2 were simultaneously added, the
oxidation peak of [Cp2Ti]+ vanished again and [Cp2TiCl]
appeared as only visible species.[20,21] Hence, the combination of
Coll•HCl and ZnCl2 led to a shift of the equilibria connecting all
species towards the active catalyst monomer. We attributed this
to the formation of [CollH]+ [ZnCl3(THF)]– from Coll•HCl and
ZnCl2(THF)2, which was calculated to release DG = –8.5 kcal
mol–1.[16] Whether the addition of ZnCl2 also has a beneficial
effect on the catalyst reduction step, as recently found for a
titanium(III) catalysis with Cp*TiCl3,[12e] was not investigated at
this time.
Acknowledgements
This work has been supported by the Deutsche
Forschungsgemeinschaft (DFG)
position to J.S.).
– 408295365 (Heisenberg
Keywords: C-C activation • catalysis • cyanides • electron
A first mechanistic proposal was derived (Scheme 6), in
which the geminal dinitrile coordinated two equivalents of in situ
generated [Cp2TiCl] (A), giving complex B. Then, the single-
electron-transfer (SET) from both titanium(III) centers triggered
the C-C cleavage, giving one equivalent of ketenimine-
titanium(IV) complex C and N-coordinated titanium(IV) cyanide
complex D or, alternatively, its C-coordinated isomer. The
participation of two titanium(III) species in the C-CN scission
was in agreement with the observed second order in catalyst
and the non-free radical behavior. A protonation of C by Coll•HCl
then released the nitrile product and Cp2TiCl2 (E). The reaction
of D with TMSCl simultaneously liberated the second equivalent
of E and formally one equivalent of TMSCN. The formation of
cyanide [presumably TMSCN or Zn(CN)2] was confirmed by an
ion chromatographic analysis of the aqueous layer obtained
from the workup. Finally, zinc regenerated the titanium(III)
catalyst.
transfer • titanium
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3
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10.1002/anie.201908372
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COMMUNICATION
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[21] Owing to the presence of added ZnCl2, the lower limit of the
measurement was reduced to E = –1.2 V, which prevented the
detection of [Cp2TiCl2]–.
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[23] [Cp2Ti]+ is usually not present in solutions of zinc-reduced Cp2TiCl2 but
generated during the CV measurement, see ref. 20. In our case,
however, the presence of ten catalyst equivalents of ZnCl2 led to the
exclusive formation of [Cp2Ti]+.
4
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COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Jens Weweler, Sara L. Younas, Jan
Streuff*
CN
H
TiIII
R
R
R'
CN
R'
CN
Page No. – Page No.
• mild conditions • scalable • high FG compatibility • no free radicals •
Titanium(III) Catalyzed Reductive
Decyanation of Geminal Dinitriles by
a Non-Free Radical Mechanism
A titanium(III) catalyzed reductive C-CN cleavage of geminal dinitriles is reported.
The reaction proceeds under unusually mild reaction conditions and tolerates
numerous common functional groups. Mechanistic studies support a catalyst-
controlled C-CN scission instead of a free radical pathway.
5
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