Regioselective Cyclotrimerization of Terminal Alkynes Using a Digermyne

Article abstract of DOI:10.1002/anie.201801222

The catalytic activation of small neutral molecules followed by the formation of C?C bonds is a highly important method to increase the complexity and/or value of simple starting materials. Reported is an isolable digermyne, a compound with a Ge≡Ge bond, which acts as a precatalyst for the cyclotrimerization of terminal arylacetylenes to afford the corresponding 1,2,4-triarylbenzenes with absolute regioselectivity. The results demonstrate that bespoke main-group-element compounds can catalytically activate and transform small neutral organic molecules and induce the formation of C?C bonds.

Full text of DOI:10.1002/anie.201801222

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Accepted Article
Title: Regioselective Cyclotrimerization of Terminal Alkynes Using a
Digermyne
Authors: Tomohiro Sugahara, Jing-Dong Guo, Takahiro Sasamori,
Shigeru Nagase, and Norihiro Tokitoh
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201801222
Angew. Chem. 10.1002/ange.201801222
Link to VoR: http://dx.doi.org/10.1002/anie.201801222
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Angewandte Chemie International Edition
10.1002/anie.201801222
COMMUNICATION
Regioselective Cyclotrimerization of Terminal Alkynes Using a
Digermyne
[
a]
[a]
[b]
[c]
Tomohiro Sugahara, Jing-Dong Guo, Takahiro Sasamori, * Shigeru Nagase, and Norihiro
Tokitoh
[
a]
Abstract: The catalytic activation of neutral small molecules
followed by the formation of C–C bonds is a highly important
method to increase the complexity and/or value of simple starting
materials. We discovered that an isolable digermyne, i.e., a
compound with a GeºGe triple bond, acts as a precatalyst for the
cyclotrimerization of terminal arylacetylenes to afford the
corresponding 1,2,4-triarylbenzenes with absolute regioselectivity.
Our results demonstrate that bespoke main-group-element
compounds can catalytically activate and transform small neutral
organic molecules and induce the formation of C–C bonds.
metal catalysts remain largely unknown could be due to difficulties
associated with the reductive elimination process of main-group
elements due to the strong covalent bonds between main-group
elements and carbon atoms. It is therefore hardly surprising that
only few cyclotrimerizations of alkynes catalyzed by main-group
elements have been accomplished that proceed via redox process of
[
8]
the main-group elements. For example, in the Si
2
Cl
6
-catalyzed
[8a]
cyclotrimerization of alkynes, ·SiCl
3
radicals initiate the radical
trimerization. But as such radical reactions usually require high
temperatures, the regioselectivity is easily compromised. Yet, the
creation of main-group-element catalysts for C–C coupling
reactions that proceed via redox processes similar to those in
transition-metal-based catalysts remain challenging.
Herein, we report a new transition-metal-free catalytic
system for the cyclotrimerization of terminal arylalkynes, which
includes a Ge-centered redox process. We discovered that a
Academically and industrially, the catalytic activation of
neutral organic molecules, followed by the formation of C–C
bonds is a highly important method to increase the complexity
[1]
and/or value of simple starting materials.
For example,
cyclotrimerization reactions of alkynes that afford aromatic
compounds should represent very important C–C bond formation
reactions due to the versatility of the resulting aromatic
substoichiometric amount of
a
previously reported isolable
[9,10]
digermyne
promotes the regioselective cyclotrimerization of
several terminal arylacetylenes in high yield. Our results thus
demonstrate that bespoke main-group-element compounds can
catalytically activate small neutral organic molecules and induce
[2,3]
products.
Traditionally, transition-metal catalysts have been
used for this purpose, as some transition-metal complexes based on
e.g. cobalt efficiently activate alkynes for (i) oxidative additions,
[11]
the formation of C–C bonds.
(
ii) ring-expansions, and (iii) reductive eliminations under
[3,4]
concomitant formation of C–C bonds.
The reaction mechanism
R
R
R'
underlying the transition-metal-catalyzed cyclotrimerization
Ge Ge
C
Ge Ge
+
R'
[3,5]
oxidative
addition
out-of-plane π
π*
in-plane π*
reactions of alkynes has been well established.
Several
C
R
R
H
H
transition-metal-based catalysts have been developed that are
highly efficient both with respect to chemical yields and
π
R, R': organic substituents
Ge(III) Ge(I)
favorable interactions between frontier orbitals
[3,6]
selectivity,
and these are superior to catalysts based on main-
Figure 1. Orbital interactions between digermynes and acetylenes.
group elements, although such “transition-metal-free” catalytic
systems would be highly desirable in order to avoid the use of
precious metals. The reason why similar C–C coupling reactions
of neutral small organic molecules in the absence of transition-
Transition-metal complexes readily insert oxidatively into the
reactive C–X (X = e.g. halogen) or C=C bonds of small molecules,
which results in the formation of reactive transition-metal
[
7]
complexes with
a
higher oxidation state. Subsequently,
transmetallation(s) and/or reductive elimination(s) may occur,
which enables such transition-metal complexes to act as active
catalysts for the transformation of organic compounds. The initial
activation process of the small molecules by transition metals, i.e.,
the oxidative addition, can be interpreted as a simultaneous s-/p-
donation from the small molecule to the transition metal and a p-
back-donation from the transition metal to the antibonding orbital
[
[
a]
b]
T. Sugahara, Dr. J.-D. Guo, Prof. Dr. N. Tokitoh
Institute for Chemical Research, Kyoto University
Gokasho, Uji, Kyoto 611-0011 (Japan)
Prof. Dr. T. Sasamori
Graduate School of Natural Sciences, Nagoya City University
Yamanohata 1, Mizuho-cho, Mizuho-ku, Nagoya, Aichi 467-8501
(
Japan)
Fax: (+81) 52-872-5820
E-mail: sasamori@nsc.nagoya-cu.ac.jp
Prof. Dr. S. Nagase
Fukui Institute for Fundamental Chemistry, Kyoto University
Sakyo-ku, Kyoto 606-8103 (Japan)
[12]
of the small molecule.
On the basis of a similarly simple
[
[
c]
concept, the HOMO and LUMO of compounds that contain
multiple bonds between group-14 elements such as dimetallynes
(
EºE), engage effectively in s-/p- and p-back-donation with small
[11]
molecules.
The activation of small molecules by such low-
***] This work was supported by JSPS KAKENHI grants JP15H03777,
5K13640, and JP20109013 from MEXT (Japan), Grant-in-Aid for
1
coordinated group-14 elements has been intensively investigated,
especially with regard to the development of main-group-element
catalysts for applications in organic synthesis. However, all reports
on the activation of such small molecules by low-coordinated
group-14 elements merely accomplish the activation of the small
molecules and afford the corresponding adducts as stable
compounds due to the formation of strong C–E bonds, which has
Research in Nagoya City University, and by the “Molecular Systems
Research” project of the RIKEN Advanced Science Institute. T.
Sugahara gratefully acknowledges support from a grant-in-aid for
JSPS fellows (JP16J05501). We would like to thank Mr. Toshiaki
Noda and Ms. Hideko Natsume at Nagoya University for the expert
manufacturing of custom-tailored glassware.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
[7,11,13]
hitherto prevented a truly catalytic use.
One of the essential
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Angewandte Chemie International Edition
10.1002/anie.201801222
COMMUNICATION
requirements for such prospective main-group-element catalysts is
their flexibility with respect to the oxidation state, similar to the
case of transition-metal-based catalysts. We have therefore focused
our attention on the properties of digermynes (RGeºGeR, R =
purification of these product mixtures is often nontrivial due to
contamination with small amounts of e.g. the 1,3,5-Ar
3
C
6
H
3
isomer.
Conversely, in our Ge-based catalytic reactions, by-products
such as 1,3,5-Ar were not observed, which demonstrates the
[3,15]
3
6 3
C H
[
11,13,14]
organic groups),
as: (i) we have already reported the
absolute regioselectivity of these reactions. The products of the Ge-
catalyzed reactions presented herein can thus be isolated by simply
passing the reaction mixture through a pad of silica gel.
Subsequently, we investigated this catalytic reaction in detail to
elucidate its reaction mechanism. Treatment of 1 with 2 equiv. of
PhCºCH (2a) in hexane at room temperature (r.t.) afforded 1,2-
digermabenzene 4a in 68% yield. The mechanism for the
formation of 4a should be similar to the previously reported
isolation of stable digermynes and their high reactivity toward
compounds that contain C–C multiple bonds, which is due to the
low-lying LUMO of the in-plane p*-orbital that results from the
[9,10,13b]
trans-bent structure;
(ii) the simultaneous s-/p-donation and
the p-back-donation from/toward the C–C multiple bond agree
well with the frontier orbitals of the GeºGe triple bond (Fig. 1);
[11]
(
iii) the possible mixed-valence states such as Ge(III)–Ge(I) should
[
10,11]
afford sufficient flexibility of the oxidation states.
reaction of
digermabenzene.
further equivalent of PhCºCH in C
unequivocally characterized as 1,6-digermatricyclo[3.3.0.0 ]octa-
,7-diene (5a). Independently, a C solution of 1 was treated
1
with acetylene that furnishes 1,2-Tbb
2
-1,2-
Treating isolated digermabenzene 4a with a
at r.t. afforded X, which was
[
10]
Tbb
6
D
6
Ge Ge
SiMe3
2
,6
Me Si
3
Tbb
4 mol%)
1
3
6 6
D
Ar
Ar
Tbb =
Me Si
(
with 3 equiv. of 2a at r.t. to quantitatively generate 5a. Similarly,
4b and 5b were isolated from the reaction between 1 and
stoichiometric amounts of p-Tol-CºCH (2b) (Scheme. 2).
Subsequently, we confirmed that a catalytic amount of isolated 5a
3
Ar
3
C D
6
SiMe3
6
Ar
2
60 ºC, 8 h
3
Me
products
(
10 mol%) promotes the quantitative cyclotrimerization of 2a in
at 60 ºC to afford 3a. These results suggest that 5 should be a
6 6
C D
key resting state of the active species involved in the catalytic cycle.
It can hence be concluded that digermyne 1 works as a pre-catalyst
for the cyclotrimerization of terminal arylacetylenes to afford the
corresponding 1,2,4-triarylbenzene derivatives, whereby 1,6-
digermatricyclo[3.3.0.0 ]octa-3,7-diene (5) represents a resting
state for the active catalyst, as another metastable intermediate
could not be observed throughout the catalytic reaction.
Me
Me
3
a (84%)
3b (95%)
3d (92%)
Br
OMe
2
,6
Br
Br MeO
OMe
3
c (92%, 48 h)
Ar
S
Me
Ar
Tbb
Ar Tbb
+
Tbb
2
Ge Ge
Me
Ge Ge
Tbb
Ar
C D
Ar
Ar
6
6
S
S
1
3
5
Ar
Me
e (84%, 48 h)
Ar
Ar
a: Ar = Ph, b: Ar = p-Tol (4-methylphenyl)
3
3f (74%, 48 h)
2
Scheme 1. Regioselective cyclotrimerization of terminal
arylacetylenes using a substoichiometric amount (4 mol%) of 1.
3
Ar
Ar
2
Ar
hexane
r.t.
absolute
C D selectivity
2
2
C D
6
6
6
6
r.t.
6
0 ºC
When 1 was treated for 8 h at 60 ºC with an excess of
, the majority of 2a was converted into 1,2,4-
Ph (3a, 84%). The H NMR spectrum of the crude mixture
did not show any signals associated with 1, but new signals arising
from X, which include the TbbGe- and PhCCH-units. This
unexpected result suggested that a small portion of 1 should act as
a pre-catalyst for the cyclotrimerization of arylacetylenes. In order
to explore the scope of this “Ge-catalyzed cyclotrimerization of
arylacetylenes”, optimized reaction conditions (4 mol% 1, 60 ºC,
Ar
PhCºCH (2a) in C
6
D
6
Tbb
Tbb
1
Ar
Tbb
Ar
Tbb
3
C
6
H
3
Ge Ge
2
Ge Ge
Ar
C D
6
6
r.t.
Ar
4
5
Ar
Scheme 2. Mechanistic studies on the reactions of stable digermene
1
with terminal arylacetylenes.
Ph
p-Tol
Tbb
6 6
C D ) were applied to a variety of arylacetylenes. As shown in
Figure 1, phenylacetylenes with p-methyl (2b), p-bromo (2c), p-
methoxy (2d), and o-methyl (2e) substituents, as well as m-
thienylacetylene (2f) exclusively afforded the corresponding 1,2,4-
triarylbenzenes (3b-3f) under these conditions (the formation of 3c,
Tbb
Ph
Tbb
Tbb
Ge Ge
Ge Ge
p-Tol
2
b
p-Tol
C D
6
6
40 ºC
Ph
p-Tol
5
a
5b
3
e, and 3f required longer reaction times), whereby 1 showed
+
excellent activity as a pre-catalyst (TON = 15~35). Unfortunately,
this protocol is not yet applicable to the cyclotrimerization of
internal alkynes. Accordingly, these catalytic reactions based on 1
could be termed “Ge-catalyzed Reppe reactions” (Scheme. 1).
Classic Reppe reactions, which use transition-metal catalysts
based on e.g. Mo or Co to promote the cyclotrimerization of
acetylenes, show less selectivity (< ~95%) in some cases, i.e., the
p-Tol
p-Tol
p-Tol
p-Tol
+
+
Ph
Ph
p-Tol
Ph p-Tol
3
aab
3abb
3b
Scheme 3. Reaction of 5a with p-tolylacetylene (2b).
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Angewandte Chemie International Edition
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COMMUNICATION
Ar
Ar
Ar
Tbb
Tbb
2
Tbb
Ar
Ar
Ge
Ge Ge
Ge
Ge Ge
Ar
Tbb
1
2
Tbb
Tbb
6 Ar
(vi)
4
(ii)
2
(i)
(vii)
Ar
Ar
Ar
Ar
Ar
Tbb
Tbb
Ar
Tbb
Ar
Ar
Ge Ge
Tbb
Ge
Ge
Ge
Ar
Tbb
Ge
Tbb
Ar
Ar
Ar
Ar
3
Ar
Ar
1
0
5
7
2
Ar
Ar
(v)
(iii)
Ar
(iv)
Ar
Ar
Ge
Ar
Ge Tbb
Ge
Ge Tbb
Ar
Tbb
Tbb
Ar
Ar
Ar
9
8
Ar
Scheme 4. Reaction mechanism for the cyclotrimerization of terminal arylacetylenes promoted by digermyne 1.
We monitored crossover reactions between 5a and p-TolCºCH
due to the steric constraints; (iv) an intramolecular Diels-Alder
reaction between the germirene and germole moieties could occur
(DG 8-9 =19.2 kcal/mol). This should be another key step for the
1
(
2b) by
triarylbenzene 3aab (1-p-Tol-2,4-Ph-benzene) was observed as the
major product, suggesting that the [Ph ] moiety of the five-
membered [C Ge] ring in 5a reacted with an external alkyne (2b)
H
NMR spectroscopy (Scheme. 3). Initially,
‡
2
C
4
stereoselectivity, which can be rationalized using the traditional
[
21]
4
explanation of Diels-Alder reactions; (v) intermediate 9 exhibits
several fused ring systems, including a Ge-containing three-
membered ring. The germirane moiety should subsequently
undergo a retro-[1+2]cycloaddition to release the strain energy
to give 3aab. Subsequently, the formation of 3abb was observed
under concomitant formation of trace amounts of 3b, which
eventually became the predominant product. Finally, 5a was
completely converted into 5b.
‡
[13b, 19, 22]
(DG 9-10 =8.1 kcal/mol);
(vi) the germylene moiety in 10
Considering the entirety of these results in combination with
could promote facile retro-[1+4]cycloaddition of the
a
detailed
theoretical
calculations
(TPSSTPSS-D3(BJ)/6-
germanorbornadiene moiety to furnish the 1,2-digermabenzene
together with the elimination of the benzene derivative to gain
aromatic stabilization energy. This process is highly exothermic
and without barrier; (vii) as the reaction of 4 with 2 generates 5, we
could determine the catalytic pathways: the highest barrier
throughout the reactions is the transformation of 5 into 6.
3
11+G(2d)<Ge,Si>, 6-31G(d,p) <C,H>//opt/3-21G*, Ar = Ph), the
[23]
reaction should proceed as shown in Scheme 4. We noticed that the
reaction barrier from 5 should be the highest among all catalytic
reactions, as only 5 is experimentally observed throughout the
catalytic cycle. The pathway from 5 most likely involves seven key
steps [(i)-(vii)]: (i) an equilibrium between 5 and germole-
germylene 6, (ii) a chain-extension of the germylene moiety in 6 to
give germole-germylene 7, (iii) a [1+2]cycloaddition of the
In conclusion, we have demonstrated that digermyne 1 acts
as a pre-catalyst for the formation of C–C bonds in the absence of a
transition-metal catalyst, specifically for the perfectly
regioselective cyclotrimerization of terminal arylacetylenes. In
these reactions, the two Ge atoms undergo mild redox processes
between Ge(II) and Ge(IV), which ultimately enables them to
transform small organic molecules with a performance that rivals
that of transition-metal catalysts. The absolute stereoselectivity of
the Ge-catalyst is probably due to kinetically controlled reactions
of such main-group-element compounds, while transition-metal-
based reactions are usually controlled thermodynamically.
Transition-metal catalysts for Reppe reactions promote the
cyclotrimerization based on their flexibility to switch between
oxidation states, while the combination of Ge(II) (germylene) and
Ge(IV) (germole) species stabilizes the redox process during the
germylene moiety in
7
to give 8, (iv) an intramolecular
4+2]cycloaddition (Diels-Alder reaction) to furnish 9, (v) a retro-
1+2]cycloaddition of the three-membered ring (germirane) moiety
[
[
in 9 to generate germanorbornadiene-germylene 10, (vi) a retro-
1+4]cycloaddition of the germanorbornadiene moiety in 10 to
[
release 1,2-digermabenzene 4 and 1,2,4-triarylbenzene 3, and (vii)
a regeneration of 5 from 4 and 2. Each step can be reasonably
explained based on the known reactivity of germanium and organic
compounds: (i) the structure of 5 can be considered as an
intramolecular [1+4]cycloadduct of
germole.
kcal/mol; DG = 2.5 kcal/mol) and that from 6 to 5 (DG 6-5 = 19.2
kcal/mol) suggest that 5 and 6 could be in equilibrium in solution
at higher temperature; (ii) the germylene moiety in 6 is reactive
toward arylacetylenes. The ºCH moiety could approach the
germylene moiety prior to the CºC unit inserting into the Ge–C=
bond to release thermodynamic energy due to an extension of the
a
germylene and
a
[
16,17]
‡
The calculated barriers from 5 to 6 (DG 5-6 = 21.7
‡
cyclotrimerization.
Low-coordinated
main-group-element
compounds may thus represent promising prospectives for
transition-metal-free catalysts for C–C coupling reactions.
‡
p-conjugation (6®7: DG = –10.6 kcal/mol; DG 6-7 = 14.2
kcal/mol);
Experimental Section
[
18]
(iii)
germylenes
typically
engage
in
This
‡
[19,20]
[
1+2]cycloadditions (7®8: DG 7-8 = 4.1 kcal/mol).
General Procedure for the catalytic cyclotrimerization of
process should be one of the key steps, as the geometry is
determined based on the direction of the approaching arylacetylene
phenylacetylene: In a J-Young NMR tube, a solution of digermyne 1 (14.7
6 6
mg, 14.1 µmol) in C D (0.5 mL) was treated with phenylacetylene (33.4
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Angewandte Chemie International Edition
10.1002/anie.201801222
COMMUNICATION
mg, 0.327 mmol, 23.2 equiv.). After 8 hours, the signals of 1,2,4-
1
tris(phenyl)benzene 3a (90% NMR yield) were observed in the H NMR
spectrum. Then, the mixture was filtered through a pad of silica gel (n-
hexane/CH Cl = 10/1) to give 3a (24.5 mg, 80.0 µmol, 84% yield). Further
2 2
details are shown in the Supporting Information.
Keywords: Digermyne, 1,2-Digermabenzene, Main-Group-Element
Catalyst, Cyclotrimerization, Reaction Mechanism
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Angewandte Chemie International Edition
10.1002/anie.201801222
COMMUNICATION
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COMMUNICATION
Tomohiro Sugahara, Jing-Dong Guo,
Takahiro Sasamori,* Shigeru Nagase,
and Norihiro Tokitoh
Catalytic Cyclotrimerization
by a Main-Group-Element compound
Page No. – Page No.
Tbb
Regioselective Cyclotrimerization of
Terminal Alkynes Using a Digermyne
Ge Ge
Ar
Tbb
4 mol%)
Ar
Ar
(
Ar
Ar
Ar
absolute selectivity
Tbb = 4-t-Bu-2,6-[CH(SiMe3)2]2-C6H2
An isolable digermyne, i.e., a compound with a GeºGe triple bond, acts as a
precatalyst for the cyclotrimerization of terminal arylacetylenes to afford the
corresponding 1,2,4-triarylbenzenes with absolute regioselectivity. The results
demonstrate that bespoke main-group-element compounds can catalytically
activate and transform small neutral organic molecules and induce the
formation of C–C bonds.
This article is protected by copyright. All rights reserved.