Ring-Closing Olefin Metathesis Catalyzed by Well-Defined Vanadium Alkylidene Complexes

Article abstract of DOI:10.1002/chem.202005438

Vanadium-based catalysts have shown activity and selectivity in ring-opening metathesis polymerization of strained cyclic olefins comparable to those of Ru, Mo, and W catalysts. However, the application of V alkylidenes in routine organic synthesis is limited. Here, we present the first example of ring-closing olefin metathesis catalyzed by well-defined V chloride alkylidene phosphine complexes. The developed catalysts exhibit tolerance to various functional groups, such as an ether, an ester, a tertiary amide, a tertiary amine, and a sulfonamide. The size and electron-donating properties of the imido group and the phosphine play a crucial role in the stability of active intermediates. Reactions with ethylene and olefins suggest that both β-hydride elimination of the metallacyclobutene and bimolecular decomposition are responsible for catalyst degradation.

Full text of DOI:10.1002/chem.202005438

Communication
doi.org/10.1002/chem.202005438
Chemistry—A European Journal
&
Homogeneous Catalysis
Ring-Closing Olefin Metathesis Catalyzed by Well-Defined
Vanadium Alkylidene Complexes
Dmitry S. Belov,[a] Gabriela Tejeda,[a] Charlene Tsay,[b] and Konstantin V. Bukhryakov*[a]
aryl[12–18] or 1-adamantyl group;[13,15,16,19] X is an amide,[12,15]
Abstract: Vanadium-based catalysts have shown activity
alkyl,[13] or alkoxide;[14,16–19] and L is an NHC[13] or PMe3
and selectivity in ring-opening metathesis polymerization
have been extensively explored by the Nomura group for the
of strained cyclic olefins comparable to those of Ru, Mo,
ring-opening metathesis polymerization (ROMP) of various
and W catalysts. However, the application of V alkylidenes
cyclic alkenes.[20] The critical step of the alkylidene formation,
in routine organic synthesis is limited. Here, we present
the a-hydrogen abstraction in the presence of PMe3, is shown
the first example of ring-closing olefin metathesis cata-
in Scheme 1 along with examples of highly active V complexes
lyzed by well-defined V chloride alkylidene phosphine
for ROMP of norbornene (NBE).[16,17] Although those complexes
complexes. The developed catalysts exhibit tolerance to
contain two PMe3 ligands, the dissociation of one of the phos-
various functional groups, such as an ether, an ester, a ter-
phine ligands is required to access the 14-electron catalytically
tiary amide, a tertiary amine, and a sulfonamide. The size
active species,[16] as is the case for Ru-[21] and Mo-based[22] cata-
and electron-donating properties of the imido group and
lysts. Important to mention, other phosphines have not been
the phosphine play a crucial role in the stability of active
applied for V alkylidene synthesis.[20]
[14–19]
intermediates. Reactions with ethylene and olefins suggest
that both b-hydride elimination of the metallacyclobutene
and bimolecular decomposition are responsible for cata-
lyst degradation.
Ring-closing metathesis (RCM) of dienes[1] is a widely applied
method for the synthesis of natural products and biologically
active compounds.[2] Nowadays, commonly used homogene-
ous catalysts for RCM are based on well-defined Ru,[3] Mo and
W[4] alkylidenes; some of them exhibit remarkably high activi-
ty[5] and enantioselectivity.[6] Examples of RCM catalyzed by
Scheme 1. Synthesis of V alkylidenes developed by Nomura. Bottom: turn-
over numbers (TON) for reaction with NBE are indicated.
other well-defined transition metal complexes are rather limit-
ed. Thus, to the best of our knowledge, only two Os com-
plexes capable of performing RCM were reported.[7] However,
Despite the successful application of V complexes for ROMP,
Os is among the rarest elements in the Earth’s crust,[8] which
examples of V-catalyzed olefin metathesis of acyclic olefins are
narrows its use in catalysis. Although ill-defined complexes of
Nb,[9] and Re[10] have been shown to promote RCM, the nature
of the active species remains unknown. Tebbe’s reagent,
Cp2TiCH2AlClMe2 can promote RCM,[11] but reaction requires
stoichiometric amounts of the Ti complex.
surprisingly limited. Although a few examples of cross-meta-
thesis (CM) have been reported recently utilizing Nomura’s cat-
alysts,[23,24] rapid V alkylidene decomposition precluded com-
plete conversion.
The use of abundant first-row metals, such as vanadium, to
make well-defined catalysts for RCM of olefins is highly desira-
ble to provide less expensive and greener alternatives for exist-
ing methods. V is the 20th most abundant element in the
Earth’s crust. The abundance of V is ꢀ102 times higher com-
pared to Mo and W and ꢀ105 times higher than for Ru.[25] As a
result, it is substantially less expensive than the rare metals
that are currently used. Additionally, purification, isolation, and
recycling of precious metals consume energy and generate a
significant amount of waste. Therefore, the use of V-based cat-
alysts will make valuable olefins more accessible to consumers
and decrease the human environmental footprint.
In the last decade, high-oxidation-state vanadium alkylidene
complexes of the type V(NR)(CHSiMe3)(X)(L), where R is an
[a] Dr. D. S. Belov, G. Tejeda, Dr. K. V. Bukhryakov
Department of Chemistry and Biochemistry
Florida International University
11200 SW 8th St., Miami, FL 33199 (USA)
E-mail: kbukhrya@fiu.edu
[b] Dr. C. Tsay
Department of Chemistry
University of California, Riverside, CA 92521 (USA)
Supporting information and the ORCID identification number(s) for the
author(s) of this article can be found under:
https://doi.org/10.1002/chem.202005438.
Recently, the Schrock group reported a method of promot-
ing a-hydrogen abstraction from Mo dialkyl complexes by
Chem. Eur. J. 2021, 27, 4578 –4582
4578
ꢀ 2020 Wiley-VCH GmbH

Communication
doi.org/10.1002/chem.202005438
Chemistry—A European Journal
using phosphonium chlorides to obtain Mo chloride alkylidene
phosphine complexes in good yields.[22,26] The resulting com-
plexes are versatile starting material for the synthesis of highly
active and E/Z selective catalysts.[27] Inspired by this work, we
have now prepared several V chloride alkylidene phosphine
complexes, a new class of compounds, and examined them in
RCM reactions. To our knowledge, we report the first examples
of RCM catalyzed by well-defined V alkylidene complexes.
Compounds 1a–e (Scheme 2) were each synthesized in one
pot, starting from imido trialkyl V complexes in the presence
of HCl and phosphines in Et2O.
isomer, which gives the V=C bond partial triple-bond charac-
ter; this has also been observed for Mo alkylidenes.[29]
X-ray crystal structures of anti-1c and anti-1d were also ob-
tained (see Supporting Information), though poor diffraction
and complex disorder in the structures preclude detailed dis-
cussion of the respective bond lengths and angles.
The NMR studies of 1d further support the presence of an
agostic interaction in the syn-isomer. The resonance of the al-
kylidene hydrogen Ha of the anti-isomer appears downfield of
the syn Ha resonance (Figure 2), as it does in analogous Mo
complexes.[30] The anti Ha signal gives a sharp triplet at
16.01 ppm (3JHP =8.0 Hz, 1JCH =124 Hz, 228C), suggesting
strong binding of the two PMe3 ligands. Although the ob-
1
served JCH is relatively small compared to the CH-coupling of a
typical anti-alkylidene (for Mo and W),[31] the NOESY spectrum
revealed the cross peaks between the alkylidene and imido
methine protons, with no correlation between the methine
and TMS-group protons. In contrast, the syn-alkylidene (Ha at
13.48 ppm, 3JHP =4.0 Hz at À408C) shows no correlation be-
tween the alkylidene and methine protons but does exhibit
cross peaks between the methine and TMS-group protons. The
agostic interaction makes the syn-isomer less Lewis acidic than
the anti-isomer, which leads to broadening of the syn Ha signal
since the PMe3 ligand is exchanging relatively rapidly with free
PMe3 at room temperature.[29] The syn-isomer is 5-coordinate;
thus, the addition of five equivalents of free phosphine does
not change the syn:anti ratio, which one might expect if there
Scheme 2. Prepared V chloride alkylidenes with isolated yields. Syn:anti ratio
determined in solution by H NMR at 228C (C6D6).
1
An X-ray diffraction study of 1e (Figure 1) revealed a mixture
of two isomers: anti-1e (ꢀ91%) and syn-1e (ꢀ9%).[28] anti-1e
has a distorted trigonal bipyramidal geometry with phosphines
in axial positions [VÀP1 2.4708(4) Š, VÀP2 2.4935(4) Š, P1-V-P2
170.12(2)8], similar to the structure reported for anti-V(N-2,6-
(iPr)2C6H3)(CHSiMe3)(OC6F5)(PMe3)2 (anti-A) [VÀP1 2.472 Š, VÀP2
2.480 Š, P1-V-P2 168.928].[24] The VÀN and VÀC bond lengths
and Si-C-V angles in anti-1e (1.6873(14) Š, 1.9153(14) Š,
131.64(7)8) and anti-A (1.691 Š, 1.917 Š, 132.58) are also
similar. Notably, the differences in VÀC bond distances and
Si-C-V angles between anti-1e (1.9153(14) Š, 131.64(7)8) and
[18]
the reported syn-V(N-2,6-Me2C6H3)(CHSiMe3)(OC6Cl5)(PMe3)2
(1.845(3) Š, 167.538) are more pronounced due to the agostic
interaction (electron donation from CÀH to V) in the syn-
Figure 1. X-ray crystal structure of anti-1e. Thermal ellipsoids shown at 50%
probability. Hydrogen atoms, except alkylidene hydrogen H1, have been
omitted for clarity.
Figure 2. 1H NMR spectra of the alkylidene (Ha) and imido methine (H) re-
gions of 1d at variable temperature (in [D8]toluene).
Chem. Eur. J. 2021, 27, 4578 –4582
www.chemeurj.org
4579
ꢀ 2020 Wiley-VCH GmbH

Communication
doi.org/10.1002/chem.202005438
Chemistry—A European Journal
was an equilibrium between a 5-coordinate anti-isomer and a
4-coordinate syn-isomer.
escape of ethylene gas from the reaction mixture.[34] Reactions
in closed vials exhibited a lower conversion of 2 to 3 in all
cases (entries 1–5, Table 1), confirming the catalyst’s limited
stability in the presence of ethylene. The slow addition of a
stock solution of 1e to 2 over a period of 1.7 hours in an open
vial allowed ethylene to escape from the reaction mixture. As a
result, a 95% conversion to 3 was achieved (entry 6, Table 1).
The reaction of 2b with ethylene gas revealed slow decom-
position of the alkylidene with formation of free phosphine,
The imido methine resonances of the anti-isomer appear as
two broad signals at 228C and two sharp septets at À408C
(Figure 2), suggesting that rotation of the arylimido group is
restricted at reduced temperatures. The inequivalent methine
resonances coalesce when coordinated PMe3 begins to ex-
change rapidly with free PMe3 at 808C, which results in broad-
ening of the anti-alkylidene proton. In contrast, both methine
protons in the syn-isomer appear as one sharp septet at 228C,
which broadens at À408C. We conclude that rotation of the ar-
ylimido group occurs in a four-coordinate complex,[32] which is
more accessible for the less Lewis acidic syn-isomer due to the
agostic interaction mentioned above.
vinylTMS, propene, and
a new paramagnetic complex
(Scheme 3). The formation of propene can be explained by b-
hydride elimination of the metallacyclobutane 6.[35]
The catalyst 1d is relatively stable in the solution. Thus, we
observed only 35% decomposition of 1d (0.023m, C6D6) at
558C after 11 days, showing that 1d is more thermally stable
compared to Ru(CHPh)(PCy3)2(Cl)2 (50% decomposition after 8
days under the same conditions).[33]
The metathesis activity of complexes 1a–e in the RCM reac-
tion with diallyl N-tosylamide 2 is summarized in Table 1.
Table 1. RCM of 2 catalyzed by 1a–e.
Scheme 3. The reaction of 1b with ethylene.
Notably, the stability of the active species depends on the
catalyst concentration; thus, TON for 1d increases with lower
catalyst loading (entries 4, 7–9, Table 1), suggesting that a bi-
molecular decomposition should be considered as a catalyst
degradation pathway. The methylidene 5 (Scheme 2) is argua-
bly the least sterically hindered complex and thus the most
prone to bimolecular decomposition.[36] However, a 15-fold in-
crease of the catalyst concentration did not lead to a signifi-
cant decrease of TON (9.0 vs. 5.3, entries 7 and 9, Table 1). We
conclude that b-hydride elimination of metallacyclobutane 6,
and not bimolecular decomposition, is the primary degrada-
tion pathway for our system, as is observed for V alkoxide com-
plexes.[23,24]
Entry
Cat.
Cat., mol%
Conv., %[a]
TON
1
2
3
4
5
6
7
8
9
1a
1b
1c
1d
1e
1e
1d
1d
1d
5
5
5
5
5
5
1
10
15
10 (8)[b]
63 (42)[b]
32 (29)[b]
40 (22)[b]
59 (54)[b]
95[c]
9
72
80
2.0
12.6
6.4
8.0
11.8
19
9.0
7.2
5.3
1
[a] by H NMR. [b] closed vial. [c] slow addition of 1e, 1.7 h.
We have also explored the RCM activity of 1a–e toward sub-
strates containing crotyl groups (Scheme 4). The second prod-
uct of the reactions is but-2-ene (usually, with E:Z ratio ꢀ7:3),
which does not lead to catalyst decomposition. Thus, com-
pound 3 was obtained with high conversion in all cases. The
reaction proceeds slowly at room temperature (42% conver-
sion to 3 in 24 h with catalyst 1d).
Products containing a tosylate (3, 7), an ether (7, 8), a terti-
ary amide (9), a tertiary amine (10), or an ester (11) were also
accessed. However, an alkene capable of chelating to the V
center (11) reacted with low yield in all cases. The metathesis
activity depends on both catalyst and substrate. In particular,
1a gives the highest conversion for 8 while 1c exceeds other
catalysts in the reaction to produce 10. Although 1b and 1e
have a similar activity toward 2; 1b outperforms 1e in the re-
actions containing disubstituted olefins, presumably due to
the steric hindrance resulting from two isopropyl groups and a
large phosphine in 1e. Thus, less sterically demanding 1d ex-
Vanadium chloride alkylidene phosphine complexes are
active catalysts in the RCM reaction of 2. Variations in the
imido group have a significant effect on catalytic activity. Thus,
an increase in imido group size and electron-donating proper-
ties in the order 1a-1c-1d leads to a corresponding increase
in the turnover number (TON, entries 1, 3, and 4, Table 1). An
increase in phosphine size has an even more pronounced
effect on the catalytic activity, as 1b and 1e are the most
active of the five synthesized catalysts (entries 2 and 5,
Table 1).
Following the catalytic reactions by 1H NMR (entry 9,
Table 1), we observed the formation of a new alkylidene spe-
cies (see Supporting Information). Both syn- and anti-alkylidene
signals of 1d and the new alkylidene slowly disappeared over
a few hours, suggesting decomposition of the active species.
Catalytic reactions were conducted in open vials to allow
Chem. Eur. J. 2021, 27, 4578 –4582
www.chemeurj.org
4580
ꢀ 2020 Wiley-VCH GmbH

Communication
doi.org/10.1002/chem.202005438
Chemistry—A European Journal
based upon work supported by the National Science Founda-
tion under Award No. 1919677.
Conflict of interest
The authors declare no conflict of interest.
Keywords: homogeneous catalysis · metathesis · ring-closing ·
vanadium · a-hydrogen abstraction
[1] G. C. Fu, R. H. Grubbs, J. Am. Chem. Soc. 1992, 114, 5426–5427; G. C. Fu,
S. T. Nguyen, R. H. Grubbs, J. Am. Chem. Soc. 1993, 115, 9856–9857.
[2] A. Deiters, S. F. Martin, Chem. Rev. 2004, 104, 2199–2238; S. Monfette,
D. E. Fogg, Chem. Rev. 2009, 109, 3783–3816; M. Yu, S. Lou, F. Gonza-
lez-Bobes, Org. Process Res. Dev. 2018, 22, 918–946; C. Lecourt, S.
Dhambri, L. Allievi, Y. Sanogo, N. Zeghbib, R. Ben Othman, M. I. Lannou,
G. Sorin, J. Ardisson, Nat. Prod. Rep. 2018, 35, 105–124.
[3] R. H. Grubbs, Angew. Chem. Int. Ed. 2006, 45, 3760–3765; Angew. Chem.
2006, 118, 3845–3850.
Scheme 4. Scope of RCM catalyzed by 1a–e.
hibits higher conversion than 1e in all cases shown in
Scheme 3, except 8. Generally, catalyst 1b displays the highest
(3, 7, 9, 11) or similar activity (8, 10) toward tested substrates.
We conclude that increasing the electrophilicity of the imido
group and the s-donating properties and size of the neutral
ligand is the strategy to develop reliable V-based catalysts for
olefin metathesis.
[4] R. R. Schrock, Angew. Chem. Int. Ed. 2006, 45, 3748–3759; Angew. Chem.
2006, 118, 3832–3844.
[5] K. M. Kuhn, T. M. Champagne, S. H. Hong, W.-H. Wei, A. Nickel, C. W. Lee,
S. C. Virgil, R. H. Grubbs, R. L. Pederson, Org. Lett. 2010, 12, 984–987.
[6] R. R. Schrock, J. Y. Jamieson, S. J. Dolman, S. A. Miller, P. J. Bonitatebus,
A. H. Hoveyda, Organometallics 2002, 21, 409–417; T. S. Pilyugina, R. R.
Schrock, P. Müller, A. H. Hoveyda, Organometallics 2007, 26, 831–837.
[7] R. Castarlenas, M. A. Esteruelas, E. OÇate, Organometallics 2005, 24,
4343–4346.
We have shown that V arylimido chloride alkylidene com-
plexes can be prepared in the reaction of arylimido V trialkyls
and HCl in the presence of phosphines. Our approach allows
for the synthesis of V chloride alkylidenes bearing NC5F6, N-2,6-
Me2C6H3, and N-2,6-(iPr)2C6H3; and PMe3, PEt3, and PPhMe2. All
prepared complexes are a mixture of syn- and anti-isomers in
solution. NMR studies show that syn-isomers do not strongly
bind phosphines, presumably due to an agostic interaction be-
tween H-alkylidene and V; this may result in the difference of
initiation, selectivity, and catalytic activity of two isomers.[32,37]
The catalytic activity in RCM reactions strongly depends on the
size and electron-donating properties of the imido group, as
well as the size and s-donating properties of the phosphine.
The active intermediates have limited stability toward ethylene.
Although bimolecular decomposition contributes to catalyst
degradation, the primary decomposition pathway involves b-
hydride elimination of the metallacyclobutane. We are now
confident that V-based olefin metathesis catalysts for routine
organic synthesis can be prepared. We are looking forward to
exploring V chloride alkylidenes as versatile starting materials
to alternate anionic and neutral ligands around a metal center
to develop catalysts that are stable to ethylene and tolerant of
various functional groups and to examining their reactions
with olefins in detail.
[8] W. F. McDonough, S. S. Sun, Chem. Geol. 1995, 120, 223–253.
[9] M. Fuji, J. Chiwata, M. Ozaki, S. Aratani, Y. Obora, ACS Omega 2018, 3,
8865–8873.
[10] M. F. Schneider, H. Junga, S. Blechert, Tetrahedron 1995, 51, 13003–
13014.
[11] K. C. Nicolaou, M. H. D. Postema, E. W. Yue, A. Nadin, J. Am. Chem. Soc.
1996, 118, 10335–10336.
[12] J. Yamada, M. Fujiki, K. Nomura, Organometallics 2005, 24, 2248–2250.
[13] W. Zhang, K. Nomura, Organometallics 2008, 27, 6400–6402.
[14] K. Nomura, Y. Onishi, M. Fujiki, J. Yamada, Organometallics 2008, 27,
3818–3824; X. Hou, K. Nomura, J. Am. Chem. Soc. 2016, 138, 11840–
11849; K. Nomura, X. Hou, Organometallics 2017, 36, 4103–4106.
[15] K. Nomura, B. K. Bahuleyan, K. Tsutsumi, A. Igarashi, Organometallics
2014, 33, 6682–6691.
[16] X. Hou, K. Nomura, J. Am. Chem. Soc. 2015, 137, 4662–4665.
[17] H. Hayashibara, X. Hou, K. Nomura, Chem. Commun. 2018, 54, 13559–
13562.
[18] S. Chaimongkolkunasin, K. Nomura, Organometallics 2018, 37, 2064–
2074.
[19] K. Nomura, K. Suzuki, S. Katao, Y. Matsumoto, Organometallics 2012, 31,
5114–5120; K. Hatagami, K. Nomura, Organometallics 2014, 33, 6585–
6592.
[20] K. Nomura, X. Hou, Dalton Trans. 2017, 46, 12–24.
[21] R. H. Grubbs, Tetrahedron 2004, 60, 7117–7140.
[22] K. V. Bukhryakov, S. VenkatRamani, C. Tsay, A. Hoveyda, R. R. Schrock, Or-
ganometallics 2017, 36, 4208–4214.
[23] W. S. Farrell, Organometallics 2019, 38, 3481–3485.
[24] W. S. Farrell, C. Greene, P. Ghosh, T. H. Warren, P. Y. Zavalij, Organometal-
lics 2020, 39, 3906–3917.
[25] W. M. Haynes, D. R. Lide, T. J. Bruno, CRC handbook of chemistry and
physics: a ready-reference book of chemical and physical data., 97th ed.
Florida: CRC Press, Boca Raton, 2016, 14–17.
Acknowledgements
[26] J. K. Lam, C. Zhu, K. V. Bukhryakov, P. Müller, A. Hoveyda, R. R. Schrock, J.
Am. Chem. Soc. 2016, 138, 15774–15783.
We are grateful to the Florida International University and the
New Faculty Development Grant provided by U.S. Nuclear Reg-
ulatory Commission for financial support. This material is
[27] M. J. Koh, T. T. Nguyen, J. K. Lam, S. Torker, J. Hyvl, R. R. Schrock, A. H.
Hoveyda, Nature 2017, 542, 80; T. T. Nguyen, M. J. Koh, T. J. Mann, R. R.
Schrock, A. H. Hoveyda, Nature 2017, 552, 347; Y. Mu, T. T. Nguyen, M. J.
Koh, R. R. Schrock, A. H. Hoveyda, Nat. Chem. 2019, 11, 478–487; P. E.
Chem. Eur. J. 2021, 27, 4578 –4582
www.chemeurj.org
4581
ꢀ 2020 Wiley-VCH GmbH

Communication
doi.org/10.1002/chem.202005438
Chemistry—A European Journal
Sues, K. V. Bukhryakov, R. R. Schrock, Helv. Chim. Acta 2017, 100,
e1700181.
[28] J. C. Axtell, R. R. Schrock, P. Müller, A. H. Hoveyda, Organometallics 2015,
34, 2110–2113.
[29] S. S. Zhu, D. R. Cefalo, D. S. La, J. Y. Jamieson, W. M. Davis, A. H. Hovey-
da, R. R. Schrock, J. Am. Chem. Soc. 1999, 121, 8251–8259.
[30] R. R. Schrock, A. H. Hoveyda, Angew. Chem. Int. Ed. 2003, 42, 4592–
4633; Angew. Chem. 2003, 115, 4740–4782.
[35] W. C. P. Tsang, R. R. Schrock, A. H. Hoveyda, Organometallics 2001, 20,
5658–5669; W. C. P. Tsang, J. Y. Jamieson, S. L. Aeilts, K. C. Hultzsch, R. R.
Schrock, A. H. Hoveyda, Organometallics 2004, 23, 1997–2007.
[36] S. H. Hong, A. G. Wenzel, T. T. Salguero, M. W. Day, R. H. Grubbs, J. Am.
Chem. Soc. 2007, 129, 7961–7968.
[37] H. Jeong, J. M. John, R. R. Schrock, Organometallics 2015, 34, 5136–
5145.
[31] R. R. Schrock, W. E. Crowe, G. C. Bazan, M. DiMare, M. B. O’Regan, M. H.
Schofield, Organometallics 1991, 10, 1832–1843.
[32] J. H. Oskam, R. R. Schrock, J. Am. Chem. Soc. 1993, 115, 11831–11845.
[33] M. Ulman, R. H. Grubbs, J. Org. Chem. 1999, 64, 7202–7207.
[34] A. H. Hoveyda, Z. Liu, C. Qin, T. Koengeter, Y. Mu, Angew. Chem. Int. Ed.
2020, 59, 22324–22348.
Manuscript received: December 22, 2020
Accepted manuscript online: December 22, 2020
Version of record online: February 5, 2021
Chem. Eur. J. 2021, 27, 4578 –4582
www.chemeurj.org
4582
ꢀ 2020 Wiley-VCH GmbH