Catalytic Generation of Rhodium Silylenoid for Alkene-Alkyne-Silylene [2 + 2 + 1] Cycloaddition

Article abstract of DOI:10.1021/acs.orglett.9b00326

An alkene-alkyne-silylene [2 + 2 + 1] cycloaddition takes place in the rhodium-catalyzed reaction of 1,6-enynes with borylsilanes bearing an alkoxy group on the silicon atoms, which react as synthetic equivalents of silylene. The reaction proceeds efficiently in 1,2-dichloroethane at 80-110 °C in the presence of a rhodium catalyst bearing bis(diphenylphosphino)methane (DPPM) as a ligand to afford 1-silacyclopent-2-enes in good to high yields.

Full text of DOI:10.1021/acs.orglett.9b00326

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Catalytic Generation of Rhodium Silylenoid for Alkene−Alkyne−
Silylene [2 + 2 + 1] Cycloaddition
Toshimichi Ohmura,* Ikuo Sasaki, and Michinori Suginome*
Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University, Katsura,
Nishikyo-ku, Kyoto 615-8510, Japan
S
* Supporting Information
ABSTRACT: An alkene−alkyne−silylene [2 + 2 + 1] cycloaddition
takes place in the rhodium-catalyzed reaction of 1,6-enynes with
borylsilanes bearing an alkoxy group on the silicon atoms, which
react as synthetic equivalents of silylene. The reaction proceeds
efficiently in 1,2-dichloroethane at 80−110 °C in the presence of a
rhodium catalyst bearing bis(diphenylphosphino)methane (DPPM)
as a ligand to afford 1-silacyclopent-2-enes in good to high yields.
ioisosterism utilizing silicon, a technique used for
improving the medicinal effects of bioactive organic
Transition-metal-catalyzed [2 + 2 + 1] cycloaddition
involving silylene as the one-atom-component is an attractive
method for the construction of silicon-containing five-
membered rings. Alkyne−alkyne−silylene [2 + 2 + 1]
cycloaddition is well established as an efficient route to
silacyclopentadienes (siloles).6 However, to date, limited
success has been achieved with alkene−alkyne−silylene [2 +
2 + 1] cycloaddition to afford 1-silacyclopent-2-enes.7−9 This
is in sharp contrast to the great success of the analogous three-
component cyclization, the Pauson−Khand reaction.10
B
compounds by replacing the functional group with a silicon-
containing group, has received increasing attention in the drug
discovery field.1,2 Of particular interest in the screening of
silicon-based bioisosteres is silicon-containing carbo- and
heterocyclic skeletons. Examples of the skeletons that have
been investigated in drug discovery are shown in Figure 1. 4-
We have reported on palladium-catalyzed alkyne−alkyne−
silylene [2 + 2 + 1] cycloaddition to afford silacyclopenta-
dienes, where borylsilanes bearing a dialkylamino group on the
silicon atoms react as the synthetic equivalent of silylenes.11a
This palladium catalyst system was applied to 1,3-diene-
silylene [4 + 1] cycloaddition11b and 2-alkenylindole-silylene
[4 + 1] cycloaddition.11c However, all our attempts at
achieving alkene−alkyne−silylene [2 + 2 + 1] cycloaddition
on the basis of palladium catalysis failed. We therefore changed
our strategy to the use of other transition metal catalysts. Here,
we describe a rhodium-catalyzed alkene−alkyne−silylene [2 +
2 + 1] cycloaddition. The new rhodium catalysis using a
borylsilane is applicable to the conversion of a wide range of
1,6- and 1,7-enynes to bicyclic 1-silacyclopent-2-enes.7,8
Diethyl malonate-derived 1,6-enyne 1a was reacted with
(pin)B−Si(NEt2)Me2 (2, 1.0 equiv)12 in 1,2-dichloroethane at
80 °C in the presence of [RhCl(cod)]2 (2.5 mol %) and a
phosphorus ligand (5.5 mol %) (Table 1). When the reaction
was carried out with DPPB as a ligand, 1-silacyclopent-2-ene
4a and aminoboronate 5 were formed, albeit in low yields (5%
and 20% yields, respectively), indicating that a rhodium
catalyst could promote the [2 + 2 + 1] cycloaddition of 4a with
silylene formed from 2 (entry 1). It is interesting to note that
the catalyst efficiency was significantly improved by the use of
Figure 1. Examples of silicon-containing cyclic skeletons investigated
in drug discovery.
Silapiperidines have been examined as bioisosteres of the
piperidine and morpholine moieties in haloperidol, loper-
amide, and other compounds.3 Silaproline has been studied as
a surrogate of proline in the development of bioactive
peptides.4 Silacarbocycles have been tested as analogues of
the parent carbocyclic compounds such as venlafaxine and
atipamezole.5 These examples suggest that a variety of silicon-
containing cyclic skeletons may contribute to improving the
medicinal effects of bioactive compounds. Hence, the develop-
ment of efficient methods to construct untouched silicon-
containing cyclic skeletons, including 1-silacyclopent-2-enes
(Figure 1), is highly attractive and could accelerate the
development of silicon-containing drugs.
Received: January 15, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b00326
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Table 1. Reaction Conditions
Scheme 1. Synthesis of 1-Silacyclopent-2-enes through
a
Rhodium-Catalyzed [2 + 2 + 1] Cycloaddition
yield (%)
b
c
entry
borylsilane
ligand
time (h)
4a
5 or 6
d
1
2
3
4
5
6
7
8
9
10
2 (X = NEt2)
2
2
2
2
2
3a (X = OMe)
3b (X = OEt)
3c (X = Oi-Pr)
3d (X = Ot-Bu)
DPPB
DPPP
DPPE
16
16
16
16
16
16
6
6
6
6
5
48
38
82
11
10
79
80
20 (5)
86 (5)
89 (5)
>99 (5)
22 (5)
e
f
g
DPPM
PMePh2
−
h
33 (5)
DPPM
DPPM
DPPM
DPPM
86 (6a)
97 (6b)
94 (6c)
91 (6d)
i
83 (83)
83
a
1a (0.10 mmol), 2 (0.10 mmol), [RhCl(cod)]2 (2.5 mol %), ligand
(5.5 mol %), and ClCH2CH2Cl (0.1 mL) were stirred at 80 °C for 6
b
c
d
h. Determined by GC. Determined by 1H NMR. Ph2P-
e
f
g
(CH2)4PPh2. Ph2P(CH2)3PPh2. Ph2P(CH2)2PPh2. Ph2PCH2PPh2.
h
i
10 mol %. Isolated yield in the 0.30 mmol scale reaction.
bidentate phosphines with smaller bite angles (entries 2−4). In
the reaction with DPPP or DPPE, 5 was formed in high yields
(86−89%) and the yields of 4a were improved to 38−48%
(entries 2 and 3). A rhodium catalyst bearing DPPM showed
the highest catalyst efficiency: 2 was fully converted to 5 and
the reaction afforded 4a in 82% yield (entry 4). The reaction
with PMePh2, or in the absence of phosphorus ligand, resulted
in low catalyst efficiency (entries 5 and 6), indicating that
bidentate coordination of DPPM is essential for high catalyst
efficiency. As mentioned above, the Pd/PMePh2 catalyst
system that we reported previously11 was not effective and
no formation of 4a took place.
It is interesting to note that the Rh/DPPM-catalyzed [2 + 2
+ 1] cycloaddition of 1a took place efficiently with alkoxy-
substituted (pin)B−Si(OR)Me2 (3),12 which to date have not
been used as silylene sources under the conditions for the
palladium-catalyzed reaction.11 3a (R = Me), 3b (R = Et), 3c
(R = i-Pr), and 3d (R = t-Bu) showed comparable reactivity.
Reactions afforded 4a in 79−83% yields and alkoxyboranes
6a−d were formed (entries 7−10). Since the reactions of 3
were faster than 2 (see Supporting Information for detailed
comparison), we determined 3c to be the preferred silylene
source in the reaction of 1.
The Rh/DPPM-catalyzed [2 + 2 + 1] cycloaddition using 3c
as a silylene source was subjected to various enynes (Scheme
1). Dimethyl and diisopropyl malonate derivatives 1b and 1c
reacted efficiently at 80 °C to afford the corresponding 1-
silacyclopent-2-enes 4b and 4c in 80% and 83% yields,
respectively. Highly efficient ring formation took place to
afford 4d in the reaction of 9H-fluorene-derived 1d, probably
due to the rigid fluorene backbone. Pentane-2,4-dione
derivative 1e afforded 4e in 80% yield, where the electrophilic
carbonyl and the acidic α-hydrogen of the methyl ketone did
a
1 (0.30 mmol), 3c (0.30 mmol), [RhCl(cod)]2 (2.5 mol %), ligand
(5.5 mol %), and ClCH2CH2Cl (0.3 mL) were stirred at 80 or 110 °C
b
c
d
for 6 h. dr 1.5:1. Reaction was carried out for 12 h. Obtained as a
mixture with a structural isomer (3%). dr 2:1. 3c (1.2 equiv) was
used. Reaction was carried out for 18 h with 3c (1.2 equiv). DPPP
was used instead of DPPM. (pin)B−Si(NEt2)MePh (7) was used
instead of 3c. dr 2.4:1. dr 1:1.
e
f
g
h
i
j
k
not affect the reaction. Cyano and acetal groups were tolerated,
and 4f and 4g were obtained by the [2 + 2 + 1] cycloaddition.
Substrates having more flexible tether moieties were also
acceptable: 1i, 1k, and 1l afforded the corresponding products
4i, 4k, and 4l in 47−64% yields.13 Although allyl propargyl
ether 1j was consumed prior to 3c, and therefore the yield was
low, 4j was also synthesized.
B
DOI: 10.1021/acs.orglett.9b00326
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Substrate 1m bearing a n-butyl group on the alkyne terminus
underwent the [2 + 2 + 1] cycloaddition at 80 °C to afford 4m
in 80% yield. On the other hand, elevation of the reaction
temperature (110 °C) was required for the conversion of
phenyl-, methoxycarbonyl-, and trimethylsilyl-substituted 1n−
p to 4n−p. It should be noted that when DPPP was used as a
ligand, terminal alkyne 1q also underwent the [2 + 2 + 1]
cycloaddition to afford 4q. The [2 + 2 + 1] cycloaddition was
applicable to 1,6-enynes 1r−s bearing substituents on the
double bond and 1u−v bearing methyl groups on the allylic
positions, to afford 4r−s and 4u−v in 57−74% yields.14 In
addition to dimethylsilylene, the methylphenylsilylene moiety
was introduced to form 4t, when the reaction was carried out
with (pin)B−Si(NEt2)MePh (7).15 It should also be noted
that 1,7-enyne 1w also underwent the [2 + 2 + 1]
cycloaddition to afford 4w, albeit in moderate yield.
Scheme 4. Possible Mechanism
The reaction of 1a with (pin)B−SiMe2Ph (8) having no
heteroatom functional group on the silicon atom provided
mechanistic information (Scheme 2). In the presence of Rh/
then formed through either nucleophilic substitution on the
silicon center or σ-bond metathesis between Rh−C and Si−X
bonds, accompanied by the reformation of A.
Silicon-containing amino acids have received much attention
for their application in bioactive peptides.2,4 The Rh/DPPM-
catalyzed [2 + 2 + 1] cycloaddition was applied to the
synthesis of a new class of silicon-containing amino acid
derivatives (Scheme 5). The [2 + 2 + 1] cycloaddition of α-
Scheme 2. A Mechanistic Investigation
Scheme 5. Application to Synthesis of Novel Silicon-
Containing Amino Acid Derivatives
DPPM catalyst, 1a underwent the [2 + 2 + 1] cycloaddition
even with 8, to afford 4a and 4t in 19% and 15% yields,
respectively. The ring formation took place with release of
phenyl or methyl groups through Si−C bond cleavage. Ph−
B(pin) (9) and Me−B(pin) (10) were also formed and
present in the reaction mixture. Some reports describe the
rhodium-catalyzed formation of five-membered silacarbocycles
through substitution of the methyl or phenyl groups on
silicon,6e,h,16,17 where an elementary step shown in Scheme 3 is
suggested. The formation of 4a and 4t indicates the existence
of this elementary step in the present rhodium-catalyzed [2 + 2
+ 1] cycloaddition.
Scheme 3. An Elementary Step Suggested in the Literature
for Rhodium-Mediated Cyclization through Substitution of
the Methyl or Phenyl Groups on Silicon6e,h,16
a
The diastereomers were completely separable by silica gel column
b
chromatography. At 110 °C with 3c (1.2 equiv) for the first step.
amino acid-derived 1,6-enynes 11a−d with 3c took place
efficiently at 80−110 °C to afford the corresponding 1-
silacyclopent-2-enes (dr ca. 1:1). The products were treated
with aqueous citric acid in THF, and the aminoesters 12a−d
were obtained. The diastereomers of 12a were separable by
silica gel column chromatography; i.e., both diastereomers
were readily accessible by the [2 + 2 + 1] cycloaddition.
Because the starting compound 11 was readily prepared
through two or three steps from N-(diphenylmethylene)-
glycine ethyl ester, this method is practical for the synthesis of
a variety of silicon-containing α-aminoesters.
A proposed reaction mechanism is shown in Scheme 4.
Transmetalation between [Rh]−X (A, X = Cl, NR2, OR, Ph, or
Me) and the borylsilane takes place to afford a rhodium
silylenoid B.18 Insertion of the C−C triple bond of 1,6-enyne
into the Rh−Si bond of B proceeds in a regioselective manner
to form alkenylrhodium C. Subsequent insertion of the C−C
double bond into the Rh−C bond of C takes place to form
alkylrhodium D. A silicon-containing five-membered ring is
In conclusion, we established an efficient route to 1-
silacyclopent-2-enes through Rh/DPPM-catalyzed alkene−
alkyne−silylene [2 + 2 + 1] cycloaddition, where borylsilanes
bearing an alkoxy group on the silicon atoms participated in
C
DOI: 10.1021/acs.orglett.9b00326
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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the reaction as synthetic equivalents of silylene. A mechanism
involving rhodium silylenoid is proposed based on the fact that
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borylsilanes having no heteroatom functional group on the
silicon atom.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b00326.
Experimental procedures, characterization data of
compounds, and 1H and 13C NMR spectra of new
compounds (PDF)
(5) (a) Damour, D.; Barreau, M.; Dutruc-Rosset, G.; Doble, A.; Piot,
O.; Mignani, S. 1,1-Diphenyl-3-dialkylamino-1-silacyclopentane De-
rivatives: A New Class of Potent and Selective 5-HT2A Antagonists.
AUTHOR INFORMATION
Corresponding Authors
■
̈
Bioorg. Med. Chem. Lett. 1994, 4, 415−420. (b) Heinonen, P.; Sipila,
*E-mail: ohmura@sbchem.kyoto-u.ac.jp.
*E-mail: suginome@sbchem.kyoto-u.ac.jp.
̈
H.; Neuvonen, K.; Lonnberg, H.; Cockcroft, V. B.; Wurster, S.;
Virtanen, R.; Savola, M. K. T.; Salonen, J. S.; Savola, J. M. Synthesis
and pharmacological properties of 4(5)-(2-ethyl-2,3-dihydro-2-
silainden-2-yl)imidazole, a silicon analogue of atipamezole. Eur. J.
Med. Chem. 1996, 31, 725−729. (c) Daiss, J. O.; Burschka, C.; Mills,
J. S.; Montana, J. G.; Showell, G. A.; Warneck, J. B. H.; Tacke, R. Sila-
venlafaxine, a Sila-Analogue of the Serotonin/Noradrenaline Reup-
take Inhibitor Venlafaxine: Synthesis, Crystal Structure Analysis, and
Pharmacological Characterization. Organometallics 2006, 25, 1188−
1198. (d) Wang, J.; Ma, C.; Wu, Y.; Lamb, R. A.; Pinto, L. H.;
DeGrado, W. F. Exploring Organosilane Amines as Potent Inhibitors
and Structural Probes of Influenza A Virus M2 Proton Channel. J. Am.
Chem. Soc. 2011, 133, 13844−13847. (e) Ramesh, R.; Reddy, D. S.
Zinc mediated allylations of chlorosilanes promoted by ultrasound:
Synthesis of novel constrained sila amino acids. Org. Biomol. Chem.
ORCID
Toshimichi Ohmura: 0000-0002-2534-3769
Ikuo Sasaki: 0000-0001-6612-9358
Michinori Suginome: 0000-0003-3023-2219
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was partially supported by JSPS KAKENHI Grant
Numbers 17J05734 for JSPS Research Fellow (I.S.) and
JP15H05811 for Scientific Research on Innovative Areas in
Precisely Designed Catalysts with Customized Scaffolding
(M.S.).
̈
2014, 12, 4093−4097. (f) Geyer, M.; Baus, J. A.; Fjellstrom, O.;
Wellner, E.; Gustafsson, L.; Tacke, R. Synthesis and Pharmacological
Properties of Silicon- Containing GPR81 and GPR109A Agonists.
ChemMedChem 2015, 10, 2063−2070.
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(11) (a) Ohmura, T.; Masuda, K.; Suginome, M. Silylboranes
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(13) In the reactions that afford 4 in moderate yields, starting 1 and
3c were completely consumed after the indicated reaction time.
However, we could not confirm assignable byproducts by GCMS
analyses of the reaction mixture. We presume that oligomerization of
1 competed with the cyclization under the conditions.
(14) An additional reaction example of a substrate derived from (E)-
non-2-en-7-yne is described in the Supporting Information.
(15) 7 was prepared by the reaction of Li−Si(NEt2)MePh with
(pin)B−Oi-Pr (see Supporting Information). Considering the ease of
preparation, we used this amino(boryl)silane instead of the
corresponding alkoxy(boryl)silane.
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E
DOI: 10.1021/acs.orglett.9b00326
Org. Lett. XXXX, XXX, XXX−XXX