Cycloaddition catalyzed by a new cationic rhodium-bisphosphine monooxide complex

Article abstract of DOI:10.1039/c4cc01320d

A rhodium-BozPHOS based complex is reported. This complex is competent in catalyzing the [4+2+2] cycloisomerization of cyclooctatrienes in moderate to good yields. The X-ray crystal structure of this complex is reported, along with formation of both bicyc

Full text of DOI:10.1039/c4cc01320d

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[
4+2+2] Cycloaddition Catalyzed by a New Cationic
Cite this: DOI:
10.1039/x0xx00000x
Rhodium-Bisphosphine Monooxide Complex
c
Gino Martin. R. Canlas and Scott R. Gilbertson*
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
ꢀ
A rhodium-BozPHOS based complex is reported. This
complex is competent in catalyzing the [4+2+2] Cl) that selectively provided the [4+2+2] product. Despite its
cycloisomerization of cyclooctatrienes in moderate to effectiveness in the reaction, the exact composition of the catalyst
3
6
was never identified. Furthermore, the lack of a discrete species
hindered making improvements in stereoselectivity and substrate
scope. Reported here is a rhodium-bisphosphine monooxide complex
that is catalytically comparable to the previously reported system
good yields. The x-ray crystal structure of this complex
is reported, along with formation of both bicyclic and
tricyclic cyclooctatrienes.
Over the last few years a wide variety of higher-order and is likely to be the same species. The complex is characterized by
cycloadditions have been developed. These methods have provided single crystal X-ray diffraction and NMR. Additionally, the substrate
access to a number of medium sized ring sizes with eight member scope for the reaction was expanded with this catalyst to include
1
-18
rings being one of the most common.
Structurally complex cyclic dienynes, providing fused tricyclic cyclooctanoids.
cyclooctanoid systems with interesting bioactivities are present in
numerous natural products (Figure 1). Many of these cyclooctane
systems are embedded within fused polycyclic frameworks
complicating their synthesis. A number of strategies have been
R
X
rhodium
catalyst
+
H
R
X
H
Me
Me
R = H or alkyl
X = O or NTs
1
9-35
developed to synthesize these structures.
We previously reported
an efficient, diastereoselective rhodium-catalyzed [4+2+2] Scheme 1. General scheme for the rhodium-catalyzed synthesis of
cycloaddition method which provides a functionally versatile fused polycyclic cyclooctatriene systems.
3
6, 37
bicyclic [6.3.0] core with a variety of substrates (Scheme 1).
The
Since the initial catalyst was formed by a reaction that did not
provide a discrete, isolable complex, a range of reaction conditions
were investigated. Temperature, solvent, ligand and metal were
varied. Reactions were attempted with both bis and monophosphine
original system was discovered while developing rhodium catalyzed
3
8, 39
[
4+2] cycloisomerizations.
product was observed. Eventually a catalyst system was prepared
using a [Rh(nbd)Cl] (1.0 equiv.), (S,S)-Me-DuPHOS (2.0 equiv.)
(1.0 equiv.) (note this is ½ an equivalent of Ag relative
As the catalyst aged, a [4+2+2]
2
and AgSbF
to
6
Table 1: Exploratory conditions in trying to obtain a discrete
catalyst for the [4+2+2] dimerization with acyclic substrate.
O
H
O
OH
Me
MeO
H H
OHC
O
H
O
H
O
H
Me
Me
Me
Me
O
H
conditions
O
H
HO
Me
Me
O
2
Me
Me
Me
H O
Me
O
Me
O
HO
H
Me
OH
Me Me
H
asteriscanolide
H
O
1
ophiobolin A
abyssomicin E
Entry
Complex/Catalyst Mixture
Solvent,
Conditions
%Yield
a
(2)
b
o
1
2
3
Rh/nbd/DuPHOS/SbF
6
CH
CH
2
2
Cl
Cl
2
2
, 60 C, 24 h
83
12
30
Figure 1. Cyclooctanoid natural products containing an eight-
b
o
Rh/nbd/dppf/SbF
6
, 65 C, 24 h
membered ring (blue) embedded in a fused polycyclic system.
[Rh(cod){(S,S)-Me-DuPHOS}]BF
4
6:1 CH
2 2
Cl :EtOAc,
rt, 24 h
o
Department of Chemistry, University of Houston, Houston TX 77204
Electronic supplementary information (ESI) available: Experimental
4
[Rh(nbd){(S,S)-Me-
CH
2
Cl
2
, 60 C, 24 h
80
c
BozPHOS}]SbF6,
b
a
1
13
31
procedure, characterization data, H, C and P NMR spectra of
Isolated yield of 2. Catalyst mixture was formed with ½ equivalent of
c
AgSbF
AgSbF
6
relative to Cl. Complex was synthesized using full equivalent of
relative to Cl.
compounds
6
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DOI: 10.1039/C4CC01320D
ligands, CHIRAPHOS, triphenylphosphine, DuPhos, BINAP,
RajPHOS, dppf, DPPP and MOP. An N-heterocyclic carbene as well
as rhodium with no donor ligand present were also tested. (Details
of the systems investigated can be found in the supplemental
Table 2: Synthesis of [4+2+2] cycloadducts
Secondary
Alkyne
Entry
Substrate
Product
%Yield
O
material.)
In
the
original
rhodium
catalyzed
[4+2]
cycloisomerization, it was observed that as the [Rh(cod){(S,S)-Et-
DuPHOS}]OTf complexes “aged“, [4+2+2] product was formed.
This observation led to using a DuPHOS-monooxide (BozPHOS) as
O
O
1
ꢀ
noneꢀ
noneꢀ
80ꢀ
Me
1
H
Me
Me
ꢀ
2
4
0-44
ꢀ
ꢀ
a ligand in the system.
This rhodium complex resulted in the
Ts
N
formation of the desired product in good yield (Table 1).ꢀꢀ
TsN
The structure of the BozPHOS complex was analyzed by
2ꢀ
TsN
57ꢀ
Me
3
1
4
Me
NMR. Two sets of P doublets of doublets corresponding to the Rh-
bound ligand were observed, with a P-P coupling of 14.97 Hz and
Rh-P coupling constants of 0.75 and 169.94 Hz. These values are
consistent with the phosphine directly bound to rhodium and the
phosphine oxide bound through the oxygen. Single crystal X-ray
analysis revealed a structure consistent with the NMR data, with the
phosphine and the oxygen for phosphine oxide directly bound to the
rhodium.
ꢀ
ꢀ
H
9
Me
O
O
3
4
ꢀ
ꢀ
H
H
H
H
45ꢀ
71ꢀ
Me
ꢀ
ꢀ
Me
1
10
TsN
11
H
ꢀ
ꢀ
ꢀ
ꢀ
TsN
Me
Me
4
H
OBn
O
5
6
ꢀ
ꢀ
OBn
53ꢀ
97ꢀ
Me
O
ꢀ
Figure 2. Structure of the Rh-BozPHOS complex,
1
Me
1
2
H
ꢀ
ꢀ
[
Rh(nbd){(S,S)-Me-BozPHOS}]SbF6, 3.
O
O
H
H
H
Me
ꢀ
H
Me
1
3
5
Me
ꢀ
Me
P
Me
Rh+
O
6
5ꢀ
O
P
7ꢀ
8ꢀ
Me
H
O
ꢀ
H
dꢀ
6
:1
Me
H
Me
Me
5
5
5
Me
Me
Me
ꢀ
ꢀ
O
O
O
O
O
16ꢀ
C
H
Rh
P
O
H
ꢀ
ꢀ
Me
H
15
ꢀ
ꢀ
ꢀ
ꢀ
O
!
O
Fur-2
Me
O
9
ꢀ
O
H
21ꢀ
82ꢀ
2
0
H
16
O
O
H
1
1
0ꢀ
1ꢀ
H
H
H
ꢀ
H
1
7
6
ꢀ
ꢀ
ꢀ
O
O
H
H
67ꢀ
ꢀ
H
1
8
Me
7
ꢀ
Me
O
O
H
1
2ꢀ
Me
Me
H
H
H
Me
Me
Me
85ꢀ
ꢀ
1
9
8
Me
ꢀ
ꢀ
!
a
Numbers refer to isolated yields for each entry. Reactions were performed in CH
2
Cl
2
using 4
The counterion and hydrogen atoms (except those in stereogenic centers) were omitted
mol% complex 3 as catalyst The solutions were degassed using freeze-pump-thaw degassing
3
1
b
for clarity. The P NMR spectrum for the compound shows the two distinct phosphorus
atoms and the expected P-O as the ligating atoms to Rh, as shown by the coupling
constants. The complex obtained after crystallization was catalytically competent in the
dimerization, further supporting its activity for the [4+2+2] cycloaddition.
method. Acetylene and propyne were bubbled onto the solution right after degassing, wherein
the solutions were still partially frozen. The reaction mixture was then allowed to warm up to 0
c
˚C before sealing the storage tube. Nongaseous secondary alkynes were added in five-fold
d
1
excess before degassing. 6:1 regioisomers at the alkyne addition, based in H NMR integration
of a inseparable mixture.
2
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DOI: 10.1039/C4CC01320D
products (C) observed in our previous work. Alternatively, the
weakly coordinating phosphine oxide can be displaced with a second
alkyne of give complex D which following migratory insertion and
reductive elimination yields the cyclooctatriene product E.
As seen in Table 2, the Rh-BozPHOS complex provides the [4+2+2]
products with a number of substrates. Both acyclic and cyclic
dieneynes, where the diene is part of a five-member ring, provide the
cyclooctatriene products, with the latter case providing tricyclic
hexahydro-2-oxa-1H-cycloocta[cd]pentalene core. Both terminal
alkynes and acetylene proceed to give desired product with acetylene
providing the higher yields. The yields were comparable to those
obtained with the previously system.
In addition to providing a catalyst system for the [4+2+2]
reaction discussed above, the discovery of a new catalyst based on
the phosphine-phosphine oxide combination illustrates the potential
for this type of ligand system, where the dative phosphine oxide can
fill the role as a labile ligand that can provide an open coordination
site as needed. We have begun to investigate this ligand type in a
number of other ligand and reaction systems, including other
versions of this type of cycloisomerization.
The initial system was optimized by following the formation of
the [4+2+2] dimer product that is observed when a second alkyne is
not present. Following that, reactions with a second alkyne were run.
The alkynes used were terminal as well as acetylene gas. We have
observed that internal alkynes will not participate in the reaction.
The reaction with propyne (Table 2 entry 7) appears to provide two
regioisomers where the alkyne inserts in two orientations.
This work was supported by the NSF CHE-0953083. Support
from the Robert A Welch Foundation is also acknowledged.
Notes and references
While the complex used for the cycloaddition is chiral the
reaction with the acyclic achiral substrates gave the corresponding
1
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R
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A
O
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elimination
oxidative
cyclization
Me
P
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P
reductive
elimination
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Rh3+
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2
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