Versatility in the Brook Rearrangement for the Selective Ring-Opening of Three-Membered Rings

Article abstract of DOI:10.1002/chem.201805006

From a single α-silylated carbinol intermediate, easily accessible by carbometallation of cyclopropenes, various scaffolds featuring a quaternary carbon stereocenter could be obtained selectively. The selectivity towards these different products was achieved by either changing the experimental conditions or the nature of the organometallic species involved.

Full text of DOI:10.1002/chem.201805006

A Journal of
Accepted Article
Title: Versatility in the Brook Rearrangement for the Selective Ring-
Opening of Three-Membered Rings
Authors: Ilan Marek, Coralie Tugny, and Fa-Guang Zhang
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Versatility in the Brook Rearrangement for the Selective Ring-
Opening of Three-Membered Rings
Coralie Tugny, Fa-Guang Zhang and Ilan Marek*[a]
Dedication ((optional))
found that a simple variation in the nature of the organometallic
species used leads to a completely different outcome of the ring
opening as described in Scheme 1e.
Abstract: From a single a-silylated carbinol intermediate, easily
accessible by carbometallation of cyclopropenes, various
scaffolds featuring a quaternary carbon stereocenter could be
obtained selectively. The selectivity towards these different
products was achieved by either changing the experimental
conditions or the nature of the organometallic involved.
a, Asymmetric conjugate addition to an extended system
R1
R3
1,6-addition
favored
R3[M]/L*
then H3O+
R2
O
O
R1
R2
O
R1
1,4-addition
R2 R3
1
The preparation of acyclic quaternary carbon stereocenters with
the concomitant creation of several carbon-carbon bonds in a
single-pot operation represents a valuable approach to the
construction of complex molecular architectures.[1] Even more
appealing would be the synthesis of these scaffolds with
additional chemical handles such as a carbonyl group and/or a
double bond for potential subsequent modifications. In this
context, an allylic quaternary carbon stereocenter remotely
positioned from a carbonyl group as in (1), abundant sub-structure
in biomolecules and typical building block in natural product
synthesis,[2] is highly desirable as it represents a difficult motif to
reach by classical synthetic routes.[3] As an illustrative example of
the difficulty to reach these scaffolds, the asymmetric 1,4-addition
to an extended conjugated system[4] remains challenging due to
the thermodynamic preference for the conjugated 1,6-addition
over the 1,4-addition (Scheme 1a).[5] We have therefore disclosed
b, Zinc homologation as the trigger for the ring-opening of three-membered rings
RO
R1
O
CO2R
CO2R
RO
CH2
R2[Cu]
R1
R2
CH2I2
Et2Zn
[Cu]
ZnEt
R1 R2
1
O
then
R1
C
R2
H3O+
H2
c, Brook rearrangement as the trigger for the ring-opening of three-membered rings
Me2N
R1
O
O
NMe2
O
CONMe2
R2[M]
CuX cat
then
O
NMe2
O
R1
[M]
OSiMe3
Ar
[M]
SiMe3
O
Me2N
OSiMe3
R1
R2
R1 R2
R1 R2
R2
Ar
O
Ar
2
3
Ar
Ar
SiMe3
d, Brook rearrangement as the trigger for the ring-expansion of three-membered rings
O
R3
R1
R3
OMe
Ar
OMe
O
R3
R1
OMe
Ar
1 R3
R
alternative
carbometallation of
approaches
where
the
diastereoselective
R3
OMe
R4[M]
CuX cat
OMe
R4
R1
R4
SiMe3
cyclopropene,[6] followed by
a
zinc
[M]
a
R1
[M]
[M] = MgBr, Li, ZnR
R4
R4
homologation triggering a selective ring-fragmentation[7] yields the
desired enantiomerically enriched acyclic product 1 possessing
the allylic quaternary carbon stereocenter in excellent yield
(Scheme 1b).[8] Inspired by the latter ring fragmentation and by
the creation of multiple carbon-carbon bonds in a single-pot
reaction, we have then extended our research to the
enantioselective preparation of either silyl enol ether 2 or 1,4-
dicarbonyl derivative 3 through a combined copper-catalyzed
carbomagnesiation followed by a Brook-rearrangement to trigger
the ring-opening of the three-membered ring (Scheme 1c).[9]
Interestingly, subtle variations of substituents on the cyclopropyl
ring changed completely the chemical outcome and
enantiomerically enriched polysubstituted cyclobutenes 5 were
obtained as a single diastereoisomer when 4 was treated by the
addition of a Grignard reagent in the presence of a catalytic
amount of copper salt followed by the addition of acylsilanes
(Scheme 1d).[10] This different reactivity underlines the peculiar
behavior of the Brook rearrangement as the origin of distinct
reactive intermediates (carbanion versus carbene).[11] This
discrepancy can also be found by simply changing the nature of
the organometallic used. In the course of the present study, we
Ar
SiMe3
Ar
5
4
e, This work
R2
R2
OMe
R4
One pot reaction
R1
R1 R3
4
O
er 94:6
8
er 94:6
1) R3MgBr, LiCl
CuBr.SMe2 (5 mol%)
toluene, -50 °C, 1 h
2) R4(CO)SiMe3,
-30 to -20 °C, 2.5 h
then H3O+
H3O+
R2
R1
R2
R2
OMe
0 to 50 °C,
12 h
nBu3MgLi
THF, -20 to 0 °C, 1.5 h
R4
OSiMe3
R4
+ MeOSiMe3
O
H
R1 R3
R1 R3
R3
dr 98:2:0:0:0:0:0:0
Bu2Mg (1.1 equiv.)
OMgBu2Li
R4
SiMe3
6
7
+ MeOMgBu2Li
E/Z: 98;2
stereodefined
disubstituted
silyl enol ether
50 °C, 12 h
R2
R4
R1 R3
Bu
9
E/Z: 2/98
Scheme 1. Various ring-opening reactions of three-membered rings
When 4 was subjected to the syn-directed copper-catalyzed
carbomagnesiation, the resulting stereodefined cyclopropyl
magnesium species could react with acylsilanes to give the
corresponding cyclopropyl a-silylated carbinol 6 in uniformly high
diastereoselectivities.[9,10] The stereochemical identity of the
major isomer was assigned by comparing 6m (R1 = Bu, R2 = Ph,
R3 = Me, R4 = Ph) with an authentic sample, the configuration
being determined by X-ray crystallography.[10] Treatment of 6 with
[a]
Dr. Coralie Tugny, Dr. Fa-Guang Zhang, Prof. Dr. Ilan Marek
The Mallat Family Laboratory of Organic Chemistry
Schulich Faculty of Chemistry
Technion – Israel Institute of Technology
Technion City 3200009, Haifa. Israel
E-mail: chilanm@technion.ac.il
Supporting information for this article is given via a link at the end of
the document. ((Please delete this text if not appropriate))
a
lithium magnesiate (easily prepared by addition of one
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equivalent of n-BuLi to the commercially available Bu2Mg solution
in THF) generates the corresponding alcoholate ate-complex that
undergoes the initial Brook-rearrangement followed by a ring-
fragmentation reaction to provide the silyl enol ether 7 in good
yield and as a single geometrical Z-isomer as illustrated in
Scheme 2a. The reaction proceeds smoothly for a large variety of
aromatic alkoxysilanes (formation of 7a-e, Scheme 2), at the
exception of the para-trifluoroaryl alkoxysilane that leads to the
decomposition of the starting material under the reaction
conditions. The nature of the substituent on the quaternary carbon
stereocenter is rather flexible as various primary, secondary and
even functionalized groups could be equally introduced (7f-7j’,
Scheme 2a). The degree of substitution of the double bond can
also be varied as mono- and di-substituted alkenes could be
formed (7a-j’ and 7k,l respectively, Scheme 2a). Interestingly,
when the reaction mixture leading to 7 is not hydrolyzed at 0 ºC
but rather heated to 50 ºC overnight, the MeOMgBu2Li resulting
from the ring-fragmentation reacts with the silyl enol ether 7 to
provide the corresponding enolate (accompanied by MeOSiMe3)
that can be subsequently hydrolyzed (or deuterolyzed leading to
8ad) into the desired ketones 8 possessing the quaternary carbon
stereocenter and the terminal double bond as described in
Scheme 2b.
diastereoisomers of 6 at C4 gave the same Z-silyl enol ether 7
(Equation 1), suggesting that the stereochemistry of the a-
hydroxysilane (C4) is irrelevant to the stereochemistry of the
formed double bond. The in-situ formed magnesiate carbenoid is
therefore configurationally unstable leading to
a
rapid
equilibration[12] into the most stable diastereoisomer before ring-
fragmentation (equation 1).
Diastereisomer at C4
OMe
OMe
HO
HO
SiMe3
Ph
Me
Bu
Me
Bu
3
4
4
Ph
OSiMe3
2
1
H
H
H
H
(1R*, 2R*, 3S*, 4S*)-6a
(1R*, 2R*, 3S*, 4R*)-6a’
Metalation nBu3MgLi
Metalation
Me
nBu3MgLi
Me
Me
Me
O
O
O
O
MgBu2Li
MgBu2Li
MgBu2Li
OSiMe3
O
O
MgBu2Li
Ph
SiMe3
Ph
Me
Bu
Me
Bu
Me
Me
Bu
Ph
OSiMe3
O
SiMe3
Ph
Bu
H
H
H
H
H
H
H
H
[1,2]-Brook
rearrangement
[1,2]-Brook
rearrangement
Epimerization
Fragmentation
Ph
OSiMe3
Bu Me
a,
7a
R2
R1
R2
R1
R2
OMe
O
OMe
nBu3MgLi
THF, -20 to 0 °C, 1.5 h
Equation 1. Mechanistic hypothesis for the formation of the single Z-
R4
H
MgBu2Li
R4
OSiMe3
isomer
R3
R3
R1 R3
OSiMe3
R4
SiMe3
7
6
Z:E 98:2
As each independent step of the transformation was now fully
controlled (complete diastereoselectivity for the carbometalation
of cyclopropenyl methyl ethers, of the addition of the acyl silane
and of the selective ring fragmentation), we focused our attention
to the direct one-pot transformation of 4 into 7 or 8 (Scheme 3).
The sequence started similarly with a diastereoselective copper-
catalyzed carbometallation reaction of 4 in toluene at -50 °C,
followed by addition of the acylsilane. Once the magnesium a-
silylcarbinolate 6MgBr was obtained, two equivalents of nBuLi were
added to generate the corresponding “ate complex” that
undergoes a selective ring-fragmentation to produce either the Z-
silyl enol ether 7a upon mild hydrolysis (Scheme 3a) or to the
ketones 8 in moderate overall yields after overnight heating of the
reaction mixture at 50 °C (based on 4, Scheme 3b).
Enantiomerically enriched cyclopropenes can be readily
prepared[13] (4e, R1 = Bu, er 94:6) and the product 8e could be
obtained in a single-pot operation from 4e with the same
enantiomeric ratio of 94:6, thus demonstrating the usefulness of
this strategy for the formation of fully alkylated carbon
stereocenters.
OMe
OSiMe3
OSiMe3
OSiMe3
OSiMe3
Bu Me
Bu Me
Bu Me
Bu Me
7a
78%
7b
81%
7c
80%
7d
69%
Cl
OSiMe3
OSiMe3
OSiMe3
OSiMe3
Me
Bu Me
Hex Me
Cy Me
Cl
7e
60%
7f
73%
7g
75%
7h
83%
Me
OSiMe
3
OSiMe
3
OSiMe
3
OSiMe
3
Me
Me
Me
Bu Me
Ph
TIPSO
HO
7i
81%
7j
80%
7j'
58%
7k
95%
Ph
Unsuccesful attempt
CF3
OSiMe3
Bu Me
7l
95%
OSiMe3
Bu Me
b,
R2
R1
OMe
R2
H3O+
(D3O+)
R2
nBu3MgLi
THF, -20 to 0 °C, 1.5 h
H
0 to 50 °C
12 h
H(D)
O
R4
7
R4
O
Z:E 98:2
1) MeMgBr, LiCl, CuBr.SMe2 (5 mol%)
toluene, -50 °C, 1 h
R3
R1
R3
O
MgBu2Li
R1 R3
R4
SiMe3
6
2) Ph(CO)SiMe3, -30 to -20 °C, 2.5 h
8
a,
OMe
3) 2 nBuLi (2 equiv), THF, -20 to 0 °C, 1,5 h
then H3O+
Ph
OMe
One pot reaction
OSiMe3
Bu
Bu Me
4a
7a
50%
O
O
O
O
O
O
Bu Me
Bu Me
Bu Me
Bu Me
8a
69%
8ad
8b
90%
8c
89%
8d
69%
1) MeMgBr, LiCl, CuBr.SMe2 (5 mol%)
toluene, -50 °C, 1 h
Cl
69%
b,
2) R2(CO)SiMe3, -30 to -20 °C, 2.5 h
3) 2 nBuLi (2 equiv), THF, -20 to 50 °C, 12 h
then H3O+
OMe
R2
O
O
Bu Me
Hex Me
Cy Me
Me
R1
One pot reaction
R1
Cl
O
Me
8e
60%
er 94:6
8f
95%
8g
66%
8h
83%
4
8
OMe
Me
Ph
Ph
O
O
O
O
O
Bu Me
Me
8i
81%
Me
8j
85%
Bu Me
Bu Me
O
O
O
O
Hex Me
Bu Me
Bu Me
Bu Me
Ph
TIPSO
8m
76%
8k
95%
8l
95%
8a
43%
8b
47%
8c
55%
8f
60%
Scheme 2. Combined [1,2]-Brook rearrangement – ring fragmentation
Ph
It should be noted that this alcoholate-ate complex intermediate
allows aliphatic alkoxysilanes, that were completely unreactive in
our previously described transformations (Scheme 1c,d) to
undergo the Brook rearrangement (Scheme 2b, 8m). Both
O
O
O
O
Bu Me
Me
8h
44%
Me
Me
8j
52%
Cl
Ph
TIPSO
8m
53%
8i
47%
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Scheme 3. One-pot transformation of cyclopropenes to acyclic molecular
demonstrates the striking dichotomy between magnesiate and
dialkylmagnesium species when added to silylated carbinol.
scaffolds
It is remarkable to note the importance of the metal species used
for the selective transformation of 6. When lithium, zinc or
magnesium alcoholates are used, only cyclobutenes 5 are formed
whereas magnesiate alcoholates are producing the Z-silyl enol
ether 7 (equation 2). This discrepancy of reactivity with the
organometallic species was confirmed further as a third different
chemical outcome was obtained when 6 was treated with
dialkylmagnesium compounds. In this case indeed, the skipped
diene 9 was obtained instead (Equation 2).
a,
R2
R1
OMe
O
R2
Bu2Mg (1.1. equiv)
THF, 50 °C, 12 h
then H3O+
R4
Bu
H
R3
R1 R3
R4
SiMe3
9
6
E:Z 98:2
OMe
Bu
Bu
Bu
Bu
Bu Me
Bu Me
Bu Me
Bu Me
9a
65%
9b
64%
9c
66%
9d
71%
R3
R1
OMe
R3
1 R3
R
OMe
R4
Ph
R[M]
nBu3MgLi
O
H
R4
Me
Me
9i
67%
M = MgBr, Li, ZnR
Bu
Bu
Bu
Bu
Hex Me
Cy Me
R4
R1
OSiMe3
SiMe3
Ar
Ar
Cl
Ph
9f
64%
9g
63%
9h
69%
7
6
5
Bu2Mg
Ph
Me
Ph
R3
Bu
Bu
Bu
Me
9j
68%
Bu Me
Bu Me
TIPSO
R4
R1
9k
68%
9l
Bu
61%
9
Equation 2. Different chemical outcome in the ring fragmentation of 6 with
b, Mechanistic hypothesis
the organometallic used
OMe
HO
OMe
OMgBu
SiMe3
Ph
OMe
4
MgBu
nBu2Mg
THF, 50 °C, 12 h
SiMe3
Ph
Me
R1
Me
Me
OSiMe3
Ph
Knowing the importance of skipped dienes, structures found in a
number of natural products,[14] and the challenges associated with
4
4
- MeOMgBu
- BuH
R1
H
R1
H
H
H
H
H
Deprotonation
Brook rearrangement
α-elimination
their preparation, particularly when
a quaternary carbon
stereocenter is present,[11a,15] we decided to extend our strategy
to their synthesis. When one equivalent of commercially available
Bu2Mg was added to 6 at room temperature in THF, a mixture of
cyclobutene 5 and skipped diene 9 was obtained. The latter,
though, was preferred at higher temperatures. It is worthy of note
that less polar solvents such as toluene or dioxane led to the
exclusive formation of the cyclobutene product 5 whereas a more
polar solvent such as HMPA only produces the Brook
rearrangement without ring-fragmentation. Similarly, no reaction
was observed when trialkylaluminum derivative was added to 6.
The reaction scope is rather large as various aromatic
substituents R4 (compare 9a with 9b-d, Scheme 4a) as well as
various primary, branched and functionalized groups R1could be
present (9a,f, 9g and 9h-j respectively, Scheme 4a). The nature
of R2 could be either a hydrogen, an alkyl or an aryl group (9a-j,
9k and 9l respectively, Scheme 4a). Although the mechanistic
picture of this dialkylmagnesium-promoted rearrangement is not
fully understood, we hypothesize that the Brook rearrangement
might give, after an a-elimination, a carbene intermediate[11] as
already demonstrated in our previous study and described[10] in
Scheme 4b. Instead of undergoing a ring-expansion towards the
cyclobutene (Scheme 1d), the in-situ generated BuMgOSiMe3
may react with the carbene to produce the corresponding
intermediate I before ring-opening. If the proposed intervention of
a carbene as intermediate is valid, both diastereoisomers of 6f
should lead to the same carbene and then to the same
geometrical isomer of 9. Therefore, both diastereoisomers of 6f
were prepared and treated under similar experimental conditions
(Scheme 4c). In both cases, only the E-configurated product 9f
was obtained, as determined by NOE experiments (see
Supporting Information). Hence, the formation of a carbene prior
to the addition and β-fragmentation might be likely in this case.
In conclusion, the combined regio- and diastereoselective
carbometallation of cyclopropenes and subsequent addition of
acylsilane led to the formation of either γ,δ-unsaturated ketones
possessing a quaternary carbon center or to the disubstituted Z-
silyl enol ethers, depending on the reaction conditions.
Alternatively, the use of dibutylmagnesium leads to the selective
formation of skipped dienes via a postulated carbene species
intermediate. Remarkably, the cleanliness of those reactions
Me
OMe
O
MgOSiMe3
Ph
Me
R1
R1
Ph
Ar
Bu
R1
9
Bu
Me
Me
H
H
Addition
Fragmentation
I
MeOMg-Bu
c,
OMe
HO
OMe
HO
Ph
SiMe3
Me
Hex
Me
Hex
Metalation
4
4
Ph
OSiMe3
[1,2]-Brook rearrangement
Carbene formation
Addition of BuMgOSiMe3
Epimerization
H
H
H
H
nBu2Mg
THF, 50 °C, 12 h
6f
6f’
(R*,R*,S*,S*)
(R*,R*,S*,R*)
Fragmentation
Ph
Bu
Hex Me
9f
Scheme 4. Selective formation of skipped dienes possessing a quaternary
carbon stereocenter
Acknowledgements
This research was supported by a grant from the European
Research Council (ERC Grant Agreement 338912). IM is holder
of the Sir Michael and Lady Sobell Academic Chair.
Keywords: Brook rearrangement• fragmentation • quaternary
carbon • selectivity • cyclopropane
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[13] Enantiomerically enriched cyclopropenyl ester precursors are readily
accessible through the asymmetric metal-catalyzed decomposition of
diazoesters with alkynes using the chiral catalyst Rh2(OAc)(R,R-DPTI)3:
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10.1002/chem.201805006
Chemistry - A European Journal
COMMUNICATION
Layout 2:
COMMUNICATION
Coralie Tugny, Fa-Guang Zhang and
Ilan Marek*
R3
R1
OMe
O
R3
R3
Ar
Bu2Mg
Ar
nBu3MgLi
H
R4
R1 R4
Page No. – Page No.
Bu
R1 R4
OSiMe3
SiMe3
Ar
Versatility in the Brook
Rearrangement for the Selective
Ring-Opening of Three-Membered
Rings
This article is protected by copyright. All rights reserved.