Interrupting Nazarov Reaction with Different Trapping Modality: Utilizing Potassium Alkynyltrifluoroborate as a σ-Nucleophile

Article abstract of DOI:10.1021/acs.orglett.6b01606

The putative oxyallyl cation intermediate generated following Nazarov cyclization of dienone has been successfully intercepted with potassium alkynyltrifluoroborates which act as σ-nucleophiles in the presence of BF₃·Et₂O. This new t

Full text of DOI:10.1021/acs.orglett.6b01606

Letter
pubs.acs.org/OrgLett
Interrupting Nazarov Reaction with Different Trapping Modality:
Utilizing Potassium Alkynyltrifluoroborate as a σ‑Nucleophile
Ronny William,† Siming Wang,† Asadulla Mallick, and Xue-Wei Liu*
Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University,
Singapore 637371, Singapore
S
* Supporting Information
ABSTRACT: The putative oxyallyl cation intermediate
generated following Nazarov cyclization of dienone has been
successfully intercepted with potassium alkynyltrifluoroborates
which act as σ-nucleophiles in the presence of BF3·Et2O. This
new trapping modality allowed unprecedented introduction of
an alkynyl moiety to the cyclopentanone framework by means
of an interrupted Nazarov reaction. The α-alkynyl cyclopentanone product can be further transformed into an array of densely
functionalized cyclic compounds.
azarov cyclization has long been recognized as a powerful
tool for the construction of cyclopentanoid compounds.1
Scheme 1. Trapping of Nazarov Intermediate with Acetylene
and Potassium Alkynyltrifluoroborate
N
Recent interests to expand the synthetic utility of Nazarov
cyclization revolve around developing catalytic asymmetric
variants,2 discovering surrogate pentadienyl cation precursors,3
and exploring cascade processes initiated by electrocyclic ring
closure via interception of a cyclic oxyallyl cation.4 With regard to
the latter, a diverse range of nucleophiles including hydride-,5
carbon-,6 and heteroatom-based7 nucleophiles have been
employed to render an elaborated cyclopentanoid framework.
While there are ample precedents of carbon-based nucleophiles
participating in interrupted Nazarov reactions, almost all
reported reactions involve the use of π nucleophiles such as
alkenes, dienes, enol derivatives, electron-rich arenes, and
heteroaromatics. Recently, West’s group reported a novel
mode of oxyallyl cation trapping in which aryl acetylenes
functioned as a competent π nucleophile furnishing α-phenacyl
cyclopentanones via carboalkoxylation of an alkyne (Scheme 1,
eq 1).6f In contrast, the use of a σ carbon-based nucleophile in
interrupting Nazarov cyclization has not been extensively
reported.6g
Organoboron compounds have gained considerable attention
as privileged reagents for C−C bond formation in organic
synthesis following the discovery of the Suzuki−Miyaura
coupling reaction.8 While trivalent boronic acids and esters
constitute the most common organoboron reagents utilized in
organic synthesis, the tetravalent potassium organotrifluorobo-
rate salts which exhibit exceptional stability toward air and
moisture have increasingly found widespread application in
transition-metal-catalyzed reactions.9 Apart from their pervasive
use in cross-coupling reactions with a plethora of coupling
partners,10 potassium organotrifluoroborates have also been
demonstrated to undergo nucleophilic addition to an iminium
ion in a Petasis borono-Mannich reaction.11 In addition, the
nucleophilic properties of organotrifluoroborates have also been
exploited in reactions with oxocarbenium12 and acylium13 ion
intermediates in the presence of a Lewis acid.
In view of the competence of organotrifluoroborates to
participate in nucleophilic addition to a stabilized carbocation, we
envisioned that treatment of dienone 1 and potassium
alkynyltrifluoroborates 2 with a suitable Lewis acid would result
in initial Nazarov cyclization which in turn initiates a cascade C−
C bond-forming event via fortuitous trapping of oxyallyl cation
intermediate I to ultimately furnish α-alkynyl cyclopentanone 3
(Scheme 1, eq 2). It is noteworthy that if successful the overall
process occurs with formation of two new C−C bonds along with
four contiguous stereocenters. Additionally, alkynyl functionality
incorporated into a cyclopentanone framework could serve as a
versatile handle for further functional group transformations.
At the outset of this study, we focused on treatment of
symmetrically substituted 1,4-dien-3-one 1a and potassium
phenylethynyltrifluoroborate 2a with a Lewis acid to effect the
proposed intermolecular interrupted Nazarov reaction. A
preliminary experiment in which dienone 1a was treated with 2
equiv of BF3·Et2O in the presence of 2 equiv of alkynyl-
Received: June 3, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b01606
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
trifluoroborate 2a in acetonitrile at room temperature did not
furnish the desired α-alkynyl cyclopentanone product. Instead,
complete consumption of dienone 1a after stirring for 1 h was
accompanied by formation of a regioisomeric mixture of
cyclopentenone 4a and methylidene cyclopentanone 5a
(Scheme 2).
anhydrous propionitrile/anhydrous dichloromethane in the
presence of 3 Å molecular sieves, the yield can be significantly
increased to 92% (Table 1, entry 6). The relative stereochemical
assignment of the newly formed stereogenic centers in
cyclopentanone 3aa was unambiguously determined by single
crystal X-ray diffraction analysis.14
Having established the optimized conditions, the scope of
potassium alkynyltrifluoroborate applicable to the present
interrupted Nazarov reaction was scrutinized by performing
the reaction with a diverse range of substituted potassium
alkynyltrifluoroborate derivatives (Scheme 3). In general, a
Scheme 2. Initial Attempt to Interrupt Nazarov Intermediate
Scheme 3. Scope of Alkynyltrifluoroborates
The outcome of our preliminary study suggested that at
ambient temperature the competing termination pathways via
proton elimination prevail over the anticipated interception of an
oxyallyl cation by nucleophilic attack of the alkynyl moiety. Based
on this initial observation, we postulated that varying the reaction
temperature could potentially exert significant influence on the
fate of the oxyallyl cation, as heat tends to favor elimination over
an addition reaction. Initially, lowering the reaction temperature
to −40 °C for reaction in acetonitrile proved to be futile,
affording only noninterrupted products 4a and 5a (Table 1, entry
a
Table 1. Screening of Reaction Parameters
b
entry
LA
solvent
CH3CN
t (°C)
yield (%)
c
1
2
3
4
5
BF3·Et2O
BF3·Et2O
BF3·Et2O
BF3·Et2O
BF3·Et2O
BF3·Et2O
BF3·Et2O
BF3·Et2O
AlCl3
rt
−
c
CH3CN
−40
−78
−78
−78
−78
−78
−78
−78
−78
−78
−78
−
d
CH2Cl2
traces
EtCN
41
63
multitude of phenylethynyltrifluoroborates containing electron-
donating or weakly electron-withdrawing substituents on the
phenyl ring efficiently intercepted a Nazarov intermediate
resulting from cyclization of dienone 1a to furnish corresponding
α-alkynyl cyclopentanones 3ab−3ag in yields ranging from 57%
to 83% as single diastereomers. It is notable that electron-
withdrawing phenylalkynyltrifluoroborates afforded lower yields
of interrupted products 3ad−3af owing to the diminished
migratory aptitude of the alkynyl moiety. A more sterically
hindered naphthylethynyltrifluoroborate also reacted smoothly
to afford cyclopentanone 3ah in 78% yield. In addition,
alkynyltrifluoroborates bearing a cyclopropyl or cyclohexyl
substituent also proved to be viable nucleophiles in intercepting
the oxyallyl cation providing cyclopentanones 3ai and 3aj in 71%
and 72% yield, respectively. Similarly, alkynyltrifluoroborates
substituted with an acyclic hydrocarbon chain also furnished the
desired cyclopentanones 3ak and 3al in good yields. In all cases,
it is notable that the reactions ensued with complete
diastereoselectivities in which the α-alkynyl cyclopentanones
were isolated as single diastereomers.
EtCN/CH2Cl2 (1:1)
EtCN/CH2Cl2 (1:1)
EtCN/CH2Cl2 (1:1)
EtCN/CH2Cl2 (1:1)
EtCN/CH2Cl2 (1:1)
EtCN/CH2Cl2 (1:1)
EtCN/CH2Cl2 (1:1)
EtCN/CH2Cl2 (1:1)
e
f
6
92 (90)
g
7
h
52
26
47
44
32
22
8
9
10
11
12
SiCl4
SnCl4
TiCl4
a
Reactions were carried out using dienone 1a (0.1 mmol, 1 equiv) in
b
solvent (0.1 M in 1a) for 15 min. NMR yields with CH2Br2 as
internal standard. 4a and 5a were obtained. 4a was obtained as
major product. Anhydrous propionitrile/dichloromethane with
anhydrous BF3·Et2O in the presence of 3 Å MS (200 wt %) was
used. Yield in parentheses is isolated yield. Performed at 0.05 M in
1a. Performed using 1.1 equiv of 2a and BF3·Et2O.
c
d
e
f
g
h
2). Later when the reaction was carried out at −78 °C in
dichloromethane as solvent in the presence of 2 equiv of BF3·
Et2O, we observed a trace amount of alkynyl cyclopentanone 3aa
resulting from interruption of the Nazarov reaction along with
substantial byproduct cyclopentenone 4a (Table 1, entry 3). To
our surprise, when a combination of propionitrile and dichloro-
methane in a 1:1 ratio was employed as the reaction solvent, the
reaction yield improved noticeably to 63% (Table 1, entry 5).
When the reaction was performed under rigorously dry
conditions with anhydrous BF3·Et2O in a 1:1 mixture of
With the scope of potassium alkynyltrifluoroborates defined,
we shifted our attention to demonstrate the general utility of this
reaction by employing dienone 1b−1f. For unsymmetrically
substituted dienones, th eregioselectivity aspect of the trapping
event can potentially raise an issue to be addressed (Table 2,
entries 3−5). In the case of dienone 1d, the reaction proceeded
B
DOI: 10.1021/acs.orglett.6b01606
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
diastereofacial selectivity from the face opposite to the bulky
adjacent substituent to minimize developing torsional strain in
the transition state leading to boron enolate C. Ultimately,
diastereoselective protonation of enolate C selectively provides a
diastereomer in which all the sterically demanding substituents
on the cylopentanone scaffold are in the trans configuration.
To exemplify the synthetic utility of an α-alkynyl cyclo-
pentanone, we have carried out a number of functional group
manipulations to afford a series of densely functionalized
structures (Scheme 5). First, the alkyne moiety can be partially
Table 2. Scope of Divinyl Ketones
b
c
entry
1
R1
R2
R3
R4
yield (%)
1
2
3
4
5
1b
1c
1d
1e
1f
Me
Me
n-Pr
Me
Me
Me
Me
Me
Me
H
4-MeOC6H4
4-ClC6H4
Ph
4-MeOC6H4
88 (3ba)
75 (3ca)
51 (3da)
72 (3ea)
4-ClC6H4
Ph
Ph
Ph
t-Bu
d
Ph
−
Scheme 5. Synthetic Transformation of 3aa
a
Reactions were carried out using dienone 1 (0.1 mmol, 1 equiv) in
solvent (0.1 M in 1a). Isolated yields. 3ca was isolated as an
inseparable mixture of regioisomers/diastereomers in a 9:1 ratio. No
b
c
d
reaction at −78 °C; at −40 °C only noninterrupted cyclopentenone
product was obtained.
in a regioselective manner in which trapping occurs preferentially
at the α-carbon substituted with a methyl group to afford 3da in
51% yield along with a minor amount of inseparable
regioisomers/diastereomers in a 9:1 ratio. For dienone 1e, the
sterically demanding tert-butyl group at the β-position dictates
that the addition of an alkyne moiety to the oxyallyl cation takes
place exclusively at the α-carbon distal to the tert-butyl group to
avoid eclipsing torsional strain in the alternate transition state
leading to the other regioisomer. Unsymmetrical dienone 1f,
however, was found to be unreactive at −78 °C even after a
prolonged reaction time. When the reaction was performed at
−40 °C, the Nazarov cyclization ensued to give the non-
interrupted product without formation of any desired
interrupted product. The apparent lower reactivity of dienone
1f which lacks a methyl group at one of the α-positions can be
ascribed to an increase in proportion of molecules adopting the
unreactive s-trans/s-cis conformer.
reduced to afford alkenyl substituted cyclopentanone 6 in the
presence of a Lindlar catalyst or completely reduced to
cyclopentanone 7 with Pd/C in methanol. In addition,
compound 8 containing a 1,4-dicarbonyl group can be readily
prepared in 81% yield through AuCl3-catalyzed hydration of 3aa.
Sodium borohydride reduction of cyclopentanone 3aa led to
formation of cyclopentanol 9 in 97% yield with a diastereomeric
ratio of 10:1. Furthermore, Baeyer−Villiger oxidation of 3aa
resulted in ring expansion to furnish highly functionalized δ-
lactone 10.
In conclusion, we have demonstrated the reactivity of
potassium alkynyltrifluoroborates as a σ-nucleophile in trapping
the oxyallyl cation generated through Nazarov cyclization of
dienones to furnish α-alkynyl cyclopentanones with high
diastereoselectivity. The present method represents the first
example in which an sp-hybridized alkyne group has been
successfully introduced to a cyclopentanone scaffold via
interrupted Nazarov reaction.
A plausible reaction mechanism for the formation of α-alkynyl
cyclopentanone 3 can be proposed as illustrated in Scheme 4. In
Scheme 4. Proposed Mechanism for Trapping
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.6b01606.
Experimental procedures and characterization data for
new compounds (PDF)
the initial step, upon mixing of potassium alkynyltrifluoroborate
2 with BF3·Et2O, the corresponding Lewis acidic organodifluoro-
borane I is generated in situ. The organodifluoroborane would
then act as the effective Lewis acid through complexation with
the carbonyl group of dienone 1, thereby forming zwitterionic
pentadienyl cation intermediate A. A facile 4π conrotatory
electrocyclic ring closure results in the formation of the oxyallyl
cation intermediate B in which the R3 and R4 substituents are
disposed trans to each other. Migration of the alkynyl group from
the boron center to the oxyallyl cation occurs with remarkable
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: xuewei@ntu.edu.sg.
Author Contributions
†R.W. and S.W. contributed equally.
Notes
The authors declare no competing financial interest.
C
DOI: 10.1021/acs.orglett.6b01606
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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Montalbano, F.; Cal, P. M. S. D.; Gois, P. M. P. Chem. Rev. 2010, 110,
6169.
ACKNOWLEDGMENTS
■
We gratefully acknowledge Nanyang Technological University
and the Ministry of Education, Singapore (RG6/13 and RG132/
14) for the financial support of this research.
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Org. Lett. XXXX, XXX, XXX−XXX