Spontaneous Dimerization of Allenes in Situ: An Efficient Synthesis of Substituted 1,2-Dimethylenecyclobutanes with High Regio- and Stereoselectivities

Article abstract of DOI:10.1002/chem.201502503

An intermolecular [2+2] cycloaddition reaction for the synthesis of 1,2-dimethylenecyclobutane derivatives from the commercially available starting materials aryl acetylenes, nBuLi, formamides, and trimethylsilyl cyanide (TMSCN) has been achieved. This reaction displays high regio- and stereoselectivities due to the captodative effect. The mechanism of the reaction has been investigated by the deuterium labeling experiments and DFT calculations. Cycloaddition of captodative allenes: A regio- and stereoselective intermolecular [2+2] cycloaddition of captodative allenes (in situ generated) for the efficient synthesis of substituted 1,2-dimethylenecyclobutanes is disclosed (see scheme; TMS = trimethylsilyl). The mechanism of the reaction has been investigated by deuterium labeling experiments and the selectivity has been studied by DFT calculations.

Full text of DOI:10.1002/chem.201502503

DOI: 10.1002/chem.201502503
Full Paper
&
Cycloaddition
Spontaneous Dimerization of Allenes in Situ: An Efficient
Synthesis of Substituted 1,2-Dimethylenecyclobutanes with High
Regio- and Stereoselectivities
Maozhong Miao,[a] Wenmin Wang,[b] Weijun Yang,[a] Lijun Xu,[a] Jing Ma,*[b] and
Hongjun Ren*[a]
Abstract: An intermolecular [2+2] cycloaddition reaction for
the synthesis of 1,2-dimethylenecyclobutane derivatives
from the commercially available starting materials aryl acety-
lenes, nBuLi, formamides, and trimethylsilyl cyanide (TMSCN)
has been achieved. This reaction displays high regio- and
stereoselectivities due to the captodative effect. The mecha-
nism of the reaction has been investigated by the deuterium
labeling experiments and DFT calculations.
Introduction
dimethylencyclobutanes from commercial materials is still in
urgent demand.
The substituted 1,2-dimethylenecyclobutanes are useful build-
ing blocks in the formation of the complex structures by un-
dergoing various fragmentation and ring-expansion reactions.[1]
Several complementary methods enable efficient access to
substituted 1,2-dimethylenecyclobutanes; in particular, the
[2+2] cyclization of allenes represents an important strategy
for the formation of cyclobutane derivatives with high efficien-
cy.[2] Although allene dimerization, which results in the forma-
tion of substituted 1,2-dimethylenecyclobutanes, has been
studied thoroughly, the stereoselectivity and regioselectivity
are poor especially with the reaction of an unsymmetrical
allene moiety.[3] Metal-catalyzed (such as Ni,[4] Pd,[5] Au,[6] and
Ru[7]-catalyzed) [2+2] dimerization of allenes takes place and
provides substituted 1,2-dimethylenecyclobutanes with high
regioselectivity. However, the thermal [2+2] cycloaddition of
non-symmetrical-substituted allenes is notorious for forming
a mixture of several dienes: tail to tail, head to head, and head
to tail adducts along with their corresponding cis-, trans-, Z-,
and E isomers.[8] Only few examples of high selectivity were
observed with the dimerization of 3,3-dimethyl-1-cyanoallene[9]
and phosphorous-derived allenes.[10] Thus, the practical syn-
thetic method for the stereoselective synthesis of 1,2-
The interest of our group in [2+2] chemistry led us to con-
sider the case of olefins and allenes of type 1a–c (Scheme 1),
which bear an electron-releasing and an electron-withdrawing
group on one carbon center. Such compounds of 1a–c are
named captodative olefins (1a), captodative diene (1b), and
captodative allenes (1c), respectively, according to the original
studies of Viehe and co-workers.[11] Captodative olefins (1a)
and captodative diene (1b) exhibit increased reactivity and se-
lectivity as p-2 components in Diels–Alder and [2+2] reac-
tions.[12] Because of the captodative effect, a-(alkylthio) acrylo-
nitrile reacts much faster with CF2=CFCl as compared with
acrylonitrile.[13] The dimerization of a-(alkylthio) acrylonitrile
leads to the head-to-head product.[14] To our best knowledge,
the reactivity and selectivity of captodative allenes (1c) has
not been largely investigated; especially the regioselectivity of
captodative allenes in [2+2] cycloaddition reactions.[15] Herein,
we wish to report our recent observation of the intermolecular
[2+2] cycloaddition reaction involving captodative allenes,
which provides a facile synthesis of 1,2-dimethylenecyclobu-
tanes.
Results and Discussion
[a] Dr. M. Miao,+ Dr. W. Yang, L. Xu, Prof. Dr. H. Ren
Department of Chemistry, Zhejiang Sci-Tech University
Hangzhou, Zhejiang 310018 (P.R. China)
E-mail: renhj@zstu.edu.cn
Initially, we intended to prepare the captodative allenes (with
acceptor group CN and donor group OTMS) from 3-phenylpro-
piolaldehyde and TMSCN. However, the reaction failed to pro-
duce the captodative allenes, but gave the cinnamonitrile in-
stead, which is consistent with the reported literature[16]
(Scheme 2).
[b] Dr. W. Wang,+ Prof. Dr. J. Ma
Collaborative Innovation Center of Chemistry for Life Sciences
School of Chemistry and Chemical Engineering
Key Laboratory of Mesoscopic Chemistry of MOE
Nanjing University, Nanjing 210093, (P.R. China)
E-mail: majing@nju.edu.cn
Then, we turned to synthesize the NR2- and CN-substituted
allene from phenyl acetylene, DMF, and TMSCN. Treatment of
phenyl acetylene with iPrMgCl at À308C resulted in deproto-
nation. After completion, anhydrous DMF and TMSCN were
added successively, and the reaction mixture was slowly
[+] These authors contributed equally to this work.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201502503.
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Scheme 1. Captodative olefins, dienes and allenes; selected typical examples
of dimerization of allenes and our strategy for the formation of 1,2-dimethyl-
enecyclobutanes with allenes in situ.
Scheme 3. The scope of the synthesis of 1,2-dimethylenecyclobutane deriva-
tives from terminal alkynes, DMF, and TMSCN by using an intermolecular
[2+2] cycloaddition approach. All reactions were carried out with aryl acety-
lene (2.0 mmol), nBuLi (2.4 mmol), formamide (5.0 mmol), and TMSCN
(3.0 mmol) in THF (5.0 mL) at À788C to room temperature. The yield of the
isolated product after flash column chromatography is shown in parenthe-
ses. Product 2 f was achieved by using iPrMgCl instead of nBuLi.
DMF (2.5 equiv) was added followed by TMSCN, giving the de-
sired corresponding 1,2-dimethylenecyclobutane derivatives in
90, 98, and 70% yield, respectively (2b–d, Scheme 3). When
2,4-dimethyl phenyl acetylene was used, the desired inter-
molecular [2+2] cycloaddition product was obtained in 78%
(2g). When good electron-donating groups, such as OMe, pyr-
role, and pyrrolidine, were introduced to the substrates, the re-
actions performed well and provided 2e, 2k, and 2j in 81, 89,
and 96% yield, respectively. The a- and b-substituted naphthyl
alkynes can also be converted to the corresponding products
in 55–77% yields (2h and i). Furthermore, the electron-with-
drawing group, such as CO2Me, was also tolerated and gave
the desired product in 57% yield (2 f) when iPrMgCl was used
instead of nBuLi. Then different substituted formamides were
also examined. When the reaction was carried out with N,N-di-
Scheme 2. Planned reactions and the unexpected result of the reaction.
warmed to room temperature. Surprisingly, the [2+2] cycliza-
tion product cyclobutane was obtained in 45% yield rather
than the expected captodative allene, presumably because this
allene is much too reactive under the reaction conditions.
When nBuLi was used instead of iPrMgCl, the yield was in-
creased to 100% (2a, Scheme 3).
Following the success of the formation of the 1,2-dimethyl-
enecyclobutane derivative 2a, the scope of this transformation
was studied by using various substituted terminal alkynes. The
results are summarized in Scheme 3. After deprotonation of p-
Me, p-Et, and p-tBu phenyl acetylenes with nBuLi, anhydrous
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ethylformamide, piperidine-1-carbaldehyde, and morpholine-4-
carbaldehyde, the corresponding 1,2-dimethylenecyclobutane
derivatives (2l–n) were obtained in 82, 70, and 56% yields, re-
spectively.
The reaction exhibited high regio- and stereoselectivities. In
all cases only the trans-3,4-diastereomers were observed. The
structures of these compounds were determined by 1H-and
13C NMR spectroscopy, IR, and HRMS. The structure of 2m was
further confirmed by using single-crystal X-ray diffraction anal-
ysis (Figure 1). We also compared the crystal structure of 2m
with the optimized structure from density functional theory
(DFT) at B3LYP/6-311+G(d,p) level. A good agreement is ach-
ieved between the experiment and calculation with the root-
mean-square deviation (RMSD) of 0.37 Š, as shown in Figure 1.
Scheme 4. An alternative approach to 1,2-dimethylenecyclobutanes. All reac-
tions were carried out with phenylpropiolaldehyde (2.0 mmol), secondary
amine (2.4 mmol), nBuLi (2.4 mmol), and TMSCN (3.0 mmol) in THF (5.0 mL)
at À788C to room temperature. The yield of the isolated product after flash
column chromatography is shown in parentheses.
Scheme 5. Mechanistic study by using a deuterium labeling experiment.
yield as well as cyclobutane ring protonated compound D-2a-
1 in 14% yield. When the reaction was quenched with electro-
phile, such as CH3I, the same result was obtained. This result
suggested that the lithium intermediate Li-2a-3 was not
formed in the reaction and that the deuterium on the cyclobu-
tane ring comes from the active deuterium of [D7]DMF
through dimerization of deuterium allene intermediate.
Accordingly, we proposed a plausible pathway for the for-
mation of product 2a, as shown in Scheme 6. Treating phenyl
acetylene with nBuLi and DMF successively gave the hemiami-
nal intermediate A, which can be alternatively formed from
phenylpropiolaldehyde and lithium amides (Scheme 6). Hemi-
aminal intermediate A further transformed to cyano-substitut-
ed intermediate B upon addition of TMSCN. Promoted by the
base, the cyano-substituted intermediate B can be easily iso-
merized into captodative allene intermediate C, which under-
went highly regio- and stereoselective [2+2] cyclization to give
the final 1,2-dimethylenecyclobutane 2a.
To better understand the mechanism and origins of regio-
and stereoselectivities in [2+2] cyclization of captodative
allene, DFT calculations were also carried out by using the
Gaussian 09 program.[17] Molecular geometries of the reactants,
intermediates, transition states, and products were optimized
at the (U)B3LYP/6-311 +G (d, p) level. The optimized structures
of all stationary points are presented in Figure S1 (see the Sup-
porting Information). All the stationary points were tested by
Figure 1. ORTEP representation of 2m.
Although the result was encouraging, the relatively inacces-
sible substituted formamides are not ideal. To further expand
the scope of the reaction, the other set of reaction conditions
resulting in product formation were explored. Thus, the reac-
tions of different secondary amines with nBuLi formed the lithi-
um amides, which were subsequently treated with phenylpro-
piolaldehyde and then TMSCN to give the 1,2-dimethylene-
cyclobutane derivatives in 49–80% yields (Scheme 4).
Although none of the captodative allene intermediates was
detected, we conducted control experiments that might be
helpful for supporting the reaction pathway (Scheme 5). Treat-
ing phenyl acetylene with nBuLi, [D7]DMF, and TMSCN gave
the cyclobutane ring deuterated compound D-2a-2 in 86%
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the frequency calculation, and the relative free energies (DG)
with the zero-point energy (ZPE) corrections were reported in
Figure 2.
There are two possible reaction mechanisms, concerted
p2s+p2a (Scheme 7a) and stepwise pathway (Scheme 7b).
Two allenes (Reactant) may dimerize through concerted p2s+
p2a or the stepwise [2+2] cycloaddition reactions to form the
substituted dimethylenecyclobutanes. As shown in Scheme 7a,
the dimerizations of two reactants in the two concerted p2s+
p2a paths, Concerted-1 and Concerted-2, are nearly identical
in both the reaction intermediates (IM-1 vs. IM-2) and products
(2a vs. 2a-3), indicating the insensitivity to the orientations of
two allenes when they are approaching with each other. On
the other hand, the [2+2] stepwise mechanism involves two
steps: the first step is dimerization of allenes to give diradical
intermediates (as illustrated in the spin density of Figure S2;
see the Supporting Information), then the intermediates under-
go ring-closure to yield the products (Scheme 7b). The relative
energies (DE, in units of kcalmolÀ1) of intermediates (with the
reference of the most stable D1) indicate that the intermedi-
ates D1 (on Stepwise-1) and D2 (Stepwise-2) are much more
Scheme 6. A plausible pathway for the formation of dimethylenecyclobu-
tanes.
Figure 2. Free energy surface for allene (reactant) dimerization through the [2+2] cycloaddition and concerted p2s+p2a mechanisms to form substituted di-
methylenecyclobutanes.
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stable than those D3 (Stepwise-
3) and D4 (Stepwise-4). For Step-
wise-1 and 2, the relative ener-
gies of the products are also
lower than those of Stepwise-3
and 4. Thus, Stepwise-1 and
Stepwise-2 are thermodynami-
cally more favorable than Step-
wise-3 and 4. Both Stepwise-
1 and Stepwise-2 begin with the
C2-C2’ bond formations. The
only difference between D1 and
D2 lies in that the dimerization
of D1 is in a face-to-face manner
whereas the dimerization of D2
occurs through the opposite di-
rection.
Figure 2 displays the potential
energy curves along the Con-
certed-1 (Figure 2a), Concerted-2
(Figure 2b), Stepwise-1 (Fig-
ure 2c), and Stepwise-2 (Fig-
ure 2d) reaction paths. The con-
certed p2s+p2a mechanism is
favored according to orbital
symmetry. The formation of
p2s+p2a intermediate IM-1 (IM-
2) only needs small energy barri-
ers of 10.6 (13.4) kcalmolÀ1. The
ring-closure transition state TSI-
a (TSII-a) is the rate-determining
step with
a barrier of 15.8
(19.7) kcalmolÀ1 to produce
products 2a and 2a-3. But the
formation of other product 3a
(II-b) from IM-1 (IM-2) needs
a much higher energy barrier of
47.2 (59.7) kcalmolÀ1
.
In the
stepwise mechanism, the forma-
tion of dimerization intermediate
D1 (D2) through TS1a (TS2a) is
the rate-determining step with
a barrier of 30.1 (34.8) kcalmolÀ1
.
In the optimized structure of
TS1a, the bond length of C2ÀC2’
is 2.3 Š and the dihedral angle
of
C1-C2-C2’-C1’
is
1028
(Figure 3). Subsequently, the
ring-closure of intermediate D1
to produce products 2a, 2a-1,
and 2a-2 only needs small
energy barriers of 13.4, 9.6,
7.8 kcalmolÀ1, respectively. Step-
wise-2 is similar to Stepwise-1.
For these two paths, product 2a
is the most stable one among
six products. The free energy of
Figure 3. B3LYP/6-311+G(d,p)-optimized structures (bond lengths in Š).
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tional Basic Research Program (Grant No. 2011CB808604), the
Science Foundation of Zhejiang Sci-Tech University (Grant No.
13062121-Y), as well as the Zhejiang Provincial Top Key Aca-
demic Discipline of Chemical Engineering and Technology of
Zhejiang Sci-Tech University. We are grateful to the High Per-
formance Computing Center of Nanjing University for perform-
ing the quantum chemical calculations in this paper on its IBM
Blade cluster system.
Keywords: allenes
·
cycloaddition
·
density functional
calculations · reaction mechanisms · stereoselectivity
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Acknowledgements
We thank the financial support from National Nature Science
Foundation of China (21272003), (Grant No. 13062066-A), Na-
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Received: June 27, 2015
Published online on August 21, 2015
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