An opened route to 1,3-dimethylenecyclobutanes via sequential ruthenium-catalyzed [2 + 2] cycloaddition of allenyl boronate and palladium suzuki coupling

Article abstract of DOI:10.1021/ja051930r

The regioselective head-to-head [2 + 2] cyclodimerization of allenyl boronate catalyzed by the ruthenium catalyst [Cp*RuCl(COD)] leads to a novel diboronated 1,3-dimethylenecyclobutane. Consecutive palladium-catalyzed C-C couplings open a route to novel d

Full text of DOI:10.1021/ja051930r

Published on Web 07/30/2005
An Opened Route to 1,3-Dimethylenecyclobutanes via Sequential
Ruthenium-Catalyzed [2 + 2] Cycloaddition of Allenyl Boronate and Palladium
Suzuki Coupling
Emilio Bustelo,† Carine Gue´rot,† Alain Hercouet,† Bertrand Carboni,† Lo¨ıc Toupet,‡ and
Pierre H. Dixneuf*,†
Institut de Chimie de Rennes, UMR 6509 UniVersite´ de Rennes 1 - CNRS, Organome´talliques et Catalyse, and
UMR 6626 UniVersite´ de Rennes 1 - CNRS, Groupe Matie`re Condense´e et Mate´riaux, Campus de Beaulieu,
35042 Rennes, France
Received March 25, 2005; E-mail: pierre.dixneuf@univ-rennes1.fr
Scheme 1
1,3-Dialkylidenecyclobutanes are key precursors for the forma-
tion of non-Ke´kule´ hydrocarbon diradicals with triplet ground
state: the 1,3-dialkylidenecyclobutane-2,4-diyls.1,2 This transforma-
tion attracts interest because of the large positive bond enthalpy
additivity deviation3 and because diradicals have potential applica-
tions in material science4 as paramagnetic building blocks for the
Scheme 2
formation of organic magnetic materials and polymers.1,2 1,3-
Dimethylenecyclobutanes lead to 1,3-diradicals in low-temperature
matrixes,5 in gas phase with atomic oxygen anion,6 and from bicyclo
[1.1.0] butanes.7
However, their synthesis is not straightforward. The current
preparation of the simplest member involves a six-step synthesis
from allene and acrylonitrile.8 The simplest access would result
Scheme 3
from [2 + 2] allene thermal cyclodimerization. However, it leads
to a 1,2-isomer with small amounts of the 1,3-isomer (85:15).9
Dimerization of substituted allenes gives mixtures of a number of
dimers and oligomers.10 Only some specific examples of substituted
1,3-dimethylenecyclobutanes are known, namely the 2,4-disulfone11
and 1,3-bispyridinylidene derivatives.7
Recently, Ni(PPh3)4 was revealed to catalyze the [2 + 2]
cycloaddition of electron-deficient allenes to give only conjugated
1,2-dimethylenecyclobutane isomers.12
of the regioselective head-to-head dimerization of 1 by [2 + 2]
coupling of the terminal double bonds.
Among functional allenes, allenylboronates have never been
involved in catalytic transformations despite their potential for
formation of small cycle intermediates, with retention of the sp2
C-B bond, allowing consecutive C-C bond formation. Our interest
in regioselective catalytic oxidative couplings of functional alkynes
and alkenes at electron-rich ruthenium precatalysts13 led us to
explore the allenylboronate reactivity.
The isolated yield of 3a + 3b varies between 40 and 60%.
Decomposition of 3a + 3b with concomitant release of pinacol is
always observed, hindering the separation of the two isomers. In
search of more stable products, we reacted the mixture of 3a + 3b
with iodobenzene in the presence of Pd(PPh3)4 under Suzuki
reaction conditions.
To prevent yield loss during workup and overcome the low
stability of boronate dimers 3a + 3b, both reactions were carried
out sequentially in one-pot: first the ruthenium catalysis to prepare
the dimeric product was performed, and then the palladium catalyst,
aryl iodide, and a base were added.
Taking advantage of the good stability of pinacol boronate esters
in water, we employed an aqueous solution of K2CO3 as a base,
with catalyst Pd(PPh3)4 (1%) and phenyl iodide in THF. After
heating for 2 h at 80 °C, 1H NMR analysis of the reaction mixture
showed the presence of the two phenyl-substituted isomers 4a +
4b (Scheme 3). Recrystallization allowed us to isolate the major
isomer 4a in 34% global yield.
The X-ray structure of 4a was determined, establishing the 1,3-
dialkylidenecyclobutane nature with an E,E-configuration of the
two exocyclic double bonds (Figure 1).
The dimethylenecyclobutane framework is planar (torsion angles
C1′C2C1C3 ) 179.2°, C1C2C1′C2′ ) 0.0°), whereas the phenyl rings
only slightly deviate from the planarity (torsion angle C4C3C1C2,
We wish to report the first direct access to rigid 1,3-dimethyl-
enecyclobutane derivatives via an original head-to-head [2 + 2]
cyclodimerization of allenyl boronate, catalyzed by a ruthenium
catalyst. Although global yields are still moderate, consecutive
palladium-catalyzed C-C couplings open a route to substituted 1,3-
dimethylenecyclobutanes (Scheme 1).
The allenyl boronate 1, CH2dCdCHB(pin) (pin ) pinacol),12
was reacted in the presence of 5 mol % of [Cp*RuCl(COD)] (2) in
refluxing dioxane at 100 °C. The reaction proceeds in 2 h to reach
the full conversion of the allene, affording [2 + 2] cyclodimerization
products (Scheme 2).
GC mass of the crude product confirms the formation of two
isomeric products 3a + 3b (m/z ) 332) in a 2:1 ratio, corresponding
to the double mass of the starting allenyl boronate. NMR analysis
shows the existence of dimethylenecyclobutane structures, a result
† CNRS, Organome´talliques et Catalyse.
‡ CNRS, Groupe Matie`re Condense´e et Mate´riaux.
9
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J. AM. CHEM. SOC. 2005, 127, 11582-11583
10.1021/ja051930r CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
The observed allene dimerization regioselectivity benefits from
both pinacol boronate and bulky C5Me5 group nature. A direct
access to 4a from phenylallene dimerization catalyzed by 2 gives
a mixture of 4a, 4b, and 1,2-diphenyl-3,4-dimethylenecyclobutane
717 (in 2:1:3 ratio), showing a lack of regioselectivity (head-to-
head versus tail-to-tail coupling) with respect to allenyl boronate
1.
To obtain a better regioselectivity with phenylallene dimerization,
the use of a more sterically hindered ruthenium site was attempted
(Scheme 4). The reaction of phenylallene in the presence of 10
mol % [Cp*Ru(PPh3)(MeCN)2][PF6] (8) in DMF at 110 °C for 6
h gave the tail-to-tail coupling compound 7, isolated in 44% yield.
In conclusion, a new route to the previously inaccessible
disubstituted 1,3-dimethylenecyclobutanes has been opened by
sequential ruthenium/palladium catalysis from readily available
materials by a simple experimental protocol. Ongoing studies aim
to develop the functionalization of dimer 3 through the rich boron
chemistry and search the access to new polymeric materials bearing
rigid cyclobutane units.
Figure 1. ORTEP drawing of the molecular structure of 4a.
Scheme 4
Acknowledgment. We are grateful to the E.U. (COST D17)
for support and for the “Marie Curie” Fellowship to E.B.
Supporting Information Available: Crystallographic data of 4a
and experimental procedures for all new compounds (CIF, PDF). This
material is available free of charge via the Internet at http://pubs.acs.org.
3.3°). This is in agreement with the early studies on gas-phase
electron diffraction of 1,3-dimethylenecyclobutane.8
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symmetry plane perpendicular to the molecular plane supports a
1,3-(Z,Z)-dimethylenecyclobutane structure, discarding the presence
of the 1,2-isomer.12
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