Synthesis and structure of conjugated molecules with the benzofulvene core

Article abstract of DOI:10.1021/ol5015366

A general method to synthesize conjugated molecules with a benzofulvene core is reported. Up to four conjugated substituents have been introduced via a three-step sequence including (1) synthesis of 1,2-bis(arylethynyl)benzenes; (2) exo-dig electrophilic cyclization promoted by iodine; and (3) cross-coupling reaction of the resulting bis-iodobenzofulvenes with organoboron, organotin, or ethynyl derivatives under Pd catalysis. Structural aspects of the new compounds are discussed.

Full text of DOI:10.1021/ol5015366

Letter
pubs.acs.org/OrgLett
Synthesis and Structure of Conjugated Molecules with the
Benzofulvene Core
Carmela Martinelli,† Antonio Cardone,*,† Vita Pinto,‡ Maurizio Mastropasqua Talamo,‡
Maria Luisa D’arienzo,‡ Ernesto Mesto,§ Emanuela Schingaro,§ Fernando Scordari,§ Francesco Naso,‡
Roberta Musio,*,‡ and Gianluca M. Farinola*,‡
†Istituto di Chimica dei Composti OrganoMetallici (ICCOM) − Consiglio Nazionale delle Ricerche CNR, Via Orabona, 4 - 70125
Bari, Italy
‡Dipartimento di Chimica, Universita
̀
degli Studi di Bari “Aldo Moro”, Via Orabona, 4 - 70125 Bari, Italy
§Dipartimento di Scienze della Terra e Geoambientali, Universita
̀
degli Studi di Bari “Aldo Moro”, Via Orabona, 4 - 70125 Bari, Italy
S
* Supporting Information
ABSTRACT: A general method to synthesize conjugated
molecules with a benzofulvene core is reported. Up to four
conjugated substituents have been introduced via a three-step
sequence including (1) synthesis of 1,2-bis(arylethynyl)-
benzenes; (2) exo-dig electrophilic cyclization promoted by
iodine; and (3) cross-coupling reaction of the resulting bis-
iodobenzofulvenes with organoboron, organotin, or ethynyl
derivatives under Pd catalysis. Structural aspects of the new
compounds are discussed.
enzofulvenes and related compounds have elicited
functionalization of the benzofulvene core, but it is limited to
the production of 3-substituted derivatives. More recently, Ye
et al.7i reported a Suzuki coupling of 3-chlorobenzofulvene with
various arylboronic acids, using Pd(OAc)2 and S-Phos as the
catalytic system. The fulvene unit possesses a cross-conjugated
system which was exploited in cycloaddition reactions10 and as
a building block for the preparation of metallocene catalysts.11
In continuation of our studies on organometallic methods for
the synthesis of conjugated molecules,12 here we report a
general route to obtain conjugated molecules containing the
benzofulvene core. This method is based on the Pd-catalyzed
cross-coupling reaction of the bis-iodo derivatives resulting
from the iodine promoted 5-exo-dig cyclization of diaryl
enediynes with conjugated organometallic reagents. The
versatility of the method is given by the possibility of
introducing up to four conjugated substituents on the
benzofulvene (two in the synthesis of the bis-ethynylbenzene
and two in the subsequent cross-coupling reactions of the
resulting cyclized intermediates).
1
Bextensive attention as biologically active molecules,
as
precursors of functionalized indenes, which are interesting
pharmaceutical agents,2 and as molecular materials.3 Despite
their importance, a straightforward method for the synthesis of
benzofulvene derivatives is still lacking. Different approaches to
the synthesis of the fulvene skeleton have been reported, based
on radical cyclization of enediynes,4 thermal5 and photo-
chemical6 cyclization of enyne-allenes, and transition-metal-
catalyzed cyclization,7 but most of them suffer from harsh
reaction conditions, poor selectivity, or a limited scope. An
interesting synthetic route is represented by a variation of the
Bergman cyclization reaction8 and consists of a 5-exo-dig
cyclization of aryl enediynes promoted by electrophiles,9
nucleophiles,9c or radicals.4b−d When bromine or iodine is
used as the electrophile to promote the cyclization, the
resulting bis-halogenated benzofulvenes appear to be very
attractive intermediates for subsequent transformations,9c
particularly via cross-coupling reactions. Despite the great
synthetic potentiality of this approach, only one example has
been reported so far, as a short end note, describing a Suzuki
coupling reaction of the dibromo-diphenyl-benzofulvene with
phenylboronic acid.7k Kovalenko et al.4c reported the synthesis
of an organometallic benzofulvene derivative using an opposite
approach: the treatment of aromatic enediynes with Bu3SnH
generates a 3-tributylstannyl-substituted benzofulvene, which is
then used in a Stille cross-coupling reaction with aryl iodides
under the Corey conditions. This method evidences the
versatility of cross-coupling organometallic reactions for the
The 1,2-bis(arylethynyl)benzenes 7−12 were synthesized by
the sequence reported in Scheme 1.
1,2-Diiodobenzene or its derivatives 1−3 were reacted with
ethynyltrimethylsilane in the Cassar−Heck−Sonogashira re-
action conditions, affording the corresponding bis-ethynyl
intermediates. These were submitted without purification to a
basic hydrolysis to remove the trimethylsilyl group. The
Received: February 21, 2014
© XXXX American Chemical Society
A
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Letter
Diiodobenzofulvene 13 and the diiodobenzofulvene inter-
mediates with R = OC8H17 16 and 17 were isolated and
characterized prior to use in the subsequent cross-coupling
reactions. In contrast, the diiodobenzofulvenes 14, 15, and 18
showed low stability and, after evaporation of the reaction
solvent, yielded a black insoluble powder. Therefore, to prevent
decomposition, they were not isolated after the cyclization step,
but were reacted in situ with thiophen-2-ylboronic acid and 5-
(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-2,2′-bithiophene,
yielding 22, 23, and 30. All the benzofulvenes were obtained as
two stereoisomers on the exocyclic double bond, with the
exception of the compounds 19, 22, 23, 25, and 30, which can
exist as only one stereoisomer since Ar = Ar1.
The structures of benzofulvenes 19 and 22 were studied by
theoretical calculations. The structure of 19 was also
investigated by X-ray analysis. All calculations were performed
on the ground electronic state in vacuo at the DFT13 level of
approximation, using the functional B3PW9114 with the 6-
311+G(2d,p) basis set for all atoms. The GAUSSIAN 0315
program was used. Molecular geometries were fully optimized
starting from standard bond lengths and angles as input data.
NBO (Natural Bond Orbital) analysis16 was used to calculate
atomic charges and provided preliminary information about the
intramolecular interactions characterizing the compounds
considered. Further computational details and tables with X-
ray and computed data supporting the following discussion are
reported in the Supporting Information (Tables SI1−SI11).
In the case of compound 19, starting from geometries with
different orientations of the phenyl rings with respect to the
benzofulvene plane, the optimization process converged in all
cases toward the same structure, reported in Figure 1. The
Scheme 1. Synthesis of 1,2-Bis(arylethynyl)benzenes 7−12
resulting 1,2-bis(ethynyl)benzenes 4−6 were finally converted
into the 1,2-bis(arylethynyl)benzenes 7−12 by Cassar−Heck−
Sonogashira reactions with three different aryl bromides, i.e.
bromobenzene, 2-bromothiophene, and 5-bromo-2,2′-bithio-
phene.
The second step of the sequence consists of a nearly
quantitative 5-exo-dig cyclization of the 1,2-bis(arylethynyl)-
benzenes 7−12, promoted by iodine in chloroform or toluene
affording derivatives 13−18. These diiodobenzofulvenes are
coupled with organoboronate, organostannane, or ethynyl
reagents under Pd-catalysis, yielding compounds 19−30
(Scheme 2).
Scheme 2. Synthesis and Reactivity of Diiodobenzofulvenes
Figure 1. Most stable conformers of 19 and 22, calculated in vacuo at
the B3PW91/6-311+G(2d,p) level of approximation.
calculated molecular geometry is in satisfying agreement with
the experimental X-ray structure (Figure 2), thus suggesting the
validity of the approximation used for calculations.
In the following discussion, the four aryl rings on the
benzofulvene moiety have been numbered as 1 (ring C11−C16
in Figure 2), 2 (C17−C22), 3 (C23−C28), and 4 (C29−C34).
X-ray analysis revealed that 19 crystallizes in the C2/c space
group with the phenyl rings significantly twisted with respect to
the plane of the benzofulvene unit. The values of the dihedral
angles between the planes through the benzofulvene core and
the phenyl rings are ϕ1 = 77.05(4)°, ϕ2= 48.84(5)°, ϕ3=
55.31(4)°, and ϕ4= 55.38(4)° (the calculated values are 55°,
50°, 44°, and 54°, respectively). The inter-ring dihedral angle
between the two central phenyls 2 and 3 is 33.98(5)° (the
calculated value is 22°), and the distance between their centers
is 3.822 Å (calculated value 3.9 Å). This nearly face-to-face
B
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same level of approximation in vacuo, some interesting
observations are possible. The introduction of phenyl and
thiophene rings on benzofulvene does not induce any
significant change in the molecular geometry, with the
exception of small variations of the C−C distances in the
fulvene moiety. In contrast, significant variations of the atomic
charges on the carbon atoms on the fulvene unit are calculated.
The analysis of hyperconjugative π → π* interactions involving
the orbitals of the benzofulvene indicates that both tiophene
and phenyl substituents induce a remarkable increase in the π-
delocalization between the fused benzene and fulvene rings of
the benzofulvene system, and within the fulvene unit, but have
only a small influence on the delocalization within the benzene
ring. In compound 19 the energy of delocalization (hyper-
conjugative interactions) increases about 7.2 kcal mol−1 with
respect to the unsubstituted benzofulvene nucleus; in 22 the
thienyl units exert a stronger effect, producing an increase of
about 11.3 kcal mol−1.
Figure 2. X-ray structure of compound 19.
arrangement, together with the calculated atomic natural
charges and the short inter-ring distance, suggests the existence
of π−π interactions17 between the two aromatic rings.
Moreover, the values of the natural atomic charges and
intramolecular C−H···C distances between adjacent phenyl
rings suggest that the structure could be stabilized by weak
intramolecular interactions of electrostatic nature and non-
classical C−H···π hydrogen bonds.
In summary, we have developed a general and straightfor-
ward method for the synthesis of substituted benzofulvenes. X-
ray analysis and computational studies performed on
structurally simple benzofulvenes 19 and 22, for which only
one stereoisomer exists, evidenced a molecular geometry in
which all rings are twisted with respect to the benzofulvene
plane.
The structural study of benzofulvene 22 was more
complicated. In principle, considering the relative positions of
the sulfur atoms of the thiophene rings, at least 16 conformers
should be taken into account. However, the conformational
analysis of the benzofulvene moiety substituted with one and
two thienyl units at the exocyclic double bond of benzofulvene
indicated that the sulfur atom of the thienyl unit is
preferentially in anti with respect to the benzene ring of the
benzofulvene core. As a consequence, only eight conforma-
tional minima were considered. The energy difference between
these minima is less that 2 kcal mol−1, but 22 is not
characterized by high conformational mobility, since the high
rotational barrier does not allow an easy interconversion
between different conformers. The thienyl units are signifi-
cantly tilted with respect to the benzofulvene plane, with the
inter-ring planes being in the range 40°−60° for all the
conformers examined. In the most stable conformer, the thienyl
units are twisted with sulfur atoms anti to each other (Figure 1)
and the calculated dihedral angles are ϕ1 = 48°, ϕ2 = 40°, ϕ3 =
54°, and ϕ4 = 49°. This conformation significantly reduces the
electrostatic repulsive interactions between the four positively
charged sulfur atoms (the charges on the four sulfur atoms are
in the range from 0.461 to 0.478) and the hydrogen atoms on
the adjacent thienyl rings. As already determined for 19, the
two central thienyl rings tend to be nearly parallel, the inter-
ring dihedral angle being 15° ca., and the calculated distance
between their centers is 3.8 Å, thus suggesting, also in this case,
the possible existence of stacked π−π interactions. The
arrangement of thienyl units does not allow the existence of
C−H···π interactions between adjacent rings to be considered.
However, a fundamental role could be played by attractive
electrostatic interactions between positively charged sulfur
atoms and negatively charged carbon atoms on adjacent thienyl
rings (the distances between the sulfur atoms and ring carbon
atoms are 3.2−4.9 Å).
ASSOCIATED CONTENT
* Supporting Information
Detailed experimental procedures, NMR spectra, X-ray analysis,
and computational data. This material is available free of charge
via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: cardone@ba.iccom.cnr.it.
*E-mail: roberta.musio@uniba.it.
*E-mail: gianlucamaria.farinola@uniba.it.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was financially supported by Italian National
Council of Research CNR through the project “EFOR-Energia
da Fonti Rinnovabili” (Iniziativa CNR per il Mezzogiorno
L.191/2009 art. 2 comma 44); Regione Puglia (APQ-Reti di
Laboratorio, Project PHOEBUS, cod. 31-FE1.20001); MIUR
̀
with Universita degli Studi di Bari “Aldo Moro” (Progetto
PRIN2009 prot. 2009PRAM8L and Progetto PON
02_00563_3316357 “Nanotecnologie Molecolari per la Salute
dell’Uomo e l’Ambiente_ MAAT”).
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