Revisiting Cyclobutadienemetal Complexes: High-Content Dewar Benzene Polymers and a One-Pot Synthesis of Permethylated Ladderanes

Article abstract of DOI:10.1055/s-2003-43348

High-content Dewar benzene polymers and sterically congested [n] -ladderanes have been synthesized from [4π_s+2π_s] cyclobutadienemetal complex cycloaddition chemistry. In the former case, tetramethylcyclobutadiene-AlCl₃ is reacted with an acetylene-containing precursor polymer to yield > 95% Dewar benzene incorporation. In the latter case, [n]-ladderanes containing up to thirteen rings are prepared via a one-pot procedure from 2-butyne.

Full text of DOI:10.1055/s-2003-43348

192
CLUSTER
Revisiting Cyclobutadienemetal Complexes: High-Content Dewar Benzene
Polymers and a One-Pot Synthesis of Permethylated Ladderanes
R
evisiting Cyclobuta
d
i
ieneme
c
tal
C
omple
h
xes ael J. Marsella,* Samia Estassi, Li-Sheng Wang, Kunsang Yoon
Department of Chemistry, University of California at Riverside, Riverside, California, 92521-0403 USA
Fax +1(909)7872435; E-mail: Michael.marsella@ucr.edu
Received 11 September 2003
To date, our attempts to directly polymerize a Dewar ben-
Abstract: High-content Dewar benzene polymers and sterically
congested [n]-ladderanes have been synthesized from [4ps+2ps] cy-
clobutadienemetal complex cycloaddition chemistry. In the former
case, tetramethylcyclobutadiene–AlCl3 is reacted with an acetyl-
zene monomer have been unsuccessful. This may be due,
at least in part, to the intrinsic reactivity of the Dewar ben-
zene system. Further frustrating our goal is the aforemen-
ene-containing precursor polymer to yield > 95% Dewar benzene tioned limitations to the synthesis of a large library of
incorporation. In the latter case, [n]-ladderanes containing up to
thirteen rings are prepared via a one-pot procedure from 2-butyne.
Dewar benzene monomers via CBD–AlCl3 synthetic
methodology. As such, we focused our attention on post-
Key words: polymer synthesis, Dewar benzene, [n]-ladderanes,
cycloadditions, cyclobutadiene
polymerization methods that employ a CBD–AlCl3-medi-
ated alkyne cyclotrimerization route to Dewar benzenes.
This reaction, shown in Scheme 1, is presumed to be a
[4ps+2ps] cycloaddition between a transient tetrasub-
The reaction between anhydrous AlCl3 and an internal stituted cyclobutadiene and an alkyne dienophile. In our
alkyne, such as 2-butyne, is a simple and efficient method hands, a 1:1 reaction between tetramethylcyclobutadiene
to produce the corresponding tetraalkylcyclobutadiene- (TMCBD) and dimethylacetylene dicarboxylate (DMAD)
AlCl3 complex (Scheme 1).1,2 This complex has been yields ca. 70% of isolated Dewar benzene 1, with the
utilized extensively in the synthesis of substituted Dewar remaining side products being unreacted DMAD and
benzenes and tricyclo[4.2.0.00,0]oct-3-enes.2–5 In almost octamethyl-tricyclo[4.2.0.00,0]octa-3,7-diene (tetrameth-
all reported cases, the reacting partners are electron-defi- ylcyclobutadiene dimer, TMCBDD, Scheme 1). Noting
cient p-systems, such as acetylene dicarboxylic acid di- that the only transformation of DMAD in this reaction is
esters. The accepted mechanism for this reaction is to product, we realized the applicability of this synthetic
thermal [4ps+2ps] cycloaddition between liberated cy- method to post-polymerization conversion of a polymeric
clobutadiene and an electron-deficient 2p component. DMAD analog [i.e., a poly(dieneophile)] to the corre-
Yields vary considerably for this reaction, being highly sponding poly(Dewar benzene).
dependant upon the nature of the 2p system. Unsuccessful
O
O
cycloadditions typically yield cyclobutadiene dimer (oc-
taalkylyl-tricyclo[4.2.0.00,0]octa-3,7-diene) as the major
product (i.e., TMCBDD, Scheme 1).
AlCl3
MeO
OMe
AlCl3
i. DMAD
ii. DMSO
CH2Cl2
0 °C
1
Having previously reported that Dewar benzene can serve
as a photolabile supramolecular protecting group capable
of inhibiting solid-state aryl-aryl stacking interactions,6
we remain interested in translating this concept to poly-
mer chemistry. For example, the ability to photochemical-
ly mask and unmask interchain aryl-aryl stacking
interactions may influence properties such as polymer sol-
ubility and crystallinity. A first step in achieving this goal
is the synthesis of high-content Dewar benzene polymers
by utilizing the limited repertoire of reactions associated
with cyclobutadiene–AlCl3 (CDB–AlCl3) chemistry.
Herein we report the chemistry of two organometallic–
CBD complexes (Al3+ and Fe2+), both capable of yielding
polymers of interest. More specifically, both the desired
high-content Dewar benzene polymer (synthesized via
post-polymerization methods) and permethyleated
ladderanes7 are reported.
TMCBDD
O
O
MeO
OMe
hν or ∆
1
2
Scheme 1
Polymeric dienophile, poly-3, was prepared by the acid-
catalyzed esterification of tetraethylene glycol and
acetylene dicarboxylic acid (neat, 50 °C, 70 mTorr). The
reaction was halted when the mixture became too viscous
to be stirred by a magnetic stir bar driven by a magnetic
stir-plate. Isolated polymers were subjected to gel per-
meation chromatography (GPC) analysis and were found
to have molecular weights of ca. 2700 (vs polystyrene
standards). Given the proof-of-concept nature of these
studies, no efforts were made to obtain higher molecular
weight polymers.
SYNLETT 2004, No. 1, pp 0192–0194
0
5.
0
1.
2
0
0
4
Advanced online publication: 26.11.2003
DOI: 10.1055/s-2003-43348; Art ID: C00703ST
© Georg Thieme Verlag Stuttgart · New York

CLUSTER
Revisiting Cyclobutadienemetal Complexes
193
Conversion of poly-3 to poly-4 was carried out under con- Poly-3 exhibits one major exotherm centered at 206 °C
ditions similar to the preparation of 2 (Scheme 2). It was (DH = 35 kcal/mol). We propose this event corresponds to
found that as the ratio of diene to dienophile increased thermal decomposition of the pre-polymer.
from 1:1 to 2.5:1, Dewar benzene incorporation increased
from ca 60% to >95%. Thus, >95% of acetylene sites in
poly-3 were converted to Dewar benzene moieties via cy-
cloaddition with TMCBD. As before, TMCBDD was the
Similar to compound 1, poly-4 exhibits one major exo-
therm (62 kcal/mol) during the heating scan. This event is
centered at ca. 157 °C. Subsequent heating/cooling scans
show no additional exothermic processes corresponding
only other major product in the reaction mixture. The iso-
to thermal isomerization. Thermal gravimetric analysis
lated crude polymer was washed with pentane to remove
(TGA) reveals that only a slight mass loss (2.4%) takes
place over the temperature range corresponding to ther-
mal isomerization.
TMCBDD impurities. No further purification of poly-4
was necessary.
AlCl3
hν or ∆
DMSO
poly(3)
poly(4)
poly(5)
Me
O
O
O
O
O
O
O
TEG
O
TEG
TEG
n
n
n
poly(3)
poly(4)
O
O
O
O
TEG = -CH2(CH2OCH2)4CH2-
poly(5)
Scheme 2
Figure 1 1H NMR spectra of polymers 3–5
Both the thermal and photochemical isomerization of
poly-4 to poly-5 were monitored by 1H NMR. For the lat-
ter experiment, a solution of poly-4 in CDCl3 was com-
pletely converted to poly-5 by irradiation with a longwave
UV lamp (2.2 W) for ca 60 minutes. For the former exper-
iment, a solution of poly-4 in toluene-d8 yielded 50% con-
version upon heating at 100 °C for 15 hours. In both cases,
1H NMR peaks associated with the aromatic isomer were
identical, with no significant signs of side reactions such
as isomerization to prismane. GPC analysis of pre- and
post-isomerized polymer (polymers 4 and 5, respectively)
show slightly different retention times (7.69 and 7.84 min,
respectively). This difference most likely results from
isomerization, as it is not of the magnitude expected if
cross-linking via undesirable side reactions had occurred.
Note that the longer retention time of poly-5 may result
from increased interaction between aryl moieties of poly-
mer and those present within the GPC stationary phase
column packing material (cross-linked divinylbenzene).
1H NMR spectra for polymers 3–5 are shown in Figure 1.
In an effort to screen the chemistry of alternative cyclo-
butadiene sources, we attempted to prepare and utilize
the known tricarbonylcyclobutadiene iron complex
[R4C4H4Fe(CO)3], specifically TMCBD–Fe(CO)3.1 Our
intent was to modify the preparation of this complex by an
in situ exchange reaction of TMCBD–AlCl3 with Fe(CO)5
to yield TMCBD–Fe(CO)3 (Scheme 3).
AlCl3
AlCl3
Fe(CO)5
CH2Cl2
0 °C
n
ladderane 6
ladderane 6 n=11
Scheme 3
Neat samples of compounds 1–5 were analyzed by differ-
ential scanning calorimetry (DSC). The thermal isomer-
ization of compound 1 to 2 corresponds to an exotherm
centered at 147 °C during the heating scan. The DH for
this process is 60 kcal/mol. This experimental value is in
good agreement with the calculated value of 62 kcal/mol
(B3LYP/6-31G[d]8). High purity of the thermal product,
compound 2, can be assessed from the observation of its
crystallization at 95 °C during the cooling scan (sharp
exothermic peak centered at 95 °C; DH = 7.4 kcal/mol).
Under such conditions, the desired iron complex, if
formed, underwent rapid decomposition to yield a mixture
of hydrocarbons differing by mass units equivalent to
C8H12 (FAB+ MS: C16H24, C24H36, C32H48, C40H60, C48H72,
C56H84). 1H NMR spectroscopy revealed multiple singlet
peaks of differing intensity clustered about the two re-
1
gions associated with the H singlets of TMCBDD (1.44
and 0.95 ppm). The combined analyses, coupled with
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194
M. J. Marsella et al.
CLUSTER
existing literature precedent of ladderane9 synthesis via conditions and a more comprehensive analysis of the
CBD–Fe(CO)3 complexes,7,10 implicate oligomerization physical properties associated with the aforementioned
of cyclobutadiene, likely by [4p+2p] cycloaddition, to polymeric materials.
yield an overall one-pot synthesis of sterically-crowded
permethyated ladderanes starting with the C4-building
block, 2-butyne. Note that the transformation of C4H6 →
C56H84 corresponds to creation of thirteen annulated rings,
Acknowledgment
This work is supported by the National Science Foundation
(CHE-0138143).
compound 6 (n = 11, Scheme 3).
The isolated mixture of ladderanes exhibited only moder-
ate stability, with thermal/photochemical decomposition
affecting work-up, purification, and isolation. The gradual
appearance of hexamethylbenzene along with an increas-
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Synlett 2004, No. 1, 192–194 © Thieme Stuttgart · New York