Cation radical cycloaddition polymerization: Diels-Alder copolymerization

Article abstract of DOI:10.1002/(SICI)1099-1395(1998110)11:11<825::AID-POC73>3.0.CO;2-U

The Diels-Alder cycloaddition copolymerization of a bis(diene) with ionizable bis(dienophiles) via a cation radical mechanism has been accomplished using tris(4-bromophenyl)aminium hexachloroantimonate as a catalyst in dichloromethane solvent. The reactions occur at 0°C and yield Diels-Alder polymers of M_w up to ca. 10,000 and a polydispersity index ca. 2.

Full text of DOI:10.1002/(SICI)1099-1395(1998110)11:11<825::AID-POC73>3.0.CO;2-U

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, VOL. 11, 825–830 (1998)
Cation radical cycloaddition polymerization: Diels±Alder
copolymerization
Nathan L. Bauld,* J. Todd Aplin, Wang Yueh and Stephanie Endo
Department of Chemistry and Biochemistry, University of Texas, Austin, Texas 78712, USA
Received 1 December 1997; revised 18 February 1998; accepted 19 February 1998
ABSTRACT: The Diels–Alder cycloaddition copolymerization of a bis(diene) with ionizable bis(dienophiles) via a
cation radical mechanism has been accomplished using tris(4-bromophenyl)aminium hexachloroantimonate as a
catalyst in dichloromethane solvent. The reactions occur at 0°C and yield Diels–Alder polymers of MW up to ca.
10,000 and a polydispersity index ca. 2. 1998 John Wiley & Sons, Ltd.
KEYWORDS: cation radical cycloaddition polymerization; Diels–Alder copolymerization
INTRODUCTION
dienophile functionalities. Further, several bis(dieno-
phile) monomers were already available from a previous
study of cation radical poly(cyclobutanation).2 Previous
research had suggested that cation radical Diels–Alder
reactions generally occur more efficiently when the
reactive cation radical is the ionized dienophile.3
Consequently, a bis(dienophile), 3, was selected which
Addition polymerization is well known to occur via
radical, anionic and cationic mechanisms,1 but only very
recently has polymerization via reactive cation radical
intermediates been demonstrated.2 Especially novel
aspects of this new polymerization mechanism are its
propensity for cycloaddition and its apparent preference
for non-vinyl (e.g. propenyl) monomers. The cation
radical cycloaddition polymerization of the dipropenyl
monomer 1 (Scheme 1), e.g. initiated by the stable cation
radical salt tris(4-bromophenyl)aminium hexachloroanti-
‡
Á
was known to be readily ionizable by 2 (Scheme 2),
while the preferred bis(diene), 4, was known to be
‡
Á
resistant to ionization by 2 . Finally, propenyl rather
than vinyl moieties were selected for the dienophile
functionality because terminal methyl groups sharply
enhance the ionizability of the alkene functions and
because they also tend to suppress the acid-catalyzed side
reactions which sometimes compete with the cation
radical reactions when unsubstituted vinyl groups are
present. The bis(dienophile) comonomer 3 was readily
prepared from diphenyl ether in three steps, while the
bis(diene) comonomer 4 was obtained from 5-bromo-1,3-
pentadiene via copper catalyzed coupling of the corre-
sponding Grignard reagent.
‡
Á
monate (2 ), has been shown to proceed via a cation
radical chain mechanism to afford the novel macro-
molecule cyclobutapoly (1). In recent years, a variety of
cation radical cycloaddition reaction types have been
explored in these and other laboratories.3 These include
cyclobutanation, Diels–Alder cycloaddition and cyclo-
propanation. The present paper describes the develop-
ment of cation radical Diels–Alder cycloaddition as a
novel approach to polymerization.
Copolymerization of 3 and 4
RESULTS AND DISCUSSION
Both homopolymerization and copolymerization formats
are potentially available for Diels–Alder cycloaddition
polymerization. The copolymerization format was se-
lected for this initial study because it appeared that the
synthesis of symmetrical bis(diene) and bis(dienophile)
comonomers might be more facile than the synthesis of
unsymmetrical monomers containing both diene and
A dichloromethane solution of comonomers 3 (0.04 M)
and 4 (1.1-fold excess) was subjected to cation radical
‡
Á
Diels–Alder copolymerization at 0°C by adding 2
(30 mol%) dropwise over a period of 10 mins. The
reaction was quenched after 20 min by adding excess
methanolic potassium carbonate, followed by the addi-
tion of water and methylene chloride and separation of
the organic phase. Alumina chromatography yielded a
polymer (82%) having Mw = 10800 and a polydispersity
index (PDI) of 2.1. Structural characterization of the
polymer was achieved by comparing the 1H NMR
*Correspondence to: N. L. Bauld, Department of Chemistry and
Biochemistry, University of Texas, Austin, Texas 78712, USA.
Contract/grant sponsor: National Science Foundation; contract grant
number: CHE-9610227.
1998 John Wiley & Sons, Ltd.
CCC 0894–3230/98/110825–06 $17.50

826
N. L. BAULD ET AL.
Scheme 1
Scheme 2
spectrum of the polymer with that of a model compound
the copolymerization of 3 and 4 is considered to be regio-
and stereospecific. However, since cation radical Diels–
Alder cycloadditions are not highly endo diastereoselec-
tive, especially where acyclic dienes are involved, the
present polymer structure is not completely tactic, but
occurs with the ethano bridges cis and trans to the aryl
substituent on the cyclohexene ring as indicated in the
structure of copoly-(3,4).
The bis(dienophile) comonomer 1, an even closer
analogue of trans-anethole, was also examined. This
monomer has previously been observed to undergo
especially facile cation radical cyclobutane homopoly-
merization.2 In the presence of a slight excess of the
bis(diene) 4, copolymerization again occurs to form
exclusively the Diels–Alder copolymer (75% yield of
chromatographed polymer) of MW = 2520 and a PDI of
2.3. Interestingly, whereas 1 undergoes homopolymer-
ization much more efficiently than does 3, the latter is a
significantly more effective partner for 4 in copolymer-
ization.
‡
Á
(5; Scheme 3) prepared by the aminium salt (2 )-
catalyzed Diels–Alder reaction of 3 with 2 mol of (E)-
1,3-pentadiene. The 1H NMR spectra of the model
compound 5 and the polymer are virtually superimpo-
sable and consist of cyclohexene type protons (ꢀ 4.9 and
5.05), benzylic methine protons (ꢀ 2.7), non-benzylic
methine protons (ꢀ 1.7), methylene protons (ꢀ 2.2–2.0)
and methyl doublets (ꢀ 0.9). In particular, the existence of
cyclobutane linkages which could have resulted from the
homopolymerization of 3 or from the incorporation of
two or more consecutive molecules of monomer 3 into
the polymer chain in substantial amounts can be ruled
out. Such cyclobutane linkages are known to exhibit
absorptions at ꢀ 1.2, corresponding to methyl groups
attached to a cyclobutane ring.2 The cation radical Diels–
Alder reactions of various dienophiles including trans-
anethole, a dienophile which is structurally very similar
to 3 are regiospecific and stereospecific. Consequently,
Mechanistic considerations
The observation that alternating Diels–Alder copolymer-
ization of 3 with 4 and of 1 with 4 dominates over the
cyclobutane (CB) homopolymerization of 3 or 1 indicates
‡
Á
‡
Á
that the Diels–Alder addition of 3 and 1 to 4 is
substantially faster than the competing additions of these
same cation radicals to neutral 3 or 1. While this
circumstance was obviously not unexpected, it should not
Scheme 3
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JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, VOL. 11, 825–830 (1998)

DIELS–ADLER COPOLYMERIZATION
827
Scheme 4
necessarily be considered to imply a strong inherent
preference for DA over CB cycloaddition. In fact, both
CB and DA cycloadditions of cation radicals are well
known to be extremely facile, and instances when these
two reaction modes are competitive are now familiar.3 It
appears much more likely that the preference for addition
to 4 as opposed to 3 (or 1) is based upon the now well
established steric preference of cation radicals for
addition to an unsubstituted alkene (vinyl) terminus over
addition to a monosubstituted alkene (propenyl) termi-
nus.4
served CB homopolymerizations (e.g. 1) is also of
mechanistic interest. An inspection of the detailed
mechanism of copolymerization (Scheme 4) reveals a
major obstacle to the crucial intramolecular hole transfer
reaction (step 4 of Scheme 4), which is required in order
for the polymerization to propagate as a chain reaction.
‡
Á
The reaction of 3 with 4 is expected to proceed
‡
Á
efficiently to the 1:1 adduct cation radical 6 (Scheme
4). The cation radical moiety in this adduct is expected
initially to reside on that functionality in direct conjuga-
tion with the pericyclic (DA) transition state which is
most readily ionizable. The two logical candidates are the
cyclohexene double bond and the 4-propenylphenoxy-
The significantly lower molecular weights observed in
these DA colypolymerizations than in previously ob-
Scheme 5
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JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, VOL. 11, 825–830 (1998)

828
N. L. BAULD ET AL.
phenyl function, and the obvious preference in terms of
ionizability is the latter function. Reaction of 6 with 4
Cation radical Diels±Alder homopolymerization
‡
Á
can then proceed directly to the 2:1 adduct cation radical
The clear implication of the preceding analysis is that
cation radical DA homopolymerization should be a much
more efficient process than copolymerization. In the case
of the polymerization of a monomer which has both diene
and ionizable dienophile functionalities, a readily ioniz-
able dienophilic moiety is available in each propagation
step to facilitate intramolecular hole transfer. This
concept is currently being investigated.
‡
Á
7 . At this point, however, significant problems with
continuing the chain propagation process become
evident. First, hole transfer from the diphenyl ether
‡
‡
Á
Á
function of 7 to the diene function of 7' is required
for further chain propagation. This hole transfer is not
obviously exergonic and may therefore be relatively
‡
Á
slow. Intermolecular hole transfer from 7 to the neutral
triarylamine 2 or to the monomer, in contrast, is highly
exergonic and may result in the preferential neutraliza-
tion of 7 (step 5). The net result is that a potentially
relatively efficient chain growth polymerization is
converted, at least partially, into a less efficient step
growth process.
‡
Á
CONCLUSIONS
The cation radical Diels–Alder copolymerizations of two
ionizable, difunctional dienophiles with a simple, acyclic
bis(diene) have been demonstrated. The resulting poly-
mers are of modest molecular weight (MW = 10000 in the
best instance). These Diels–Alder polymerizations thus
appear to be substantially less efficient than the cation
radical homocyclobutanation polymerizations of ioniz-
able bis(dienophiles). Nevertheless, the rections occur
under very mild conditions (0°C) and are regio- and
stereospecific. The reactions appear to occur by a step
growth mechanism (PDI ꢀ 2), rather than the more
efficient cation radical chain mechanism proposed for
poly(cyclobutanation). The apparent suppression of the
chain mechanism is viewed as an inherent problem with
the copolymerization format of cation radical Diels–
Alder polymerization.
A second potential problem with propagation from the
2:1 adduct cation radical is that even if intramolecular
‡
Á
hole transfer were to occur, affording 7' , the reaction of
this diene cation radical with neutral 3 is not of the type to
be preferred in most cation radical DA cycloadditions
‡
Á
(i.e. dienophile cation radical/neutral diene). In fact 7'
is unlikely to react with 3 to yield a DA adduct at all,
since it should be formed preferentially in the s-trans
conformation and would be unable to generate the s-cis
diene cation radical required for DA reaction.
The even further diminished efficiency of DA copoly-
merization of 1 with 4 can be readily understood on the
same basis. Since 1 has an oxidation potential which is
about 0.1 V less than that of 3, it is not unreasonable to
‡
Á
assume that the hole transfer from 8 (Scheme 5) to the
diene moiety is even less energetically favorable than that
‡
‡
Á
Á
in 7 . This proposal is further supported by viewing 7
EXPERIMENTAL
as a benzene-type cation radical having a stabilizing
phenoxy substituent, while 8 is a benzene-type cation
radical having a stabilizing alkoxy substituent. Since the
value of methoxy ( 0.78) is much more negative than
‡
Á
Reagents. Dried solvents were obtained by distillation
under nitrogen immediately prior to use. Reagent-grade
dichloromethane and acetonitrile were distilled from
phosphorus pentoxide. Pyridine was distilled from
potassium hydroxide. Tetrahydrofuran (THF) and diethyl
ether (Et2O) were distilled from a blue solution of sodium
(or potassium) and benzophenone. All other reagents
were used as received unless specified otherwise.
Alumina (neutral) TLC plates and alumina preparative-
scale TLC (PTLC) plates (Analtech, 1.5 mm layer thick-
ness) were washed with a 1:1 solution of ethyl acetate
(EtOAc) and dichloromethane, then dried in an oven at
110°C prior to use. Reagent-grade lithium perchlorate
used for electrochemisty was dried by heating at 180°C
for 24 h under an N2 purge and stored in a desiccator
containing Drierite. All moisture-sensitive reactions were
carried out in oven-dried glassware which had been
flushed with dry nitrogen. All organic product solutions
were dried over magnesium sulfate unless idicated
otherwise.
‡
that of phenoxy ( 0.50), the cation radical moiety should
‡
‡
Á
Á
be more stabilized in 8 than in 7 . This view is
supported by the substantially lower oxidation potential
of 1 than 3 :EOX(1) = 1.327, EOX(3) = 1.424 V vs SCE.
Hence it does not appear that the delocalization of the
cation radical moiety over the second phenyl ring in 3
provides greater stabilization than that available in 1 .
Consequently, hole transfer to give 8' should be ener-
getically even less favorable than from 7 to give 7' .
Evidence has previously been presented that the CB
homopolymerization of 1 occurs via a chain growth
process, at least in the first and most efficient stage of
reaction. It is therefore expected, if DA copolymerization
is constrained to proceed via step growth, that this type of
polymerization will be less efficient. The PDIs of the
presently observed DA copolymerizations, being ca 2 at
essentially 100% monomer conversion, are in good
accord with the proposal of a step growth mechanism.
‡
Á
‡
Á
‡
Á
‡
Á
‡
‡
Á
Á
1
Analysis. Room temperature H NMR and 13C NMR
spectra were recorded on a Bruker AC 250 spectrometer
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JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, VOL. 11, 825–830 (1998)

DIELS–ADLER COPOLYMERIZATION
829
as solutions in CDCl3 at 55°C. Solid-state 13C NMR
spectra were recorded on a Chemagentics CMX-300
spectrometer as fine powders. Chemical shifts are
reported in parts per million (ppm) downfield from a
tetramethylsilane (TMS) reference. Splitting patterns are
designated as follows: s, singlet; d, doublet; t, triplet; q,
quartet; m, multiplet; dd, doublet of doublets; dt, doublet
of triplets; dq, doublet of quartets; br, broad. Solution-
solution on the plate) or as solutions in CCl4 using an
NaCl solution cell. Melting-points were determined on a
Mel-Temp capillary melting-point apparatus and are
uncorrected.
Bis[4-(propionyl)phenyl] ether. To a dry round-bottomed
flask equipped with a magnetic stirrer, AlCl3 (500 g,
0.38 mol) was added, followed by dichloromethane
(100 ml). After lowering the temperature of the reaction
to 20°C (by means of a dry-ice–acetone bath, a solution
of diphenyl ether (25.6 g, 0.15 mol) in dichloromethane
(40 ml) was added slowly. A 1:1 (v/v) solution of
propionyl chloride (41.6 g, 0.45 mol) in dichloromethane
was then added dropwise over a period of 15 min. After
the addition was complete, the dry-ice–acetone bath was
replaced with an ice bath, and the reaction mixture was
stirred for an additional 3 h, then carefully poured into a
separating funnel containing crushed ice. The aqueous
layer was separated and extracted twice with dichloro-
methane (2 Â 100 ml). The combined dichloromethane
layers were washed with water (500 ml), saturated aqueos
NaHCO3 (2 Â 500 ml) and water (2 Â 500 ml) prior to
drying (MgSO4) and solvent removal by a rotary
evaporator. The crude product was recrystallized from
heptane, yielding 38.1 g (90%) of the pure diketone
(>95% purity, by GC): m.p. 98.5–99.8°C (lit.5 100°C);
1H NMR, ꢀ 1.2 (t, 6H), 3.0 (q, 4H), 7.1 (d, 4H), 8.0 (d,
4H); 13C NMR, ꢀ 8.2, 31.7. 118.8, 130.4, 160.2, 199.5;
1
state H and 13C and solid-state 13C NMR spectra of
polymer samples gave broad signals with little fine
structure, so chemical shifts of such samples are reported
as the mid-point of the broad signals. Gas chromato-
graphic (GC) analyses were performed on a Varian
Model 3700 chromatograph equipped with a flame
ionization detector and a DB-1 capillary column (J&W
Scientific, 15 m  0.25 mm i.d., 1 mm film thickness)
using helium as a carrier gas. Low-resolution mass
spectra (LRMS) were obtained on a Hewlett-Packard
Model 5890 gas chromatograph equipped with a DB-1
capillary column (15 m  0.25 mm i.d., 1 mm film
thickness) and a Model 5971A mass selective detector.
Low-resolution chemical ionization mass spectra (CIMS)
were recorded on a Finigan MAT TSQ-70 mass spectro-
meter. High-resolution mass spectra (HRMS) were
recorded on a DuPont (CED) 21-110B mass spectro-
meter. Cyclic voltammetric (CV) measurements were
performed using a BAS Model 100 electrochemical
analyzer at a scan rate of 100 mV 1s. The CV measure-
ments were carried out using a divided cell equipped with
a platinum disk working electrode (anode) and a
reticulated vitreous carbon counter electrode (cathode)
attached to copper wire which was separated from the
‡
LRMS, m/z 282 (M ), 271, 253, 197, 196, 139, 120, 92
(base); HRMS, m/z calculated for C18H18O3 282.1256;
found 282.1261.
‡
working electrode by a glass frit. AnAg/Ag reference
Bis[4-(1-hydroxypropyl)phenyl] ether. The diketone pre-
pared in the previous procedure (2028 g, 0.072 mol) was
dissolved in 200 ml of a 3:1 solution of ethanol–THF,
followed by the addition of 5.44 g (0.14 mol) of sodium
borohydride. After 1.5 h at room temperature, the
reaction was quenched with 10% acetic acid at 0°C.
The quenched solution was then extracted with dichloro-
methane (3 Â 100 ml) and the combined extracts were
washed with aqueous NaHCO3 (2 Â 100 ml) and water
(2 Â 100 ml), prior to drying (MgSO4) and solvent
removal (rotary evaporator). The oily product diol
(20.5 g, 99.7%) was used without further purification:
1H NMR, ꢀ 0.9 (t, 6H), 1.6–1.9 (m, 4H), 2.6 (s, 2H), 4.5
electrode (silver wire immersed in an acetonitrile solution
0.1 M in AgNO3 and LiClO4), calibrated against ferro-
‡
cene/ferrocene , was placed in the anode compartment
and separated from the bulk solution by a Vycor frit. The
peak oxidation potential (EpOX) measurements vs
‡
Ag/Ag were converted to vs SCE by adding 0.3 V to
each value. A blank CV trace of the electrolyte solution
was recorded prior to analyzing the substrate. The
substrate was then added to the cell as a solution in the
electrolyte (concentration ca 4 mM) and its CV response
recorded. Gel permeation chromatography (GPC) was
1
carried out in dichloromethane or THF (1 ml min
)
using a Waters Model 550 HPLC pump, a Waters Model
410 differential refractometer and a Waters Model 745
(t, 2H), 6.9 (d, 4H), 7.2 (d, 4H); 13C NMR, ꢀ 10.2, 31.9.
1
75.4, 118.6, 127.4, 139.5, 156.5; IR (OH), 3360 cm
;
‡
˚
data module with an Ultrastyragel 500 A column
LRMS, m/z 286 (M ), 254, 250, 115 (base); HRMS, m/z
4
˚
connected in series with a mStyragel 10 A column. The
calculated for C18H22O3 286.1569; found 286.1576.
GPC analyses were calibrated with a polystyrene
standard. UV–visible spectra were taken on a Hewlett-
Packard Model 845A spectrophotometer. Fourier trans-
form infrared (FT-IR) spectra were recorded on a Nicolet
205 FT-IR spectrometer using polystyrene as a standard.
Liquid samples were run as thin films between NaCl
plates. Solid samples were run as thin films on NaCl
plates (generated by evaporation of a dichloromethane
Bis[4-(1-propenyl)phenyl ether. To a solution of the diol
obtained in the previous procedure (6.31 g, 0.022 mol) in
pyridine (25 ml) was added a slight excess of phosphorus
oxytrichloride (7.5 g) at room temperature. The reaction
mixture was refluxed for 2 h, then cooled in an ice bath.
Water was then added slowly to quench any excess
POCl3. The quenched reaction mixture was then trans-
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830
N. L. BAULD ET AL.
ferred to a separating funnel, washed with water and
dried (MgSO4) prior to solvent removal (rotary evapora-
tor). The crude product (3) was subjected to column
chromatography (9:1 hexane–ethyl acetate), yielding
3.96 g (72%) of 3 which was 99% pure but consisted of
an 87:11:2 mixture of the E,E-, E,Z- and Z,Z-isomers,
respectively, of 3: m.p. 117–119°C; ꢁmax 270 nm, log "
2H), 6.0–6.1 (m, 2H), 6.25–6.38 (m, 2H); 13C NMR, ꢀ
32.2, 115, 131.4, 134.2, 137.1; LRMS, m/z 134 (M ),
119, 106, 92, 79, 67 (base); HRMS, m/z calculated for
C10H14 134.1096; found 34.110.
‡
‡
Á
Copolymerization of 3 with 4 using 2 . To a solution of
3 (100 mg, 0.4 mmol) and a 1.1-fold excess of 4 (59 mg)
1
2.24; H NMR, ꢀ 1.9 (t, 6H), 6.1 (dq, 2H, Jtrans = 15.8),
in 2 ml of dichloromethane at 0°C was added a solution
6.4 (d, 2H, Jtrans = 15.8), 6.9 (d, 4H), 7.4 (d, 4H); 13C
NMR, ꢀ 18.4, 118.8, 124.7, 130.2, 137.2, 156.1; IR (C=C)
of 2 (0.098 g, 30%) in 8 ml of dichloromethane, drop-
‡
Á
wise, over a period of 10 min. After 20 min, the reaction
was quenched, followed by the addition of water (20 ml)
and dichloromethane (20 ml). The combined organic
layers were dried and the solvent was removed. Column
chromatography gave 124 mg (82%) of polymer whose
1H NMR spectrum was essentially superimposable on
that of the cross-adduct between 3 and trans-1,3-
pentadiene: MW = 10 800; PDI 2.1.
‡
1590; LRMS, m/z 250 (M ), 207, 179, 165. 133. 115
(base), 91; HRMS, m/z calculated for C18H18O 250.1358;
found 250.1369; EpOX 1.36 V.
(E,E)-1,3,7,9-Decatetraene (4). A dry 500 ml round-
bottomed flask equipped with a reflux condenser, stirrer,
nitrogen inlet and addition funnel was placed in an ice
bath and 200 ml of 1.0 M vinylmagnesium bromide in
THF were added. A solution of 13 ml (0.19 mol) of
freshly distilled 2-propenal in 20 ml of THF was then
added dropwise. The reaction was stirred for 2 h after the
addition was complete. The reaction was then quenched
by the addition of saturated NaHCO3. The layers were
separated and the aqueous layer was extracted with
pentane (3 Â 100 ml). The combined organic extracts
were dried (MgSO4) and the solvent removed (rotary
evaporator). Distillation gave 8.1 g (50%) of 1,4-
pentadiene-3-ol: b.p. 115–120°C. The crude alcohol
was placed in a 250 ml round-bottomed flask equipped
with a stirrer, ice bath and addition funnel. With stirring,
25 ml of 48% HBr were added slowly and the reaction
was allowed to stir overnight. Once the reaction was
complete, the aqueous solution was added with Et2O
(3 Â 100 ml). The combined ethereal solutions were
dried and the solvent was removed. Distillation afforded
pure 1-bromo-2,4-pentadiene (43% yield, b.p. 28–32°C
at 50 Torr).
‡
Á
Copolymerization of 1 with 4 using 2. . This
polymerization was performed exactly as for 3 with 4
except that 15 ml of dichloromethane were used to
dissolve 1: MW = 2520; PDI 2.3.
Reaction of 3 with (E)-1,3-pentadiene. To a dry, 25 ml
round-bottomed flask containing 0.198 g (0.792 mmol) of
3 and 0.120 g (1.77 mmol) of (E)-1,3-pentadiene dis-
solved in 10 ml of dichloromethane at 0°C was added
‡
Á
0.052 g (8.1%) of 2 . The reaction was quenched
(K2CO3–CH3OH) after 10 min and worked up in the
same manner as described for the polymerization of 3.
The pure Diels–Alder adduct 5 was obtained after
1
chromatography on alumina: H NMR, ꢀ 0.75 (d, 3H),
0.85 (d, 3H), 2.2–2.4 (m, 3H), 2.7 (m, 2H), 5.6 (m, 1H),
5.7 (m, 1H), 6.9 (d, 2H), 7.1 (d, 2H).
Acknowledgment
Finally, a dry 250 ml round-bottomed flask containing
2.0 g (82 mmol) of magnesium and 50 ml of dry THF was
placed in an ice bath and equipped with a reflux con-
denser, stirrer, nitrogen inlet and addition funnel. Then, a
solution of 10 g (68 mmol) of 5-bromo-1,3-pentadiene in
30 ml of THF was added dropwise. After the addition was
complete, the reaction mixture was stirred for 3 h at room
temperature. The Grignard solution was added via a
syringe to a vigorously stirred suspension of anhydrous
CuCl (7.3 g, 75 mmol) in 30 ml of THF at 0°C over a
period of 30 min. The resulting solution was filtered and
work-up performed by addition of 5 ml of 6 M HCl and
100 ml of H2O. The aqueous layer was extracted with
Et2O and the combined ethereal solutions were dried.
After solvent removal, the crude product was purified by
distillation at reduced pressure to give 8.24 g (75% yield)
The authors thank the National Science Foundation for
support of this research (CHE-9610227).
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JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, VOL. 11, 825–830 (1998)