Preparation of 1,3-dienyl organotrifluoroborates and their Diels-Alder/cross-coupling reactions

Article abstract of DOI:10.1016/j.tet.2007.08.063

2-BF₃-substituted 1,3-butadienes with potassium and tetrabutyl ammonium counterions have been prepared in gram quantities from chloroprene via a simple synthetic procedure. The potassium salt of this new main group element substituted diene has

Full text of DOI:10.1016/j.tet.2007.08.063

Tetrahedron 63 (2007) 10939–10948
Preparation of 1,3-dienyl organotrifluoroborates and
their Diels–Alder/cross-coupling reactions
*
Subhasis De, Cynthia Day and Mark E. Welker
Department of Chemistry, Wake Forest University, PO Box 7486, Winston-Salem, NC 27109, USA
Received 23 July 2007; revised 16 August 2007; accepted 20 August 2007
Available online 25 August 2007
Abstract—2-BF3-substituted 1,3-butadienes with potassium and tetrabutyl ammonium counterions have been prepared in gram quantities
from chloroprene via a simple synthetic procedure. The potassium salt of this new main group element substituted diene has been characteri-
1
zed by H, 13C, 11B, and 19F NMR and the tetra n-butyl ammonium salt was also characterized by X-ray crystallography. Diels–Alder
reactions of these dienes with dienophiles such as ethyl acrylate, methyl vinyl ketone, and N-phenylmaleimide are reported as well as
subsequent Pd-catalyzed cross-coupling reactions of those Diels–Alder adducts. 4-Phenyl-2-BF3-substituted 1,3-diene was prepared by
magnesium–halogen exchange from the corresponding 2-bromo and iodo dienes. The 4-phenyl-2-bromo-1,3-butadiene was also characteri-
zed by X-ray crystallography. 4-Phenyl-2-BF3-1,3-butadiene was used in Diels–Alder/cross-coupling reactions and the product of a
Diels–Alder reaction with N-phenylmaleimide followed by cross-coupling with 4-bromo-benzonitrile was also characterized by X-ray
crystallography.
Ó 2007 Elsevier Ltd. All rights reserved.
1. Introduction
the laboratories of Vaultier,12–15 Lallemand,13,15–18 and
others.19–21 Most of these reports use the dienylboronates
in [4+2]/allylation tandem reactions (1–4), and this sequence
is now often called the Vaultier tandem sequence (Scheme
1). In general, the regioselectivities and endo/exo selectiv-
ities of the Diels–Alder reactions of these 1,3-dienyl-1-
boronates are in the 4:1–9:1 range. One report also exists
for a Suzuki coupling of a 1,3-dienyl-1-boronate (1)22 and
very recently a Diels–Alder/allylation using a silyl alkyne
as dienophile23 and a cycloisomerization/Diels–Alder/
allylation sequence involving 1,3-dienyl-1-boronates have
been reported.24
Reports of main group element substituted 1,3-dienes and
their reaction chemistry are still fairly rare in organic chemi-
stry. 2-Triethylsilyl-1,3-butadiene and a few of its Diels–
Alder reactions were reported by Ganem and Batt in
1978.1 Fleming and co-workers reported 1-trimethylsilyl-
1,3-butadiene in 19762 and its Diels–Alder dimerization
in 1981.3 Paquette and Daniels reported some 2-silyl-
substituted 1,3-cyclohexadienes in 1982 but none of their
Diels–Alder chemistry.4 Silyl-substituted diene chemistry
was reviewed in 1993.5 While not containing a diene carbon
to silicon bond, related 2-trimethylsilyloxy-1,3-dienes have
also been transmetallated to zirconium.6 A 2-phenylseleno
and 2-trialkylstannyl-1,3-butadiene and their Diels–Alder
reactions were reported by Bates and co-workers in 1987.7
Much less has been reported previously about aluminum-
substituted 1,3-dienes. Eisch8 and Hoberg9 reported the
preparation of alumina-1,3-cyclopentadienes decades ago,
but very little has been done with them synthetically.10
B(OR)2
EWG
B(OR)2
EWG
+
R1
R1
1
2
3
R2CHO
R2
R1
EWG
4
Most work reported with main group-substituted dienes has
been done with the 1-(dialkoxyboryl)-1,3-butadienes (1),
sometimes termed 1,3-dienyl-1-boronates. These com-
pounds were reported by Vaultier in 1987,11 and numerous
reports of their Diels–Alder chemistry have appeared from
OH
Scheme 1. Reactions of 1,3-dienyl-1-boronates.
In contrast to the 1,3-dienyl-1-boronates, few report the
preparation and Diels–Alder chemistry of 1,3-dienyl-2-
boronates (6).25–28 Limited use of this class of compounds
is presumably due to their high affinity toward dimerization,
even at room temperature.29 Suzuki and co-workers first
* Corresponding author. Tel.: +1 336 758 5758; fax: +1 336 758 4656;
e-mail: welker@wfu.edu
0040–4020/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2007.08.063

10940
S. De et al. / Tetrahedron 63 (2007) 10939–10948
Cl
Cl
RCHO
DMF, r.t. 12 h
85% (R =Ph)
OH
B(OR)2
CrCl2
R
(RO)2B
DMF, r.t.
(RO)2B
(RO)2B
(RO)2B
75%
5
6
8
7
E:Z = 1:1
R1
R2
CH2Cl
C
C
CH2Cl
CH2Cl
dienophile
toluene, 50 °C
31-94%
Zn, THF
O
B
O
O
O
B
O
B
O
CH2Cl
9
10
11
12
Scheme 2. Prior boron diene syntheses.
synthesized the unsubstituted diene (11), which could be iso-
lated in a fast trap-to-trap distillation under high vacuum.26
This diene (11) showed reasonable reactivity with both
mono and disubstituted dienophiles at 50 ꢀC (Scheme 2).
reaction of diene (18) was performed with diethyl acetylene
dicarboxylate to afford the cycloadduct (19) in 67% yield as
a single diastereomer.31 Diboronyl dienes (22) were synthe-
sized by Shimizu et al. in a one-pot reaction.32 Surprisingly,
these dienes showed higher reactivity in cycloaddition than
the corresponding mono-substituted boron diene (11). Car-
bene coupling to boryl alkynes has also been reported
(Scheme 4).33
Related chiral diene (14) was synthesized in high yield
(81%) via a free-radical addition of bromomethane sulfonyl
bromide to 13 followed by vinylogous Ramberg–Backlund
reaction (at room temperature slow dimerization of 14
occurred).30 An attempted asymmetric version of a Diels–
Alder reaction of 14 with N-phenylmaleimide produced 15
in a 70% yield with no diastereoselectivity (Scheme 3).
Recently, Renaud and co-workers have synthesized highly
functionalized 1,3-dienyl-2-boronates (24) using enyne
ring-closing metathesis reactions of boronate-substituted
alkynes. These dienes undergo cycloaddition reactions
with electron-deficient dienophiles (nitroethylene, acrolein,
MVK) with high regio- and stereoselectivity. Unfortunately,
many of these dienes turned out to be unstable due to
Following a Negishi-type cross-coupling reaction, Knochel
and co-workers reported the synthesis of a stereochemically
pure 2-boron functionalized 1,3-diene (18). The Diels–Alder
O
N-Ph
O
1. BrCH2SO2Br
CH2Cl2, r.t.
O
N-Ph
(OR)2B
(*OR)2B
2. Et3N, CHCl3
3. t-BOK, t-BuOH
81%
CH2Cl2, r.t.
(RO)2B
O
13
14
70%, dr = 50:50
15
O
B
(*OR)2B =
O
Scheme 3. A chiral boron diene synthesis.
Bu
Bu
(Pin)B
CO2Et
CO2Et
Bu
(Pin)B
Pd0
I
EtO2C
CO2Et
(Pin)B
toluene reflux, 3.5 h
67%
77%
Hex
ZnI
Hex
19
Hex
18
16
17
R1
O
R1, R2 = H
toluene, 20 °C
(Pin)B
(Pin)B
Br
1. LiTMP
(Pin)B
R2
N
Ph
(Pin)B
R1
R2
2.
Ph
N
(Pin)B
O
O
O
(Pin)B
93%
23
20
Scheme 4. Pinacol boron diene reactions.
21
22

S. De et al. / Tetrahedron 63 (2007) 10939–10948
10941
dimerization.34 In 2005, Lee and Kim reported the prepara-
tion of a number of 2-boron substituted 1,3-dienes by enyne
cross-metathesis.35 A Diels–Alder reaction of one of these
dienes was reported but the stereochemistry of the product
of that reaction was not defined (Scheme 5).
TBA salts of other trifluoroborates have been shown to
improve cross-coupling yields considerably, presumably
due to their greater organic solvent solubility.47 The bulkier
ammonium salt should also increase organic solvent solubil-
ity of this class of dienes and may drive their solution confor-
mation more toward s-cis and increase their Diels–Alder
reactivity. The structure of the tetra n-butyl ammonium
salt diene (29) was also confirmed by X-ray crystallography
(Fig. 1).
Z
H
Ts
N
N
Ts
Z
+
minor isomers
O
O
benzene
B
nBu4N+OH-
B
O
O
77-89%
CH2Cl2/H2O
KF3B
rt, 10 min
Bu4NF3B
29 (85%)
Z = NO2, CHO, COCH3
25
24
27
Scheme 5. Prior substituted boron diene Diels–Alder chemistry.
Scheme 7. Cation exchange.
Because so much is now known about cross-coupling/
transmetallation reactions of boron, aluminum, and silicon
substituted alkenyl compounds, we were convinced that
when we found a synthetic route to stable compounds in
the 1,3-dienyl-2-main group element family, then these com-
pounds would prove useful to synthetic organic chemists.
Our experience in transition-metal dienyl complex chemis-
try had also been that 2-metal substituted 1,3-dienes were
vastly superior to 1-metal substituted 1,3-dienes36,37 both
in rate enhancement and in stereoselectivity so we expected
the same to be true for main group-substituted dienes if we
could develop this chemistry. Our results in this area using
boron substituted dienes are described below.
2. Results and discussion
2.1. 2-Boron substituted 1,3-butadiene preparation
Potassium organotrifluoroborates were first introduced as al-
ternatives to boronic esters and acids in 1995.38 Since then,
many have reported on their utility and advantages such as
atom economy compared to boronic acids and esters, their
ease of purification and disposal, their monomeric rather
than trimeric nature, and their air stability.39,40 Given the
reported stability and utility of this class of compounds,41
we recently set out to prepare the first 1,3-dienyl-2-trifluoro-
borates.42 We chose to prepare the butadiene initially and
used a route that involved preparing the Grignard reagent
of chloroprene (26),43,44 followed by its quenching with
trimethylborate (B(OMe)3) and aqueous KHF2. This new
boron substituted dienyl (27) is a white, air stable solid, and
shows no propensity to dimerize.45 It has now been prepared
on several gram scale (78%), characterized by 1H, 13C, 11B,
and 19F NMR, and appears by NOESY to be predominantly
in a solution conformation close to s-trans-(28) (Scheme 6).
Figure 1. View of a molecule of 29.
2.2. Diels–Alder reactions and Diels–Alder/cross-
coupling tandem reactions of simple boron dienes
We first tried Diels–Alder reactions of diene (27) with
N-phenylmaleimide (30) in toluene and ethyl acrylate and
methyl vinyl ketone (MVK) in ethanol/methanol and found
that boron containing cycloadducts (31–35) could be
isolated in high yields. Those cycloadducts could then
subsequently be cross-coupled using Pd catalysis to yield
organic cycloadducts (36 and 37) (Scheme 8).
1) Mg, cat ZnCl2
2) B(OMe)3
3) aq KHF2
Cl
KF3B
27 (78%)
KF3B
26
28
We then performed a series of tandem Diels–Alder/cross-
coupling reactions without isolating and characterizing bo-
ron intermediates as shown in Table 1. We first heated the
boron diene (27 or 29) and dienophile, then added Pd(OAc)2
(0.5 mol %) and 3 equiv K2CO3, and refluxed in EtOH or
Scheme 6. BF3 diene preparation.
We have also prepared the tetra n-butyl ammonium (TBA)
salt of the BF3-substituted diene (29) (85%) (Scheme 7).46

10942
S. De et al. / Tetrahedron 63 (2007) 10939–10948
O
O
rather than MVK adducts (entries 1–7 versus 8–13). Phenyl
halides with electron withdrawing groups (entries 1, 2, 6, 12,
and 13) typically produce products in 5–10% higher isolated
yield than those with electron donating groups (entries 4, 7,
and 9). The preference for the para over meta regioisomer in
these initial experiments ranges from 3 to 5:1. We wondered
if the tetrabutyl ammonium counterion might have some
effect on isolated yields (due to increased solubility) or
regiochemical outcomes (due to steric effects) but compari-
son of entries 4 and 7 indicates that dienes 27 and 29 are
almost identical in product outcome. Performing these reac-
tions in a commercial microwave reactor drastically reduces
the time required and produces a product in almost identical
yield and regiochemistry to the one obtained from a classical
thermal reaction (entries 1 and 2 and 10 and 11). Even in its
limited form, Table 1 demonstrates that 27 or 29 can serve as
a synthon for a host of 2-substituted 1,3-butadienes. We have
not worried about regiochemistry here in these early studies,
since we ultimately plan to transmetallate boron dienes to Rh
prior to cycloadditions and we have already demonstrated
that low valent transition metal substituted dienes participate
in Diels–Alder reactions with excellent regio- and stereo-
selectivity (Scheme 9).48
95-100 °C
toluene
+
N
Ph
N
Ph
KF3B
27
KF3B
O
30
O
31 (91%)
EWG
EWG
+
KF3B
EWG
95-100 °C
ROH, 36 h
KF3B
KF3B
F3C
EWG = CO2Et, 3.3:1, 85%, 32/33
EWG = COMe, 3.7:1, 83%, 34/35
27
EWG
I
EWG
and
F3C
Pd(OAc)2 cat.
EtOH, K2CO3
F3C
EWG = CO2Et 4.4:1, 73% 36/37
Scheme 8. Preliminary Diels–Alder chemistry followed by cross-coupling.
MeOH for 5 h. The sequence appears useful for unsubsti-
tuted phenyl halides (entries 3 and 8), phenyl halides
substituted by electron donating (entries 4, 7, and 9) or with-
drawing groups (entries 1, 2, 6, 12, and 13), and hetero-
aromatic halides (entries 5, 10, and 11). The yields for this
tandem sequence are generally slightly higher for acrylate
EWG
EWG
1)
+
Table 1. Tandem Diels–Alder/cross-coupling reactions of boron dienes
EWG
R1
R1
2) R1X, Pd(OAc)2 (0.5%)
K2CO3 (3 eq), ROH, Δ
KF3B
major
38
minor
39
Entry Diene a,b-Unsaturated
carbonyl
Aryl halide Products Ratios Yields
27
Scheme 9. Sequential Diels–Alder/cross-coupling chemistry.
1
2
3
27
27
27
CH2]CHCO2Et
CH2]CHCO2Et
CH2]CHCO2Et
36/37
36/37
2.9:1 62
3.1:1 64
F3C
I
a
2.3. Preparation of 4-aryl-2-boron substituted
1,3-butadiene and its Diels–Alder/cross-coupling
reactions
F3C
I
38/39a 2.5:1 60
38/39b 3.9:1 50
I
OMe
I
Following these proof of principle experiments with the sim-
plest 1,3-diene, we wanted to expand the scope of this chem-
istry to include more highly substituted boron dienes. Phenyl
substituted halo dienes (43–45) were directly generated from
the corresponding allenic alcohol (41) using the established
protocol of Ma and Wang (Scheme 10).49 We found that the
temperature of the rearrangement reaction has significant
impact on the product yield for the formation of the iodo-
substituted diene (43). In the presence of 1.5 mol % Pd
catalyst the reaction could be performed at 40 ꢀC for both
the bromo- and the chloro-substituted dienes (44, 45).50
Attempts to run the rearrangement reaction at temperatures
even 10 ꢀC higher than that for the iodo-substituted diene
(43) resulted in product yields of <10% yield. Higher dia-
stereoselectivity (>98–99% E-isomer) was also observed
when these reactions were carried out at the lower tempera-
ture. The 2-bromo-4-phenyl-1,3-diene (44) was obtained as
a crystalline solid and its structure was confirmed by X-ray
crystallography (Fig. 2).
4
27
CH2]CHCO2Et
I
5
6
7
8
9
27
27
29
27
27
CH2]CHCO2Et
CH2]CHCO2Etb
CH2]CHCO2Et
CH2]CHCOMe
CH2]CHCOMe
38/39c 3.8:1 55
38/39d 5.7:1 60
38/39b 2.3:1 53
38/39e 2.7:1 56
38/39f 5.1:1 48
S
NC
Br
OMe
I
I
OMe
I
I
10
27
27
CH2]CHCOMe
CH2]CHCOMe
38/39g 5.2:1 50
38/39g 4.8:1 54
S
a
I
11
12
S
2.3.1. Initial attempts to make 4-substituted 1,3-dienyl-2-
trifluoroborates. Newly formed halo dienes (43–45) were
first subjected to the protocol (used for the formation of
the unsubstituted 2-BF3 diene (27)) to attempt to prepare
the corresponding BF3-substituted dienes. The first step
was to generate the corresponding Grignard reagent fol-
lowed by addition of trimethoxy borate and quenching by
27
27
CH2]CHCOMe
CH2]CHCOMeb
38/39h 2.8:1 57
38/39i 3.9:1 41
F3C
NC
I
13
Br
a
Reactions run in a microwave.
Pd catalyst of 1.5% was used.
b

S. De et al. / Tetrahedron 63 (2007) 10939–10948
10943
HO
R'
HO
R'
R
a
c
R
C
R = Ph
R
X
41 R = Ph, R' = H, 73%
43 R = Ph, X = I, 50%
44 R = Ph, X = Br,62%
45 R = Ph, X = Cl, 66%
b
40 R' = H, R = Ph
c
AcO
R'
R
C
42 R = Ph, R' = H, 80%
Scheme 10. Halo diene preparation. Reagents: (a) paraformaldehyde, diisopropylamine, CuBr; (b) Ac2O, pyridine; (c) LiX in acetic acid.
reaction with pinacolborane following a protocol similar to
the one that Colobert and co-workers used in their arylboro-
nates synthesis.51 Reduced diene (47) was obtained as the
major product in this process instead of the expected product
(48) (Scheme 11).
In a second attempt to overcome this problem, a diboryl re-
agent (49) was employed with ligandless palladium catalyst
conditions, similar to the approach taken by Zhang and
co-workers in their arylboronate synthesis.52 But in this
case, a homocoupled dimer (50) of the diene was obtained
(Scheme 12).
Figure 2. View of a molecule of 44.
Recently, Kabalka et al. reported the preparation of allyl-
trifluoroborate salts using Pd-catalyzed cross-coupling of
Baylis–Hillman acetate adducts with bis(pinacolato)-
diboron.53 When 1-phenylbuta-2,3-dienyl acetate (42) was
treated with bis(pinacolato)diboron (49) in the presence of
palladium catalyst followed by quenching with aqueous
KHF2 this reaction also produced dimer (50) (Scheme 13).
KHF2/H2O. Surprisingly, the desired products were not ob-
tained via this route, instead a yellow solid was isolated.
This solid was soluble in CDCl3 but contained no alkenyl
proton resonances in the 1H NMR. This material had a com-
plicated aromatic 1H NMR spectrum, which could be due to
the polymerization of the starting material, but further chara-
cterization was not continued.
After these initial failures to make more highly substituted
boron dienes from halo dienes, we tried milder procedures
for preparing Grignard reagents from the corresponding al-
kenyl halides. Halogen–magnesium exchange reactions,
mostly pioneered by Knochel and co-workers, have become
Instead of making 2-BF3-substituted dienes, efforts were ini-
tially made to prepare 2-substituted boronate dienes (such as
48) via a cross-coupling route. First a phenyl substituted
2-halo diene (44) was subjected to the cross-coupling
Ph
Ph
H
O
1. Pd(OAc)2, NEt3,
DPEphos, dioxane, 100 °C
56%
47
H
B
+
O
Ph
Br
O
O
44
46
B
48
expected product
Scheme 11. Halo diene and borohydride.
Ph
Ph
O
O
O
O
1. Pd(OAc)2
+
B
B
2. K2CO3, DMF
46%
50
I
Ph
43
49
48
expected product
Scheme 12. Halo diene and diboron reagent.

10944
S. De et al. / Tetrahedron 63 (2007) 10939–10948
Ph
O
C
O
[Pd]cat
O
O
O
O
+
B
B
Ph
50 68%
THF, 50 °C, Aq.
KHF2
42
49
48
expected product
Scheme 13. Allenic acetate and diboron reagent.
the method of choice for the preparation of organomagne-
sium reagents with high functional group tolerance.54,55
Kabalka have shown that organotrifluoroborates generally
cross-couple well with a variety of coupling reagents under
conventional Pd catalyst conditions in THF/H2O, toluene/
H2O, alcohol/H2O, or dioxane/H2O.59,60 We selected di-
oxane/H2O as a choice of solvent system for the cross-
coupling reaction with PdCl2(dppf)$CH2Cl2 as catalyst
and Cs2CO3 as a base. We performed the Diels–Alder re-
action in dioxane and therefore the following step could be
performed in the same pot without removing the solvent.
A sequence of reactions have been performed to demonstrate
the successful extension of the methodology as shown in
Scheme 2. Interestingly, the reaction resulted in the forma-
tion of only the endo adduct, as presumably the BF3 group
is not bulky enough to direct the dienophile to approach in
an exo fashion. The structure was confirmed both by NMR
and by X-ray crystallography (Scheme 15).
Several different reagents are commonly employed (e.g.,
iPrMgCl, iPrMgBr, sec-BuMgBr, iPr2Mg$LiCl, sec-Bu2Mg$
LiCl), which react interchangeably with the halo species to
make the corresponding Grignard reagent.55,56 In theory
only 1 equiv of external Grignard reagent is required in a typ-
ical experimental procedure for halogen–Mg exchange reac-
tions, but very slow transformations were observed to
happen until excess reagents (1.6–2 equiv) were used. Sim-
ilar observations were also reported by other workers in
some related alkenyl halides–magnesium exchange reac-
tions.57,58 Moreover a competing side reaction, i.e., incorpo-
ration of the alkyl moiety from the external Grignard reagent
into the 2-position of the diene was also observed during this
process, which could be minimized with shorter reaction
times. The conditions were optimized by employing 1.8–
2 equiv of Grignard reagents to minimize the side reactions
(<5%). It was also observed that the iodo-substituted diene
(43) reacted faster than the corresponding bromo-diene (44)
as expected. But with higher amounts of external Grignard
reagent (1.8 or greater) the bromo-diene could also be con-
verted in a similar fashion. It is noteworthy to mention that
these reactions were only possible in THF since no transfor-
mation occurred when an external Grignard reagent in di-
ethyl ether was used or diethyl ether was used as solvent
for the reaction (Scheme 14).
The structure of 53 was first postulated by means of NMR
spectroscopy. A COSY experiment allowed the unambigu-
ous assignment of all the signals in the 500 MHz H NMR
1
spectrum of 53. Proton H2 appeared at 6.63 ppm as a doublet
of doublets (J¼5.2 and 1.9 Hz) with the corresponding car-
bon (C2) signal at 129.2 ppm as confirmed by HMQC. The
H2 proton showed two cross-peaks, one at 4.07 and the other
at 2.84 ppm in the COSY. The proton at 2.84 ppm showed
a strong cross-peak with a resonance at 3.54 ppm and both
were attached to the same carbon atom (25.87 ppm) as con-
firmed by HMQC. It was then concluded that these two pro-
tons were the diastereotopic pair, H7 and H70. The peak at
4.07 ppm was confirmed as H3 (corresponding carbon ap-
peared at 42.66 ppm) as it showed three cross-peaks with
H2, H4a, and one of the H7 protons. Bridgehead protons
(H4a and H4b) appeared at the same chemical shift in the
proton NMR but correlated clearly with two different carbon
signals in the HMQC. Furthermore, one of the bridgehead
protons showed a cross-peak with a proton at 2.84 ppm
(H7) in the COSY as expected.
Isolating boron substituted diene (51) in analytically pure
form, free of any 52, was proved impossible even under op-
timized reaction conditions. Recognizing that the purity of
diene (51) obtained from this sequence was inconsequential
as long as 52 had limited impact on possible subsequent
Diels–Alder cross-coupling chemistry, we decided to forge
ahead and use this diene (51) in consecutive reactions. We
also wanted to perform both the cycloaddition and the
cross-coupling reactions in one-pot and thereby slightly
modified our initial protocol used for the unsubstituted di-
enes (27, 29) reported above. Recently, Molander and
The endo structure of the molecule was predicted on the ba-
sis of the coupling constant between H3 and H4a (triplet)
Ph
Ph
R'MgX, THF, rt
1) B(OMe)3
2) aq KHF2
Ph
OR
R'BF3K
+
Sec-BuLi + R'MgX, rt
XMg
X
KF3B
44
Scheme 14. Halogen–magnesium exchange followed by boron electrophile.
51
52

S. De et al. / Tetrahedron 63 (2007) 10939–10948
10945
1. R'MgBr
3. Conclusion
2. B(OMe)3
R'BF3K
R
R
BF3K
X
3. Aq. KHF2
In summary, we have prepared new, stable, monomeric
dienyl trifluoroborates in high yield and find that they readily
participate in Diels–Alder/cross-coupling tandem reactions.
We will report the rhodium catalyzed reaction chemistry of
these main group element substituted dienes in due course.
43
51
R = Ph
X = I
O
Dioxane 5 ml
Reflux, 10 h
( 0.8 Eq)
Ph
Br
N
O
One-pot Reaction
4. Experimental section
4.1. General procedures
2.5 mol % PdCl2(dppf),
CH2Cl2, Cs2CO3 (3 eq),
H2O (0.5 ml), Reflux, 4 h
( 0.6 Eq)
CN
The proton nuclear magnetic resonance (1H NMR) spectra
were obtained using a 300 MHz spectrometer operating
at 300.13 MHz or a 500 MHZ spectrometer operating at
500.13 MHz. 13C NMR spectra were obtained using a
300 MHz spectrometer operating at 75.48 MHz. 11B and
19F NMR spectra were recorded on a 300 MHz spectrometer
Ph
O
H
3
5
2
4a
N
Ph
13
10
4b
6
12
1
8
9
7
at 96.29 and 282.38 MHz, respectively. H and 13C NMR
1
H
O
14
NC
11
spectra were referenced to the residual proton or carbon sig-
nals of the respective deuterated solvents. In the 13C NMR
spectrum, the signal of the quaternary carbon a to the tetra-
valent boron was not observed in organotrifluoroborates type
of compounds as expected.63 11B NMR chemical shifts were
referenced to external BF3$OEt2 (0.0 ppm). All 19F NMR
chemical shifts were referenced to external CFCl3
(0.0 ppm). All elemental analyses were performed by Atlan-
tic Microlabs Inc., Norcross, GA. High-resolution mass
spectrometry was performed at the Duke Mass Spectrometry
Facility, Duke University in Durham, NC.
53
Only detected endo isomer
25% Overall Yield (based on limiting reagent)
with an average yield of 63%/step over the 3 steps.
Scheme 15. Sequential halogen–magnesium exchange, boron electrophile
quench, Diels–Alder/cross-coupling.
(J¼5.4 Hz) at the H3 proton. Using the experimental J3–4a of
5.4 Hz in the electronegativity adjusted Karplus equa-
tion61,62 predicted a dihedral angle between H3 and H4a of
45ꢀ. This closely matches the expected dihedral angle for
the endo structure in molecular models. This was further
confirmed by an X-ray crystallographic analysis where the
observed dihedral angle was 57.5ꢀ (Fig. 3).
All reactions were carried out under an atmosphere of nitro-
gen. Methylene chloride was distilled from CaH2; ether,
THF, and pentane were distilled over Na. Water was deioni-
zed and distilled. Absolute ethanol, methanol, and dioxane
(HPLC quality) were used without further purification. Deu-
terated solvents were dried over molecular sieves. Magne-
sium sulfate, magnesium small turnings, zinc chloride,
1,2-dibromoethane, iodobenzene, 4-bromobenzonitrile,
4-iodobenzotrifluoride, 2-iodoanisole, 2-iodothiophene,
potassium hydrogenfluoride, cuprous bromide, diisopropyl-
amine, potassium carbonate, [1,10-bis(diphenylphosphino)-
ferrocene]dichloropalladium(II), complex with CH2Cl2,
cesium carbonate, pinacolborane, bis(pinacolatodiboron),
lithium chloride, N-phenylmaleimide, methyl vinyl ketone,
and ethyl acrylate were purchased from Aldrich Chemical
Company and used as received. Palladium acetate, bis(2-
diphenylphosphinophenyl)ether, 98% DPEphos, and tri-
methylborate were purchased from Strem Chemicals and
used as received. 1-Phenyl-2-propyn-1-ol, lithium iodide,
trihydrate, and lithium bromide were purchased from GFS
Chemicals and used as received. 2-Chloro-1,3-butadiene
50% in xylene (chloroprene) was purchased from Pafltz &
Bauer, Inc. and used as received. Complete experimental
details for the preparation of 27, 29, and 32–39 are available
as supplementary material that accompanies Ref. 42.
4.1.1. Preparation of 5-potassiumtrifluoroborato-3a,4,
7,7a-tetrahydro-2-phenyl-2H-isoindole-1,3-dione (31).
Potassium 1,3-dienyl trifluoroborate (27) (1.1 equiv, 176 mg,
1.1 mmol), N-phenylmaleimide (30) (173 mg, 1 mmol),
and toluene (2.5 mL) were added to a thick walled pressure
Figure 3. View of a molecule of 53.

10946
S. De et al. / Tetrahedron 63 (2007) 10939–10948
tube equipped with a magnetic stirring bar. The tube was
heated to 95–100 ꢀC for 16 h in a silicon oil bath. The re-
action mixture was then cooled to room temperature and sol-
vent was removed by rotary evaporation. The residual solid
was washed with acetone and the acetone extracts were
transferred to a round bottom flask. The solvents were re-
moved by rotary evaporation and high vacuum. Compound
31 was obtained as a white solid in 91% yield (303 mg,
0.91 mmol). 1H NMR (300 MHz, CDCl3, d): 7.43–7.31
(m, 3H), 7.25 (dt, J¼7.3, 1.7 Hz, 2H), 5.86 (br s, 1H),
3.18–3.04 (m, 2H), 2.64 (dd, J¼14.5, 3.2 Hz, 1H), 2.46–
2.37 (m, 1H), 2.25–2.10 (m, 2H). 13C NMR (75.4 MHz,
CDCl3, d): 180.9, 180.6, 134.5, 129.4, 128.6, 128.1, 124.4,
41.1, 40.8, 27.3, 24.8 (due to quadrupolar relaxation, the car-
bon attached to the boron atom was not detected). Negative
ion ESI-MS: m/z calculated for C14H12BF3KNO2: (Mꢁ)
333.2, found: 294.0 (MꢁKꢁ), 274.0 (MꢁKꢁHFꢁ), 253.9
(MꢁKꢁ2HFꢁ).
4.1.4. (E)-1-Phenyl-3-bromo-1,3-butadiene (44). The title
compound was prepared in 62% yield (2.25 g, 10.8 mmol)
from allene 41 (2.55 g, 17.4 mmol) as a yellow solid follow-
ing the general procedure outlined above with substitution of
1
LiBr for LiI. H NMR (300 MHz, CDCl3, d): 7.45 (d, J¼
7.4 Hz, 2H), 7.38–7.24 (m, 3H), 6.94 (d, J¼15.1 Hz, 1H),
6.72 (d, J¼15.1 Hz, 1H), 5.92 (s, 1H), 5.70 (s, 1H). 13C
NMR (75.4 MHz, CDCl3, d): 135.9, 135.7, 129.9, 128.7,
128.4, 127.1, 126.7, 120.2. GC/MS: m/z (relative, %): 210
(10), 208 (11) [M+], 129 (100), 128 (74), 127 (29), 102
(8), 77 (7), 64 (7), 51 (8).
4.1.5. (E)-1-Phenyl-3-chloro-1,3-butadiene (45). The title
compound was prepared in 66% yield (1.89 g, 11.5 mmol)
from allene 41 (2.55 g, 17.4 mmol) as a yellow solid follow-
ing the general procedure outlined above with substitution of
LiCl for LiI. NMR spectroscopic data were comparable to
those previously reported.65
4.1.2. 1-Phenylbuta-2,3-dien-1-ol (41).64 Following a litera-
ture procedure with slight modification, 1-phenyl-2-propyn-
1-ol (40) (12.6 g, 95.6 mmol), paraformaldehyde (3.30 g,
109.9 mmol), cuprous bromide (6.88 g, 48.0 mmol), and di-
isopropylamine (16.2 mL, 114.8 mmol) were refluxed in dry
dioxane (350 mL) overnight. The mixture was allowed to
cool to room temperature and the dioxane was carefully re-
moved by rotary evaporation. The residue was then filtered
through Celite, with addition of water (200 mL) and diethyl
ether (250 mL). The filtrate was washed with water, and the
collected organic layer was dried over MgSO4. The solvent
was evaporated under reduced pressure to afford 1-phenyl-
buta-2,3-dien-1-ol (41) (10.2 g, 69.8 mmol) (73% yield) as
a brown liquid. The alcohol was used in the next step without
4.1.6. Attempted preparation of boronate diene (48) via
cross-coupling reaction between a halo diene and pina-
colborane. Diene 44 (208 mg, 1 mmol) was taken up in
dioxane (2 mL), and triethylamine (558 mL, 4 mmol), palla-
dium(II) acetate (11.2 mg, 0.05 mmol), DPEphos (53.8 mg,
0.10 mmol), and pinacolborane (3.00 mL, 1.0 M solution in
THF, 3 mmol) were added slowly under nitrogen. The reac-
tion mixture was heated at 100 ꢀC for 5 h. Afterwards it was
cooled to room temperature and quenched by a satd solution
of NH4Cl (10 mL). The aqueous phase was extracted with
diethyl ether (3ꢂ15 mL) and dried over MgSO4. The solu-
tion was filtered and removed under reduced pressure to
obtain 47 (73 mg, 0.56 mmol) in 56% yield as a colorless
liquid. NMR spectroscopic data were comparable to those
previously reported.66
1
further purification. H NMR (300 MHz, CDCl3, d): 7.43–
7.28 (m, 5H), 5.45 (q, J¼6.6 Hz, 1H), 5.28 (pentet, J¼
2.8 Hz, 1H), 4.94 (m, 2H). 13C NMR (75.4 MHz, CDCl3,
d): 207.1, 142.8, 128.5, 127.8, 126.0, 95.2, 78.2, 71.9. GC/
MS: m/z (relative, %): 146 (3) [M+], 145 (18), 128 (6), 117
(6), 107 (100), 79 (51), 77 (38), 51 (10).
4.1.7. Attempted preparation of boronate diene (48) via
cross-coupling reaction between a halo diene and bis-
(pinacolato)diboron. In a dry and nitrogen flushed 25-mL
2-neck flask were charged diene (43) (128 mg, 0.5 mmol),
bis(pinacolatodiboron) (140 mg, 0.55 mmol), potassium
carbonate (207.3 mg, 1.5 mmol), palladium acetate
(17 mg, 0.05 mmol), and DMF (5 mL). The mixture was de-
gassed by gently bubbling nitrogen for 10 min and then
heated at 80 ꢀC for 5 h in an oil bath. The reaction mixture
was cooled to room temperature and diluted with water
(50 mL). It was then extracted with ethyl acetate (3ꢂ
25 mL). The organic layer was washed with brine (2ꢂ
10 mL) and dried over MgSO4. The solution was filtered
and the solvent was removed under reduced pressure to ob-
tain a light yellowish solid. The product was purified by flash
chromatography on silica gel (elution with hexane/ethylace-
tate 5:1) to yield a white solid (50) in 46% yield (59 mg,
4.1.3. (E)-1-Phenyl-3-iodo-1,3-butadiene (43). A round
bottom flask containing 1-phenylbuta-2,3-dien-1-ol (41)
(2.55 g, 17.4 mmol) was charged with Pd(OAc)2
(1.5 mol %) (58.6 mg, 0.26 mmol) and LiI$3H2O (3 equiv)
(9.85 g, 52.3 mmol) in acetic acid (50 mL). The mixture
was heated at 40 ꢀC with stirring for 1 h (reaction was moni-
1
tored either by H NMR or by GC/MS). Water was added
(125 mL) and the aqueous phase was extracted with pentane
(2ꢂ150 mL). The organic layer was thoroughly washed with
water (2ꢂ150 mL), satd NaHCO3 (4ꢂ125 mL), and brine
(2ꢂ50 mL) solution and was then dried over MgSO4. The
solvent was removed under reduced pressure and the crude
product obtained was passed through silica in a fritted funnel
by eluting with pentane. Compound 43 was obtained as a
light yellow crystalline solid after removal of pentane
1
23 mmol). H NMR (300 MHz, CDCl3, d): 7.41–7.27 (m,
5H), 6.93 (d, J¼16.0 Hz, 1H), 6.53 (d, J¼16.2 Hz, 1H),
5.42 (d, J¼1.5 Hz, 1H), 5.18 (d, J¼2.3 Hz, 1H). 13C NMR
(75.4 MHz, CDCl3, d): 146.6, 137.1, 131.5, 129.6, 128.5,
127.5, 126.5, 118.2. GC/MS: m/z (relative, %): 258 (48)
[M+], 243 (16), 228 (14), 215 (11), 202 (6), 129 (18), 165
(41), 154 (100), 141 (16), 128 (45), 115 (31), 102 (7), 91
(29), 77 (14), 63 (4), 51 (7).
1
under reduced pressure (2.23 g, 8.7 mmol, 50%). H NMR
(300 MHz, CDCl3, d): 7.45 (d, J¼7.9 Hz, 2H), 7.37–7.23
(m, 3H), 6.75 (d, J¼15.3 Hz, 1H), 6.44 (s, 1H), 6.27 (d, J¼
15.3 Hz, 1H), 6.03 (s, 1H). 13C NMR (75.4 MHz, CDCl3, d):
139.2, 135.9, 129.6, 128.9, 128.7, 128.3, 127.2, 107.9. GC/
MS: m/z (relative, %): 256 (21) [M+], 129 (100), 128 (75),
127 (31), 102 (9), 77 (7), 63 (5), 51 (6). Anal. Calcd for
C10H9I: C, 46.90; H, 3.54. Found: C, 47.00; H, 3.59.
4.1.8. Attempted preparation of 2-BF3-substituted diene
via Pd-catalyzed cross-coupling of allenic acetate with

S. De et al. / Tetrahedron 63 (2007) 10939–10948
10947
bis(pinacolato)diboron. Acetic acid 1-phenyl-buta-2,3-di-
enyl ester (42) (188 mg, 1.0 mmol), bis(pinacolato)diboron
(49) (280 mg, 1.1 mmol), and Pd(OAc)2 (5 mol %) were
taken in a dry nitrogen flushed 50 mL 2-neck flask equipped
with a Schlenk tube, a magnetic stirrer, and a septum. THF
(4 mL) was added using a syringe at room temperature.
The mixture was heated at 50 ꢀC for 4 h and the color of
the solution changed to blackish red. The solution was
then cooled to 0 ꢀC and excess KHF2 (468 mg, 6 mmol)
was added all at once. Water (2 mL) was added drop by
drop and the suspension was allowed to stir for an additional
20 min at room temperature. The solvents were removed on
a rotary evaporator and the resulting white solids were sub-
jected to high vacuum until dried completely. The solids
were extracted with dry acetone (2ꢂ25 mL), and the acetone
was evaporated to obtain a white solid (50) in 68% yield
(175 mg, 0.68 mmol). Spectroscopic data matched with
data reported above for this compound.
4.1.10. One-pot tandem Diels–Alder/cross-coupling
reaction: 4-(1,3-dioxo-2,7-diphenyl-2,3,3a,4,7,7a-hexa-
hydro-1H-isoindol-5-yl)-benzonitrile (53). 2-Substituted
BF3 phenyl diene (51) was first prepared from 2-iodo phenyl
diene (43) (starting with 512.0 mg in a 2 mmol scale) as
described above. The BF3-substituted diene obtained was
then dissolved in dioxane (5 mL), N-phenylmaleimide
(277.0 mg, 1.6 mmol, 0.8 equiv with respect to the halo di-
ene) was added, and refluxed for 10 h. The solution was
then cooled down to room temperature and 4-bromobenzo-
nitrile (218.4 mg, 1.2 mmol, 0.6 equiv with respect to the
starting halo diene) was added with PdCl2(dppf)$CH2Cl2
(19.6 mg, 0.024 mmol) and Cs2CO3 (1173.6 mg, 3.6 mmol)
followed by addition of H2O (0.5 mL). The reaction mixture
was refluxed for 4 h and then cooled to room temperature.
Water (10 mL) was added and extracted with CH2Cl2
(3ꢂ20 mL). The extracts were dried over MgSO4 and con-
centrated under reduced pressure. The crude residue was pu-
rified by flash chromatography (EtOAc/hexane 5:1) to afford
53 as a white solid (120 mg, 0.3 mmol) in 25% overall yield
with an average yield of 63% over each of the three steps. 1H
NMR (500 MHz, CDCl3, d): 7.67 (d, J¼8.3 Hz, 2H), 7.57 (d,
J¼8.3 Hz, 2H), 7.37–7.26 (m, 8H), 6.78 (dd, J¼6.7, 1.6 Hz,
2H), 6.63 (H2) (dd, J¼3.2, 1.9 Hz, 1H), 4.07 (H3) (t, J¼
5.1 Hz, 1H), 3.59 (H4a and H4b) (m, 2H), 3.48 (H7) (dd, J¼
16.1, 1.9 Hz, 1H), 2.84 (H7) (m, 1H). 13C NMR (75.4 MHz,
CDCl3, d): 177.9, 175.5, 144.5, 137.9, 137.7, 132.5, 131.4,
129.2, 129.1, 129.0, 128.6, 128.5, 127.7, 126.3, 126.1,
118.7, 111.4, 45.1, 42.6, 39.5, 25.8. HRMS (EI): m/z calcu-
lated for C27H20N2O2: (M+) 404.1525, found: 404.1539.
4.1.9. (E)-4-Phenyl-1,3-butadienyl-2-potassium trifluoro-
borate (51). Diene (43) (256 mg, 1 mmol) was dissolved in
THF (5 mL) in a dry nitrogen flushed 50 mL 2-neck flask.
Secondary butyl magnesium bromide (0.9–1.0 mL, 1.8–
2 mmol, 2 M solution in THF) was added drop by drop
(for over 5 min) using a syringe at room temperature. The
color of the solution changed from light yellow to dark red
and the mixture was stirred for 2/2.5 h depending upon the
amount of Grignard reagent used at that temperature. The
completion of the halogen–magnesium exchange was
checked either by 1H NMR or by GC/MS analysis of reac-
tion aliquots. The reaction mixture was cooled to ꢁ78 ꢀC
and trimethylborate (w557 mL, 5 mmol) was added very
slowly and stirring was continued at that temperature for
30 min. The solution was allowed to warm gradually to am-
bient temperature over a period of an hour (with slow change
in color to white) and stirred for 30 min. The mixture was
then cooled to 0 ꢀC with additional stirring (30 min) and
KHF2 (930 mg, 12 mmol) was added all at once. Water
(1.25 mL) and methanol (0.75 mL) were slowly added and
the temperature of the reaction mixture was maintained
around 0 ꢀC. The resulting white suspension was allowed
to warm to room temperature and stirred for half an hour.
The solvents were removed on a rotary evaporator and the
resulting white solids were subjected to high vacuum until
dried completely. The solids were extracted with dry acetone
(4ꢂ100 mL), and the acetone was evaporated to obtain
a mixture of 51 and 52 as white solid (0.3 g). Diagnostic
1H and 13C NMR data are listed below.
4.1.11. Crystallographic data details. Crystallographic
data for the structures of compounds 29, 44, and 53 have
been deposited with the Cambridge Crystallographic Data
Centre as supplementary publication nos. 655091–655093.
Copies of these data can be obtained, free of charge, on appli-
cation to CCDC, 12 Union Road, Cambridge CB2 1#Z, UK.
Acknowledgements
We thank the National Science Foundation for their support
of this work (CHE-0104083 and 0450722) and the NMR in-
strumentation used to characterize the compounds reported
here. The Duke University Center for Mass Spectrometry
performed high-resolution mass spectral analyses. We thank
Marcus Wright for assistance with the structure determina-
tion for compound 53.
Compound 51: 1H NMR (300 MHz, C3D6O, d): 7.39 (dd, J¼
7.4, 1.3 Hz, 2H), 7.25 (t, J¼7.5 Hz, 2H), 7.11 (tt, J¼7.4,
1.3 Hz, 1H), 6.91 (d, J¼16.0 Hz, 1H), 6.83 (d, J¼16.0 Hz,
1H), 5.31 (d, J¼5.1 Hz, 1H), 5.21 (br s, 1H). 13C NMR
(75.4 MHz, CDCl3, d): 138.3, 134.9, 128.2, 127.3, 125.2,
125.0, 120.3 (due to quadrupolar relaxation, the carbon
attached to the boron atom was not detected).63 LRMS
(FABꢁ) observed 236.15, (MꢁK)11B 197.12, (MꢁK)10B
196.12.
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