Cobalt-catalyzed arylation and alkenylation of alpha-bromo eneformamides and enecarbamates by cross-coupling with organic bromides: Application to the synthesis of functionalized piperidines and azepanes

Article abstract of DOI:10.1016/j.jorganchem.2015.01.001

The synthesis of α-arylated and alkenylated piperidine and azepane derivatives has been accomplished through cross-coupling of α-bromo eneformamides or enecarbamates with feedstock organic halides such as aryl and vinyl bromides, under cobalt catalysis. The coupling products, which themselves are synthetic intermediates for accessing other functionalized piperidines and azepanes are obtained in good to excellent yields.

Full text of DOI:10.1016/j.jorganchem.2015.01.001

Journal of Organometallic Chemistry 780 (2015) 6e12
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Cobalt-catalyzed arylation and alkenylation of alpha-bromo
eneformamides and enecarbamates by cross-coupling with organic
bromides: Application to the synthesis of functionalized piperidines
and azepanes
Daniel P. Bassler, Amir Alwali, Laura Spence, Oliver Beale, Timothy K. Beng*
Department of Chemistry, Susquehanna University, Selinsgrove, PA 17870, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis of
through cross-coupling of
such as aryl and vinyl bromides, under cobalt catalysis. The coupling products, which themselves are
synthetic intermediates for accessing other functionalized piperidines and azepanes are obtained in good
to excellent yields.
a
-arylated and alkenylated piperidine and azepane derivatives has been accomplished
Received 5 November 2014
Received in revised form
2 January 2015
Accepted 3 January 2015
Available online 10 January 2015
a
-bromo eneformamides or enecarbamates with feedstock organic halides
© 2015 Elsevier B.V. All rights reserved.
Keywords:
Piperidine and azepane alkaloids
Eneformamides and enecarbamates
Arylation and vinylation
Cobalt-catalyzed cross-coupling
Introduction
The piperidine and azepane structural motifs constitute the core
of several alkaloids and pharmaceuticals (selected examples of
which are depicted in Fig. 1) that represent over thirty therapeutic
areas including antidepressants and analgesics [1].
Fittingly, these motifs continue to inspire researchers toward
the development of increasingly more efficient strategies for their
construction and functionalization [2e8]. One such strategy that
provides efficient access to functionalized piperidine and azepane
derivatives is to employ an eneformamide or enecarbamate as a
substrate for subsequent functionalization. In addition to offering
latent functionality at the
a and b positions (see VI, Fig. 2), cyclic
enamides or enecarbamates offer several other advantages as a
starting point for access to functionalized azaheterocycles [9e19].
As detailed in many previous reports, C-2 functionalization has
primarily been achieved by utilizing the corresponding vinyl tri-
flate [20e22], phosphate [23], stannane [24], or boronate [21,25] in
cross-coupling manifolds (Fig. 2, top). The double bond of the
* Corresponding author. Tel.: þ1 4042736793.
E-mail address: beng@susqu.edu (T.K. Beng).
Fig. 1. Examples of bioactive piperidines and azepanes.
http://dx.doi.org/10.1016/j.jorganchem.2015.01.001
0022-328X/© 2015 Elsevier B.V. All rights reserved.

D.P. Bassler et al. / Journal of Organometallic Chemistry 780 (2015) 6e12
7
simplicity of the procedure employed in entry 4 still endows it with
a practical advantage. In somewhat unsurprising results, chloro-
benzene and chloro eneformamide 1b are incompetent reactive
partners when respectively coupled with eneformamide 1a or
bromobenzene (entries 15 & 16).
With satisfactory conditions in hand (Table 1, entry 4 or 14), the
scope of the
partner was briefly explored (Scheme 1). Sterically demanding,
electron-rich, electron-deficient, and -deficient aryl bromides
a-arylation with respect to the aryl bromide coupling
p
were surveyed. A highly electron-rich aryl bromide undergoes
faster and more efficient coupling compared to an electron-neutral
aryl bromide (6 vs 3). Of note, ortho substitution is tolerated on the
aryl bromide (see 4). Not surprisingly,
mides are less competent coupling partners and afford the products
in modest yields (see 7). The -alkenylation of 1a also proceeds
satisfactorily as coupling with -bromostyrene affords N-formyl
p-deficient heteroaryl bro-
a
b
amino diene 8 in good yield. Vinylated adducts of dehydropiper-
idines such as 8 are highly desired since they serve as a diene
component in [4 þ 2]-cycloaddition reactions [42].
We have found that the Co-catalyzed reductive cross-coupling
of two electrophilic organic bromides is not limited to the piperi-
dine heterocycle. For example, homologous azepane bromo ene-
Fig. 2. C-2 functionalization strategies.
enamide or enecarbamate may also be reduced or oxidized
[15,16,26,27], cyclopropanated [28], or engaged in allylic function-
alization protocols [29].
Seeking a complementary approach to these established reac-
tivity modes for the synthesis of a-arylated and vinylated N-het-
erocycles, we reasoned that cross-coupling of two readily available
electrophilic organic bromides such as VI and VII (Fig. 2, bottom),
catalyzed by an inexpensive and nontoxic metal such as cobalt,
offered an attractive and conceptually sagacious approach [30].
Communicated herein is the Co-catalyzed cross-coupling of bromo
eneformamides and enecarbamates with organic bromides to afford
formamide 2 also undergoes efficient
a
-arylation and vinylation to
afford the corresponding
a-functionalized azepenes in good yields
(see 9ꢁ12).
When Co-catalyzed arylation of bromo enecarbamate 13a with
2-bromonaphthalene was attempted, slow reactivity was encoun-
tered. Gratifyingly, heating to 40 ꢀC for 4 h affords enecarbamate 14
in good yield (Scheme 2). Consistent with observations made when
bromo eneformamide 1a was employed, coupling with electron-
rich aryl bromides proceeds highly efficiently (see 15 and 16).
Furthermore, regioselective
a-alkenylation of 13a with 1-
bromopropene proceeds satisfactorily and furnishes conjugated
diene 17.
In a finding of great significance, particularly in the context of
stereoselective construction of polysubstituted piperidines, these
a-substituted dehydropiperidines and azepenes, which are further
elaborated to other synthetically useful intermediates.
Results and discussion
Gleaning from Gosmini's insightful reports on Co-catalyzed
Table 1
cross-coupling of aromatic halides with
a-halostyrenes [31], we
Optimization of the Co-catalyzed coupling of bromo eneformamide 1a with phenyl
commenced studies on the -arylation of halo eneformamides by
a
bromide.
coupling bromo eneformamide 1a [32] with bromobenzene in the
presence of CoBr2, a phosphine ligand, and a reductant such as
manganese. Starting with a monodentate phosphine ligand such as
triphenylphosphine [31,33,34], and after performing the reaction
using the conditions described in Table 1, it was ascertained that
acetonitrile was the most suitable solvent (see entries 1e3);
consistent with Gosmini's findings [31]. In lieu of previous obser-
vations that the bite angle of a Co-bound phosphine ligand can have
a dramatic effect on the efficiency of cross-couplings [33,35e41],
several bidentate ligands were also evaluated (entries 4e12).
Fortuitously, resounding success was achieved when bis-1,2-
diphenylphosphinopropane (dppp) was utilized (entry 4). Ligands
Entry
Ligand
Solvent
% Yield (by GC)
1
2
3
4
5
6
7
8
PPh3
PPh3
PPh3
dppp
dppe
dppm
dppb
dpph
dppf
rac-BINAP
bipyridine
none
DMF
MeCN
THF
30
61
0
86
71
46
67
22
36
0
0
0
80
94
26
<5
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
with
smaller
bite
angles
such
as
bis-1,2-
diphenylphosphinomethane (dppm, entry 6) or larger bite angles
such as bis-1,2-diphenylphosphinohexane (dpph, entry 8) have an
adverse effect on the efficacy of the coupling.
9
10
11
12
13a
14b
15c
16d
Furthermore, no coupling is observed in the presence of rac-
BINAP (entry 10), bipyridine (entry 11), or in the absence of any
ligand (entry 12). Additionally, shortening the reaction time by
performing the coupling at 40 ꢀC has no noticeable beneficial effect
on the yield (entry 13). Although the efficiency of the arylation
marginally improves when bromo eneformamide 1a is slowly
introduced over the course of an hour (presumably due to the
minimization of dimerization events, entry 14), the operational
dppp
dppp
dppp
dppp
(c) using PhCI, (d) using chloro eneformamide 1b.
a
at 40 ꢀC for 1 h.
1a added slowly over 60 min.
b

8
D.P. Bassler et al. / Journal of Organometallic Chemistry 780 (2015) 6e12
The utility of
a-vinylated adducts of dehydropiperidines in
hexannelation reactions has been demonstrated. When diene 17 is
subjected to a [4 þ 2] cycloaddition reaction with bromomaleic
anhydride (21) at room temperature in benzene, a single stereo-
and regioisomer of the isomerized bromo adduct (not shown) is
formed, which slowly dehydrobrominates upon standing. As such,
this intermediate was converted to tricycle 22 upon treatment with
DBU (Scheme 3). Treatment of diene 17 with activated ester
quinone 23a [59] gives rise to a single isomer of [6-6-6] tricycle 24a,
bearing four contiguous stereocenters, one of which is quaternary.
As a testament to the challenges associated with the formation of
vicinal quaternary centers in sterically congested environments, we
find that methyl-bearing quinone 23b fails to react with diene 17,
even after 3 days at 50 ꢀC.
The a-arylated cyclic enecarbamates prepared in Scheme 2 may
be further elaborated to other synthetically useful piperidine de-
rivatives. For example, hydroboration-oxidation of crude 25 using
conditions first introduced by Dhimane and coworkers [60] affords
vicinally functionalized piperidine derivative 26 as a single dia-
stereomer (Scheme 4). Of note, 2-aryl-3-hydroxypiperidines such
as 26 serve as important intermediates in the synthesis of com-
modity chemicals such as 3-hydroxypipecolic acid [61e63] and
alkaloids such as epi-L-773,060 [64,65].
Scheme 1.
a-Functionalization of eneformamides 1a and 2.
The synthesis of 2,6-disubstituted dehydropiperidines such as
20 sets the stage for accessing
Illustratively, catalytic hydrogenation of enecarbamate 27 over
Pearlman's catalyst, then benzylic lithiation-transmetalation and
copper-mediated allylation [66] affords a single diastereomer of
a,a,a
0-trisubstituted piperidines.
studies have revealed that
mates such as 13b/c/d undergo efficient
g- and a
0-substituted bromo enecarba-
-arylation under similar
a
reaction conditions to afford 2,4- and 2,6-disubstituted dehy-
dropiperidines (see 18ꢁ20). The utility of protected dihydropyr-
idones such as 18 in alkaloid synthesis has been amply
demonstrated [43e58].
While the detailed mechanistic features of the cobalt-catalyzed
reductive cross-coupling described herein have not been fully
investigated at this point, we suspect that a mechanism analogous
to that first proposed by Gosmini [31] is operative. The general
features include manganese-mediated reduction of Co(II) to Co(I),
oxidative addition of one of the organic bromide coupling partners,
a second reduction from Co (III) to Co (I), oxidative addition of the
other bromide coupling partner, and subsequent reductive
elimination.
trisubstituted piperidine 27, bearing two
one of which is quaternary (Scheme 5). NOESY experiments
established the relative configuration of 27 as having the -aryl
group and the
0-methyl group cis. In this two-step sequence,
a-amino stereocenters,
a
a
highly diastereoselective reduction of 27 and subsequent net ster-
eoretentive allylation are implicated. Instructively, the presence of
an allyl moiety on a piperidine skeleton offers several advantages
for the synthesis of piperidine alkaloids and pharmaceuticals since
the double bond may be oxidized [67e72], reduced [69,72,73],
carbolithiated [74e77], engaged in metathesis reactions [78], or
hydrozirconated [79].
The synthesis of
a,a-disubstituted azepanes and of g
0-func-
tionalized azepane derivatives is possible through careful manip-
ulation of the coupling products obtained using methodology
described herein. For example, base-mediated cleavage of the
formyl group in arylated azepene 11 followed by addition of allyl-
magnesium bromide to the resulting imine tautomer affords
Scheme 2. Cobalt-catalyzed
a
-functionalization of enecarbamates 15a/b/c/d.
Scheme 3. Hexannelation of a-vinylated enecarbamate 17.

D.P. Bassler et al. / Journal of Organometallic Chemistry 780 (2015) 6e12
9
further purification. TMEDA was distilled on a short path, over
CaH2. Column chromatography was performed on silica gel
(230e400 mesh). Thin-layer chromatography (TLC) was performed
on silica plates. Visualization of the TLC plates was aided by UV
irradiation at 254 nm or by KMnO4 staining. Unless otherwise
indicated, 1H, 13C, DEPT-135, COSY 45, HMQC, and NOESY NMR
spectra were acquired using C6D6 or CDCl3 as solvent at room
temperature. Chemical shifts are quoted in parts per million (ppm).
Scheme 4. Hydroboration-oxidation of enecarbamate 25.
General procedure A. Arylation of halo eneformamides
To a solution of CoBr2 (22 mg, 0.10 mmol, 10 mol%), bis-1,2-
diphenylphosphinopropane (41.2 mg, 0.10 mmol, 10 mol%), and
manganese powder (165 mg, 3 mmol, 3 equiv) in acetonitrile (5 mL)
was added the aryl bromide (2 mmol, 2 equiv) at the desired
temperature (rt for the bromo eneformamides or 40 ꢀC for the
bromo enecarbamates). A solution of the bromo eneformamide
(1 mmol, 1 equiv) in acetonitrile (5 mL) was added slowly. After
completion (as judged by TLC and GCeMS), the reaction mixture
was treated with a mild acid such as 10% H3PO4 (aq) and extracted
with CH2Cl2. The combined organic layers were dried over MgSO4,
filtered and concentrated under reduced pressure to afford the
desired coupling product as an oil. Purification was carried out by
flash chromatography on silica (pretreated with 1% Et3N).
Scheme 5. Stereocontrolled preparation of saturated trisubstituted piperidine 19.
Compound 3
Prepared from 1a (190 mg, 1.0 mmol) and phenyl bromide
(314 mg, 2.0 equiv), using General Procedure A. T ¼ 23 ꢀC,
time ¼ 4 h; Purification: Flash chromatography on silica (pretreated
with 1% Et3N) eluting with hexane/EtOAc (70:30). Yield ¼ 135 mg,
72%. 1H NMR (400 MHz, C6D6)
d 8.39 (1H), 7.07 to 6.98 (5H), 4.76
(1H), 3.66 to 3.63 (2H), 1.72 to 1.68 (2H), 1.37 to 1.28 (2H). 13C NMR
(101 MHz, C6D6) 160.4, 138.5, 137.0, 128.5, 128.0, 127.8, 111.4, 39.0,
Scheme 6. 1,2- and 1,4-addition of allylmagnesium bromide to cyclic amines and
imino dienes, respectively.
d
23.0, 21.9. HRMS calc for C12H13NO 187.0997, found 187.0995.
quaternary cyclic amine 29 (Scheme 6). When the same sequence
of events is repeated on vinylated azepene 12, conjugate addition
out-competes 1,2-addition and furnishes g’-allylated cyclic imine
Compound 4
Prepared from 1a (190 mg, 0.10 mmol) and o-nitro-
bromobenzene (400 mg, 2 equiv) using General Procedure A at
room temperature for 12 h. Purification: Flash chromatography on
silica (pretreated with 1% Et3N) eluting with hexane/EtOAc (20:80).
30.
Conclusion
Yield ¼ 14.5 mg, 62%. 1H NMR (400 MHz, C6D6)
d 8.09 (1H), 7.30 to
7.28 (1H), 6.70 to 6.54 (3H), 4.40 to 4.38 (1H), 3.71 to 3.68 (2H), 1.60
In summary, cobalt-catalyzed
a-arylation and alkenylation of
to 1.56 (2H), 1.34 to 1.25 (2H). 13C NMR (101 MHz, C6D6)
d 160.0,
cyclic -bromo eneformamides and enecarbamates with vastly
a
134.6, 132.9, 132.0, 131.4, 129.7, 124.7, 111.7, 39.9, 23.3, 21.6. . HRMS
calc for C12H12N2O3 232.0848, found 232.0844.
available organic bromides have been accomplished on two
different classes of azaheterocycles, The 2-substituted enecarba-
mates and eneformamides are amenable to further functionaliza-
tion under different modes of reactivity including cycloaddition,
hydroboration-oxidation, stereoselective benzylic lithiation/allyla-
tion, 1,2- or 1,4-addition of nucleophiles to their corresponding
imine tautomers. This short synthetic sequence sets the stage for
Compound 5
Prepared from 1a (190 mg, 1.0 mmol) and 1-bromo-3-chloro-4-
fluorobenzene (418 mg, 2 equiv) using General Procedure A at rt
for 8 h. Purification: Flash chromatography on silica (pretreated
with 1% Et3N) eluting with hexane/EtOAc (90:10). Yield ¼ 18 mg,
the synthesis of a- and a,b-functionalized piperidine or azepane
68%. 1H NMR (400 MHz, C6D6)
d 8.04 (1H), 6.97 to 6.95 (1H), 6.47 to
alkaloids and pharmaceuticals. Studies aimed at fully understand-
ing the mechanistic details of the Co-catalyzed cross-coupling are
underway.
6.45 (2H), 4.48 to 4.46 (1H), 3.53 to 3.50 (2H), 1.64 to 1.60 (2H), 1.28
to 1.25 (2H). 13C NMR (101 MHz, C6D6)
160.3, 136.1, 134.3, 129.9,
127.4, 127.3, 116.6, 116.4, 112.6, 39.0, 22.8, 21.7. HRMS calc for
12H11ClFNO 239.0513, found 239.0511.
d
C
Experimental
Compound 6
Prepared from 1a (190 mg, 1.0 mmol) and 4-bromoanisole
General information
(374 mg, 2 equiv) using General Procedure A. Purification: Flash
chromatography on silica (pretreated with 1% Et3N) eluting with
hexane/EtOAc (50:50). Yield ¼ 173.6 mg, 80%. 1H NMR (400 MHz,
All experiments involving air and moisture-sensitive reagents
such as cobalt precatalysts, phosphine ligands, and organolithium
reagents were carried out under an inert atmosphere of nitrogen
and using freshly distilled solvents. All electrophiles such as aryl
and vinyl bromides were newly purchased and used without
C6D6)
d 8.43 (1H), 7.04 to 7.01 (2H), 6.68 to 6.65 (2H), 4.81 to 4.79
(1H), 3.69 to 3.66 (2H), 3.33 (3H), 1.82 to 1.78 (2H), 1.43, 1.36 (2H).
13C NMR (101 MHz, C6D6)
d
160.6, 159.7, 138.2, 133.5, 129.1, 129.1,

10
D.P. Bassler et al. / Journal of Organometallic Chemistry 780 (2015) 6e12
114.0, 110.4, 54.6, 39.1, 23.0, 22.1. HRMS calc for C13H15NO2 217.1103,
found 217.1106.
Purification: Flash chromatography on silica (pretreated with 1%
Et3N) eluting with PE/DCM (1:3). Yield ¼ 25.2 mg; 88%. Data as
reported [81]. 1H NMR (400 MHz, C6D6)
d 7.26, 7.23 (2H), 6.77, 6.75
Compound 7
(2H), 5.07 to 5.05 (1H), 3.70 to 3.67 (2H), 3.33 (3H),1.87 to 1.82 (2H),
Prepared from 1a (190, 1.0 mmol) and 2-bromopyridine
(320 mg, 2 equiv) using General Procedure A. Purification: Flash
chromatography on silica (pretreated with 1% Et3N) eluting with
hexane/EtOAc (20:80 to 0:100). Yield ¼ 9.6 mg, 51%. 1H NMR
1.55 to 1.46 (2H), 1.14 (9H). 13C NMR (101 MHz, C6D6)
d
158.98,
153.81, 140.30, 134.07, 126.54, 114.22, 113.20, 79.33, 54.51, 44.33,
27.54, 23.52, 23.45. Note: The aromatic region is contaminated with
the byproduct of biaryl coupling.
(400 MHz, CDCl3)
d 8.83 (1H), 8.81 (1H), 8.34 to 8.32 (1H), 7.56 to
7.44 (1H), 7.25 to 7.23 (1H), 5.17 to 5.15 (1H), 3.72 to 3.68 (2H), 2.18
Compound 16
to 2.13 (2H), 1.73 to 1.66 (2H). 13C NMR (101 MHz, CDCl3)
d
162.5,
Prepared from 13a (26.1 mg, 0.1 mmol) and 5-bromobenzo[d]
[1,3]dioxole (2 equiv) using General Procedure A at 40 ꢀC for 2 h.
Purification: Flash chromatography on silica (pretreated with 1%
Et3N) eluting with PE/DCM (1:3). Yield ¼ 26 mg; 86%. 1H NMR
150.0, 141.9, 138.8, 128.3, 125.0, 122.7, 109.5, 41.2, 23.7, 21.0. HRMS
calc for C11H12N2O 188.0950, found 188.0953.
Compound 8
(400 MHz, C6D6)
d 6.94 (1H), 6.79 to 6.63 (1H), 5.33 (1H), 5.02 to
5.00 (2H), 3.65 to 3.62 (2H), 1.80 to 1.76 (2H), 1.52 to 1.19 (11H). 13
NMR (101 MHz, C6D6) 154.37,148.29,147.34,140.91,136.14,119.52,
C
Prepared from 1a (190 mg, 1.0 mmol) and
b-bromostyrene
(2 equiv) using General Procedure A. Purification: Flash chroma-
d
tography on silica (pretreated with 1% Et3N) eluting with hexane/
114.04,109.08,107.07,101.27, 80.15, 44.96, 28.31, 24.13, 24.08. HRMS
calc for C17H21NO4 303.1471, found 303.1466.
EtOAc (70:30). Yield ¼ 173 mg, 811H NMR (400 MHz, CDCl3)
d 8.79
(1H), 7.65 to 7.32 (6H), 7.14 & 7.12 (1H), 5.13 to 5.10 (1H), 3.68 to 3.64
(2H), 2.13 to 2.09 (2H), 1.73 to 1.70 (2H). 13C NMR (101 MHz, CDCl3)
Compound 17
d
160.0, 145.3, 132.7, 129.5, 129.2, 109.5, 41.2, 23.8, 21.1. HRMS calc
Prepared from 13a (261 mg, 1.0 mmol) and 1-bromopropene
(2 equiv) using General Procedure A at 40 ꢀC for 3 h. Purifica-
tion: Flash chromatography on silica (pretreated with 1% Et3N)
eluting with PE/DCM (2:1). Data as reported [82]. Yield ¼ 175 mg;
78%.
for C14H15NO 213.1154, found 213.1157.
Compound 9
Prepared from 2 (20.5 mg, 0.1 mmol) and bromobenzene
(2 equiv) using General Procedure A. Purification: Flash chroma-
tography on silica (pretreated with 1% Et3N) eluting with hexane/
EtOAc (70:30). Yield ¼ 16 mg, 80%. Data as reported [80].
Compound 18
Prepared from 13b (32.0 mg, 0.10 mmol) and 5-bromobenzo[d]
[1,3]dioxole (2 equiv) using General Procedure A at 40 ꢀC for 2 h.
Purification: Flash chromatography on silica (pretreated with 1%
Et3N) eluting with DCM/MeOH (95:2). Yield ¼ 28 mg; 77%. 1H NMR
Compound 10
Prepared from 2 (20.5 mg, 0.1 mmol) and 4-bromoanisole
(2 equiv) using General Procedure A. Purification: Flash chroma-
tography on silica (pretreated with 1% Et3N) eluting with hexane/
EtOAc (50:50). Yield ¼ 203.5 mg, 80%. Data as reported [80].
(400 MHz, C6D6)
d 6.85 (1H), 6.77 to 6.71 (1H), 6.54 to 6.52 (1H),
5.32 to 5.23 (3H), 4.05 to 4.02 (1H), 3.53, 3.52, 3.50, 3.49, 3.49, 3.48,
3.47, 3.46, 3.44, 3.44, 3.43, 3.43, 3.42, 3.41, 3.37, 3.34, 1.98, 1.97, 1.95,
1.95, 1.54, 1.48, 1.48, 1.47, 1.44, 1.44, 1.41, 1.41, 1.39, 1.38, 1.33, 1.33,
Compound 11
1.30, 1.28, 1.26, 1.11, 1.10. 13C NMR (101 MHz, C6D6)
d 154.15, 153.10,
Prepared from
2 (102.5 mg, 0.5 mmol) and 1-bromo-3-
147.54, 147.31, 143.16, 134.37, 119.69, 112.79, 107.64, 106.91, 104.91,
100.70, 80.08, 78.66, 64.07, 63.98, 63.88, 44.97, 35.46, 28.10, 27.40.
HRMS calc for C19H23NO6 361.1525, found 361.1534.
chlorobenzene using General Procedure A. Purification: Flash
chromatography on silica (pretreated with 1% Et3N) eluting with
hexane/EtOAc (80:20). Yield ¼ 88 mg, 75%. Data as reported [80].
Compound 19
Compound 12
Prepared from 13c (27.6 mg, 0.10 mmol) and 2-
bromonaphthalene (2 equiv) using General Procedure A at 40 ꢀC
for 4 h. Purification: Flash chromatography on silica (pretreated
with 1% Et3N). PE/DCM (2:1). Yield ¼ 25.6 mg; 79%. 1H NMR
Prepared from 2 (102.5 mg, 0.5 mmol) and
b-bromostyrene
(2 equiv) using General Procedure A. Purification: Flash chroma-
tography on silica (pretreated with 1% Et3N) eluting with hexane/
EtOAc (90:10). Yield ¼ 80 mg, 70%. Data as reported [80].
(400 MHz, C6D6, mixture of rotamers)
d 8.03 to 7.84 (1H), 7.76 to
7.21 (6H), 5.17 & 5.16 (1H), 3.81 to 3.76 (2H), 2.16 to 2.09 (1H), 1.74
Compound 14
to 1.66 (1H), 1.52 to 0.98 (11H), 0.82 to 0.65 (3H). 13C NMR
Prepared from 13a (26.1 mg, 0.10 mmol) and 2-
bromonaphthalene (41.4 mg, 2 equiv) using General Procedure A
but at 40 ꢀC for 4 h. Purification: Flash chromatography on silica
(pretreated with 1% Et3N) eluting with PE/DCM (2:1).
(101 MHz, C6D6)
d 153.65, 139.51, 138.81, 133.59, 132.81, 127.79,
127.57, 127.20, 126.02, 125.40, 124.68, 123.95, 120.72, 79.65, 42.91,
31.62, 29.04, 27.36, 21.18. HRMS calc for C21H25NO2 323.1885, found
323.1879.
Yield ¼ 21.6 mg; 70%. 1H NMR (400 MHz, C6D6)
d 8.03 (1H), 7.81 to
7.20 (6H), 5.18 to 5.17 (1H), 3.75 to 3.73 (2H), 1.88 to 1.83 (2H), 1.55
to 1.52 (2H), 0.98 (9H). 13C NMR (101 MHz, C6D6)
d
153.71, 140.51,
Compound 20
138.91, 133.61, 132.79, 128.52, 128.27, 127.81, 127.56, 127.22, 126.00,
125.37, 124.51, 123.77, 114.90, 79.58, 44.22, 27.37, 23.62, 23.45.
HRMS calc for C20H23NO2 309.1729, found 309.1733. Note: The ar-
omatic region is contaminated with the byproduct of biaryl
coupling.
Prepared from 13d (27.6 mg, 0.10 mmol) and 5-bromobenzo[d]
[1,3]dioxole (2 equiv) using General Procedure A at 40 ꢀC for 2 h.
Purification: Flash chromatography on silica (pretreated with 1%
Et3N). PE/DCM (1:3). Yield ¼ 26.3 mg; 83%. 1H NMR (400 MHz, C6D6,
mixture of rotamers)
d 6.94 to 6.91 (1H), 6.79 to 6.76 (1H), 6.66
(1H), 5.37 to 5.32 (2H), 5.02 to 5.01 (1H), 4.90 to 4.72 (1H), 1.83, to
Compound 15
1.01 (16H). 13C NMR (101 MHz, C6D6, mixture of rotamers)
d
153.54,
Prepared from 13a (26.1 mg, 0.1 mmol) and 4-bromoanisole
148.28, 147.62, 146.94, 146.55, 137.10, 136.84, 135.43, 120.28, 118.58,
113.11, 108.37, 107.69, 107.52, 106.22, 100.72, 100.57, 98.30, 80.17,
(2 equiv) using General Procedure A but at 40 ꢀC for 2 h.

D.P. Bassler et al. / Journal of Organometallic Chemistry 780 (2015) 6e12
11
79.37, 48.84, 46.80, 27.97, 27.79, 27.64, 27.36, 19.83, 18.19, 15.47,
15.33. HRMS calc for C18H23NO4 317.1627, found 317.1630.
52.8 mg of the trans-26 in 84% yield. 1H NMR (400 MHz, CDCl3)
7.27 to 7.23 (1H), 6.83 to 6.75 (3H), 5.35 to 5.33 (1H), 4.48 to 4.46
(1H), 4.12 to 4.06 (1H), 3.80 to 3.77 (3H), 3.13 (1H), 2.92 to 2.85 (1H),
2.04 to 1.25 (13H). 13C NMR (101 MHz, CDCl3)
159.9, 156.6, 140.2,
d
Compound 22
d
Cycloaddition. A 5 mL tube was flame-dried, evacuated and flushed
with nitrogen. A solution of the dienophile (1.0 mL, 0.10 M in
benzene) was added to a solution of the diene (1.0 mL, 0.10 M in
benzene) under nitrogen. After 10 min, the solvent was removed in
vacuo to obtain the bromo adduct (35 mg, 87%). No further purifi-
129.6, 118.6, 112.5, 111.7, 79.9, 67.3, 60.1, 55.1, 40.1, 28.5, 28.4, 26.0,
18.8. HRMS calc for C17H25NO4 307.1784, found 307.1780.
Compound 28
Hydrogenation. Pd(OH)2 (102 mg, 0.4 mmol) was added to a solu-
tion of crude enecarbamate 27 (27.3 mg, 0.1 mmol) in freshly
distilled MeOH (4 mL) under hydrogen (1 atm) at room tempera-
ture. After complete consumption of the 27 (based on LC-MS and
TLC monitoring, ~48 h), the mixture was filtered through a plug of
Celite, concentrated under reduced pressure, and dried extensively
on a high vacuum line.
cation is necessary. 1H NMR (400 MHz, C6D6)
d 3.65 to 3.60 (1H),
3.46 (1H), 2.91 to 2.85 (1H), 2.55 to 2.41 (2H), 2.22 to 2.10 (1H), 1.84
to 1.75 (1H), 1.49 to 1.29 (14H), 1.24 (3H). 13C NMR (101 MHz, C6D6)
d
168.21, 167.79, 153.27, 136.42, 111.36, 81.31, 60.14, 58.94, 44.61,
37.29, 35.31, 28.51, 25.73, 23.19, 17.30.
Caution. The product is acid (and base) sensitive and should not be
put on silica (unless pretreated with 1% Et3N). The adduct also
readily eliminates HBr upon standing.
Benzylic lithiation. To an oven-dried, septum-capped round bottom
flask equipped with a stir bar, was added freshly distilled TMEDA
(0.02 mL, 0.12 mmol, 1.2 equiv) and a solution of the reduced
enecarbamate (0.1 mmol, 1.0 equiv) in THF (2 mL) under nitrogen.
The mixture was cooled to ꢁ60 ꢀC and a solution of n-BuLi (0.10 mL,
2.0 M in hexanes, 0.2 mmol, 2.0 equiv) was added slowly. After 1 h
at this temperature, a solution of ZnCl2 (0.13 mL, 1.0 M in THF,
1.3 equiv) was added slowly. After 30 min, a solution of CuCN$2LiCl
[prepared from CuCN (1.2 equiv) and LiCl (2.5 equiv)] in THF
(1.0 mL) was added. After 30 min, allyl bromide (1.1 equiv) was
added. The mixture was allowed to stir for 10 h at this temperature
prior to the addition of MeOH (0.2 mL) and warming to room
temperature. A solution of NH4Cl (2 mL) was added and the
aqueous layer was extracted with Et2O. The combined organic
layers were dried over Na2SO4 and evaporated to give the crude
product. Purification by flash chromatography on silica eluting with
hexane-EtOAc (98:2) afforded 23 mg of 28 in 73% yield from 13d. 1H
Dehydrobromination. To the bromo adduct (20 mg, 0.05 mmol)
dissolved in distilled CH2Cl2 (5 mL) was added DBU (3 equiv)
dropwise. After 3 h at room temperature, the reaction was
quenched by addition of sat. aqueous NH4Cl. The mixture was
diluted with DCM and the layers were separated. The aqueous layer
was further extracted with DCM and the combined organic layers
were washed with brine, dried over Na2SO4 (30 min), filtered, and
concentrated under reduced pressure. Purification by flash chro-
matography on silica (pretreated with 1% Et3N) eluting with PE/
DCM (1:3) afforded 11.32 mg of the desired product in 79% yield
(from diene 17). 1H NMR (400 MHz, C6D6)
to 3.16 (1H), 2.69 to 2.09 (6H), 1.47 to 1.20 (10H), 0.91 & 0.89 (3H).
13C NMR (101 MHz, C6D6)
163.45, 152.15, 143.29, 140.42, 134.18,
d 3.38 to 3.22 (1H), 3.20
d
110.47, 81.30, 44.65, 35.04, 27.74, 25.31, 22.02, 21.99, 16.17. HRMS
calc for C17H21NO5 319.1420, found 319.1418.
NMR (400 MHz, CDCl3)
(2H), 4.45 (1H), 3.17 to 2.96 (2H), 2.14 to 0.85 (18H). 13C NMR
(100 MHz, CDCl3) 155.5, 150.4, 136.2, 128.0, 125.8, 125.5, 117.3,
79.5, 62.4, 48.2, 45.3, 37.3, 28.3, 27.3, 22.5, 14.5. HRMS calc for
20H29NO2 315.2198, found 315.2194.
d 7.45 to 7.12 (5H), 6.05 (1H), 5.19 to 5.08
Compound 24a
d
A 5 mL tube was flame-dried, evacuated and flushed with ni-
trogen. A solution of dienophile 23a (1.0 mL, 0.1 M in benzene) was
added to a solution of diene 17 (1.0 mL, 0.1 M in benzene) under
nitrogen. After 5 min, the solvent was removed in vacuo to obtain
C
the crude product (35.1 mg, 93%). 1H NMR (400 MHz, C6D6)
d 6.26
Compound 29
Imine formation. To eneformamide 11 (59 mg, 0.25 mmol,1.0 equiv)
dissolved in freshly distilled THF (5 mL) was added n-BuLi (0.15 mL,
0.30 mmol, 2.0 M in hexanes, 1.2 equiv) at ꢁ78 ꢀC. After complete
deprotection of the eneformamide (~30 min, as indicated by TLC
and LCMS-monitoring), the mixture was quenched with H2O and
diluted with EtOAc. It was washed with sat. NaHCO3 and then with
brine. The organic layer was dried over Na2SO4 and concentrated
under reduced pressure to give the crude product as an oil.
(1H), 6.06 (1H), 5.69 to 5.56 (1H), 4.15 (1H), 3.82 (1H), 3.29 to 3.19
(6H), 2.85 to 2.79 (1H), 2.61 to 2.54 (1H), 2.41 to 2.34 (1H), 1.46 to
1.09 (13H). 13C NMR (101 MHz, C6D6)
d 197.74, 195.10, 171.80, 154.19,
142.87, 140.18, 137.68, 124.86, 79.92, 61.97, 54.83, 52.87, 46.67,
36.78, 35.70, 30.14, 28.73, 26.10, 17.12. HRMS calc for C21H27NO6
389.1838, found 389.1845.
Caution. The product is acid (and base) sensitive and should not be
put on silica (unless pretreated with 1% Et3N). Even residual acid in
CDCl3 destroys the compound.
Grignard addition. To the crude imine dissolved in freshly distilled
THF (5 mL), was slowly added allyl magnesium bromide (0.5 mL,
1.0 M solution in Et2O, 2 equiv) under nitrogen at 0 ꢀC. The mixture
was stirred for 10 min at 0 ꢀC, then warmed slowly to room tem-
perature. After complete consumption of the imine (as indicated by
TLC and LC-MS, ~12 h), the mixture was cooled to 0 ꢀC, diluted with
Et2O. Water was then added slowly and the layers were separated.
The aqueous layer was extracted twice with Et2O and the combined
organic layers were dried over Na2SO4 for 30 min, filtered, and
concentrated under reduced pressure to give the desired product.
Purification: Flash chromatography on silica (pretreated with 1%
Et3N) eluting with Hexane/EtOAc (1:2). Yield ¼ 52.3 mg; 84%. 1H
Compound 26
To a solution of the crude enecarbamate 25 (58 mg, 0.20 mmol,
1.0 equiv) in freshly distilled CH2Cl2 (4 mL) at ꢁ78 ꢀC was added
dropwise a 2 M solution of BH3$SMe2 in THF (100 mL, 1 equiv). After
a few minutes, the mixture was warmed to room temperature and
stirring was continued for 6 h. It was then cooled to 0 ꢀC and a few
drops of 10% NaOH (aq) were added slowly followed by 0.5 mL of
30% H2O2. The mixture was returned to room temperature and
stirred for 3 h. Water was added and the mixture was extracted
with CH2Cl2. The combined organic layers were washed with brine,
dried over MgSO4, filtered and concentrated under reduced pres-
sure to afford crude product as an oil. Purification by flash chro-
matography on silica eluting with hexanes:EtOAc (40:60) gave
NMR (400 MHz, C6D6)
(1H), 4.75 to 4.71 (2H), 2.63 to 2.57 (1H), 2.29 to 2.06 (2H), 1.84 to
0.75 (10H). 13C NMR (101 MHz, CDCl3)
151.1, 134.3, 134.0, 129.2,
d 7.63 (1H), 7.27 to 6.87 (3H), 5.42 to 5.20
d

12
D.P. Bassler et al. / Journal of Organometallic Chemistry 780 (2015) 6e12
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Compound 30
Prepared in the same way as 29 starting from diene 12 (57 mg,
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Yield ¼ 46.5 mg; 77%. 1H NMR (400 MHz, CDCl3)
d 7.46 to 7.04 (5H),
5.83 to 5.55 (1H), 5.19 to 4.91 (2H), 3.38 to 1.21 (15H). 13C NMR
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116.78, 48.63, 43.14, 40.87, 40.72, 39.64, 27.35, 25.93, 22.77. . HRMS
calc for C17H23N 241.1830, found 241.1827.
d 144.06, 136.19, 128.95, 128.47, 127.48, 126.44,
Acknowledgments
We are grateful to the Chemistry Department and the Com-
mittee for Faculty Scholarship at Susquehanna University for
funding. Professor Bill Dougherty is greatly acknowledged for
making it logistically possible for us to perform experiments
involving air- and moisture-sensitive reagents.
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