Rhodium- N-Heterocyclic Carbene Catalyzed Hydroalkenylation Reactions with 2-Vinylpyridine and 2-Vinylpyrazine: Preparation of Nitrogen-Bridgehead Heterocycles

Article abstract of DOI:10.1021/acs.organomet.8b00149

Dinuclear rhodium-NHC complexes of formula [Rh(μ-Cl)(NHC)(η²-coe)]₂ react with 2-vinylpyridine to yield the chelate compounds RhCl(NHC)(κ-N,η²-CH₂=CHC₅H₄N) {NHC = IPr, 1,3-bis(2,6-diisoprop

Full text of DOI:10.1021/acs.organomet.8b00149

Article
Cite This: Organometallics XXXX, XXX, XXX−XXX
Rhodium−N‑Heterocyclic Carbene Catalyzed Hydroalkenylation
Reactions with 2‑Vinylpyridine and 2‑Vinylpyrazine: Preparation of
Nitrogen-Bridgehead Heterocycles
Ramon Azpíroz,† Andrea Di Giuseppe,† Vincenzo Passarelli,†,‡ Jesus J. Perez-Torrente,†
́
́
́
Luis A. Oro,† and Ricardo Castarlenas*,†
†Departamento de Química Inorgan
CSIC, C/Pedro Cerbuna 12, 50009 Zaragoza, Spain
‡Centro Universitario de la Defensa, Ctra Huesca S/N, 50090 Zaragoza, Spain
́
ica − Instituto de Síntesis Química y Catal
́
isis Homogen
́
ea-ISQCH, Universidad de Zaragoza −
S
* Supporting Information
ABSTRACT: Dinuclear rhodium−NHC complexes of formula [Rh(μ-
Cl)(NHC)(η2-coe)]2 react with 2-vinylpyridine to yield the chelate
compounds RhCl(NHC)(κ-N,η2-CH2CHC5H4N) {NHC = IPr, 1,3-
bis(2,6-diisopropylphenyl)imidazolin-2-carbene; IMes, 1,3-bis(2,4,6-
trimethylphenyl)imidazolin-2-carbene}. The strained metallacycle can
be opened by substitution of the pyridine ring by the small electron-rich
PEt3 to give RhCl(IPr){η2-CH2CH(C5H4N)}(PEt3), whereas π
ligands such as olefins or alkynes undergo C−C coupling to yield 2-
(butenyl)pyridine or 2-(butadienyl)pyridine RhCl(NHC){κ-N,η2-CH-
(R)CH(C5H4N)} complexes by formal hydroalkenylation of the
unsaturated bond by vinylpyridine. Reaction of the dinuclear precursors
with 2-vinylpyrazine in the presence of pyridine affords the η2 derivative
RhCl(IPr){η2-CH2CH(C4H3N2)}(py). Compound RhCl(IMes)(κ-N,η2-CH2CHC5H4N) is an efficient catalyst for the
hydroalkenylation of a range of alkenes and alkynes with 2-vinylpyridine and 2-vinylpyrazine. The butadienyl-heterocycle
derivatives resulting from coupling of 2-vinylazines with alkynes undergo a thermal 6π-electrocyclization to yield 4H-quinolizines
or 6H-pyrido[1,2-a]pyrazines depending on the nature and position of the substituent of the butadienyl fragment. DFT
calculations reveal that fused N-bridgehead heterocycles are more stable than opened butadienylazine derivatives, in spite of the
dearomatization of the azine moiety.
most typical is that both the nitrogen and the double bond
coordinate in a chelate fashion (a)7 or as a bridge between two
metals (b).7a,8 In some cases, depending on the metal and the
coligands, monocoordination by the nitrogen atom (c)9 or the
double bond (d)10 has also been observed. Notably,
dearomatization of the pyridine ring to an alkyl-amido ligand
has been described in bis-cyclopentadienyl zirconium com-
plexes (e).11 In spite of the chelate coordination of 2-
vinylpyridine in species a, it does not bind strongly to the
metal center, mainly due to the high strain of the metallacycle
formed, therefore allowing it to participate in catalytic
transformations.
Fused heterocycles containing a bridgehead nitrogen are
widespread in natural and synthetic alkaloids.12 Extensive
research has been carried out for the development of efficient
synthetic protocols, among which organometallic catalysts have
played a prominent role.13 Quinolizine-based derivatives belong
to this interesting family.14 However, the usual instability of
these species precludes their isolation,15 and consequently, they
INTRODUCTION
■
Transition metal mediated C−C coupling via C-H activation
has become nowadays a selective and atom-economical tool for
synthetic chemists.1 In general, a nucleophilic directing group
facilitates the activation process. Pyridine scaffolds constitute a
prevalent family of these auxiliaries which have been
successfully applied in the activation of aromatic C(sp2)−H2
or even C(sp3)−H bonds.3 However, functionalization of
olefinic C−H bonds is more problematic owing to the
dichotomic behavior of alkenyl-substituted pyridines in C−C
coupling transformations (Scheme 1). Alkenylpyridines can be
involved in Heck-type processes due to the π electrons of the
double bond4 or, alternatively, undergo direct C−H cleavage by
the metal center. In the latter case, depending on the coupling
partner and the catalyst, two main processes can take place. A
nonoxidative base-promoted C-H activation of the alkenylpyr-
idine and subsequent coupling with an electrophile5 or a Murai-
type hydroalkenylation of unsaturated substrates.6
Regardless of the coupling mechanism, coordination of the 2-
vinylpyridine moiety to the metallic center is essential for the
catalytic activity. The versatility of vinylpyridine as ligand is
reflected in the variety of coordination modes (Scheme 2). The
Received: March 9, 2018
© XXXX American Chemical Society
A
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cation to 2-vinylpyrazine, which have led to the preparation of
pyrido[1,2-a]pyrazines.
Scheme 1. Functionalization of the Olefinic C−H Bond of
Pyridine-Substituted Olefins
RESULTS AND DISCUSSION
■
Synthesis and Reactivity of 2-Vinylpyridine and 2-
Vinylpyrazine Rh−NHC Complexes. Dinuclear rhodium−
NHC complexes of formula [Rh(μ-Cl)(NHC)(η2-coe)]2 (1)
(coe = cyclooctene) {NHC = IPr (1a), IPr = 1,3-bis(2,6-
diisopropylphenyl)imidazolin-2-carbene; IMes (1b), IMes =
1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-carbene} are ad-
equate precursors for the synthesis of mononuclear Rh−
NHC species by chlorido-bridge cleavage with nitrogenated
ligands.22 Thus, treatment of a toluene solution of 1 with 2-
vinylpyridine for 1 h at room temperature afforded the chelate
mononuclear derivatives RhCl(NHC)(κ-N,η2-CH2CH-
C5H4N) {NHC = IPr (2a), IMes (2b)} in good yields
(Scheme 4).20
Scheme 4. Preparation of RhICl(NHC)(κ-N,η2-
Vinylpyridine) Complexes
Scheme 2. Coordination Modes of 2-Vinylpyridine
have been alternatively obtained as quinolizidines,16 quinolizi-
nium salts,17 or quinolizinones.18 Moreover, examples of
formation of pyrido[1,2-a]pyrazines are even more scarce.19
In this context, we have described in a previous report that the
quinolizine skeleton can be formed by thermal 6π-electrocyclic
rearrangement within a butadienylpyridine framework.20
However, that synthetic pathway presents an important
handicap, the lack of an efficient and straightforward access
to butadienylpyridines. With the aim to circumvent this
problem, we designed a synthetic approach consisting in the
C−C coupling between 2-vinylazines and alkynes promoted by
a rhodium−N-heterocyclic carbene (NHC) catalyst (Scheme
3). This atom-economical reaction proceeds under mild
Single crystals of 2a were grown by slow diffusion of hexane
into a saturated toluene solution of the complex. Figure 1
shows a view of the crystal structure of 2a, and Table 1 contains
selected bond lengths and angles. A distorted square planar
Scheme 3. Synthetic Approach for the Access to 4H-
Quinolizines
conditions with high chemoselectivity, since neither alkyne
dimerization nor polymerization has been observed.21
Although, it has been previously described that rhodium−
phosphane catalysts efficiently promote the C−C coupling
between alkenylpyridines and olefins,6 we were the first to
describe the coupling with terminal alkynes. Now, herein we
report on the isolation and characterization of key organo-
metallic intermediates in these catalytic processes as well as new
alkene hydroalkenylation reactions. Notably, we extend the
alkyne hydroalkenylations reported in our previous communi-
Figure 1. View of the crystal structure of 2a. Most hydrogen atoms are
omitted for clarity. Brandl−Weiss parameters of the C(36)-H(36)···π-
{C(6)-C(7)-C(8)-C(9)-C(10)-C(11)} interaction are dCX 3.54 Å; α
137.4°, dHpX 0.75 Å. X, centroid of the π system; Hp, projection of the
interacting hydrogen atom onto the plane containing the π system.
B
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In view of the high strain of the 2-vinylpyridine ligand in
Rh−NHC complexes, we carried out some experiments in
order to assess its lability. After treatment of 2a at room
temperature with σ-donors, such as pyridine or triphenylphos-
phine, any reactivity was observed. Heating of the samples
resulted in a mixture of unidentified compounds. However,
reaction of 2a with the small electron-rich triethylphosphine, in
C6D6 at −30 °C, led to the in situ formation of the tetraligated
species RhCl(IPr){η2-CH2CH(C5H4N)}(PEt3) (3), as a
result of the opening of the chelate ligand by substitution of
the heterocyclic moiety of 2-vinylpyridine by the phosphine
(Scheme 5). The η2-olefin coordination is confirmed by the
Table 1. Selected Bond Lengths (Å) and Angles (deg) of
RhCl(IPr)(κ-N,η2-CH2CHC5H4N) (2a), RhCl(IPr){κ-
N,η2-(Z)-CH(Et)CH(C5H4N)} (4), and RhCl(NHC)[κ-
N,η2-(Z)-CH{(E)-C(Et)CH(Et)}CH(C5H4N)] (6a)
2a
4
6a
Rh−Cl
2.3603(7)
1.9837(17)
2.0914(16)
1.9680(5)
1.409(3)
2.1021(19)
2.078(2)
92.24(5)
167.79(7)
73.52(5)
25.0
2.3721(5)
1.9884(18)
2.0771(16)
1.9729(16)
1.424(3)
2.0950(19)
2.100(2)
91.11(5)
176.41(7)
74.21(4)
24.5
2.3486(15)
1.989(5)
2.087(4)
1.9646(6)
1.429(8)
2.103(5)
2.078(5)
91.28(14)
170.67(18)
73.76(12)
24.3
Rh−C(1)
Rh−N(30)
a
Rh−ct
C(36)−C(37)
Rh−C(36)
Rh−C(37)
C(1)−Rh−Cl
C(1)−Rh−N(30)
N(30)−Rh−ct
ψpy
θpy
ψim
θim
Scheme 5. Reaction of 2a with PEt3
a
13.7
12.8
12.2
4.2
−2.0
2.8
8.2
11.1
10.8
a
ct: centroid of C(36) and C(37).
coordination polyhedron was observed at the metal center with
a cis arrangement of the NHC and the chlorido ligands [Cl−
Rh−C(1) 92.24(5)°]. The remaining coordination sites are
occupied by 2-vinylpyridine exhibiting a κ-N,η2-CC′ coordina-
tion mode. Notably, the small bite angle of vinylpyridine
[N(30)−Rh−ct 73.52(5)°] renders a severely distorted
arrangement of this ligand. Accordingly, the pitch (θpy 13.7°)
and yaw (ψpy 25.0°) angles of the heterocyclic moiety indicate
that it largely deviates from its ideal position with respect to the
rhodium−nitrogen bond.23 Further, the dihedral angle C(37)−
C(36)−Rh−N(30) [−110.39(13)°] reveals that the coordi-
nated CC bond, namely, C(36)−C(37), largely deviates
from the perpendicular arrangement with respect to the
coordination plane, observed in related square planar complex-
es.22,23b,24 As for the NHC ligand, the Rh−C(1) bond length
[1.9837(17) Å] nicely fits in with a single metal−carbon
bond.22,23b,24 Notably, a CH···π25 interaction between the π
system C(6)-C(7)-C(8)-C(9)-C(10)-C(11) and the C(36)-
H(36) group is observed (Figure 1), along with a concomitant
deviation from the ideal arrangement of the imidazole with
respect to the Rh−C(1) bond (θim 8.2°, ψim 4.2°). For the sake
appearance of shielded signals at δ 4.89, 2.97, and 2.58 ppm in
the low temperature H NMR spectrum. In addition, two
1
doublets at δ 53.9 (JC‑Rh = 15.2 Hz) and 30.0 (JC‑Rh = 14.7 Hz)
ppm are observed in the 13C{1H}-APT NMR spectrum. The
carbene carbon atom appears at 187.8 ppm as doublet of
doublets (JC‑P = 136.7 and JC‑Rh = 43.9 Hz). The 31P{1H} NMR
spectrum of 3 shows a doublet at δ 16.1 ppm with a JP‑Rh
coupling constant of 121.0 Hz. The Rh−P and C−P coupling
constants are in agreement with a phosphine-carbene trans
disposition.22c Indeed, the longe-range 1H−15N HMQC
spectrum is indicative of a η2-olefin coordination of the 2-
vinylpyridine ligand with uncoordinated pyridine since a cross-
peak appears at δ 305.5 ppm for the nitrogen atom, which is
deshielded with regard to that found in the κ-N,η2 ligand in 2a
(249.2 ppm) and lies in the range of the free ligand (311.2
ppm). This fact excludes the possibility for 3 to be a
rhodium(I)-pentacoordinated species. The stability of complex
3 is limited. Warming of the toluene solution of 3 to room
temperature caused the releasing of 2-vinylpyridine to yield
RhCl(IPr)(PEt3)2 in addition to traces of RhCl(PEt3)3 and
other unidentified rhodium species. Moreover, Rh-IMes
counterpart 2b is more reactive. Treatment of 2b with PEt3
resulted in the formation of bis-phosphine complexes even at
low temperature.
In view that a powerful σ-donor can promote the
decoordination of the nitrogen atom of the 2-vinylpyridine
ligand, we next explored the possible decoordination of the
vinyl fragment by reactivity with π-donor ligands. Thus, a
Young-type NMR tube containing a C6D6 solution of 2a was
charged with 2 bar of ethylene and heated at 80 °C for 2 h. The
expected η2-ethylene derivative was not observed, but in
contrast, the 2-butenylpyridine complex RhCl(IPr){κ-N,η2-(Z)-
CH(Et)CH(C5H4N)} (4) was obtained as a result of the
formal hydroalkenylation of ethylene with 2-vinylpyridine
(Scheme 6). Complex 4 could not be isolated in pure form
since it evolves to [Rh(μ-Cl)(IPr)(η2-ethylene)]222a and free 2-
{1-(Z)-butenyl}pyridine, likely due to ethylene excess. Indeed,
the same reaction starting from 2b gave directly the coupled
of comparison, the related RhCl(IPr)(κ-N,η2-vpzMe) (vpzMe
=
5-dimethyl-1-vinylpyrazole) features a very similar CH···π
interaction.23b
The solid state structure of 2a is maintained in solution. The
1H NMR spectra of 2a and 2b show three shielded signals
around δ 3.4−2.1 ppm which corroborate the coordination of
the vinyl fragment of the 2-vinylpyridine ligand. The trans and
cis proton−proton couplings of around 9.7 and 6.8 Hz,
respectively, are significantly smaller than the typical values of a
vinyl moiety. Indeed, the trans-to-proton terminal hydrogens of
2a and 2b feature a scalar coupling with the rhodium atom of
about 3 Hz. Moreover, the 13C{1H}-APT NMR spectra display
two doublets around δ 46 and 45 ppm with JC‑Rh about 12−16
Hz, ascribed to the coordinated alkene. The carbene carbon
atom was observed at 183.3 (2a) and 181.1 (2b) ppm as a
1
doublet with JC‑Rh around 57 Hz. The long-range H−15N
HMQC NMR spectrum of 2a displays a cross-peak at δ 249.2
ppm for the nitrogen atom which lies in the range observed for
a pyridine fragment coordinated trans to a carbene ligand.21
C
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Scheme 6. Preparation of RhICl(IPr)(κ-N,η2-
Butenylpyridine) Derivatives by Hydroalkenylation
organic product and [Rh(μ-Cl)(IMes)(η2-ethylene)]2. On the
other hand, reaction of 2a with the stoichiometric amount of
styrene at 80 °C for 48 h afforded RhCl(IPr){κ-N,η2-
(Z)CH(CH2CH2Ph)CH(C5H4N)} (5), as a result of C−C
coupling of styrene and 2-vinylpyridine, which could be isolated
as an orange solid in 58% yield.
Single crystals of 4 were grown by slow diffusion of hexane
into a saturated C6D6 solution of the complex. A view of the
crystal structure of 4 is shown in Figure 2, and Table 1 contains
selected bond lengths and angles. Similarly to 2a, the metal
center exhibits a distorted square planar coordination
polyhedron (Figure 2) with a cis arrangement of the NHC
and the chlorido ligands [Cl−Rh−C(1) 91.11(5)°] and a Rh−
C(1) bond length [1.9884(18) Å] indicative of a single metal−
carbon bond (vide supra). The remaining coordination sites are
occupied by 1-Z-butenylpyridine exhibiting a κ-N,η2-CC′
coordination mode. Notably, the Rh(κ-N,η2-CC′-alkenylpyr-
idine) moiety of 4 and the related fragment Rh(κ-N,η2-CC′-
vinylpyridine) of 2a are virtually superimposable (Figure 2). As
a confirmation, the pitch (θpy 12.8°) and yaw (ψpy 24.5°) angles
of the heterocyclic moiety and the dihedral angle C(37)−
C(36)−Rh−N(30) [−111.30(13)°] of 4 are similar to the
corresponding angles of 2a (vide supra). At variance of 2a, a
CH···π interaction exists between the π system C(18)-C(19)-
C(20)-C(21)-C(22)-C(23) and the allylic C(38)-H(38b)
group (Figure 2). Accordingly the C(1)−Rh−N(30) angle is
wider in 4 [176.41(7)°] than in 2a [167.79(7)°] and the yaw
angle of the imidazole ring of 4 (ψim −2.0°) is opposite to that
of 2a (ψim 4.2°) (cf. Figure 2, bottom). This observation reflects
the flexibility of an NHC ligand in a square planar environment.
The NMR spectra of 4 and 5 corroborate the η2-olefin
coordination, since a set of shielded signals between 3.4 and 2.7
Figure 2. (top) View of the crystal structure of 4. Most hydrogen
atoms are omitted for clarity. Brandl−Weiss parameters (cf. Figure 1)
of the C(38)-H(38b)···π{C(18)-C(19)-C(20)-C(21)-C(22)-C(23)}
interaction are dCX 3.76 Å; α 148.2°, dHpX 0.34 Å. (bottom) Structure
overlay of 4 (green) and 2a (orange).
13C{1H}-APT NMR spectra display two doublets around δ 65
and 44 ppm with JC‑Rh about 18 and 12 Hz, respectively.
The unexpected hydroalkenylation reaction observed in the
formation of complexes 4 and 5 led us to explore the reactivity
with other π donors such as alkynes. Thus, treatment of a
toluene solution of 2a or 2b with 3-hexyne or diphenylacety-
lene resulted in the formation of RhCl(NHC)[κ-N,η2-CH{C-
(R)CH(R)}CH(C5H4N)] (6a,b−7a,b) {NHC = IPr (6),
IMes (7); R = Et (a), Ph (b)} in good yields (Scheme 7).
Slow diffusion of hexane into a saturated toluene solution of
6a provided single crystals suitable for X-ray diffraction
measurements. Two rotamers around the C(37)−C(38)
bond were observed in the crystal structure of 6, and they
were refined as complementary components of a static
positional disorder (Figure 4A,B). Both rotamers formally
contain conjugated double bonds, one Z [C(36)−C(37)] and
the other E [C(38a/b)−C(39a/b)], and differ as for the
dihedral angles C(36)−C(37)−C(38a/b)−C(39a/b) [a,
56(1)°; b, −135(1)°]. Nevertheless, similar to 2a, the rhodium
center exhibits a square planar coordination polyhedron with a
1
ppm were observed in the H NMR spectra. The olefinic
proton proximal to the pyridine fragment (H7) resonates as a
doublet with JH‑H = 7.2 Hz, in agreement with a cis
configuration of the double bond (Figure 3). In contrast, the
alkenyl proton H8 is observed as complex resonance which can
be described as a doublet of doublets of doublets. Besides the
cis CHCH coupling, a JH‑H = 10.4 Hz and a JH‑Rh = 3.4 Hz are
observed. The former interaction is ascribed to the coupling
with only one of the diastereotopic methylene protons, whereas
the latter reflects the coordination to the metal center. The
D
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is virtually superimposable to that of 2a (Figure 4C) exhibiting
C(37)−C(36)−Rh−N(30) [−110.65(34)°], pitch (ψpy 24.3°),
and yaw (θpy 12.2) angles similar to those of 2a and 4 (vide
supra). Finally, it is worth mentioning that in each rotamer, the
π system C(18)-C(19)-C(20)-C(21)-C(22)-C(23) establishes
a CH···π interaction with either C(42a)-H(42a) or C(39b)-
H(39b) (Figure 4A,B). In this connection, like for 2a and 4, the
pitch and yaw angles of the imidazole ring (Table 1) reasonably
should be a consequence of the CH···π interaction.
The NMR spectra of 6−7 are similar to those of the
previously described κ-N,η2-olefin complexes. Additional 2D
1H−1H-NOESY NMR experiments were recorded in order to
confirm the stereochemistry of the double bonds. Coordinated
olefin presents a Z configuration in 6−7, as observed in the X-
ray structure of 6a, since a NOE cross-peak is observed
between both olefinic protons. Moreover, correlation between
the C(R)H proton of the unbonded olefin and one proton of
the coordinated alkene confirms the mutually cis disposition of
the substituents in the noncoordinated olefin. This config-
uration arises from a typical syn-hydroalkenylation to the triple
bond.
The reactivity of 1 with 2-vinylpyrazine is somewhat different
to that observed for 2-vinylpyridine. Treatment of 1a or 1b
with the vinylheterocycle resulted in a mixture of unidentified
compounds, including some Rh-hydrides. However, when the
reaction was performed in the presence of 2 equiv of pyridine at
−30 °C, complex RhCl(IPr){η2-CH2CH(C4H3N2)}(py) (8)
was formed and characterized by multinuclear low temperature
NMR experiments (Scheme 8). Coordination of the olefin
moiety of 2-vinylpyrazine in 8 is supported by the appearance
of three shielded resonances at δ 4.53, 3.10, and 2.86 ppm and
the occurrence of carbon−rhodium scalar coupling around 16
Hz for the corresponding olefinic carbon atoms in the 1H NMR
and 13C{1H}-APT NMR spectra, respectively. Moreover, a
long-range 1H−15N-HMQC NMR experiment corroborates the
presence of an uncoordinated pyrazine fragment and the
coordination of a pyridine ligand in 8 (Figure 5). Accordingly,
the N4 pyrazine atom resonates at δ 331.6 ppm, only slightly
shifted from that of the free pyrazine (336.5 ppm). Indeed, the
nitrogen atom of the pyridine ligand is observed at 269.5 ppm,
shielded almost 50 ppm compared to free pyridine (318.6
ppm), reflecting its coordination to the metal center. The
structure of a related Rh−NHC complex containing an opened
η2-vinylpyrazol and pyridine ligands has been previously
described by X-ray diffraction analysis.23b
Figure 3. Comparison of the 1H NMR resonances of the vinyl moiety
for 2a (top) and 4 (bottom).
Scheme 7. Synthesis of RhCl(NHC)(κ-N,η2-
Butadienylpyridine) Complexes
Catalytic Hydroalkenylation. Catalytic addition of an
alkenyl fragment to an unsaturated bond is a powerful tool for
synthetic chemists.26 These so-called hydroalkenylation reac-
tions can be efficiently promoted by transition metal catalysts
using a range of olefins or internal alkynes as acceptor
substrates, but the hydroalkenylation of terminal alkynes is
more challenging.27 Particularly, C−C coupling between
alkenylpyridines and terminal alkynes was unprecedented up
to our previous communication.20 In view of the formation of
the butenylpyridine complex 4 as a result of stoichiometric
hydroalkenylation of ethylene (vide supra), we sought to study
the catalytic hydroalkenylation of alkenes with 2-vinylazines.
Preliminary catalyst optimization showed that complex 2b
displayed slightly better catalytic results than 2a; thus it was
used thereafter. The addition of 2-vinylpyridine (9a) or 2-
vinylpyrazine (9b) to alkenes were monitored in Young-Type
NMR tubes containing 0.5 mL of C6D6 and 2b as catalyst. A
1:1 ratio was chosen for the substrates or, alternatively, 2 bar of
cis arrangement of the NHC and the chlorido ligands (Table 1).
Furthermore, the Rh(κ-N,η2-CC′-alkenylpyridine) moiety of 6a
E
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Figure 4. (A, B) Views of the crystal structure of the two rotamers of 6a, refined as complementary components of a static positional disorder. Most
hydrogen atoms are omitted for clarity. Brandl−Weiss parameters (cf. Figure 1) of the C−H···π interactions are C(42a)-H(42a)···π{C(18)-C(19)-
C(20)-C(21)-C(22)-C(23)}, dCX 3.63 Å, α 132.6°, dHpX 0.87 Å; C(39b)-H(39b)···π{C(18)-C(19)-C(20)-C(21)-C(22)-C(23)}, dCX 3.44 Å, α
134.4°, dHpX 0.62 Å. (C) Structure overlay of 6a (both rotamers, purple) and 2a (orange).
Scheme 8. Preparation of RhICl(IPr)(η2-vinylpyrazine)(py)
Complex
Figure 5. 2D long-range 1H−15N-HMQC NMR correlation for 8,
pyridine, and vinylpyrazine (inset boxes).
pressure of the olefin was charged in the NMR tube in the case
of gaseous substrates, ethylene and propylene, which are
involved in this transformation for the first time (Scheme 9,
Scheme 9. Catalytic Hydroalkenylation of Olefins
Table 2). At the end of the reaction, the organic products were
purified by column chromatography. Catalyst 2b efficiently
catalyzed the hydroalkenylation of ethylene with 2-vinyl-
pyridine at 2 mol % loading (entry 1). The main product is
the Z-butenyl derivative which further undergoes a common
metal-mediated olefin isomerization to the compound with E
configuration.28 Increasing of the catalyst loading to 5 mol %
resulted in the selective formation of the thermodynamic
product which has been isolated in 92% yield (entry 2).
Propylene and 2-vinylpyridine reacted slowly to give the linear
(E)-1-pentenyl-pyridine product (entry 3). Notably, the
branched product resulting from Markonikov-type addition
was not detected. However, concomitant to the cross-coupling
product, (E)-1,4-dipyridyl-1-butene arising from the competing
homocoupling of 2-vinylpyridine, was also formed. 3,3-
Dimethyl-1-butene afforded lower conversions with similar
selectivity, whereas styrene was almost unreactive (entries 4
and 5). Dimerization of 2-vinylpyridine was also slow (entry 6).
The reaction of alkenes with 2-vinylpyrazine followed a similar
trend (entries 7−10). It is worth mentioning that alkene
hydroalkenylation reactions catalyzed by RhCl(PPh3)3, Ru-
F
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a
a
Table 2. Olefin Hydroalkenylation with 2-Vinylazines
Table 3. Alkyne Hydroalkenylation with 2-Vinylpyridine
a
a
Reaction conditions: 0.2 mmol of 2-vinylazine, and 2 bar of pressure
Reaction conditions: 0.2 mmol of 2-vinylpyridine, 0.2 mmol of
b
b
or 0.2 mmol of alkene in C6D6 (0.5 mL) at 80 °C. Catalyst loading in
alkyne, and 0.01 mmol of 2b in C6D6 (0.5 mL) at 40 °C. NMR
c
c
mol %. NMR conversion determined from 2-vinylazine consumption.
conversion determined from 2-vinylpyridine consumption. 1E,3Z
d
e
d
e
Isolated yield. (E/Z) styrene/2-vinylpyridine cross-coupling and (E/
isomer. 4-Phenyl. 4-Methyl.
Z) 2-vinylpyridine homocoupling products were observed by GC−MS.
f
(E/Z)-1,4-dipyridyl-1-butene selectivity.
for an intermediate species. According to the alkene hydro-
alkenylation (vide supra), the kinetic products are the 1Z-
butadienyl derivatives (13 and 15), which can be isomerized to
1-E products. The absence of 3Z-butadienyl isomers accounts
for a syn addition of C-H alkenyl over the triple bond of the
alkyne. 4H-Quinolizines derivatives could not be isolated by
column chromatography,20,29d,31c but 3-phenyl-4H-quinolizine
(17a) could be purified by washing with n-hexane due to their
different solubility with regard to butadienylpyridine isomers.
Compound 2b also catalyzed the alkyne hydroalkenylation
reaction with 2-vinylpyrazine (Table 4). The reactions were
slower; thus a higher temperature (60 °C) was required to
achieve acceptable conversions. Formation of the N-bridgehead
heterocycles 6H-pyrido[1,2-a]pyrazines was unambiguously
(H)2(H2)2(IMes)(PCy3), or the catalytic system [RhCl(coe)2]2
+ PR3 required higher catalyst loading (10 mol %) and more
elevated temperatures.6a−e
As we have described in our previous communication,20 the
hydroalkenylation of alkynes promoted by 2b is more efficient
than that of olefins (Scheme 10, Table 3). The catalytic
Scheme 10. Catalytic Hydroalkenylation of Alkynes and
Formation of N-Bridgehead Heterocycles
a
Table 4. Alkyne Hydroalkenylation with 2-Vinylpyrazine
reactions of 2-vinylpyridine with alkynes were performed at 40
°C in C6D6. In addition to several butadienylpyridine isomers
(13−16), 4H-quinolizine derivatives (17) were also observed.
The formation of these heterocycles can be rationalized by a
thermal noncatalyzed 6π-electrocyclization reaction within the
butadienyl fragment and a CN double bond of the pyridine
moiety. Similar rearrangements within diene frameworks have
been observed from imines29 or oximes,30 but dearomatization
of pyridine is much more challenging.31 Catalyst 2b is
chemoselective for the hydroalkenylation, since competitive
dimerization of terminal alkynes to give enyne 18 was not
detected. It is noticeable that only the 1Z,3gem butadienylpyr-
idine isomers 15 rearrange to the corresponding 4H-
quinolizines. This transformation is very fast for aromatic
alkynes; thereby 15 could not be detected. However, the 6π-
electrocyclization is slower for aliphatic alkynes; thus 1Z,3gem
butadienyl isomers were obtained in significant amounts. In
addition, these compounds displayed the typical kinetic profile
a
Reaction conditions: 0.2 mmol of 2-vinylpyrazine, 0.2 mmol of
b
alkyne, and 0.01 mmol of 2b in C6D6 (0.5 mL) at 60 °C. NMR
conversion determined from 2-vinylpyrazine consumption. Calculated
from alkyne consumption. (E)-enyne. 3-Z isomer. 6H-6-methyl-7-
phenylpyrido[1,2-a]pyrazine. 6H-6-phenyl-7-methylpyrido[1,2-a]-
c
d
e
f
g
pyrazine.
G
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Article
1
determined by NMR, particularly the 2D long-range H−15N-
HMQC experiment (Figure 6). As a drawback, gem-enynes
Figure 7. Isomerization of butadienyl- to 2,4-pentadienyl-heterocycles
via an N-bridgehead heterocyclic intermediate. Optimized structures
and relative energies for pyridine-based derivatives (ΔG, 353 K,
toluene).
Figure 6. 2D long-range H−15N-HMQC NMR correlation for 6,7-
1
A mechanistic proposal for the formation of N-bridgehead
heterocycles is presented in Scheme 11. The first step consists
in the C(sp2)-H activation of the vinylazine directed by the
coordination of the nitrogen atom to form a hydride-alkenyl-
RhIII intermediate. Although we fail to unambiguously detect
such species in Rh−NHC systems, we have recently reported
the structural characterization of a related phosphine-based Rh-
hydride-alkenyl derivative.32 Subsequent insertion of the alkyne
can occur into the Rh−H or the Rh−alkenyl bond with 1,2 or
2,1 regioselectivity. Migratory insertion in the Rh−H bond is
more likely, although alkyne insertion into Rh−alkenylpyridine
bond has also been described.33 Then, reductive elimination
within the coordination sphere of the metal will result in
butadienyl heterocycle derivatives which can then undergo,
depending on the substitution pattern, a thermal metal-free 6π-
electrocyclization process to yield the N-bridgehead hetero-
cycles. Substitution on the 3-position of the butadienyl
fragment as well as a cisoidal configuration seems to be a
prerequisite for efficient cyclization; thus 4-substituted
derivatives or 1-E,3gem isomerized products do not rearrange.
diethyl-6H-pyrido[1,2-a]pyrazine.
(18) were obtained as byproducts, as a result of the
homodimerization of the terminal alkyne, particularly for
phenylacetylene (entry 1). For aliphatic alkynes such as 1-
hexyne or cyclopropylacetylene, dimerization was considerably
reduced and formation of the fused heterocycle reached around
30% selectivity (entries 2 and 3). The bulky tert-butyl- and
trimethylsilylacetylene reacted slowly. In these cases, the E-
enyne was mainly obtained as byproduct as described
previously with related Rh−NHC catalysts.21 Formation of
the N-bridgehead heterocycles 6H-pyrido[1,2-a]pyrazine was
favored for internal alkynes reaching up to 85% selectivity for 3-
hexyne (entry 6), which can be purified by precipitation with n-
hexane. Diphenylacetylene displayed almost half selectivity
between the butadienyl- and the N-bridgehead heterocycle
(entry 7). Both regioisomeric fused heterocycles were obtained
for 1-phenylpropyne in a 74:9 ratio with 6H-6-methyl-7-
phenylpyrido[1,2-a]pyrazine as the major isomer (entry 8).
The formation of N-bridgehead heterocycles entails the
intriguing dearomatization of pyridine within a butadienylpyr-
idine fragment. It would be expected that conjugation of two
double bonds with an aromatic heterocyclic fragment provided
stability to the butadienyl-heterocyclic derivatives. DFT
calculations on ground state minimum energy confirmed the
experimental results (Figure 7; see the Supporting Information
for pyrazine-based derivatives). The fused heterocycles are 0.8
(py, 17j) and 2.2 (pz, 17g) kcal mol−1 more stable than the
opened compounds 13. Likely, a substituent on position 3 of
the butadienyl backbone hinders the coplanarity of the
pyridinic ring with the two double bonds, thus enforcing a
helical disposition and hampering conjugation. Moreover, N-
bridge heterocycles 17 are not very stable either. Heating a
toluene solution of 17 at 80 °C for 12 h yielded 2-(3-ethyl-2,4-
hexadienyl)heterocycle {pyridine (19), pyrazine (20)} as a
mixture of (2Z,4E):(2E,4E) isomers (71:29 for 19, 74:26 for
20) as a result of a formal isomerization of two double bonds
within 13.6c,28 Although pyridine and the new pentadienyl
fragment are not mutually conjugated in 19−20, rearomatiza-
tion of pyridine and conjugation of two double bonds likely
resulted in net stabilization of 19 with regard to 17. Theoretical
calculations confirm this point, since the 2,4-pentadienyl
derivatives lie 3.2 (py, 19a) and 1.3 (pz, 20a) kcal mol−1
below 17.
CONCLUSION
■
An efficient Rh−NHC catalyst for alkene and alkyne hydro-
alkenylation with vinylazines is presented. Key organometallic
intermediates have been characterized. 2-Vinylpyridine coor-
dinates to a RhI-NHC fragment in a typical chelate fashion.
However, the high strain of the metallacycle, reflected by the
short bite angle as well as the observed pitch and yaw angles,
allows for its opening by substitution of the pyridine ring with
PEt3. In addition, reaction with π-donor ligands such as olefins
or alkynes gave κ-N,η2-2-(butenyl)pyridine or 2-(butadienyl)-
pyridine rhodium complexes as a result of the stoichiometric
C−C coupling by formal hydroalkenylation of both types of
substrates with 2-vinylpyridine. CH···π interactions involving
one aromatic ring of the wingtip of the NHC ligand provide
additional stability to the complexes. Notably, an olefinic and
an allylic proton participate in the CH···π interaction in the
vinylpyridine and the butenylpyridine complexes, respectively.
This bonding scheme results in an opposite tilting of the NHC
reflecting its flexibility as a ligand.
The NHC ligand plays an essential role in the catalytic
activity, since catalyst loading (5 mol %) and temperature
reaction (80 °C) for olefin hydroalkenylations are reduced with
regard to previous Rh-phosphino catalysts. Notably, gaseous
H
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Scheme 11. Mechanistic Proposal for the Formation of N-Bridgehead Heterocycles
vacuo. Yield: 250 mg (84%). Anal. Calcd for C34H43N3ClRh: C, 64.61;
H, 6.86; N, 6.65. Found: C, 64.92; H, 6.89; N, 6.62. H NMR (400
alkenes and 2-vinylpyrazine participate in this catalytic
transformation for the first time. Alkynes react faster than
olefins to form butadienyl-heterocycle derivatives. The 1Z,3gem
isomers undergo a thermal noncatalyzed 6π-electrocyclization
to yield 4H-quinolizines or 6H-pyrido[1,2-a]pyrazines. As
revealed by DFT calculations, fused heterocycles are more
stable than the butadienyl derivatives, in spite of the
dearomatization of the nitrogenated ring. The steric demand
of the substituent in position 3 of the butadienyl fragment
hampers the coplanarity of the pyridinic ring and the two
double bonds, thus accounting for the lack of conjugation.
Moreover, if an alkyl substituent is located in position 3, the N-
bridgehead heterocycle can be reopened, yielding 2,4-
pentadienyl-heterocycle derivatives in which rearomatization
of the nitrogenated ring and unconnected conjugation of two
double bonds account for their higher stability.
1
MHz, C6D6, 298 K): δ 8.09 (d, JH‑H = 5.3, 1H, H6‑py), 7.27 (t, JH‑H
=
7.6, 2H, Hp‑Ph‑IPr), 7.2 (m, 4H, Hm‑Ph‑IPr), 6.55 (s, 2H, CHN), 6.49
(dd, JH‑H = 7.8, 7.4, 1H, H4‑py), 6.09 (d, JH‑H = 7.8, 1H, H3‑py), 5.94
(dd, JH‑H = 7.4, 5.3, 1H, H5‑py), 4.02 and 2.96 (both br, 4H, CHMeIPr),
3.28 (dd, JH‑H = 9.8, JH‑Rh = 2.9, 1H, CHCH2), 2.73 (d, JH‑H = 6.7,
1H, CHCH2), 2.12 (dd, JH‑H = 9.8, 6.7, 1H, CHCH2), 1.11 and
1.06 (both d, JH‑H = 7.12, 24H, CHMeIPr). 13C{1H}-APT NMR (100.4
MHz, C6D6, 298 K): δ 183.3 (d, JC‑Rh = 57.4, Rh-CIPr), 168.2 (d, JC‑Rh
= 4.7, C2‑Py), 147.8 (s, C6‑py), 147.3 and 146.5 (both s, Cq‑IPr), 137.3 (s,
CqN), 134.4 (s, C4‑py), 129.6 (s, Cp‑Ph‑IPr), 124.1 and 124.0 (both s,
Cm‑Ph), 123.9 (s, CHN), 119.4 (s, C5‑py), 116.3 (s, C3‑py), 46.4 (d,
JC‑Rh = 12.3, CH2CH), 45.3 (d, JC‑Rh = 16.0, CH2CH), 29.2 and
29.0 (both s, CHMeIPr), 26.8, 25.8, 23.7, and 23.2 (all s, CHMeIPr).
1H−15N HMQC NMR (40.5 MHz, toluene-d8, 243 K): δ 249.2 (Npy),
191.0 (NIPr).
Preparation of RhCl(IMes)(κ-N,η2-CH2CH-C5H4N) (2b). The
complex was prepared as described for 2a starting from 1b (300 mg,
0.271 mmol) and 2-vinylpyridine (58 μL, 0.542 mmol). Yield: 260 mg
(87%). Anal. Calcd for C28H31N3ClRh: C, 61.38; H, 5.70; N, 7.67.
Found: C, 61.05; H, 5.80; N, 7.46. 1H NMR (400 MHz, C6D6, 298 K):
δ 8.16 (d, JH‑H = 5.3, 1H, H6‑py), 6.94 and 6.87 (both s, 4H, Hm‑Ph),
6.56 (dd, JH‑H = 7.7, 7.7, 1H, H4‑py), 6.30 (s, 2H, CHN), 6.22 (d,
JH‑H = 7.7, 1H, H3‑py), 6.01 (ddd, JH‑H = 7.7, 5.3, 1H, H5‑py), 3.39 (dd,
JH‑H = 9.6, JH‑Rh = 3.2, 1H, CHCH2), 2.90 (d, JH‑H = 6.9, 1H, CH
CH2), 2.72 and 2.24 (both s, 18H, MeIMes), 2.13 (dd, JH‑H = 9.6, 6.9,
1H, CHCH2). 13C{1H}-APT NMR (100.4 MHz, C6D6, 298 K): δ
181.1 (d, JC‑Rh = 57.1, Rh-CIMes), 168.1 (d, JC‑Rh = 4.4, C2‑py), 147.3 (s,
C6‑py), 137.9 and 137.1 (both s, Cq‑IMes), 136.2 (s, CqN), 134.6 (s,
C4‑py), 129.3 and 128.7 (both s, Cm‑Ph), 122.4 (s, CHN), 119.4 (s,
C5‑py), 116.5 (s, C3‑py), 46.0 (d, JC‑Rh = 15.3, CH2CH), 45.8 (d, JC‑Rh
= 12.4, CH2CH), 20.8 and 19.4 (both s, MeIMes).
EXPERIMENTAL SECTION
■
General Considerations. All reactions were carried out with
rigorous exclusion of air using Schlenk-tube techniques. The reagents
were purchased from commercial sources and were used as received,
except for phenylacetylene and 2-vinylpyridine that were distilled and
stored over molecular sieves. Organic solvents were dried by standard
procedures and distilled under argon prior to use or obtained oxygen-
and water-free from a Solvent Purification System (Innovative
Technologies). The organometallic precursors [Rh(μ-Cl)(NHC)(η2-
coe)]2 (1a,b)34 were prepared as previously described in the literature.
Chemical shifts (expressed in parts per million) are referenced to
residual solvent peaks (1H, 13C{1H}) or external H3PO4 (31P) and
liquid NH3 (15N). Coupling constants, J, are given in Hz. Spectral
assignments were achieved by combination of H−1H COSY, 13C-
1
In Situ Formation of RhCl(IPr){η2-CH2CH(C5H4N)}(PEt3) (3).
A solution of 2a (30 mg, 0.023 mmol) in toluene-d8 (0.5 mL, NMR
tube) at 243 K was treated with triethylphosphine (4 μL, 0.060 mmol).
1
{1H}-APT, and H−13C HSQC/HMBC experiments. C, H, and N
analyses were carried out in a PerkinElmer 2400 CHNS/O analyzer.
High-resolution electrospray mass spectra (HRMS) were acquired
using a MicroTOF-Q hybrid quadrupole time-of-flight spectrometer
(Bruker Daltonics, Bremen, Germany). GC−MS analyses were
recorded on an Agilent 5973 mass selective detector interfaced to an
Agilent 6890 series gas chromatograph system, using an HP-5MS 5%
phenyl methyl siloxane column (30 m × 250 mm with a 0.25 mm film
thickness).
1
The spectra were recorded immediately at low temperature. H NMR
(300 MHz, toluene-d8, 243 K): δ 8.07 (d, JH‑H = 4.4, 1H, H6‑py), 7.42
and 7.33 (both t, JH‑H = 7.6, 2H, Hp‑IPr), 7.08 and 7.0 (both d, JH‑H
=
7.6, 4H, Hm‑IPr), 6.96 (dd, JH‑H = 7.6, 6.0, 1H, H4‑py), 6.60 and 6.54
(both s, 2H, CHN), 6.49 (dd, JH‑H = 6.0, 4.4, 1H, H5‑py), 6.14 (d,
JH‑H = 7.6, 1H, H3‑py), 4.89 (dd, JH‑H = 8.9, 7.7, 1H, CHCH2), 4.65,
Preparation of RhCl(IPr)(κ-N,η2-CH2CH-C5H4N) (2a). A
yellow solution of 1a (300 mg, 0.235 mmol) in 10 mL of toluene
was treated with 2-vinylpyridine (50 μL, 0.470 mmol) and was stirred
at room temperature for 1 h. After filtration through Celite, the solvent
was evaporated to dryness. Addition of hexane caused precipitation of
a yellow solid, which was washed with hexane (3 × 4 mL) and dried in
3.74, 2.99, and 2.40 (all sept, JH‑H = 6.6, 4H, CHMeIPr), 2.97 (d, JH‑H
=
8.9, 1H, CHCH2), 2.58 (d, JH‑H = 7.7, 1H, CHCH2), 1.78, 1.66,
1.56, 1.21, 1.16, 1.11, 1.04, and 0.96 (all d, JH‑H = 6.6, 24H, CHMeIPr),
1.40 (dq, JH‑P = 7.8, JH‑H = 7.6, 6H, PCH2CH3), 0.65 (dt, JH‑P = 14.2,
JH‑H = 7.6, 9H, PCH2CH3). 13C{1H}-APT NMR (75 MHz, toluene-d8,
243 K): δ 187.8 (dd, JC‑P = 136.7, JC‑Rh = 43.9, Rh-CIPr), 165.7 (s,
I
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C2‑py), 148.6, 147.8, 146.1, and 145.6 (all s, Cq‑Ph), 148.1 (s, C6‑py),
137.4 and 137.0 (both s, CqN), 134.7 (s, C4‑py), 129.8 and 129.5 (both
s, Cp‑IPr), 125.3, 124.6, 123.8, and 122.6 (all s, Cm‑IPr), 125.2 (s, C3‑py),
(m, 2H, CH2CH), 1.66, 1.58, 1.18, and 1.15 (all d, JH‑H = 6.6, 24H,
CHMeIPr), 1.09 (overlapped, 3H, CH3CH2Cq), 0.62 (t, JH‑H = 6.7,
3H, CH3CH2CH). 13C{1H}-APT NMR (100.4 MHz, C6D6, 298
K): δ 184.2 (d, JC‑Rh = 57.9, Rh-CIPr), 166.1 (s, C2‑py), 147.0 and 146.7
(both s, Cq‑IPr), 145.6 (s, C6‑py), 139.9 {s, (Et)Cq}, 137.5 (s, CqN),
134.2 (s, C4‑py), 131.9 (s, (Et)CH), 129.6 (s, Cp‑IPr), 124.4 and
123.7 (both s, Cm‑IPr), 123.9 (s, CHN), 118.5 (s, C5‑py), 117.2 (s,
124.6 and 123.7 (both s, CHN), 119.3 (s, C5‑py), 53.9 (d, JC‑Rh
=
15.2, CHCH2), 30.0 (d, JC‑Rh = 14.7, CHCH2), 29.3, 28.9, 28.8,
and 28.4 (all s, CHMeIPr), 27.2, 26.7, 26.6, 26.2, 23.8, 23.3, 23.2, and
23.1 (all s, CHMeIPr), 13.7 (d, JC‑P = 22.0, PCH2CH3), 7.86 (s,
PCH2CH3). 31P{1H} NMR (121.4 MHz, toluene-d8, 243 K): 16.1 (d,
JP‑Rh = 121.0). 1H−15N HMQC NMR (40.5 MHz, toluene-d8, 243 K):
δ 305.5 (Npy), 193.1 and 190.2 (NIPr).
C3‑py), 64.8 (d, JC‑Rh = 21.0, CH(CEt)CH(py)), 43.0 (d, JC‑Rh
=
11.3, CH(CEt)CH(py)), 29.0 and 28.7 (both s, CHMeIPr), 26.4,
26.2, 23.0, and 22.8 (all s, CHMeIPr), 24.9 (s, CH3CH2Cq), 21.6 (s,
CH3CH2CH), 14.6 (s, CH3CH2Cq), 13.8 (s, CH3CH2CH).
Preparation of RhCl(IMes)[κ-N,η2-(Z)-CH{(E)-C(Et)CH(Et)}
CH(C5H4N)] (6b). A solution of 2b (100 mg, 0.18 mmol) in 10 mL of
toluene was treated with 3-hexyne (23 μL, 0.20 mmol) and stirred at
313 K for 12 h. After filtration through Celite, the solvent was
evaporated to dryness. Addition of hexane induced the precipitation of
a yellow solid, which was washed with hexane (3 × 4 mL) and dried in
vacuo. Yield: 100 mg (88%). Anal. Calcd for C34H41N3ClRh: C, 64.81;
In Situ Formation of RhCl(IPr){κ-N,η2-(Z)-CH(Et)-CH(C5H4N)}
(4). A solution of 2a (30 mg, 0.023 mmol) in C6D6 (0.5 mL, NMR
tube) was treated with ethylene (2 bar) and was heated at 353 K for 2
1
h. H NMR (400 MHz, C6D6, 298 K): δ 8.14 (d, JH‑H = 5.4, 1H,
H6‑py), 7.3−7.1 (m, 6H, HPh‑IPr), 6.63 (s, 2H, CHN), 6.60 (dd, JH‑H
= 7.7, 7.4, 1H, H4‑py), 6.21 (d, JH‑H = 7.7, 1H, H3‑py), 5.94 (dd, JH‑H
=
7.4, 5.4, 1H, H5‑py), 3.53 and 3.42 (both br, 4H, CHMeIPr), 3.33 {ddd,
JH‑H = 10.4, 7.2, JH‑Rh = 3.4, 1H, CH(Et)CH(py)}, 2.51 {d, JH‑H
=
1
7.2, 1H, CH(Et)CH(py)}, 1.95 and 1.22 (both m, 2H, CH2CH3),
1.59, 1.54, 1.09, and 1.07 (all d, JH‑H = 6.8, 24H, CHMeIPr), 0.75 (t,
JH‑H = 7.2, 3H, CH2CH3). 13C{1H}-APT NMR (125.6 MHz, C6D6,
298 K): δ 184.5 (d, JC‑Rh = 57.0, Rh-CIPr), 165.0 (br, C2‑py), 146.1 (s,
C6‑py), 146.9 and 146.1 (both s, Cq‑IPr), 137.7 (s, CqN), 134.5 (s,
C4‑py), 129.5 (s, Cp‑IPr), 124.2 (s, CHN), 123.9 and 123.8 (both s,
Cm‑IPr), 119.0 (s, C3‑py), 117.2 (s, C5‑py), 66.6 {d, JC‑Rh = 18.5,
CH(Et)CH(py)}, 43.9 {d, JC‑Rh = 12.3, CH(Et)CH(py)}, 28.9
and 28.7 (both s, CHMeIPr), 26.2, 26.0, 23.2, and 23.0 (all s,
CHMeIPr), 26.5 (s, CH2CH3), 16.1 (s, CH2CH3).
H, 6.56; N, 6.67. Found: C, 65.03; H, 6.43; N, 6.51. H NMR (400
MHz, C6D6, 298 K): δ 8.27 (d, JH‑H = 5.4, 1H, H6‑py), 6.96 and 6.89
(both s, 4H, Hm‑IMes), 6.62 (dd, JH‑H = 7.9, 7.1, 1H, H4‑py), 6.27 (s, 2H,
CHN), 6.24 (d, JH‑H = 7.9, 1H, H3‑py), 6.15 (t, JH‑H = 7.0, 1H,
CH2CH), 6.04 (dd, JH‑H = 7.1, 5.4, 1H, H5‑py), 3.98 {d, JH‑H = 6.7,
1H, CH(CEt)CH(py)}, 2.84 {d, JH‑H = 6.7, 1H, CH(CEt)
CH(py)}, 2.60, 2.43, and 2.25 (all s, 18H, MeIMes), 2.03 (m, 2H,
CH3CH2Cq), 1.85 (m, 2H, CH3CH2CH), 1.17 (t, JH‑H = 7.0, 3H,
CH3CH2Cq), 0.75 (t, JH‑H = 7.6, 3H, CH3CH2Cq). 13C{1H}-APT
NMR (100.4 MHz, C6D6, 298 K): δ 182.2 (d, JC‑Rh = 60.4, Rh-CIMes),
166.3 (s, C2‑py), 145.6 (s, C6‑py), 139.8 {s, (Et)Cq}, 138.1 (s,
Cq‑IMes), 137.6 (s, CqN), 134.4 (s, C4‑py), 132.9 {s, (Et)CH}, 129.4
and 129.0 (both s, Cm‑IMes), 122.9 (s, CHN), 119.5 (s, C5‑py), 117.6
(s, C3‑py), 65.8 {d, JC‑Rh = 18.3, CH(CEt)CH(py)}, 43.1 {d, JC‑Rh
= 13.2, CH(CEt)CH(py)}, 25.4 (s, CH3CH2Cq), 21.8 (s,
CH3CH2CH), 21.1, 19.3, and 19.1 (all s, MeIMes), 14.7 (s,
CH3CH2Cq), 14.3 (s, CH3CH2CH).
Preparation of RhCl(IPr){κ-N,η2-(Z)-CH(CH2CH2Ph)CH-
(C5H4N)} (5). A solution of 2a (100 mg, 0.16 mmol) in 10 mL of
toluene was treated with styrene (20 μL, 0.17 mmol) and heated at
353 K for 48 h. After filtration through Celite, the solvent was
evaporated to dryness. Addition of hexane induced the precipitation of
an orange solid, which was washed with hexane (3 × 4 mL) and dried
in vacuo. Yield: 68 mg (58%). Satisfactory elemental analysis could not
be obtained. HRMS (ESI) m/z Calcd for C42H51N3Rh (M − Cl−):
Preparation of RhCl(IPr)[κ-N,η2-(Z)-CH{(Z)-C(Ph)CH(Ph)}
CH(C5H4N)] (7a). The complex was prepared as described for 6a
starting from 2a (100 mg, 0.16 mmol) and diphenylacetylene (32 mg,
0.18 mmol). Yield: 91 mg (70%). Anal. Calcd for C48H53N3ClRh: C,
1
700.3138. Found: 700.3133. H NMR (400 MHz, C6D6, 298 K): δ
8.21 (d, JH‑H = 4.7, 1H, H6‑py), 7.33 (t, JH‑H = 7.2, 2H, Hp‑IPr), 7.28 (d,
JH‑H = 7.2, 4H, Hm‑IPr), 7.16 (dd, JH‑H = 8.4, 7.7, 2H, Hm‑Ph), 7.09 (t,
JH‑H = 7.7, 1H, Hp‑Ph), 6.99 (d, JH‑H = 8.4, 2H, Ho‑Ph), 6.71 (s, 2H, 
CHN), 6.61 (dd, JH‑H = 8.1, 6.8, 1H, H4‑py), 6.09 (d, JH‑H = 8.1, 1H,
H3‑py), 5.98 (dd, JH‑H = 6.1, 4.7, 1H, H5‑py), 3.60 and 3.55 (both sept,
JH‑H = 6.7, 4H, CHMeIPr), 3.43 {ddd, JH‑H = 10.4, 7.2, JH‑Rh = 3.4, 1H,
CH2CHCH(py)}, 2.70 {d, JH‑H = 7.2, 1H, CH2CHCH(py)},
2.68 and 2.35 (both m, 2H, CH2Ph), 2.34 and 1.57 (both m, 2H
CH2CH), 1.71, 1.66, 1.21, and 1.17 (all d, JH‑H = 6.7, 24H,
CHMeIPr). 13C{1H}-APT NMR (125.6 MHz, C6D6, 298 K): δ 184.6
(d, JC‑Rh = 59.0, Rh-CIPr), 165.4 (d, JC‑Rh = 4.5, C2‑py), 146.9 and 146.7
(both br, Cq‑IPr), 146.1 (s, C6‑py), 142.5 (s, Cq‑Ph), 137.9 (s, CqN),
134.6 (s, C4‑py), 129.8 (s, Cp‑IPr), 128.8 (s, Co‑Ph), 128.3 (s, Cm‑Ph),
125.7 (s, Cp‑Ph), 124.5 (s, CHN), 124.2 and 124.0 (both s, Cm‑IPr),
119.2 (s, C5‑py), 117.5 (s, C3‑py), 64.1 {d, JC‑Rh = 18.4, CH2CH
CH(py)}, 44.6 {d, JC‑Rh = 12.4, CH2CHCH(py)}, 38.4 (s, CH2Ph),
35.5 (s, CH2CH), 29.1 and 29.1(both s, CHMeIPr), 26.6, 26.2, 23.3,
and 23.2 (all s, CHMeIPr).
1
71.15; H, 6.59; N, 5.19. Found: C, 71.30; H, 6.88; N, 5.07. H NMR
(400 MHz, C6D6, 298 K): δ 8.08 (d, JH‑H = 4.8, 1H, H6‑py), 7.65 (s,
1H, CHPh), 7.23 (t, JH‑H = 8.0, 2H, Hp‑IPr), 7.20 and 7.14 (both d,
JH‑H = 8.0, 4H, Hm‑IPr), 6.97 and 6.89 (both d, JH‑H = 8.2, 4H, Ho‑Ph),
6.87 (dd, JH‑H = 8.2, 7.7, 4H, Hm‑Ph), 6.86 and 6.81 (both t, JH‑H = 7.7,
2H, Hp‑Ph), 6.62 (s, 2H, CHN), 6.36 (dd, JH‑H = 7.9, 7.2, 1H, H4‑py),
5.92 (d, JH‑H = 7.9, 1H, H3‑py), 5.81 (dd, JH‑H = 7.2, 4.8, 1H, H5‑py),
4.39 {dd, JH‑H = 7.5, JH‑Rh = 1.3, 1H, CH(CPh)CH(py)}, 3.75 and
3.30 (both d, JH‑H = 6.7, 4H, CHMeIPr), 2.43 {d, JH‑H = 7.5, 1H,
CH(CPh)CH(py)}, 1.59, 1.50, 1.08, and 1.05 (all d, JH‑H = 6.7,
24H, CHMeIPr). 13C{1H}-APT NMR (100.4 MHz, C6D6, 298 K): δ
183.5 (d, JC‑Rh = 57.5, Rh-CIPr), 165.7 (d, JC‑Rh = 4.6, C2‑py), 147.1 and
146.9 (both s, Cq‑IPr), 146.0 (s, C6‑py), 143.7 and 138.7 (both s, Cq‑Ph),
142.7 (s, PhCq), 137.7 (s, CqN), 134.9 (s, PhCH), 134.5 (s,
C4‑py), 129.9 (s, Cp‑IPr), 129.4 and 129.4 (both s, Co‑Ph), 128.1 and
128.0 (both s, Cm‑Ph), 126.2 and 126.1 (both s, Cp‑IPr), 124.6 (s, 
CHN), 124.2 and 124.1 (both s, Cm‑IPr), 119.5 (s, C5‑py), 117.5 (s,
Preparation of RhCl(IPr)[κ-N,η2-(Z)-CH{(E)-C(Et)CH(Et)}
CH(C5H4N)] (6a). A solution of 2a (100 mg, 0.16 mmol) in 10 mL
of toluene was treated with 3-hexyne (20 μL, 0.18 mmol) and stirred
at 333 K for 12 h. After filtration through Celite, the solvent was
evaporated to dryness. Addition of hexane induced the precipitation of
an orange solid, which was washed with hexane (3 × 4 mL) and dried
in vacuo. Yield: 105 mg (92%). Anal. Calcd for C40H53N31ClRh: C,
67.27; H, 7.48; N, 5.88. Found: C, 67.22; H, 7.25; N, 5.71. H NMR
(400 MHz, C6D6, 298 K): δ 8.24 (d, JH‑H = 5.1, 1H, H6‑py), 7.40 (t,
JH‑H = 7.6, 2H, Hp‑IPr), 7.30 (d, JH‑H = 7.6, 4H, Hm‑IPr), 6.75 (s, 2H, 
CHN), 6.70 (dd, JH‑H = 7.4, 7.3, 1H, H4‑py), 6.27 (d, JH‑H = 7.3, 1H,
H3‑py), 6.13 (dd, JH‑H = 7.4, 5.1, 1H, H5‑py), 5.82 (t, JH‑H = 6.4, 1H,
CH2CH), 4.06 {d, JH‑H = 6.0, 1H, CH(CEt)CH(py)}, 3.75
and 3.37 (both sept, JH‑H = 6.6, 4H, CHMeIPr), 2.51 {d, JH‑H = 6.0, 1H,
CH(CEt)CH(py)}, 2.07 and 1.86 (both m, 2H, CH2Cq), 1.77
C3‑py), 65.8 {d, JC‑Rh = 18.3, CH(CPh)CH(py)}, 42.6 {d, JC‑Rh
=
11.8, CH(CPh)CH(py)}, 29.3 and 29.1 (both s, CHMeIPr), 26.6,
26.4, 23.4, and 23.1 (all s, CHMeIPr).
Preparation of RhCl(IMes)[κ-N,η2-(Z)CH{(Z)C(Ph)CH(Ph)}
CH(C5H4N)] (7b). The complex was prepared as described for 6b
starting from 2b (100 mg, 0.18 mmol) and diphenylacetylene (36 mg,
0.20 mmol). Yield: 82 mg (63%). Anal. Calcd for C42H41N3ClRh: C,
1
69.47; H, 5.69; N, 5.79. Found: C, 69.75; H, 5.93; N, 5.54. H NMR
(400 MHz, C6D6, 298 K): δ 8.06 (d, JH‑H = 4.3, 1H, H6‑py), 7.85 (s,
1H, CHPh), 7.05 and 6.88 (both d, JH‑H = 8.1, 4H, Ho‑Ph), 6.95 and
6.89 (both dd, JH‑H = 8.1, 7.6, 4H, Hm‑Ph), 6.93 and 6.84 (both t, JH‑H
7.6, 2H, Hp‑Ph), 6.84 and 6.74 (both s, 4H, Hm‑IMes), 6.36 (dd, JH‑H
=
=
7.9, 7.6, 1H, H4‑py), 6.22 (s, 2H, CHN), 5.99 (d, JH‑H = 7.9, 1H,
H3‑py), 5.78 (dd, JH‑H = 7.6, 4.9, 1H, H5‑py), 4.26 {dd, JH‑H = 7.6, JH‑Rh
=
J
DOI: 10.1021/acs.organomet.8b00149
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
1.7, 1H, CH(CPh)CH(py)}, 2.85 {d, JH‑H = 7.6, JH‑Rh = 1.2, 1H,
CH(CPh)CH(py)}, 2.51, 2.32, and 2.08 (all s, 18H, MeIMes).
Crystal Data for 4. C36H47ClN3Rh, M = 660.12 g mol−1; T =
100(2) K; monoclinic; P21/n, a = 10.3033(5) Å, b = 29.2535(14) Å, c
= 11.0594(5) Å, β = 96.2640(10)°, V = 3313.5(3) Å3; Z = 4; Dcalc
=
13C{1H}-APT NMR (100.4 MHz, C6D6, 298 K): δ 181.4 (d, JC‑Rh
=
1.323 Mg m−3; μ = 0.624 mm−1; red prism, 0.190 × 0.120 × 0.090
mm3; θmin/θmax 1.392/26.371°; reflections collected/unique 71588/
6769 [R(int) = 0.0436]; data/restraints/parameters 6769/0/379; GoF
= 1.064; R1 = 0.0267 [I > 2σ(I)], wR2 = 0.0631 (all data); largest diff.
peak/hole 0.438/−0.355 e Å−3.
55.7, Rh-CIMes), 165.2 (d, JC‑Rh = 4.8, C2‑py), 145.3 (s, C6‑py), 142.6 (s,
PhCq), 138.3 and 138.0 (both s, Cq‑Ph), 137.9 (s, Cq‑IMes), 137.0 (s,
CqN), 134.9 (s, PhCH), 134.2 (s, C4‑py), 129.3 and 128.8 (both s,
Cm‑IMes), 129.2 and 128.9 (both s, Co‑Ph), 127.9 and 127.6 (both s,
Cm‑Ph), 126.0 and 125.7 (both s, Cp‑Ph), 122.7 (s, CHN), 119.4 (s,
C5‑py), 117.4 (s, C3‑py), 66.8 {d, JC‑Rh = 18.3, CH(CPh)CH(py)},
42.0 {d, JC‑Rh = 12.3, CH(CPh)CH(py)}, 20.7, 18.9, and 18.8 (all s,
MeIMes).
Crystal Data for 6a. C40H53ClN3Rh, M = 714.21 g mol−1; T =
170(2) K, monoclinic, C2/c, a = 39.994(14) Å, b = 10.187(4) Å, c =
18.141(6) Å, β = 93.078(5)°, V = 7380(4) Å3; Z = 8; Dcalc = 1.286 Mg
m−3; μ = 0.566 mm−1; red prism, 0.190 × 0.080 × 0.035 mm3; θmin
/
In Situ Formation of RhCl(IPr){η2-CH2CH(C4H3N2)}(py) (8).
A solution of 2a (30 mg, 0.023 mmol) in toluene-d8 (0.5 mL, NMR
tube) was treated with 2-vinylpyrazine (5 μL, 0.049 mmol) and
pyridine (6 μL, 0.070 mmol) at 253 K. The spectra were recorded
immediately at low temperature. 1H NMR (500 MHz, toluene-d8, 253
θmax 2.040/25.350°; reflections collected/unique 29939/6748 [R(int)
= 0.0608]; data/restraints/parameters 6748/12/422; GoF = 1.054; R1
= 0.0556 [I > 2σ(I)], wR2 = 0.1603 (all data); largest diff. peak/hole
2.882/−1.020 e Å−3.
Computational Details. All calculations were performed with the
Gaussian09 package40 at the M06-2X level41 and the Dunning-type
basis sets cc-pVTZ.42 Full optimizations of geometry without any
constraint were performed, followed by analytical computation of the
Hessian matrix to confirm the nature of the stationary points as
minima on the potential energy surface. Gibbs free energies at 353 K
were calculated on the basis of the rigid rotor-harmonic oscillator
approximation. The solvent effects (toluene) were calculated by the
SMD method.43
K): 7.86 (d, JH‑H = 5.6, 2H, H2‑py), 7.71 (s, 1H, H3‑pz), 7.62 (d, JH‑H
=
2.2, 1H, H6‑pz), 7.4−7.1 (6H, HPh), 7.00 (d, JH‑H = 2.2, 1H, H5‑pz), 6.68
and 6.60 (both br, 2H, CHN), 6.36 (t, JH‑H = 7.1, 1H, H4‑py), 5.88
(dd, JH‑H = 7.1, 5.6, 2H, H3‑py), 4.63, 3.71, 3.05, and 2.40 (all sept, JH‑H
= 6.6, 4H, CHMeIPr), 4.53 (dd, JH‑H = 10.2, 7.2, 1H, CHCH2), 3.10
(d, JH‑H = 10.2, 1H, CHCH2), 2.86 (d, JH‑H = 7.2, 1H, CHCH2),
1.88, 1.70, 1.67, 1.25, 1.19, 1.14, 1.08, and 1.06 (all d, JH‑H = 6.6, 24H,
CHMeIPr). 13C{1H} NMR (100.4 MHz, toluene-d8, 253 K): δ 181.3
(d, JC‑Rh = 52.6, Rh-CIPr), 161.3 (s, C2‑pz), 151.8 (s, C2‑py), 148.6, 148.5,
145.8, and 145.7 (all s, Cq‑IPr), 146.2 (s, C3‑pz), 142.8 (s, C5‑pz), 138.3
(s, C6‑pz), 134.1 (s, C4‑py), 130.4 and 130.0 (both s, Cp‑IPr), 125.1,
124.6, 123.6, and 122.5 (all s, Cm‑IPr), 124.7 and 123.8 (both s, 
CHN), 122.7 (s, C3‑py), 50.4 (d, JC‑Rh = 15.7, CHCH2), 35.0 (d,
JC‑Rh = 16.5, CHCH2), 29.2, 29.1, 28.8, and 28.7 (all s, CHMeIPr),
27.0, 26.6, 26.5, 26.0, 23.8, 23.7, 22.6, and 22.4 (all s, CHMeIPr).
1H−15N HMQC NMR (40.5 MHz, toluene-d8, 233 K): δ 331.6
(N4‑pz), 319.4 (N1‑pz), 269.5 (Npy), 193.2 and 192.7 (NIPr).
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.8b00149.
NMR data for organometallic and organic products
Standard Catalytic Reactions. An NMR tube containing a
solution of 0.01 mmol of catalyst in 0.5 mL of C6D6 under an argon
atmosphere was treated with 0.20 mmol of heteroarene and 0.20 mmol
of substrate or pressurized to 2 bar of gas (ethylene and propene).
Then, the NMR tube was sealed under an argon atmosphere and
heated at the corresponding temperature for the determined time. The
(PDF)
Optimized coordinates for the computed compounds
(XYZ)
Accession Codes
CCDC 1825029 and 1825032−1825033 contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge via www.ccdc.cam.ac.uk/data_request/
cif, or by emailing data_request@ccdc.cam.ac.uk, or by
contacting The Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
1
reaction course was monitored by H NMR, and the conversion and
selectivities were determined by integration of the corresponding
resonances of the 2-vinylazine and the reaction products.
1
6,7-Diethyl-6H-pyrido[1,2-a]pyrazine. H NMR (500 MHz, C6D6,
298 K): δ 7.61 (s, 1H, H1), 6.46 (d, JH‑H = 4.9, 1H, H3), 5.67 (d, JH‑H
=
6.1, 1H, H8), 5.50 (d, JH‑H = 4.9, 1H, H4), 4.77 (d, JH‑H = 6.1, 1H, H9),
3.53 (dd, JH‑H = 8.1, 3.8, 1H, H6), 1.73 (m, 2H, H12), 1.55 and 1.09
(both m, 2H, H10), 0.90 (t, JH‑H = 7.2, 3H, H13), 0.70 (t, JH‑H = 7.3, 3H,
H11). 13C{1H}-APT NMR (125.6 MHz, C6D6, 298 K): δ 151.3 (s, C1),
133.3 (s, C9a), 127.2 (s, C4), 123.9 (s, C7), 121.9 (s, C3), 117.7 (s, C8),
93.3 (s, C9), 64.8 (s, C6), 27.0 (s, C12), 24.5 (s, C10), 12.4 (s, C13), 10.5
(s, C11). 15N−1H HMBC (40 MHz, C6D6, 298 K): δ 307.3 (s, N2),
109.8 (s, N5). HRMS (ESI) m/z Calcd for C12H17N2 (M + H+):
189.1386. Found: 189.1384.
AUTHOR INFORMATION
Corresponding Author
*E-mail: rcastar@unizar.es.
ORCID
Andrea Di Giuseppe: 0000-0002-3666-5800
Vincenzo Passarelli: 0000-0002-1735-6439
■
Crystal Structure Determinations. X-ray diffraction, data were
collected on APEX SMART (4) or APEX−DUO SMART (2a, 6a)
Bruker diffractometers with graphite-monochromated Mo−Kα
radiation (λ = 0.71073 Å) using narrow ω rotations (0.3−0.6°).
Intensities were integrated and corrected for absorption effects with
SAINT-PLUS,35 and SADABS36 programs, both included in the
APEX2 package. The structures were solved by the Patterson method
with SHELXS-201337 and refined by full-matrix least-squares on F2
with SHELXL-201438 under WinGX.39
Jesus J. Perez-Torrente: 0000-0002-3327-0918
́
́
Luis A. Oro: 0000-0001-7154-7239
Ricardo Castarlenas: 0000-0003-4460-8678
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Crystal Data for 2a. C34H43ClN3Rh; M = 632.07 g mol−1; T =
100(2) K; monoclinic, P21/n, a = 15.998(4) Å, b = 11.453(3) Å, c =
18.434(5) Å, β = 107.862(3)°, V = 3214.8(14) Å3; Z = 4; Dcalc = 1.306
́
Financial support from the Spanish Ministerio de Economıa y
Competitividad (MINECO/FEDER) under the Projects
CTQ2013-42532-P and CTQ2016-75884-P, the CSIC under
the Project Proyectos Intramurales Especiales (201680I011),
and the Diputacion
gratefully acknowledged. A.D.G. thanks the Spanish Ministerio
Mg m3; μ = 0.640 mm−1; red prism, 0.263 × 0.225 × 0.120 mm3; θmin
/
θmax 2.022/26.372°; reflections collected/unique 49810/6578 [R(int)
= 0.0457], data/restraints/parameters 6578/0/360, GoF = 1.065; R1 =
0.0245 [I > 2σ(I)], wR2 = 0.0649 (all data); largest diff. peak/hole
0.372/−0.251 e Å−3.
́ ́
General de Aragon (DGA/FSE-E07) is
́
de Economıa y Competitividad (MINECO) for the post-
K
DOI: 10.1021/acs.organomet.8b00149
Organometallics XXXX, XXX, XXX−XXX

Organometallics
doctoral grant Juan de la Cierva - Incorporacion
Article
(c) Dauth, A.; Rigling, C.; Tsoung, J.; Love, J. A. Chem. - Eur. J. 2013,
19, 17180.
(11) Thomas, D.; Baumann, W.; Spannenberg, A.; Kempe, R.;
́
2015 (IJCI-
2015-27029). The authors would like to acknowledge the use
of Servicio General de Apoyo a la Investigacio
Universidad de Zaragoza.
́
n-SAI,
Rosenthal, U. Organometallics 1998, 17, 2096.
(12) (a) Michael, J. P. Nat. Prod. Rep. 2008, 25, 139. (b) Madapa, S.;
Gadhiya, S.; Kurtzman, T.; Alberts, I. L.; Ramsey, S.; Reith, M.;
Harding, W. W. Eur. J. Med. Chem. 2017, 125, 255.
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