Stereocontrolled Synthesis of Benzo[k]fluoranthenes - An Unexpected Isomerization Mediated by Rhodacyclopentadiene

Article abstract of DOI:10.1002/chem.201405145

Herein, a Rh^III-catalyzed stereocontrolled synthesis of benzo[k]fluoranthenes is reported. It was found that the unexpected E/Z isomerization was highly sensitive to the electronic effects of the substituents on the aryl groups. Theoretical calculations revealed that this controllable stereochemistry originates from the mediation of rhodacyclopentadiene intermediates during the isomerization. The fact that similar stereochemistry was observed when using an Ir^III catalyst further suggests a certain generality of this discovery toward some other transition metals.

Full text of DOI:10.1002/chem.201405145

DOI: 10.1002/chem.201405145
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Synthetic Methods |Hot Paper|
Stereocontrolled Synthesis of Benzo[k]fluoranthenes—An
Unexpected Isomerization Mediated by Rhodacyclopentadiene
Shengjie Xu,[a] Kejuan Chen,[b] Hui Chen,*[b] Jiannian Yao,[b] and Xiaozhang Zhu*[a]
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isomers 3 and 3’ were formed unexpectedly and their ratios
were highly sensitive to the relative electron-withdrawing abili-
ties of the two terminal substituents, R1 and R2. This finding
can be correlated to their s values in the Hammett equation.[11]
From the theoretical mechanism investigations, we found that
1) for substrates with electron-withdrawing groups (EWGs), the
isomers can be formed via a rhodacyclopentadiene intermedi-
ate (Scheme 1b); 2) thermodynamically, aryl groups with EWGs
are more prone to locate at the 1Z,3E-position of the Rh spe-
cies compared with those bearing electron-donating groups
(EDGs); 3) kinetically, by bearing a lower isomerization barrier,
aryl groups with EWGs are easier to isomerize than those with
EDGs. We believe that this assay will stimulate a deeper under-
standing of the stereochemistry of transition-metal-catalyzed
annulation reactions.
The RhIII-catalyzed synthesis of benzo[k]fluoranthenes 3be
from (4-methoxyphenyl)boronic acid 1b and 1,8-bis(phenyle-
thynyl)naphthalene 2e was optimized in N,N-dimethylforma-
mide (DMF) at room temperature with the RhIII catalyst
[Cp*RhCl2]2 (Cp*=h5-C5Me5) (Table 1). The use of AgF as an oxi-
dant significantly improved the yield to 92%.[12] The scope of
Abstract: Herein, a RhIII-catalyzed stereocontrolled synthe-
sis of benzo[k]fluoranthenes is reported. It was found that
the unexpected E/Z isomerization was highly sensitive to
the electronic effects of the substituents on the aryl
groups. Theoretical calculations revealed that this control-
lable stereochemistry originates from the mediation of
rhodacyclopentadiene intermediates during the isomeriza-
tion. The fact that similar stereochemistry was observed
when using an IrIII catalyst further suggests a certain gen-
erality of this discovery toward some other transition
metals.
The transition-metal-catalyzed synthesis of cyclic compounds,
which are core components in organic chemistry, materials sci-
ence, and medicinal chemistry, is a crucial issue in modern or-
ganic chemistry.[1] In this regard, carborhodation of alkynes[2]
followed by CÀE bond formation (for example, E=C, Si, N, or
O) has been developed into a powerful tool for the synthesis
of carbocycles and heterocycles.[3] Distinct from their Pd coun-
terparts,[4] the stereochemistry of the carborhodation of al-
kynes related to E/Z isomerization has rarely been reported[5]
because the final products could always be afforded with the
same configuration as the rhodium intermediates formed by
an initial syn addition (Scheme 1a).[2,6] In most cases, syn ad-
ducts promptly proceeded to the next reaction step, such as
1,4-rhodium migration[2a,7] or the formation of a rhodacycle by
CÀH activation.[8] RhIII-catalyzed synthesis of naphthalene deriv-
atives from arylboronic acids and alkynes can be viewed as
a typical example.[9] As shown in Scheme 1a, five-membered
rhodacycles were formed quickly by means of CÀH activation
following the initial carborhodation of alkynes. Afterwards, the
locked five- and seven-membered rhodacycles exclusively af-
forded naphthalenes by final reductive elimination without any
isomer observed. Thus, it is of interest to investigate what will
happen if the formation of the five-membered rhodacycles,
after the initial carborhodation of alkynes, is hindered. Herein,
we report that the RhIII-catalyzed synthesis of benzo[k]fluoran-
thenes,[10] from arylboronic acids 1 and 1,8-bis(arylethynyl)-
naphthalenes 2 (Scheme 1b), is a suitable model reaction be-
cause the intramolecular alkenylrhodium–alkyne complexation
prevents rapid formation of a rhodacycle. Benzo[k]fluoranthene
Table 1. Optimization of reaction conditions.[a]
Entry
Oxidant
Yield
[%][b]
1
2
3
4
5
6
Cu(OOCCH3)2
Cu(OOCCF3)2
AgOOCCH3
AgOOCCF3
AgF
8
50
10
36
92
57
CuF2
[a] Reaction conditions: 1b (0.40 mmol), 2e (0.20 mmol), [Cp*RhCl2]2
(4 mol% based on 2e), oxidant (0.40 mmol), DMF (2 mL), RT, 12 h. [b] Iso-
lated yield based on 2e.
the cycloaromatization reaction between arylboronic acids
1 and bis(arylethynyl)naphthalenes 2 was examined under op-
timized reaction conditions (Table 2). Compounds 1a–1d with
para-EDGs gave benzo[k]fluoranthenes 3ae–3de in excellent
yields (82–92%) with the expected molecular structures (en-
tries 1–4). In the case of 4-fluorophenylboronic acid, an unex-
pected mixture of 3 fe and 3 fe’ was formed with a total yield
of 72% (entry 6), and was unambiguously confirmed by single-
crystal X-ray analysis (Figure 1),[13] which suggested the exis-
tence of a potential E/Z isomerization at some stage of the re-
action. Arylboronic acid 1j, with a stronger EWG (NO2), could
also react with diyne 2e to give a mixture of 3je and 3je’ with
a high total yield of 96% (entry 10). The main product was
identified to be the isomer 3je’ (Figure 1).[13] The isomer ratio
[a] S. Xu, Prof. Dr. X. Zhu
CAS Key Laboratory of Organic Solids
Beijing National Laboratory for Molecular Sciences
CAS Key Laboratory of Organic Solids
Institute of Chemistry, Chinese Academy of Sciences
Beijing 100190 (P. R. China)
E-mail: xzzhu@iccas.ac.cn
[b] K. Chen, Prof. Dr. H. Chen, Prof. Dr. J. Yao
CAS Key Laboratory of Photochemistry
Beijing National Laboratory for Molecular Sciences
CAS Key Laboratory of Photochemistry
Institute of Chemistry, Chinese Academy of Sciences
Beijing 100190 (P. R. China)
E-mail: chenh@iccas.ac.cn
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201405145.
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was further determined to be
1
14:86 (3je/3je’) by H NMR anal-
ysis.[14] Arylboronic acids 1g–1i
with different EWGs similarly af-
forded the products (3ge–3ie)
as mixtures in high total yields
(85–93%)
(entries 7–9).
The
isomer ratios (E/Z) decreased
with the electron-withdrawing
ability of the para-substituents:
73:27 (F, 3 fe), 61:39 (COOMe,
3ge), 26:74 (CF3, 3he), 17:83
(CN, 3ie), and 14:86 (NO2, 3je),
corresponding to the s values:
0.15 (F), 0.45 (COOMe), 0.53
(CF3), 0.70 (CN), and 0.81 (NO2).
The isomer ratio was also found
to be solvent dependent. For
the synthesis of compound 3he,
the isomer ratio increased con-
siderably in methanol, 57:43
(MeOH) versus 26:74 (DMF)
(entry 8). Conversely, the reac-
tions between phenylboronic
acid 1e and diynes 2b, 2d, and
2 f–2j were also examined.
Diynes 2b and 2d, with the
EDGs OMe and Me, gave com-
pounds 3eb and 3ed as single
products in good yields of 60
and 88%, respectively (entries 11
and 12). However, isomers were
Scheme 1. RhIII-catalyzed synthesis of polycyclic aromatic hydrocarbons (PAHs) between aromatic substrates and
alkynes.
formed from diynes 2 f–2j bearing EWGs, but in low ratios (7–
12%, entries 13–17). No isomers were observed (entries 18–20)
from the reaction of (4-methoxyphenyl)boronic acid 1b with
diynes 2 f, 2g, and 2i, which have EWGs with different elec-
tron-withdrawing abilities. Although the cycloaromatization re-
actions between 1 f and diynes 2b with a weak EWG (F) gave
a single product (3 fb) (entry 21), isomers were obtained by ap-
plying arylboronic acids with stronger EWGs, such as COOMe,
and CN. The isomer ratios (E/Z) also depended on the electron-
withdrawing abilities of R1; 50:50 (3gb) and 24:76 (3ib) (en-
tries 22 and 23). In agreement with the above observations,
a mixture of compounds, 3ig and 3ig’, was obtained from (4-
cyanophenyl)boronic acid 1i and dimethyl 4,4’-(naphthalene-
1,8-diylbis(ethyne-2,1-diyl))dibenzoate 2g in 95% yield, with an
isomer ratio of 33:67 because the electron-withdrawing ability
of CN is stronger than that of COOMe (entry 24).[15]
A plausible reaction mechanism for the cycloaromatization
reaction of arylboronic acids and 1,8-bis(arylethynyl)naphtha-
lenes is illustrated in Scheme 2. Substrates 1 and 2a are used
for the mechanism discussion. The reaction was initiated by re-
moving chloride from the catalyst ([Cp*RhCl2]2) with AgI ions
to afford the active catalyst, [Cp*RhF2]2.[12] Benzo[k]fluoran-
thenes 3 were formed by the twofold alkyne insertions (gener-
ating intermediates A and C), CÀH activation of C, and final re-
ductive elimination of rhodabenzocycloheptatriene D. The RhIII
Figure 1. Single-crystal structures of compounds 3ae (top left), 3je’ (top
right), 3 fe (bottom left), and 3 fe’ (bottom right).
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Table 2. RhIII-catalyzed cycloaromatization of arylboronic acids 1 and 1,8-
bis(arylethynyl)naphthalenes 2.[a]
Entry
Product (R)
Yield [%]
(3:3’ ratio)[b]
1
2
3
4
5
6
7
8
3ae (NPh2)
82 (100)
92 (100)
91 (100)
89 (100)
3be (OMe)
3ce (tBu)
3de (Me)
3ee (H)
3 fe (F)
3ge (COOMe)
3he (CF3)
57 (À)
72 (73:27)
93 (61:39)
88 (26:74)
40 (57:43)[c]
85 (17:83)
96 (14:86)
Scheme 2. Plausible reaction mechanism.
observed experimentally, appears possible. Because isomers
3he’–3je’ were obtained as the main products in the experi-
mental studies, the formation of an isomerized intermediate
such as C’ is expected to be preferred over C in the presence
of strong EWGs. To shed more light on these mechanistic
issues, theoretical calculations employing density functional
theory (DFT) were conducted.
We first considered the typical EWG, NO2, as R1. Because the
E/Z isomerization should occur at the initially formed double
bond at some stage before reaching the locked structure D,
we explored the reaction pathway from A to C and the possi-
ble E/Z isomerizations, the key findings of which are depicted
in Figure 2. After the first alkyne insertion, generating A, the
exothermic alkenylrhodium–alkyne complexation from A to B,
as shown in Figure 2, prevents the rapid formation of a molecu-
lar configuration-locked five-membered rhodacycle from A by
occupying the vacant metal coordinate site, leaving open the
possibility for later E/Z isomerization. In principle, owing to
their unlocked structures, 16-electron species of either A or C
can be potential candidates accounting for E/Z isomeriza-
tion.[16] However, our calculations demonstrate that compared
9
10
3ie (CN)
3je (NO2)
11
12
13
14
15
16
17
3eb (OMe)
3ed (Me)
3ef (F)
3eg (COOMe)
3eh (CF3)
3ei (CN)
60 (100)
88 (100)
77 (88:12)
93 (88:12)
94 (92:8)
99 (93:7)
94 (93:7)
3ej (NO2)
18
19
20
21
22
23
24
3bf (OMe,F)[d]
95 (100)
97 (100)
95 (100)
56 (100)
85 (50:50)
86 (24:76)
95 (33:67)
3bg (OMe/COOMe)[d]
3bi (OMe/CN)[d]
3 fb (F/OMe)[d]
3gb (COOMe/OMe)[d]
3ib (CN/OMe)[d]
3ig (CN/COOMe)[d]
[a] General reaction conditions: 1 (0.40 mmol), 2 (0.20 mmol), [Cp*RhCl2]2
(4 mol% based on 2), AgF (0.40 mmol), DMF (2 mL) dark conditions, RT,
12 h. [b] Isomer ratios were determined by 1H NMR measurements.
[c] The reaction was performed in methanol. [d] R1/R2.
catalyst was recovered by the
oxidation of RhI by the AgI ions.
When the R1 substituents, such
as Me, tBu, MeO, and Ph2N, were
electron donating, compounds 3
with the expected molecular
configuration were obtained.
However, when the R1 substitu-
ents were EWGs, such as F,
COOMe, CF3, CN, and NO2, an E/
Z isomerization process appears
possible before reaching the
locked structure D, for example,
the isomerization of C to C’,
Figure 2. Relative energy profile from A to C, and the subsequent lowest-energy E/Z isomerization from C to C’
which results in the isomer 3’ as for R1 =NO2 calculated in DMF at 258C.
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with the second alkyne insertions through transition state stable than C.[18] Thus, there is no thermodynamic driving force
TSCÀC from A (Figure 2), E/Z isomerization from A are disfa- for isomerization with EDGs, such as OMe. Having shown the
vored in energy by approximately 18 kcalmolÀ1.[14] Thus, A can thermodynamic aspect of E/Z isomerization, we then turned to
be excluded as an isomerization intermediate and the only re- the kinetic aspect. Comparing the highest-energy isomerization
maining potential isomerization intermediate is C. We discov- transition states (TSs) with R1 =NO2 and OMe, one can see that
ered that a rhodacyclopentadiene intermediate, Ia, mediates the TS with the OMe group (TS2b) is higher in energy than that
the E/Z isomerization from C to C’, which only needs to over- with the NO2 group (TS1a) by approximately 3.8 kcalmolÀ1
.
come activation energy barriers up to 11 kcalmolÀ1. Concern- Therefore, our calculations indicate that isomerization with an
ing the necessity of mediation of Ia in isomerization, we recall NO2 substituent on the aryl group is kinetically more favorable
that C is a 16-electron species. By forming the five-membered than that with an OMe substituent on the aryl group. Together,
rhodacyclopentadiene intermediate Ia from C, a stabilized 18- our calculations show E/Z isomerization in the presence of
electron configuration of Rh can be generated, in which the EDGs is disfavored both thermodynamically and kinetically
C=C rotation can be viewed as being half-way. Compared with compared with that of EWGs. These calculations are in good
the isomerization from A via the necessary rhodacyclopropene accordance with the experiments described above, demon-
intermediate with large reaction barriers of approximately strating that isomerization is preferable in the presence of
26 kcalmolÀ1 [14]
tadiene, is much more favored. This reactivity advantage of
,
this isomerization pathway, via rhodacyclopen- EWGs.
It is interesting to explore whether this E/Z isomerization
rhodacyclopentadiene over rhodacyclopropene is mainly due process also exists for other transition metals.[19] Thus, we ex-
to the larger strain of the three-membered ring in the latter amined the synthesis of benzo[k]fluoranthenes by utilizing the
species compared to that of the five-membered ring in the iridium catalyst,[20] [Cp*IrCl2]2, which is a congener of Rh. We
former species. Therefore, rhodacyclopentadiene is more stable used exactly the same reaction conditions as that of
than rhodacyclopropene. Based on these results, we propose [Cp*RhCl2]2 for comparison. To our delight, we found that 3be
that in the current Rh system, E/Z isomerization is mediated by was also obtained as a single product, and isomers 3he and
rhodacyclopentadiene species, which differs from the ruthena- 3he’ were obtained in a 38:62 ratio,[21] suggesting that the ste-
cyclopropene mechanism previously proposed for the E/Z iso- reochemistry described in this assay might have certain gener-
merization in Ru systems.[17] To the best of our knowledge, this ality for other transition metals. Although the isomer ratio of
is the first time that the metallacyclopentadiene-mediated E/Z 3he and 3he’ was slightly different from that with Rh catalyst
isomerization mechanism is proposed.
(26:74), we conjectured that a similar isomerization process oc-
Having clarified the E/Z isomerization pathway, we com- curred for the Ir catalyst.
pared the effects of the typical EWG, NO2, and the typical EDG,
In summary, we have revealed and rationalized the controlla-
OMe, as R1 on the isomerization process. The distribution of in- ble stereochemistry in the RhIII-catalyzed synthesis of benzo[k]-
termediates C and C’ from isomerization would determine the fluoranthenes. The experimental results combined with theo-
stereochemical outcomes of the reaction. As shown in retical mechanism investigations suggested that the unexpect-
Figure 3, our comparative calculations demonstrate that for C ed E/Z isomerization process occurs for those substrates with
with R1 =OMe, the Z isomer C is more stable than the E isomer EWGs and that the reaction is mediated by the rhodacyclopen-
C’ by 1.0 kcalmolÀ1, whereas for C with R1 =NO2, the relative tadiene intermediates. To the best of our knowledge, such C=C
stability of C and C’ is reversed and C’ becomes slightly more isomerization processes, controlled by the electronic properties
of the substituents, was demon-
strated for the first time in tran-
sition-metal-catalyzed annulation
reactions. This work also unrav-
els the importance of the locked
rhodacycles on the stereochemi-
cal specificity of RhIII-catalyzed
annulation reactions. These find-
ings also have a certain generali-
ty for transition metals besides
Rh, as exemplified by an Ir cata-
lyst.
Acknowledgements
We thank the National Natural
Science Foundation of China
Figure 3. Relative energy profiles of the lowest-energy E/Z isomerization between C and C’ for R1 =NO2 and OMe
calculated in DMF at 258C.
(91333113, 21290194, 21221002,
21473215), the Strategic Priority
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[15] The complete orthogonal experiments between the representative aryl-
boronic acids 1 and 1,8-bis(arylethynyl)naphthalenes 2 were examined
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[16] It should be noted that the 18-electron species B with R1 =NO2 lacks
the prerequisite thermodynamic driving force for E/Z isomerization (the
Z isomer is more stable than the E isomer by 0.4 kcalmolÀ1). On the
contrary, B with R1 =OMe has the thermodynamic driving force for E/Z
isomerization (the Z isomer is less stable than the E isomer by 0.5 kcal
molÀ1). These inconsistencies with the experimental observations indi-
cate that B is not likely to be the intermediate for E/Z isomerization. In
addition, 18-electron species B cannot form metallacyclopropene, and
our trials to locate the transition state of rotation around the C=C bond
without involving the metallacyclopropene species turned out to be
unsuccessful for B.
Keywords: alkynes · benzo[k]fluoranthene · computational
chemistry · rhodium · stereochemistry
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Received: September 4, 2014
Published online on October 28, 2014
Chem. Eur. J. 2014, 20, 16442 – 16447
www.chemeurj.org
16447
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