Ruthenium-catalyzed dimerization of 7-oxabicyclo[2,2,1]hepta-2,5-diene-2,3- dicarboxylates

Article abstract of DOI:10.1021/jo400104q

The ruthenium catalyzed dimerization of oxanorbornadiene dicarboxylates was studied. The effects of the ester moiety and the addition of a C1 substituent to the bicyclic alkene on the reaction were explored, and moderate yields and excellent regioselectiv

Full text of DOI:10.1021/jo400104q

Note
pubs.acs.org/joc
Ruthenium-Catalyzed Dimerization of 7‑Oxabicyclo[2,2,1]hepta-2,5-
diene-2,3-dicarboxylates
Kelsey Jack and William Tam*
Guelph-Waterloo Centre for Graduate Work in Chemistry and Biochemistry, Department of Chemistry, University of Guelph,
Guelph, Ontario, Canada N1G 2W1
S
* Supporting Information
ABSTRACT: The ruthenium catalyzed dimerization of oxanor-
bornadiene dicarboxylates was studied. The effects of the ester
moiety and the addition of a C1 substituent to the bicyclic alkene
on the reaction were explored, and moderate yields and excellent
regioselectivities were obtained.
Scheme 1. Metal Catalyzed Reactions of 7-
Oxanorbornadienes
icyclic alkenes are synthetically useful molecules due to
Btheir rigid structure and high ring strain which allows for
participation in reactions not available to other alkenes.1 The
bridged structure of [2.2.1] bicyclic alkenes provides two faces
on which reactions can occur. The exo face is encompassed by
the oxygen bridgehead which provides additional electron
density. Homoconjugation of the π-orbitals of the two alkenes
acts to increase the reactivity of these olefins and the electron
density of the endo face. Norbornadienes, specifically, have been
used as intermediates toward to the natural product synthesis of
compounds such as prostaglandin endoperoxides PGH2 and
PGG2, cis-trikentrin B, and β-santalol.2−5 Oxabicyclic alkenes
are often used in the creation of highly substituted ring systems
as well as in the formation of the core structure of many natural
products.6−9 The oxygen functionality presents synthetic
opportunities for oxabicyclic alkenes that are not present in
hydrocarbon bicyclic alkenes. Our group has thoroughly
explored reactions of oxabenzonorbornadienes with much
success.9 Less work, however, has been done with 7-
oxabicyclo[2.2.1]hepta-2,5-diene-2,3-dicarboxylates 1. Insight
into the reactivity of this compound allowed our group and
others to demonstrate its versatility in a wide variety of metal
catalyzed reactions (Scheme 1).
For example, when 1 reacted with an alkyne in the presence
of Cp*Ru(COD)Cl, a [2 + 2] cycloaddition resulted in
cyclobutene 2.10,11 A Pauson−Khand [2 + 2 + 1] cycloaddition
will occur in the presence of an alkyne with Co2(CO)8 to
provide 3.12 The oxanorbornadiene can be opened and
aromatized using either an iron or iridium catalyst to give
4.13 The ring opening can also be accomplished using a
palladacycle with a benzylzinc halide to produce 514 or with a
copper catalyst and a trialkyl aluminum reagent to give 6.15
Deoxygenenation of the 7-oxa moiety to give 7 can be achieved
using a titanium catalyst with LiAlH4.16,17 Ru catalysts can
promote the cyclopropanation of 7-oxanorbornadienes using
propargylic alcohols or acetates to yield 8.18,19 The asymmetric
dimerization to provide 9 is accomplished using a rhodium
catalyst.20,21
Dimerization reactions are of great interest due to their high
atom efficiency.22 Whipple and co-workers first explored the
dimerization of norbornadiene in 1965. Although there are six
norbornadiene dimers possible (Figure 1) due to the two faces
of the bicyclic alkene (three cis and three trans), only the three
possible trans dimers 10a−c were obtained using a
Co2(CO)6(Ph3P)2 or Ni(CO)4 catalyst.23 Further studies of
this reaction have been carried out involving other Ni and Co
catalysts as well as Fe, Cr, and Rh catalysts. Regardless of the
catalyst system employed, only trans norbornadiene dimers
were obtained as products, confirmed by 1H coupling
constants.24 The dimers resulting from the Rh catalyzed
reactions were minor products in the formation of trimers and
other more complex cycloadducts.25,26 Despite the many
Received: January 16, 2013
Published: March 18, 2013
© 2013 American Chemical Society
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The Journal of Organic Chemistry
Note
Cp*Ru(COD)X variants. Ru(COD)Cl2 and (Ph3P)2CpRuCl
provided no discernible products from reaction under these
conditions. Since there is very little difference in the product
yields when Cp*Ru(COD)Cl or Cp*Ru(COD)Br was used,
Cp*Ru(COD)Cl was chosen for all further investigations due
to its commercial availability.
With our catalyst of choice, Cp*Ru(COD)Cl, in hand, we
went on to explore solvent and temperature effects on this
dimerization (Table 2). All solvents tested provided moderate
Figure 1. Possible structures of norbornadiene dimers.
Table 2. Solvent and Temperature Optimization
literature examples of the dimerization of norbornadiene,
Cheng and co-workers provided the only example of the
dimerization of an oxabicyclic alkene. It was shown that
oxabenzonorbornadiene 11 will undergo a [2 + 2] dimerization
in the presence of NiCl2(PPh3)2 and Zn to give dimer 12 in
high yield (96%) (Scheme 2). The exo-trans-exo conformation
a
entry
solvent
temp (°C)
yield (%)
1
2
3
4
5
6
7
8
DMSO
toluene
DMF
THF
60
60
60
60
60
60
45
25
39
44
45
50
51
66
54
61
1
was predicted based on the H and 13C NMR spectra and
confirmed using X-ray diffraction.24
hexane
DCE
Scheme 2. Dimerization of Oxabenzonorbornadiene
DCE
DCE
a
Isolated yields after column chromatography.
Our interest in oxabicyclic alkenes and the lack of
investigation into the dimerization of these compounds lead
to our study of the Ru catalyzed dimerization of 7-oxabicyclo-
[2,2,1]hepta-2,5-diene-2,3-dicarboxylates. Our investigations
began by screening various Ru catalysts to determine their
efficacy in this dimerization (Table 1). Initially, Cp*Ru(COD)-
yields of the dimer; however, some proved more effective than
others. DMSO provided the lowest yield of 39% (entry 1).
Toluene and DMF (entries 2 and 3) resulted in slightly higher
yields at 44% and 45%, respectively. When the reaction was ran
in THF or hexanes (entries 4 and 5) an increased yield was
again observed providing 50% and 51% of the desired product,
respectively. Utilizing DCE (entry 6) as a solvent resulted in a
much higher yield of 66% and was thus selected as our solvent
of choice. Less variability was observed when the effect of
temperature on the dimerization was explored (entries 6−8).
Running the reaction at 60 °C provided a slightly higher yield
than at 45 or 25 °C.
Table 1. Dimerization Catalyst Optimization
a
entry
catalyst
yield (%)
Having determined the optimal conditions for our desired
reaction, we moved on to explore the scope of the reaction
(Table 3). We first examined the effect of changing the alkyl
component of the ester. Increasing the size of the ester moiety
did not affect the reaction (entries 1−3). Regardless of whether
a Me, Et, or tBu ester was present on C2 and C3 of the starting
oxabicyclic alkene 1a−c, the yield of the dimer 13a−c remained
constant at 66%. These results negate any steric effect of the
ester component of the bicyclic in the reaction mechanism. The
[2 + 2] cycloaddition that takes place to produce the dimerized
product occurs between C5 and C6 of both oxabicyclic alkenes,
so it follows that added steric bulk at C2 and C3 would not
have an effect on the outcome of the reaction. The
stereochemistry of the dimerized product 13c was confirmed
by X-ray crystallography and was found to have an exo-trans-exo
geometry.27a
We have examined the effect of a substitutent at the C1
position of the bicyclic alkene on the Ru-catalyzed dimerization
reaction (Table 3, entries 4−11). Two possible exo-trans-exo
dimerization products could be formed: one with the two C1
substituents syn to each other, and another with the two C1
substituents anti to each other (Figure 2). To our delight, the
reactions were highly regioselective, and only the syn exo-trans-
exo dimers were formed in all cases. Although the syn-dimers
1
2
3
4
5
6
7
8
9
Cp*Ru(COD)Cl
CpRu(COD)Cl
66
24
32
16
70
55
58
0
CpRu(COD)Br
CpRu(COD)I
Cp*Ru(COD)Br
[Cp*Ru(CH3CN)3]+PF6
[CpRu(CH3CN)3]+PF6
−
−
Ru(COD)Cl2
(Ph3P)2CpRuCl
0
a
Isolated yields after column chromatography.
Cl, a useful catalyst in many other reactions of bicyclic alkenes,
was tested for its ability to dimerize 1a, yielding 66% of the
product 13a. The CpRu(COD)X series (where X = Cl, Br, and
I, entries 2−4) were also examined resulting in lower yields in
all three cases. Interestingly, the use of CpRu(COD)Br
produced more product than its counterparts leading us to
attempt the dimerization reaction catalyzed by Cp*Ru(COD)-
Br (entry 5). The use of the bromide ligand in this case also
lead to an increase in yield compared to the chloride ligand,
obtaining 70% conversion to the desired product. Two cationic
Ru catalysts were additionally investigated (entries 6 and 7).
Both gave good yields but were not as effective as either of the
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Note
Table 3. Dimerization Reactions of Oxanorbornadienes
Scheme 3. Proposed Dimerization Mechanism
a
entry
alkene
R
R1
yield (%)
1
1a
1b
1c
1d
1e
1f
Me
Et
H
66
66
66
57
24
23
2
H
3
tBu
Me
Me
Me
Me
Me
Me
Me
Me
H
4
Me
Et
5
6
(CH2)4CH3
tBu
b
7
1g
1h
1i
0 (68)
b
8
TMS
0 (47)
9
COOMe
Ph
53
c
10
11
1j
0
c
53% yield (Table 3, entry 9). The presence of other C1
substituents such as Ph and CH2OH (1j−1k, entries 10−11)
led to decomposition of the oxanorbornadiene and produced a
complicated mixture of products.
1k
CH2OH
0
a
b
Isolated yields. Yields of ring opened aromatized products 14 in
parentheses when the reactions were run at 80 °C for 48 h.
c
Complicated mixture of products was obtained.
Having been able to confirm the exo-trans-exo configuration
of our dimerized products, we can propose a mechanism for the
formation of these products (Scheme 3). Following dissociation
of the COD ligand from Cp*Ru(COD)Cl, there is
coordination of the new Ru complex to the C2−C3 olefin of
the bicyclics. Oxidative cyclization provides a metallacyclopen-
tane intermediate which undergoes reductive elimination to
provide the dimerized oxanorbornadiene and regenerate the
Ru-catalyst. Coordination of the catalyst preferentially takes
place on the exo face, as it is more sterically favorable.
In conclusion, we have demonstrated the ruthenium
catalyzed dimerization reaction of oxanorbornadiene dicarbox-
ylates. Dimerization of the parent compound proceeded
smoothly, with yields decreasing when substituents are added
at the C1 position. The ruthenium catalyzed dimerization
reactions of C1 substituted oxanorbornadiene dicarboxylates
were found to be highly regioselective, giving only the syn-
dimers in moderate yields.
Figure 2. Two possible exo-trans-exo dimerization products.
might be expected to be less stable due to R1−R1 interactions
that are not present in the anti-dimers,28 they are the only
isolated products and the structures of dimers 13d and 13i
were confirmed by X-ray crystallography.27b,c We do not have a
good explanation at this point why the syn-dimers were formed
preferentially, and further mechanistic studies including
computation calculations will be required. When a methyl
group was introduced at the C1 position of the bicyclic alkene
(entry 4), a slight decrease in yield to 57% was observed.
Substitution at this position does have a steric effect on the
reaction as indicated by the sharp decrease in yield when the
size of the substituent was increased from a methyl group to an
ethyl or pentyl group which produced the corresponding dimer
in a 24% and 23% yield, respectively (Table 3, entries 5 and 6).
With successful examples of primary alkyl groups, the
EXPERIMENTAL SECTION
■
General Information. All reactions were carried out in an
atmosphere of dry nitrogen or argon. 1H and 13C spectra were
recorded on a 400 MHz spectrometer. Starting alkenes were prepared
according to the following known literature procedures: 1a,29 1b,30
1c,31 1d,17 1e,32 1f,33 1h,34 1i,35 1j,36 and 1k.31
t
dimerization of a C1 Bu oxabicyclic alkene 1g was undertaken
(entry 7). To our surprise, starting material was recovered with
no observable reaction under the initial conditions. In an
attempt to try and promote the reaction to occur, the
temperature was increased to 80 °C and the reaction was
allowed to stir for 48 h instead of the usual 18 h. The change in
conditions did not produce the desired dimer but instead
resulted in ring opened aromatized product 14g in a 68% yield.
Similarly when the C1 TMS substituted oxabicyclic alkene 13h
(entry 8) was subjected to our standard conditions, no [2 + 2]
dimerization product was obtained and the ring opened
aromatized product 14h was formed in 47% yield. An increase
in in the size of the C1 substituent (from 1° alkyl groups Me,
Et, and pentyl to 3° alkyl group tBu or a TMS group) may lead
to the disfavored sterically crowded Ru-pentacycle (Scheme 3)
with two bulky R1 groups close to the Cp* ligand on the Ru,
and therefore with bulky R1 groups, only ring opened
aromatized products were obtained. Changing the electronic
nature of the C1 substituent to the electron-withdrawing
methyl ester 1i resulted in the dimerized oxanorbornadiene in
Oxanorbornadiene 1g. Dimethylacetylene dicarboxylate (0.39
mL, 3.2 mmol) was measured and added via syringe into an N2 purged
oven-dried screw cap vial. 2-tert-Butylfuran (0.50 mL, 3.5 mmol) was
added dropwise via syringe. The vial was purged with N2, and the
septum was replaced with a cap. The reaction was heated to 90 °C and
allowed to stir for 12 h. The crude product was purified by column
chromatography (EtOAc/hexanes 3:7) to give oxanorbornadiene 1g
(638 mg, 2.4 mmol, 75%) as a yellow oil. Rf 0.43 (EtOAc/hexanes
1
3:7); H NMR (CDCl3, 400 MHz) δ 7.14 (dd, 1H, J = 1.8, 5.3 Hz),
7.08 (d, 1H, J = 5.3 Hz), 5.59 (d, 1H, J = 1.8 Hz), 3.79 (s, 3H), 3.68
(s, 3H), 1.06 (s, 9H); 13C (APT, CDCl3, 100 MHz) δ 167.7, 162.1,
159.2, 149.4, 145.0, 142.7, 105.5, 82.1, 52.3, 52.0, 32.6, 26.1; IR (neat)
3434, 3095, 2957, 2911, 2846, 1732, 1634, 1560, 1436, 1399, 1248,
1222, 1146, 1075, 1035, 996, 953, 936, 885, 821, 756, 737, 709, 684
cm−1; HRMS (EI-TOF) calcd for C14H18O5 (M+): 266.1154; found:
266.1159.
General Procedure for the Ru-Catalyzed Dimerization of
Oxanorbornadienes (Table 3, Entry 1). Oxanorbornadiene 1a (44
mg, 0.21 mmol) was weighed into an oven-dried screw-cap vial. The
vial was purged with nitrogen, taken into the drybox where
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The Journal of Organic Chemistry
Note
Cp*RuCl(COD) (8 mg, 0.02 mmol) and DCE (0.3 mL) were added,
and sealed. The reaction mixture was stirred outside the glovebox at 60
°C for 16−20 h. The crude product was purified by flash
chromatography to yield the corresponding cycloadduct (ethyl
acetate/hexanes mixture).
Phenol 14g (Table 3, entry 7). The crude product was purified
by column chromatography (EtOAc/hexanes 2:3) to give phenol 14g
(45.3 mg, 0.169 mmol, 68%) as a brown oil; Rf 0.18 (EtOAc/hexanes
2:3); 1H NMR (CDCl3, 400 MHz) δ 10.81 (s, 1H), 7.60−7.63 (d, 1H,
J = 9.12), 6.98−7.01 (d, 1H, J = 9.08), 3.92 (s, 3H), 3.84 (s, 3H), 1.35
(s, 9H); 13C NMR (APT, CDCl3, 100 MHz) δ 170.6, 169.7, 159.4,
138.3, 134.5, 133.0, 118.8, 110.2, 53.0, 52.1, 35.8, 31.5; IR (CH2Cl2)
3423, 2956, 1738, 1675, 1591, 1441, 1326, 1193, 1161, 1012 cm−1;
HRMS (EI-TOF) calcd for C14H18O5 (M+): 266.1154; found:
266.1162.
Phenol 14h (Table 3, Entry 8). The crude product was purified
by column chromatography (EtOAc/hexanes 1:9) to give phenol 14h
(10.9 mg, 0.042 mmol, 47%) as a yellow oil; Rf 0.32 (EtOAc/hexanes
1:9); 1H NMR (CDCl3, 400 MHz) δ 10.84 (s, 1H), 7.61−7.63 (d, 1H,
J = 8.44 Hz), 7.03−7.05 (d, 1H, J = 8.48 Hz), 3.92 (s, 3H), 3.86 (s,
3H), 0.254 (s, 9H); 13C NMR (APT, CDCl3, 100 MHz) δ170.1,
169.6, 161.7, 140.9, 140.9, 128.1, 118.4, 110.0, 52.9, 52.1, −0.5; IR
(CH2Cl2) 3423, 2955, 1736, 1677, 1577, 1458, 1343, 1255, 1212,
1130, 1014, 895, 877, 802 cm−1; HRMS (EI-TOF) calcd for
C13H18O5Si (M+): 282.0924; found: 282.0935.
Dimer 13a (Table 3, Entry 1). The crude product was purified by
column chromatography (EtOAc/hexanes 2:3) to give the dimer 13a
(29.0 mg, 0.069 mmol, 66%) as a white solid (mp 180−182 °C). Rf 0.2
1
(EtOAc/hexanes 2:3); H NMR (CDCl3, 400 MHz) δ 5.16 (s, 4H),
3.79 (s, 12H), 2.17 (s, 4H); 13C (APT, CDCl3, 100 MHz) δ 162.6,
142.7, 82.6, 52.2, 39.6; IR (CH2Cl2) 3007, 2955, 2847, 1717, 1631,
1437, 1306, 1229, 1121, 917, 746 cm−1; HRMS (EI-TOF) calcd for
C20H20O10 (M+): 420.1056; found: 420.1073.
Dimer 13b (Table 3, Entry 2). The crude product was purified by
column chromatography (EtOAc/hexanes 3:7) to give the dimer 13b
(106.2 mg, 0.22 mmol, 66%) as a white solid (mp 173−175 °C); Rf
1
0.31 (EtOAc/hexanes 3:7); H NMR (CDCl3, 400 MHz) δ 5.14 (s,
4H), 4.20−4.26 (q, 8H, J = 7.1 Hz), 2.16 (s, 4H), 1.27−1.30 (t, 12H, J
= 6.7 Hz); 13C NMR (APT, CDCl3, 100 MHz) δ 162.3, 142.5, 82.6,
61.3, 39.7, 14.0; IR (CH2Cl2) 2941, 1707, 1630, 1333, 1227, 1122,
1022, 918 cm−1; HRMS (EI-TOF) calcd for C24H28O10 (M+):
476.1683; found: 476.1665.
ASSOCIATED CONTENT
■
Dimer 13c (Table 3, Entry 3). The crude product was purified by
column chromatography (EtOAc/hexanes 1:9) to give dimer 13c
(28.8 mg, 0.050 mmol, 66%) as a white solid (mp 120 °C dec.); Rf
S
* Supporting Information
1H and 13C NMR spectra all new compounds. This material is
available free of charge via the Internet at http://pubs.acs.org.
1
0.22 (EtOAc/hexanes 1:9); H NMR (CDCl3, 400 MHz) δ 5.07 (s,
4H), 2.14 (s, 4H), 1.49 (s, 36H); 13C NMR (APT, CDCl3, 100 MHz)
δ 161.7, 142.7, 82.7, 82.4, 40.0, 28.0; IR (CH2Cl2) 1725, 1655, 1422,
1370, 1164, 1123, 896 cm−1; HRMS (ESI-Quadrupole TOF) calcd for
C32H44O10 (M+Na+): 611.2832; found 611.2829.
AUTHOR INFORMATION
Corresponding Author
*E-mail: wtam@uoguelph.ca.
■
Dimer 13d (Table 3, Entry 4). The crude product was purified by
column chromatography (EtOAc/hexanes 2:3) to give dimer 13d
(23.7 mg, 0.053 mmol, 57%) as a white solid (mp 129−131 °C); Rf
Notes
The authors declare no competing financial interest.
1
0.29 (EtOAc/hexanes 2:3); H NMR (CDCl3, 400 MHz) δ 5.07 (s,
2H), 3.82 (s, 6H), 3.76 (s, 6H), 2.16 (s, 4H), 1.63 (s, 6H); 13C NMR
(APT, CDCl3, 100 MHz) δ 164.4, 162.2, 146.8, 140.8, 89.4, 81.32,
52.3, 52.2, 41.3, 41.2, 12.4; IR (CH2Cl2) 2955, 2846, 1721, 1634, 1437,
1389, 1200, 1063, 1001, 832 cm−1; HRMS (EI-TOF) calcd for
C22H24O10 (M+): 448.1370; found: 448.1355.
ACKNOWLEDGMENTS
■
This work was supported by the Merck Frosst Centre for
Therapeutic Research and Natural Sciences and Engineering
Research Council of Canada (NSERC). K.J. thanks the NSERC
for providing a postgraduate (CGS M) scholarship.
Dimer 13e (Table 3, Entry 5). The crude product was purified by
column chromatography (EtOAc/hexanes 2:3) to give dimer 13e
(48.4 mg, 0.086 mmol, 24%) as a white solid (mp 110−115 °C); Rf
REFERENCES
■
1
(1) Khoury, P. R.; Goddard, J. D.; Tam, W. Tetrahedron 2004, 60,
8103.
0.44 (EtOAc/hexanes 2:3); H NMR (CDCl3, 400 MHz) δ 5.10 (s,
2H), 3.83 (s, 6H), 3.76 (s, 6H), 2.14−2.20 (m, 4H), 1.96−2.10 (m,
4H), 0.99−1.03 (t, 6H, J = 7.5 Hz); 13C NMR (APT, CDCl3, 100
MHz) δ 164.9, 162.1, 146.5, 141.1, 94.1, 81.1, 52.3, 52.2, 41.6, 40.7,
19.7, 9.3; IR (CH2Cl2) 2955, 2884, 1720, 1631, 1437, 1330, 1226,
1074, 1014, 938, 896 cm−1; HRMS (EI-TOF) calcd for C24H28O10
(M+): 476.1683; found: 476.1702.
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Dimer 13f (Table 3, Entry 6). The crude product was purified by
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mg, 0.044 mmol, 23%) as a white solid (mp 71−72 °C); Rf 0.29
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1
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(EtOAc/hexanes 1:4); H NMR (CDCl3, 400 MHz) δ 5.10 (s, 2H),
3.83 (s, 6H), 3.76 (s, 6H), 2.18−2.19 (d, 2H, J = 5.80 Hz), 2.13−2.14
(d, 2H, J = 5.80 Hz), 1.95−1.96 (m, 4H), 1.48−1.49 (m, 2H), 1.29−
1.35 (m, 6H), 0.87−0.90 (t, 6H, J = 6.68 Hz); 13C NMR (APT,
CDCl3, 100 MHz) δ 165.0, 162.2, 146.6, 141.0, 93.7, 81.2, 52.3, 52.2,
41.5, 41.0, 32.1, 26.6, 24.9, 22.3, 13.9; IR (CH2Cl2) 2955, 2872, 1720,
1630, 1437, 896 cm−1; HRMS (EI-TOF) calcd for C30H40O10 (M+):
560.2622; found: 560.2636.
Dimer 13i (Table 3, Entry 9). The crude product was purified by
column chromatography (EtOAc/hexanes 1:1) to give dimer 13i
(165.5 mg, 0.617 mmol, 53%) as a white solid (mp 196−198 °C); Rf
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1
0.40 (EtOAc/hexanes 1:1); H NMR (CDCl3, 400 MHz) δ 5.25 (s,
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5.4 Hz), 2.35−2.37 (d, 2H, J = 5.4 Hz); 13C NMR (APT, CDCl3, 100
MHz) δ 165.9, 162.5, 161.6, 143.8, 140.4, 90.5, 82.0, 53.0, 52.7, 52.6,
41.8, 41.0; IR (CH2Cl2) 2956, 2850, 1743, 1438, 1331, 1204, 1158,
1082, 1036, 896 cm−1; HRMS (EI-TOF) calcd for C24H24O14 (M+):
536.1166; found: 536.1166.
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