An efficient one-pot protocol for asymmetric bifunctionalization of 5,15-disubstituted porphyrins: Direct access to meso activated alkenyl-substituted meso-formylporphyrins

Article abstract of DOI:10.1016/j.tetlet.2011.08.025

An efficient one-pot protocol for the direct conversion of free base 5,15-disubstituted porphyrins into the corresponding meso activated alkenyl-substituted meso-formylporphyrins has been developed using a sequential S_NAr reaction with PyMe

Full text of DOI:10.1016/j.tetlet.2011.08.025

Tetrahedron Letters 52 (2011) 5345–5348
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Tetrahedron Letters
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An efficient one-pot protocol for asymmetric bifunctionalization
of 5,15-disubstituted porphyrins: direct access to meso activated
alkenyl-substituted meso-formylporphyrins
⇑
Toshikatsu Takanami , Satoshi Hayashi, Kazuhiro Iso, Jun Matsumoto, Fumio Hino
Meiji Pharmaceutical University, 2-522-1, Noshio, Kiyose, Tokyo 204-8588, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
An efficient one-pot protocol for the direct conversion of free base 5,15-disubstituted porphyrins into the
corresponding meso activated alkenyl-substituted meso-formylporphyrins has been developed using a
sequential SNAr reaction with PyMe2SiCH2Li, conjugate addition to enones or alkenoates in the presence
of TMSCl, and oxidation with DDQ.
Received 6 July 2011
Revised 1 August 2011
Accepted 3 August 2011
Available online 8 August 2011
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Porphyrin
SNAr reaction
Silylmethyllithium
a,b-Unsaturated carbonyl compound
Interest in the chemistry of porphyrins and related tetrapyrrolic
the corresponding meso acyl-substituted meso-formylporphy-
macrocycles has increased greatly in recent years, since these com-
pounds have potentially important applications in many areas of
chemistry, biology, and material sciences,1 serving as, for example,
homogeneous catalysts,2 medicines for photodynamic therapy
(PDT),3 materials for nonlinear optics (NLO) and solar energy con-
version systems,4 and synthetic receptors for various organic and
inorganic species.5 Therefore, immense effort has been devoted
to the development of novel and efficient synthetic strategies
and intermediates for the preparation of porphyrin derivatives
with a variety of peripheral substituents.6,7 In this context, ‘multi-
functional porphyrins’ that possess two or more different reactive
functional groups, such as carbonyl, halogenic, alkenyl, and alkynyl
groups, on the porphyrin core would be particularly attractive
starting materials for further manipulation to construct more com-
plex porphyrin derivatives, because each functional group, which is
directly attached to the ring, can be individually replaced with
other functionalities. Despite significant advances in the synthetic
pathways and strategies in the field of porphyrin chemistry, only a
limited number of general methods have been developed for the
direct introduction to the porphyrin core of more than one reactive
functional group, even with two functionalities, giving distinct
reactivity.8
rins.7a As shown in Scheme 1, this one-pot asymmetric bifunc-
tionalization of free base porphyrins involves
a sequential
nucleophilic substitution (SNAr reaction)9 of porphyrins 1 with (2-
pyridyldimethylsilyl)methyllithium(PyMe2SiCH2Li),10 followed by
trapping of the resulting anion A with acylchlorides (R0COCl) as
an electrophile and oxidation with DDQ, where the PyMe2SiCH2
group works as a latent formyl functionality in the reaction.7b
As a follow-up of our work, we chose
a,b-unsaturated carbonyl
compounds as an electrophile in the asymmetric bifunctionaliza-
tion of porphyrins based on the SNAr strategy with PyMe2SiCH2Li.
Herein, we report the first example of an efficient direct
R
R
PyMe2SiCH2Li
NH N
N HN
NH N
N HN
SiMe2Py
H
H
R
R
1
A
R
O
O
NH N
N HN
1) R'COCl
2) DDQ
SiMe2
N
Recently, we reported an efficient one-pot procedure for the
direct conversion of 5,15-disubstituted free base porphyrins into
H
R'
Li
PyMe2SiCH2Li
R
⇑
Corresponding author. Tel./fax.: +81 42 495 8780.
E-mail address: takanami@my-pharm.ac.jp (T. Takanami).
Scheme 1. Our previous work on one-pot preparation of meso-acyl-substituted
meso-formylporphyrins.
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.08.025

5346
T. Takanami et al. / Tetrahedron Letters 52 (2011) 5345–5348
O
+ TMSCl
2a
1)
Ph
Ph
(20 equiv)
(15 equiv)
PyMe2SiCH2Li
(10 equiv)
O
O
H
NH N
N HN
NH N
N HN
°
→
rt
ether, -78
C
°
→
rt
THF, -78
C
2) 0.1M HCl, 0 °C
3) DDQ (10 eq), rt
Ph
Ph
3aa
1a
Ph
Ph
HO
O
O
H
NH N
N HN
O
H
NH N
NHN
or
Ph
Ph
5aa
4aa
Scheme 2. One-pot direct conversion of 5,15-diphenylporphyrin 1a to meso activated alkenyl-substituted meso-formylporphyrin 3aa using cyclopentenone 2a as an
electrophile.
Table 1
Scope of 5,15-disubstituted porphyrins 1 and
a
,b-unsaturated carbonyl compounds 2 for the one-pot preparation of meso activated alkenyl-substituted meso-formylporphyrins 3a
O
X
+ TMSCl
(20 equiv)
2
1)
R1
NH
R1
(15 equiv)
O
X
PyMe2SiCH2Li
(10 equiv)
O
H
N
NH N
N HN
°
→
rt
ether, -78
C
HN
R1
N
°
C
→
THF, -78
rt
2) 0.1M HCl, 0 °C
3) DDQ (10 eq), rt
R1
3
1
One-pot
Ph
Ph
Ph
Ph
Me
O
O
O
O
O
O
H
O
H
O
H
O
H
NH N
N HN
NH N
N HN
NH N
N HN
NH N
N HN
Me
Ph
3aa (71%)
Ph
Ph
Ph
Ph
b
3ab (69%)
3ac
3ad (nd)
(54%)
Ph
Ph
O
O
O
O
O
H
O
NH N
N HN
NH N
N HN
NH N
N HN
MeO
H
H
Me
Ph
Ph
Ph
b,c
3ae (nd)
3af (43%)
3ag
(41%)
p-Tol
p
-Tol
p
-Tol
O
O
O
O
H
O
O
H
NH N
N HN
NH N
N HN
NH N
N HN
Me
MeO
H
p
-Tol
p
-Tol
p
-Tol
b
b
3bb (72%)
3-MeOPh
3bc
3bg
(65%)
3-vinylPh
(47%)
3-CF3Ph
i
-Bu
O
O
O
O
O
H
O
O
H
O
NH
NH N
HN
NH
NH N
N
N
MeO
HN
N
N HN
3-MeOPh
3cg (45%)b
N
N HN
3-CF3Ph
3eb (68%)
H
H
i
-Bu
3-vinylPh
3db (66%)
3fb (74%)c
a
b
c
The numbers in the parentheses are isolated yields.
E-isomer was isolated.
Isolated yields after conversion of the crude products into the corresponding Ni(II) complexes (see, Ref. 14).

T. Takanami et al. / Tetrahedron Letters 52 (2011) 5345–5348
5347
O
R
R
+ TMSCl
2
PyMe2SiCH2Li
NH N
N HN
NH N
N HN
SiMe2Py
SNA
r
H
H
Michael addition
R
R
A
1
R
OTMS
H
O
R
O
H
NH
DDQ
N
NH N
N HN
SiMe2Py
N HN
R
Oxidation reactions
H
3
R
6
Scheme 3. Plausible reaction pathways for the one-pot direct conversion of 5,15-disubstituted porphyrins to the corresponding meso activated alkenyl-substituted meso-
formylporphyrins.
introduction of a formyl group and an activated alkenyl substitu-
ent onto the porphyrin core at the meso-position in a simple one-
pot procedure.
(Scheme 3). Thus, anionic intermediate A, generated from the SNAr
reaction of porphyrin 1 with PyMe2SiCH2Li, undergoes conjugate
addition to a,b-unsaturated carbonyl compound 2 in the presence
We began our studies by evaluating the asymmetric bifunction-
alization of 5,15-diphenylporphyrin 1a with PyMe2SiCH2Li and
2-cyclopentene-1-one 2a using similar reaction conditions to those
previously reported for the one-pot synthesis of meso acyl-
substituted meso-formylporphyrins.7a Thus, a solution of porphyrin
1a in THF was treated in the following order: PyMe2SiCH2Li
(10 equiv) at ꢀ78 °C to rt, an ethereal solution of enone 2a (15 equiv)
and TMSCl (20 equiv) at ꢀ78 °C to rt, diluted aqueous HCl at 0 °C,11
and then DDQ (10 equiv) at rt. As shown in Scheme 2, we unexpect-
edly did not obtain either the corresponding Michael type adduct
4aa via 1,4-addition or the alcohol derivative 5aa via 1,2-addition,
but did obtain the enone moiety directly coupled adduct, meso acti-
vated alkenyl-substituted meso-formylporphyrin 3aa, as the sole
isolable product in 71% yield. In this reaction, TMSCl was found to
be essential as the additive for the formation of 3aa; any attempt
without TMSCl gave only a trace amount of the Michael adduct
4aa as an isolable product, along with an inseparable complex prod-
uct mixture.12 When TMSCl was replaced by other silylating agents,
such as TESCl, TBSCl, and TMSOTf, significant deterioration in the
yields of 3aa (<30%) was observed.
of TMSCl to give the porphodimethene derivative 6,16 of which the
silylmethyl group, enol silyl ether moiety, and porphodimethene
ring are in turn oxidized with DDQ into the formyl group, the en-
one or alkenoate moiety, and the porphyrin core, respectively,
leading to the final product 3, although the precise order of these
oxidation reactions is not yet clear.17 This reaction process pro-
vides a satisfying explanation for the failure to obtain the desired
activated alkenyl-substituted product 3 in the reaction performed
without TMSCl (vide supra), which cannot create the porphodi-
methene intermediate 6 with an enol silyl ether substituent.
Further experiments are currently underway to elucidate the reac-
tion process in more detail.
In summary, direct introduction of an activated alkenyl substi-
tuent and a formyl group onto the meso carbons of free base 5,
15-disubstituted porphyrins can now be realized using a simple
one-pot procedure that involves a sequential SNAr reaction with
PyMe2SiCH2Li, conjugate addition to enones or alkenoates in the
presence of TMSCl, and oxidation with DDQ. This one-pot protocol
operates efficiently under mild conditions, can be applied to free
base porphyrins, and is suitable for cyclic and acyclic alkenoates
as well as enones. Most notably, unprecedented, formal direct cou-
plings between enones or alkenoates and porphyrins at the meso-
positions take place during the reaction to provide meso activated
alkenyl-substituted meso-formylporphyrins, which has never been
achieved by known porphyrin functionalization.18 These asymmet-
rically bifunctionalized porphyrins, of which reactive functional
groups at the meso-positions can be readily and individually
replaced by a variety of other functionalities, will serve as versatile
synthetic precursors in subsequent transformations for the con-
struction of more complex porphyrin systems that could have
potential applications. Further work to extend the porphyrin func-
tionalization based on the SNAr strategy with PyMe2SiCH2Li is
underway in our laboratory.
Next, we investigated the scope of the above procedure using a
range of 5,15-disubstituted porphyrins and
a,b-unsaturated car-
bonyl compounds as substrates, and the representative results
obtained are summarized in Table 1.13–15 It was found that not only
cyclic enones, 2a and 2b, but also an acyclic enone 2c could readily
participate in the asymmetric bifunctionalization of diphenylporph-
yrin 1a to afford the corresponding meso activated alkenyl-substi-
tuted meso-formylporphyrins 3aa, 3ab, and 3ac in good yields.
However, sterically hindered enones appear to be a current
limitation of our method, as 2-methylcyclopentenone 2d and
4-methylcyclohexenone 2e did not afford the desired products
3ad and 3ae, giving low yields (<20%) of meso-formylated diph-
enylporphyrin instead. This protocol is not limited to enones; both
cyclic and acyclic alkenoates, 2f and 2g, were also compatible with
the one-pot asymmetric bifunctionalization, furnishing the desired
products 3af and 3ag in acceptable yields. Other porphyrins
including 5,15-diarylporphyrins, 1b–1e, of which the substituent
on the phenyl ring is Me, MeO, CH2@CH, and CF3, and 5,15-
dialkylporphyrin 1f were evaluated next and also found to be good
substrates for the present one-pot protocol; the corresponding
meso-formylporphyrins substituted with enone and alkenoate moi-
eties at the meso-position were obtained in good to moderate yields.
Mechanistically, we believe that the reaction proceeds via a
porphodimethene derivative 6 bearing a PyMe2SiCH2 group and
an enol silyl ether moiety on the sp3 carbons of the ring
Acknowledgements
This work was supported by a Grant-in-Aid for Scientific
Research (KAKENHI) from JSPS and a grant from the High-Tech
Research Center Project, MEXT, Japan.
Supplementary data
Supplementary data (experimental procedures and character-
ization data for all compounds) associated with this article can
be found, in the online version, at doi:10.1016/j.tetlet.2011.08.025.

5348
T. Takanami et al. / Tetrahedron Letters 52 (2011) 5345–5348
(50 mg, 0.2 mmol), and the mixture was stirred at 140 °C for 2 h. After
References and notes
completion of the reaction (monitored by TLC), the solution was poured into
water (80 mL). The resulting precipitate was collected and washed with water
and MeOH to give the Ni(II) complex in a nearly quantitative yield.
1. For a review, see: The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard,
R., Eds.; Academic Press: San Diego, 1999–2003; Vol. 1-20,. and some recent
examples, see Refs. 2–5.
Complex Ni-3aa: 1H NMR (CDCl3) d: 11.98 (1H, s, –CHO), 9.75 (2H, d,
J = 5.1 Hz), 8.89 (2H, d, J = 5.1 Hz), 8.82 (2H, d, J = 5.1 Hz), 8.65 (2H, d,
J = 5.1 Hz), 7.95–7.89 (4H, m), 7.71–7.69 (6H, m), 7.07 (1H, s, O@C–CH@C–),
3.50–3.44 (2H, m), 2.97–2.93 (2H, m); IR (KBr): 1705, 1674, 1605, 1566, 1439,
1358, 1277, 1161, 1076, 1007, 949, 791, 752, 702 cmꢀ1; UV/vis (CH2Cl2) kmax
2. (a) Lu, H.; Zhang, X. P. Chem. Soc. Rev. 2011, 40, 1899; (b) Che, C.-M.; Lo, V. K.-Y.;
Zhou, C.-Y.; Huang, J.-S. Chem. Soc. Rev. 2011, 40, 1950; (c) Morandi, B.; Cheang,
J.; Carreira, E. M. Org. Lett. 2011, 13, 3080; (d) Dogutan, D. K.; McGuire, R., Jr.;
Nocera, D. G. J. Am. Chem. Soc. 2011, 133, 9178; (e) Usanov, D. L.; Yamamoto, H.
Angew. Chem., Int. Ed. 2010, 49, 8169; (f) Fackler, P.; Berthold, C.; Voss, F.; Bach,
T. J. Am. Chem. Soc. 2010, 132, 15911; (g) Morandi, B.; Carreira, E. M. Angew.
Chem., Int. Ed. 2010, 49, 938; (h) Che, C.-M.; Huang, J.-S. Chem. Commun. 2009,
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S.; Ng, D. K. P. Chem. Eur. J. 2011, 17, 1605; (c) Song, J.; Aratani, N.; Heo, J. H.;
Kim, D.; Shinokubo, H.; Osuka, A. J. Am. Chem. Soc. 2010, 132, 11868; (d) Bessho,
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(log e): 423 (5.2), 549 (4.1), 596 (4.2) nm; HRMS (EI): calcd for C38H24N4NiO2:
626.1253; found: 626.1252.
Complex Ni-4aa: 1H NMR (CDCl3) d: 11.88 (1H, s, –CHO), 9.68 (2H, d,
J = 5.1 Hz), 9.09 (2H, d, J = 5.1 Hz), 8.71 (2H, d, J = 5.1 Hz), 8.57 (2H, d,
J = 5.1 Hz), 7.92–7.86 (4H, m), 7.71–7.62 (6H, m), 5.31–5.20 (1H, m, O@C–
CH2–CH–Por), 3.56 (1H, dd, J = 19.5 and 11.5 Hz, O@C–CHH–CH–Por), 3.33 (1H,
dd, J = 19.5 and 10.0 Hz, O@C–CHH–CH–Por), 3.30–3.26 (1H, m), 3.09–3.02 (1H,
m), 2.97–2.90 (1H, m), 2.81–2.71 (1H, m); IR (KBr): 1742, 1673, 1541, 1441,
1354, 1236, 1162, 1077, 1009, 949, 793, 756, 704 cmꢀ1; UV/vis (CH2Cl2) kmax
(log e): 426 (5.3), 556 (4.0), 602 (4.2) nm; HRMS (EI): calcd for C38H26N4NiO2:
628.1409; found: 628.1406.
13. General procedure for the one-pot preparation of meso activated alkenyl-
substituted meso-formylporphyrins: An oven-dried 100 mL three-necked flask
equipped with a magnetic stirring bar, a reflux condenser, and rubber septum
was charged with a porphyrin 1 (0.2 mmol). The flask was evacuated and
flushed with argon (three times), and then absolute THF (80 mL) was added. To
the solution was added an ethereal solution of PyMe2SiCH2Li (prepared by
adding 1.3 mL of 1.58 M tBuLi in pentane to
a solution of 2.4 mmol 2-
pyridyltrimethylsilane in 3 mL of ether, followed by stirring at ꢀ78 °C for 2 h)5
via a cannula at ꢀ78 °C. After being stirred at ꢀ78 °C for 5 min, the cooling bath
was removed and the mixture was stirred at room temperature. The reaction
was complete within 3 h, having been monitored by TLC. Upon completion of
the reaction, the mixture was cooled to ꢀ78 °C, and then an ethereal solution
of an enone or an alkenoate (3 mmol) and TMSCl (4 mmol, 0.51 mL) was added.
After being stirred at ꢀ78 °C for 10 min, the mixture was warmed to room
temperature, stirred for 3 h, and recooled to 0 °C. To the cooled mixture was
added 10 mL of 0.1 M HCl. The resulting solution was stirred for 10 min at 0 °C,
and then DDQ (2 mmol, 454 mg) was added. After being stirred at room
temperature for 12 h, the mixture was concentrated under a reduced pressure.
The resulting residue was dissolved in CH2Cl2 (ca. 100 mL) and the solution
was poured into brine. The organic layer was separated and the aqueous layer
was extracted with CH2Cl2. The organic layers were combined, concentrated in
vacuo, and subjected to chromatography on silica gel using 0–10% hexane/
CH2Cl2 as an eluent. The eluate was evaporated and the resulting solid was
purified by recrystallization from CH2Cl2/hexane to afford the meso activated
alkenyl-substituted meso-formylporphyrin 3.
6. For recent reviews of functionalization reactions of porphyrins, see: (a) Senge,
M. O. Chem. Commun. 2011, 47, 1943; (b) Shinokubo, H.; Osuka, A. Chem.
Commun. 2009, 1011.
7. We have reported several functionalization reactions of porphyrins: (a)
Takanami, T.; Wakita, A.; Matsumoto, J.; Sekine, S.; Suda, K. Chem. Commun.
2009, 101; (b) Takanami, T.; Wakita, A.; Sawaizumi, A.; Iso, K.; Onodera, H.;
Suda, K. Org. Lett. 2008, 10, 685; (c) Takanami, T.; Matsumoto, J.; Kumagai, Y.;
Sawaizumi, A.; Suda, K. Tetrahedron Lett. 2009, 50, 68; (d) Takanami, T.;
Yotsukura, M.; Inoue, W.; Inoue, N.; Hino, F.; Suda, K. Heterocycles 2008, 76,
439; (e) Takanami, T.; Hayashi, M.; Chijimatsu, H.; Inoue, W.; Suda, K. Org. Lett.
2005, 7, 3937; (f) Takanami, T.; Hayashi, M.; Hino, F.; Suda, K. Tetrahedron Lett.
2003, 44, 7353.
8. Multi-step total syntheses of asymmetrically substituted porphyrins have been
reported, see: (a) Lindsey, J. S. Acc. Chem. Res. 2010, 43, 300; (b) Fazekas, M.;
Pintea, M.; Senge, M. O.; Zawadzka, M. Tetrahedron Lett. 2008, 49, 2236.
9. Porphyrin modification based on the SNAr strategy with organolithium
reagents was pioneered by the research group of Senge, see: (a) Senge, M. O.
Acc. Chem. Res. 2005, 38, 733; (b) Senge, M. O.; Hatscher, S. S.; Wiehe, A.;
Dahms, K.; Kelling, A. J. Am. Chem. Soc. 2004, 126, 13634; (c) Dahms, K.; Senge,
M. O.; Bakar, M. B. Eur. J. Org. Chem. 2007, 3833; (d) Senge, M. O.; Richter, J.;
Bischoff, I.; Ryan, A. Tetrahedron 2010, 66, 3508. and references cited therein.
10. Itami, K.; Kamei, T.; Mitsudo, K.; Nokami, T.; Yoshida, J. J. Org. Chem. 2001, 66,
3970.
11. Neutralization of the reaction mixture prior to the addition of DDQ is critical to
obtain the desired product 3aa: when the reaction was conducted without
neutralization, significant amounts of oligomeric side products were noted.
12. NMR data were not available for 4aa due to its poor solubility. Therefore, the
structural differences between the enone moiety directly coupled adduct 3aa
and the Michael type adduct 4aa were confirmed by the spectroscopic data of
the corresponding Ni(II) complexes Ni-3aa and Ni-4aa. The procedure for the
conversion of the free bases to the Ni(II) complexes is as follows: To a solution
of a free base porphyrin (0.04 mmol) in DMF (3 mL) was added Ni(OAc)2ꢁ4H2O
14. In the cases of the preparation of 3ag and 3fb, impurities could not be removed
from the crude products by recrystallization. These free bases, therefore, were
further converted to their Ni(II) complexes, Ni-3ag and Ni-3fb, using a similar
procedure for the preparation of Ni-3aa and Ni-4aa, see Ref. 12.
15. All the compounds reported herein showed spectral data consistent with the
assigned structures, see Supplementary data.
16. All our attempts for isolating the porphodimethene intermediates 6 were
unsuccessful probably because of their high susceptibility to moisture and
oxygen in atmosphere.
17. It is known that DDQ can serve as an oxidant for the conversion of enol silyl
ethers into enones, see: Bhattacharya, A.; DiMichele, L. M.; Dolling, U.-H.;
Grabowski, E. J. J.; Grenda, V. J. J. Org. Chem. 1989, 54, 6118.
18. Preparation of meso activated alkenyl substituted porphyrins has been
reported, see: (a) Yeung, M.; Ng, A. C. H.; Drew, M. G. B.; Vorpagel, E.;
Breitung, E. M.; McMahon, R. J.; Ng, D. K. P. J. Org. Chem. 1998, 63, 7143; (b)
Locos, O. B.; Arnold, D. P. Org. Biomol. Chem. 2006, 4, 902; (c) Horn, S.; Sergeeva,
N. N.; Senge, M. O. J. Org. Chem. 2007, 72, 5414; (d) Sergeeva, N. N.; Bakar, M. B.;
Senge, M. O. J. Org. Chem. 2009, 74, 1488.