Synthesis of porphyryl boronates with (un)saturated side-chains

Article abstract of DOI:10.1016/j.jorganchem.2008.05.006

Porphyrins with (un)saturated side-chains containing boron residues were developed as synthons for porphyrin functionalisation. Porphyrins with mono and bis-substituted unsaturated boronyl residues were prepared in good yields (52-66%) using a cross-metathesis approach in the presence of Grubbs I-generation catalysts. In all cases complete E-stereoselectivity (100%) was observed. Furthermore, formal cross-metathesis products with α,β-unsaturated chains smoothly underwent addition with bis(pinacolato)diboron [(Me₄C₂O₂)B-B(O₂C₂Me₄)] to yield the corresponding saturated boron compounds in 60-70% yields.

Full text of DOI:10.1016/j.jorganchem.2008.05.006

Journal of Organometallic Chemistry 693 (2008) 2637–2640
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Note
Synthesis of porphyryl boronates with (un)saturated side-chains
*
Natalia N. Sergeeva, Vanesa López Pablo, Mathias O. Senge
School of Chemistry, SFI Tetrapyrrole Laboratory, Trinity College Dublin, Dublin 2, Ireland
a r t i c l e i n f o
a b s t r a c t
Article history:
Porphyrins with (un)saturated side–chains containing boron residues were developed as synthons for
porphyrin functionalisation. Porphyrins with mono and bis-substituted unsaturated boronyl residues
were prepared in good yields (52–66%) using a cross-metathesis approach in the presence of Grubbs I-
generation catalysts. In all cases complete E-stereoselectivity (100%) was observed. Furthermore, formal
Received 26 February 2008
Received in revised form 30 April 2008
Accepted 6 May 2008
Available online 13 May 2008
cross-metathesis products with a,b-unsaturated chains smoothly underwent addition with bis(pinacola-
to)diboron [(Me4C2O2)B-B(O2C2Me4)] to yield the corresponding saturated boron compounds in 60–70%
yields.
Keywords:
Boron
Porphyrin
Cross-metathesis
Ru-catalyst
Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction
proach. Nevertheless, it is known CM to be limited by its lack of
predictability and stereoselectivity of the final products and empir-
Applications of metal-catalysed coupling reactions continue to
grow rapidly and require easy access to novel heteroatomic build-
ing blocks. The continuing development of new catalysts with
widespread activities expands the variety of functionalities to be
introduced into the substrates and allows the preparation of more
elaborate and multifunctional synthons. One of the best and pow-
erful examples known is boron containing compounds, which
serve as excellent precursors in many areas of synthetic chemistry
[1]. Moreover, new strategies accessing molecules of choice have
being developed in order to extent the scope of the available boron
compounds.
The many possible applications of porphyrins require the devel-
opment of new and more efficient reactions to introduce functional
groups into the macrocycle [2]. So far, only few strategies for the
synthesis of porphyrin building blocks containing boronyl residues
either directly attached at the meso/b-positions or via aromatic
linkers have been described [3].
ical models for selective CM have not yet been developed [5,6].
First attempts to use ‘‘second generation” Grubbs catalyst Ru-II
(Fig. 1) were unsuccessful, no reaction occurred. Modification of the
reaction conditions such as solvents, catalysts and temperature, fi-
nally resulted in the formation of the boronyl porphyrins 2 in good
yields (Scheme 1). The allyl-porphyrins 1a–d could be easily con-
verted under heating to reflux in CH2Cl2 into the corresponding
boronyl compounds 2 in the presence of Ru-I (0.1–0.2 equiv.) only.
In contrast to reports in the literature, modification of b-vinyl
chlorins and b-vinyl porphyrins catalysed by a ‘‘second generation”
Grubbs’ catalyst [7] involving b-substituents was successfully
accomplished. However, meso-vinyl porphyrins did not participate
in CM reactions with vinyl boronates either, in the presence of Ru-I
or Ru-II catalysts. This fact strongly underlines the differences in
the nature of the meso and b positions of the macrocycle in the cat-
alytic cycle, where electronic effects play a significant role in pro-
moting the reaction. Noteworthy, more reactive allyl boronates,
did not undergo CM reaction with the corresponding allyl or/and
vinyl porphyrins. Similarly, self-dimerisation of the vinyl-porphy-
rins did not take place with either catalyst. This is contrasted by
the reaction of b-allyl porphyrins, which could be utilised for the
preparation of benzoporphyrins via olefin ring-closure metathesis
as described by Liu et al. [8].
2. Results and discussion
Here, we present results aimed at a novel strategy for the syn-
thesis of alkenyl 2 and saturated 5 boronyl porphyrins, which were
not previously accessible. Recently, we reported on the preparation
of allyl porphyrins 1 via standard Suzuki reactions [4]. This
prompted us to investigate their utility as versatile precursors for
the synthesis of boronyl porphyrins using a cross-metathesis ap-
Significantly, the CM products 2 obtained here were all formed
exclusively as the E-isomers. This was easily confirmed by 1H NMR
spectroscopy. The J coupling constants calculated were in the range
of 15.8–17.8 Hz and strongly indicate a trans stereochemistry for
all compounds in this series.
It is well known that
1,4-addition reactions with reagents such as cuprates or boronates.
a,b-unsaturated compounds can undergo
* Corresponding author. Tel.: +353 1 896 8537; fax: +353 1 896 8536.
E-mail address: sengem@tcd.ie (M.O. Senge).
0022-328X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2008.05.006

2638
N.N. Sergeeva et al. / Journal of Organometallic Chemistry 693 (2008) 2637–2640
However, the novel metallated CM products 4 were easily
MesN
NMes
Ph
transformed into the boronates via 1,4-addition reactions
(Scheme 2). For example, compounds 4a and 4b reacted with
(pinacolato)diboron [(Me4C2O2)B-B(O2C2Me4)] in the presence of
CuCl/KOAc to yield the corresponding bisboronated adducts 5 in
up to 70% yield. Interestingly, these transformations are very sus-
ceptible to the reaction temperature. While compound 5b was eas-
ily prepared at 55 °C a further increase in temperature resulted in
rapid decomposition. An even higher sensitivity was observed in
for the zinc complex 5a. A reaction temperature of 45 °C caused ra-
pid decomposition of the products formed, while the reaction at
room temperature proceeded smoothly to give 5a in 70% yield.
The syntheses described offer a convenient approach to the
novel boronyl functionalised porphyrins 2 and 5 and expand the
repertoire of reagents to be utilised in metal-catalysed coupling
reactions for the functionalisation of the porphyrin periphery.
Based on the rich chemistry of boron derivatives of aromatic
compounds [1], the porphyrylboronates presented can serve as
versatile precursor molecules for the facile introduction of carbon-
heteroatom bonds in to a macrocycle. The boronated porphyryl
species constitute a new class of synthons for porphyrin-based
materials via metal-catalysed C–C bond-forming reactions or hyd-
roboration polymerisation. Compounds of type 5 with saturated
chains are an intriguing new class of porphyrins for transformation
into porphyrin bioconjugates, e.g., amino and hydroxy acid deriva-
tives, especially for medicinal applications.
PCy3
Ru
PCy3
Cl
Cl
Cl
Cl
Ru
PCy3
Ph
Ru-I
Ru-II
Fig. 1. Type I and II generations of Ru-catalysts used in this study.
R
R
N
N
N
N
N
N
N
N
R'
R'
M
R
M
R
B
O
O
1a R: 4-MeC6H4; R': allyl; M: 2H
1b R: 3-MeOC6H4; R': allyl; M: 2H
1c R: 3-MeOC6H4; R': allyl; M: Zn(II)
1d R, R': Ph; M: 2H
2a R': CH2CH=CHB(OCH(CH3)2)2 52 %
2b R': CH2CH=CHB(OCH(CH3)2)2 50 %
2c R': CH2CH=CHB(OCH(CH3)2)2 65 %
2d R': Ph
66 %
Scheme 1. Synthesis of compound 2 via cross-metathesis of allyl porphyrin 1.
Reaction conditions: porphyrin (1 equiv.), vinyl boronic acid pinacol ester (10–20
equiv.) and Ru–I (0.1–0.2 equiv.), CH2Cl2, reflux 12–24 h under argon.
For example, Takahashi et al. reported that a, b-unsaturated com-
pounds undergo boronation with (pinacolato)diboron [(Me4-
C2O2)B-B(O2C2Me4)] [9]. This strategy offers the possibility to
prepare saturated boronyl porphyrins from the corresponding
3. Experimental
a
,b-unsaturated porphyrins. We found that a,b-unsaturated por-
General experimental conditions were as described earlier
[10,11]. Grubbs catalysts Ru-I and Ru-II were purchased from Sig-
ma–Aldrich.
phyrins can also be prepared using a CM-approach. Porphyrins
1b and 5,15-diallyl-10,20-diphenylporphyrin 1e [4] reacted
smoothly with ethyl vinyl ketone in the presence of Grubbs II
Ru-II, forming the corresponding compounds 3a and 3b in excel-
lent yields of 94%, each.
3.1. Synthesis of compounds 1 via Suzuki reaction [4]
Compounds 1a–c,e were prepared as reported elsewhere [4].
3.1.1. Compound (1d)
R
COEt
NH
N
N
Yield: 89%. M.p. >250 °C. UV/Vis (CH3CO2Et): kmax(loge) 415
(5.5), 513 (4.3), 548 (4.1), 593 (4.0), 648 (4.0) nm. 1H NMR
(400 MHz, CDCl3): À2.69 (s, 2H), 5.22 (dd, J = 10.5 Hz, 16.9 Hz,
2H), 5.82 (d, J = 5.2 Hz, 2H), 6.89 (m, 1H), 7.78 (m, 9H), 8.25 (m,
6H), 8.84 (br, 4H), 8.95 (d, J = 4.7 Hz, 2H), 9.51 (d, J = 4.7 Hz, 2H).
13C NMR (100.6 MHz, CDCl3): 38.6, 115.8, 119.3, 119.4, 126.2,
126.3, 127.3, 127.8 (br), 130.8 (br), 134.1, 141.1, 141.6, 141.9.
HRMS (ES+): C41H30N4 calc. for [M+H+] 579.2543, found 579.2536.
HN
EtOC
R
3a R: Ph
94 %
3b R: 3-MeOC6H4 94 %
Attempts to use these porphyrins for 1,4-addition reactions to
3.2. General procedure for the synthesis of compounds 2 and 3 via
a,b-unsaturated compounds showed that the free base porphyrins
cross-metathesis
3 were very sensitive to the reaction conditions ([(Me4C2O2)B-
B(O2C2Me4)]) and fast decomposition of the boronated complexes
took place.
A
solution of porphyrin 1 (0.1 mmol) and Ru–catalysts
(0.2 mmol) in 30 mL of CH2Cl2 was flushed with argon and an
R
R
O
B
O
O
O
O
EtOC
B B
N
N
N
N
N
N
N
N
O
O
Et
Et
M
R
M
R
CuCl-KOAc, DMF
O
COEt
B
O
O
5a R: Ph; M: Zn(II)
70%
4a M: Zn(II)
4b M: Ni(II)
5b R: 3-MeOC H ; M: Ni(II) 60%
6
4
Scheme 2. Boronation of CM compounds 4. Reaction conditions: porphyrin (1 equiv., ꢀ30 mg), (pinacolato)diboron 6 (4–6 equiv.), CuCl (0.1 equiv.), KOAc (0.1 equiv.), DMF
(3–5 mL), r.t. (for 5a), 55 °C (for 5b), argon.

N.N. Sergeeva et al. / Journal of Organometallic Chemistry 693 (2008) 2637–2640
2639
appropriate olefin (1–2 mmol) was added. The reaction mixture
was flushed with argon again and refluxed under argon for 12–
24 h (TLC–control). The mixture was filtered through silica gel
and the volatiles were removed under reduced pressure. The resi-
due was purified by column chromatography on Silica with CH2Cl2/
hexanes and recrystallised from MeOH–CH2Cl2 to yield the corre-
sponding product 2. For the synthesis of compound 2 Ru-I and
for the compound 3 and 4, Ru II have been used.
CDCl3): 7.3, 33.1, 36.9, 55.1, 113.1 (m), 119.1, 120.0, 1270, 127.1,
127.4, 130.9, 131.9, 142.9, 147.4, 157.5, 200.5. HRMS (ES+):
C46H42N4O4 calc. for [M+H] 715.3284, found 715.3286.
3.2.7. Compound (4a)
Compound 4a was obtained by metallation of 3a with
Zn(OAc)2 Á 2H2O in CH2Cl2–MeOH according to standard procedure
[2].
Yield: 87%. M.p. >250 °C. UV/Vis (CH2Cl2): kmax(loge) 421 (6.0),
3.2.1. Compound (2a)
551 (4.7) nm. 1H NMR (400 MHz, CDCl3): 0.86 (t, J = 7.0 Hz, 6H),
2.31 (q, J = 7.0 Hz, 4H), 5.74 (d, J = 5.8 Hz, 4H), 6.02 (d, J = 15.8 Hz,
2H), 7.40 (dt, J = 5.8 Hz, 15.8 Hz, 2H), 7.80 (m, 6H), 8.18 (m, 4H),
8.94 (d, J = 4.7 Hz, 4H), 9.32 (d, J = 4.7 Hz, 4H). 13C NMR
(100.6 MHz, CDCl3): 7.4, 32.8, 37.4, 114.1, 120.3, 126.2, 127.2,
128.4, 130.6, 132.6, 133.9, 142.1, 147.8, 149.4, 149.8, 200.6.
Yield: 52%. M.p. >250 °C. UV/Vis (CH3CO2Et): kmax(loge) 420
(5.5), 519 (4.7) nm. 1H NMR (400 MHz, CDCl3): À2.67 (s, 2H),
1.15 (s, 24H), 2.75 (s, 6H), 5.66 (d, J = 17.6 Hz, 1H), 5.85
(d, J = 5.9 Hz, 2H), 7.58, (m, 4H), 7.59 (dt, J = 5.9 Hz, 17.6 Hz, 2H),
8.09 (m, 4H), 8.90 (d, J = 4.7 Hz, 4H), 9.41 (d, J = 4.7 Hz, 4H). 13C
NMR (100.6 MHz, CDCl3): 21.1, 24.8, 40.5, 82.6, 114.4, 118.9,
126.8, 126.3, 126.8, 133.9, 136.8, 139.1, 154.9. HRMS (ES+):
3.2.8. Compound (4b)
Compound 4b was obtained by metallation of 3b with Ni(acac)2
in toluene according to standard procedure [2].
C
52H56B2N4O4 calc. for [M+H+] 823.4560, found 823.4552.
3.2.2. Compound (2b)
Yield: 73%. M.p. 250 °C. UV/Vis (CH2Cl2): kmax(loge) 417 (5.5),
Yield: 50%. M.p. >250 °C. 1H NMR (400 MHz, CDCl3): À2.68 (br,
2H), 1.16 (s, 24H), 4.00 (s, 6H), 5.65 (d, J = 18.1 Hz, 2H), 5.85
(d, J = 4.7 Hz, 4H), 7.37 (m, 2H), 7.62 (dt, J = 4.7 Hz, 18.1 Hz, 2H),
7.68 (m, 2H), 7.82 (m, 4H), 8.91 (d, J = 4.7 Hz, 4H), 9.41
(d, J = 4.7 Hz, 4H). 13C NMR (100.6 MHz, CDCl3): 24.3, 40.4, 55.1,
82.7, 113.3, 114.1, 118.6, 119.7, 126.3, 127.1, 127.6, 131.5, 143.4,
154.9, 157.4.
533 (4.5) nm. 1H NMR (400 MHz, CDCl3): 0.99 (t, J = 4.8 Hz, 6H),
2.42 (q, J = 4.8 Hz, 4H), 3.97 (s, 6H), 5.46 (d, J = 4.0 Hz, 4H). 6.19
(d, J = 10.6 Hz, 2H), 7.31 (m, 2H), 7.55 (m, 2H), 7.61 (m, 4H), 7.65
(dt, J = 4.0 Hz, 10.6 Hz, 2H), 8.84 (d, J = 3.0 Hz, 4H), 9.18
(d, J = 3.0 Hz, 4H). 13C NMR (100.6 MHz, CDCl3): 7.7, 33.3, 36.4,
55.3, 112.0, 113.4, 118.2, 119.5, 126.5, 127.7, 129.2, 131.0, 133.1,
141.6, 141.8, 142.3, 147.2, 158.0, 200.8.
3.2.3. Compound (2c)
3.3. General procedure for the synthesis of compounds 5 via
Yield: 65%. M.p. 195 °C. UV/Vis (CH2Cl2): kmax(log
e
) 421 (6.0), 550
1,4-addition
(4.8) nm. 1H NMR (400 MHz, CDCl3): 1.16 (s, 24H), 4.01 (s, 6H), 5.65
(d, J = 17.6 Hz, 2H), 5.88 (d, J = 4.9 Hz, 4H), 7.36 (m, 2H), 7.71 (m, 8H),
9.01 (d, J = 4.0 Hz, 4H), 9.52 (d, J = 4.0 Hz, 4H). 13C NMR (100.6 MHz,
CDCl3): 24.7, 41.2, 55.5, 83.1, 113.6, 116.1, 120.0, 120.1, 127.3, 127.5,
A solution of KOAc (1 mg), CuCl (1 mg) and (pinacolato)diboron
6 (0.2335 mmol, 59.3 mg) in 2 mL of anhydrous DMF was stirred
for 10–15 min (color change to green–blue). Porphyrin
4
129.5, 132.5, 144.2, 149.5, 150.6, 155.7, 157.8. MS (ES+): C52H54
-
(0.0584 mmol) was added as a solid and the reaction was stirred
for 20 min (TLC-control). For 5a the reaction was stirred at r.t.
For 5 b the mixture was warmed up to 55 °C. The final solution
was washed with water and NH4Cl (sat), extracted with CH2Cl2
and dried with Na2SO4. The crude product was purified by column
chromatography (CH3CO2Et:hexane = 1:2) to give 5.
B2N4O6Zn calc. for [M+H3O+] 935.4, found 935.5.
3.2.4. Compound (2d)
Yield: 66%. M.p. >250 °C. UV/Vis (CH3CO2Et): kmax(loge) 415
(5.6), 513 (4.4), 548 (4.3), 593 (4.2), 650 (4.2) nm. 1H NMR
(400 MHz, CDCl3): À2.72 (s, 2H), 1.16 (s, 12H), 5.66
(d, J = 17.8 Hz, 1H), 5.91 (d, J = 5.8 Hz, 2H), 7.64 (d, J = 5.8 Hz,
17.8 Hz, 1H), 7.78 (m, 9H), 8.24 (m, 6H), 8.82 (br, 4H), 8.92
(d, J = 5.0 Hz, 2H), 9.51 (d, J = 5.0 Hz, 2H). 13C NMR (100.6 MHz,
CDCl3): 24.2, 40.6, 82.7, 115.0, 119.2, 119.3, 126.2, 126.3, 127.2,
130.5 (br), 134.0, 134.1, 141.6, 141.9, 154.9. HRMS (ES+):
3.3.1. Compound (5a)
Yield: 70%. M.p. >250 °C. UV/Vis (CH2Cl2): kmax(loge) 421 (6.2),
552 (5.0) nm. 1H NMR (600 MHz, CDCl3): d 0.91 (m, 6H), 1.28 (m,
12H), 1.39 (m, 12H), 1.49 (m, 2H), 2.09 (m, 2H), 2.41 (m, 4H),
2.55 (m, 2H), 4.93 (m, 2H), 5.45 (m, 2H), 7.80 (m, 6H), 8.22
(m, 4H), 8.96 (d, J = 4.5 Hz, 4H), 9.56 (br, 4H). 13C NMR
(150.9 MHz, CDCl3): d 7.7, 24.7, 28.0, 34.1 (d), 35.3, 43.0 (d), 83.2,
119.8, 120.0 (d), 126.3, 127.2, 129.2, 132.0, 134.1, 134.2, 134.3,
134.4, 143.0, 149.2, 150.5, 211.7. MS (ES): C56H62B2N4O8Zn calc.
for [M+OHÀ] 989.2, found 989.4.
C
47H41BN4O4 calc. for [M+H+] 705.3395, found 705.3371.
3.2.5. Compound (3a)
Yield: 94%. M.p. 193 °C. UV/Vis (CH3CO2Et): kmax(loge) 416 (5.3),
516 (4.3), 548 (4.2), 593 (4.2), 656 (4.2) nm. 1H NMR (400 MHz,
CDCl3): À2.67 (br, 2H), 0.91 (t, J = 7.6 Hz, 6H), 2.32 (q, J = 7.6 Hz,
4H), 5.92 (d, J = 5.2 Hz, 4H), 6.07 (d, J = 15.8 Hz, 2H), 7.79 (m, 8H),
8.21 (m, 4H), 8.91 (d, J = 4.6 Hz, 4H), 9.37 (d, J = 4.6 Hz, 4H).
13C NMR (100.6 MHz, CDCl3): 7.3, 33.1, 37.0, 113.1, 119.4, 126.1,
126.2, 127.3, 127.4, 130.9, 132.0, 134.0, 140.8, 141.6, 146.4,
147.4, 200.5. HRMS (ES+): C44H38N4O2 calc. for [M+H] 655.3073,
found 655.3082.
3.3.2. Compound (5b)
Yield: 60%. M.p. 255 °C. UV/Vis (CH2Cl2): kmax(loge) 418 (6.3),
533 (5.3) nm. 1H NMR (600 MHz, CDCl3): d 0.83 (m, 6H), 1.30 (br,
24H), 1.93 (m, 10H), 3.97 (s, 6H), 4.54 (m, 2H), 5.16 (m, 2H), 7.29
(m, 2H), 7.57 (m, 6H), 8.80 (d, J = 4.1 Hz, 4H), 9.29 (br, 4H). 13C
NMR (150.9 MHz, CDCl3) d = 7.6, 24.7, 25.4, 32.3 (d), 35.1, 42.3
(d), 55.3, 83.2, 113.3, 117.2, 117.4, 119.5, 126.5, 127.6, 130.0,
132.4, 141.2, 142.0, 142.5, 158.0, 211.5. MS (ES): C58H66B2N4NiO8
calc. for [M+H+] 1027.5, found 1027.9.
3.2.6. Compound (3b)
Yield: 94%. M.p. 220 °C. UV/Vis (ethyl acetate): kmax(loge) 421
(5.4), 517 (4.3), 558 (4.1), 592 (4.1), 652 (4.0) nm. 1H NMR
(400 MHz, CDCl3): 2.68 (br, 2H), 0.93 (t, J = 7.3 Hz, 6H), 2.34 (q,
J = 7.3 Hz, 4H), 4.04 (s, 6H), 5.89 (d, J = 5.8 Hz, 4H). 6.07 (d,
J = 15.8 Hz, 2H), 7.40 (m, 2H), 7.69 (m, 2H), 7.80 (m, 6H), 8.97 (d,
J = 4.7 Hz, 4H), 9.35 (d, J = 4.7 Hz, 4H). 13C NMR (100.6 MHz,
Acknowledgement
This work was generously supported by a Science Foundation
Ireland Research Professorship Grant (SFI 04/RP1/B482).

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N.N. Sergeeva et al. / Journal of Organometallic Chemistry 693 (2008) 2637–2640
[4] S. Horn, N.N. Sergeeva, M.O. Senge, J. Org. Chem. 72 (2007) 5414.
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