Microwave-promoted insertion of Group 10 metals into free base porphyrins and chlorins: Scope and limitations

Article abstract of DOI:10.1039/b716181f

The scope and limitations of microwave heating as a tool for insertion of Group 10 metals into meso-tetraphenyl-porphyrin, -porpholactone, and -2,3-dihydroxychlorin derivatives are discussed. In some cases it is possible to reduce reaction times dramatically while obtaining good yields of the metallated products while in others new issues arise relating to the metal salt used as well as acceleration not only of the metallation reaction but also of byproduct formation. The Royal Society of Chemistry.

Full text of DOI:10.1039/b716181f

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www.rsc.org/dalton | Dalton Transactions
Microwave-promoted insertion of Group 10 metals into free base porphyrins
and chlorins: scope and limitations
Michelle L. Dean, Jason R. Schmink, Nicholas E. Leadbeater* and Christian Bru¨ckner
Received 19th October 2007, Accepted 12th December 2007
First published as an Advance Article on the web 15th January 2008
DOI: 10.1039/b716181f
The scope and limitations of microwave heating as a tool for insertion of Group 10 metals into
meso-tetraphenyl-porphyrin, -porpholactone, and -2,3-dihydroxychlorin derivatives are discussed. In
some cases it is possible to reduce reaction times dramatically while obtaining good yields of the
metallated products while in others new issues arise relating to the metal salt used as well as
acceleration not only of the metallation reaction but also of byproduct formation.
somewhat milder method for the synthesis of Pd(II) chlorins was
devised (Pd(II)(acac)2 in hot toluene), though the success of the
Introduction
insertions were strongly concentration-dependent.6 Pt(II) insertion
is generally accomplished using only the harshest methods, i.e.
refluxing benzonitrile for days.2 It thus does not surprise that
reports of Pd(II) and Pt(II) chlorins are rare.7 On the other hand,
insertion of the metal ion into a robust porphyrinic precursor,
followed by the conversion of the pophyrin into a chlorin is also
not always possible or practical.8
Metalloporpyrins find use in applications as broad-ranging
as light-harvesting systems, chemotherapeutics, light-emitting
diodes, or chemical sensors.1 The overwhelming majority of
preparative routes to metalloporphyrins involve a metathesis
reaction between free base porphyrin and an appropriate metal
salt, generating the metalloporphyrin and the corresponding
conjugated anion acid (eqn (1)).2
In the organic chemistry community, the use of microwave
heating has been widely accepted as a tool for reactions that
conventionally require long times at elevated temperatures.9 It
is often possible to reduce reactions times dramatically while at
the same time improving product yields. There are many fewer
examples of the use of microwave heating as a tool for the
preparation of inorganic and organometallic complexes.10
Microwave heating occurs on a molecular level as opposed to
relying on convection currents and thermal conductivity when
using conventional heating methods.11 This offers an explanation
as to why microwave reactions are so much faster. With microwave
irradiation, since the energy is interacting with the molecules at
a very fast rate, the molecules do not have time to relax and
the heat generated can be, for short times, much greater than
the overall recorded temperature of the bulk reaction mixture. In
essence, there will be sites of instantaneous localized superheating
where reactions will take place much faster than in the bulk. This
localized superheating can be especially marked when the reaction
mixture contains highly polar reagents or metal salts.
While a range of free-base porphyrins have been prepared using
microwave heating and scattered reports have appeared using
microwaves to insert metal ions, including Group 2, 3 metals,
lanthanides, and Ni(II), we are not aware of any systematic study.12
With this in mind, we systematically tested the suitability of
microwave heating as a tool for inserting Group 10 metal ions into
meso-tetraphenyl-porphyrins and -chlorins, representatives of the
most commonly used synthetic porphyrin classes. The results of
this investigation are reported here.
Por-H2 + [MII(anion)2] ꢀ [PorMII] + 2 anion-H
(1)
Although thermodynamically favorable, metal insertions are
often kinetically inhibited due to the relative inflexibility of
the porphyrin ligand as well as the nature of the metal ion
used. Therefore, the reaction conditions may range from mild to
exceedingly harsh.1
Group 10 metalloporphyrins are a widely studied group of met-
alloporphyrins. The d8 configuration of their +II oxidation states
results in the formation of neutral metalloporphyrins containing
metal ions that are coordinated in square planar fashion.2 Ni(II)
is frequently used as a protecting group for the central nitrogens.3
The classic metal insertion conditions are using a stoichiometric
excess of Ni(II) chloride or acetate in refluxing pyridine (bp =
115 ◦C) or DMF (bp = 153 ◦C) for hours to days,2 conditions that
may not be suitable for more sensitive porphyrins. Pd(II) and Pt(II)
metalloporphyrins possess interesting luminescence properties.
Consequently, they have found use as, e. g., oxygen sensors and
pressure-sensitive paints.4,5 These applications might require the
adjustment of the optical properties of the metalloporphyrin used.
This can potentially be accomplished by modification of the
porphyrinic chromophore, for instance, by using the Pd(II) and
Pt(II) complexes of chlorins (2,3-dihydroporphyrins) and bacteri-
ochlorins (2,3,12,13-tetrahydroporphyrins). This poses, however,
a formidable problem: The classic insertion methods for Pd(II)
are refluxing of Pd(II) chloride in DMF or benzonitrile (bp =
191 ◦C), with reaction times that can stretch from 45 min to days.
Chlorins are, however, sensitive toward (Pd(II)-induced) oxidations
and, thus, may not be susceptible to these reaction conditions. A
Results and discussion
Department of Chemistry, University of Connecticut, Unit 3060, CT, 06269-
3060, USA. E-mail: nicholas.leadbeater@uconn.edu; Fax: +1 860 486 2981;
Tel: +1 860 486 5076
Most reports of using microwave-promoted metallations of por-
phyrins used domestic microwave ovens.12 However, there are
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Table 1 Microwave-promoted metallation of porphyrins and chlorins
Entry
Substrate
Metal salt
Solventa
Conditionsb
Product,% yieldc
1
2
3
1
1
2
3 equiv. Ni(OAc)2·4H2O
1 equiv. Ni(OAc)2·4H2O
3 equiv. Ni(OAc)2·4H2O
Pyridine
Pyridine
Pyridine
180 ◦C, 15 min
180 ◦C, 15 min
180 ◦C, 15 min
1Ni, 100
1Ni, 60
2Ni, ∼70
Mix of other products, ∼20–30
4
5
6
7
8
9
10
5
1
5
1
1
1
5
3 equiv. Ni(OAc)2·4H2O
Pyridine
Pyridine
Pyridine
Pyridine, DMF
PhCN
PhCN
PhCN
180 ◦C, 15 min
180 ◦C, 15 min
180 ◦C, 15 min
180–250 ◦C, 15 min
180 ◦C, 15 min
250 ◦C, 15 min
250 ◦C, 20 min
5Ni, 100
1Pd, 100
5Pd, 100
0
3 equiv. Pd(acac)2
3 equiv. Pd(acac)2
3 equiv. Pt(acac)2, K2PtCl4, PtI2
3 equiv. Pt(acac)2
3 equiv. Pt(acac)2
3 equiv. Pt(acac)2
1Pt, 75
1Pt, 100
5Pt, 100
a Sealed tubes. b Initial microwave power of 300 W. c Determined by UV-vis, TLC, or isolation.
problems associated with using domestic ovens, in particular
poor reproducibility of reactions because it is hard to precisely
control the reaction. With scientific microwave systems, such as
that used in our study, it is possible to control the temperature,
pressure, microwave power and reaction times readily and precisely
and with excellent reproducibility. We performed our experiments
in sealed tubes since this allows us to work with lower-boiling
solvents safely and easily at temperatures above their boiling point.
The disadvantage of this method is, however, that is does not
allow the equilibrium of the ligand exchange reaction (eqn (1))
to shift to the right by allowing the volatile acids that are formed
(such as HCl, acetic acid, pentane-2,4-dione) to escape from the
reaction flask. However, we deem this shifting of the equilibrium
(or the lack thereof) unimportant for the efficient formation of the
thermodynamically very stable Group 10 metal ions. In addition,
the use of solvents such as pyridine shift the equilibrium by salt
formation. The results obtained are summarized in Table 1.
Working at 180 ◦C, we probed the effects of reaction time and
the molar ratio of metal salt used. Reducing the reaction time from
15 min to 10 min has the effect of lowering the product conversion
to 70%. Keeping this longer reaction time but reducing the ratio
of 1 to Ni(OAc)2 from 1 : 3 to 1 : 2 and then 1 : 1 we observed
a decrease in product conversion to 65% and 60% respectively
(Table 1, entry 2), suggesting that a 1 : 3 porphyrin to Ni(II) ratio
was optimal. Excess metal salt is also required in classic nickel
insertion protocols.2
Recent reports appearing in the literature have suggested
that simultaneous microwave heating and cooling can result in
higher product yields and new pathways that were previously
unattainable.13 In principle, this technique allows for higher levels
of microwave energy to be introduced into a reaction while main-
taining the mixture at a particular bulk temperature by passing
a stream of compressed air over the reaction vessel. There have
been criticisms leveled at the technique, in particular focusing on
whether the measurement of the reaction temperature is accurate.
We have investigated the concept ourselves, looking specifically
at the measurement of temperature when using simultaneous
cooling and then assessing its use in different synthetic organic
transformations.14 We found that it is important to monitor the
temperature of a reaction mixture accurately and this involves
the use of a fiber-optic temperature measurement set-up. Having
modified our microwave apparatus for use in conjunction with
fiber-optic temperature measurement, we performed the Ni(II)
insertion reaction using a 3-fold stoichiometric excess of Ni(II) in
pyridine. Unfortunately, microwave irradiation with concomitant
cooling of the sample resulted, over 15 min, in little if any metal
insertion.
Insertion of Ni(II)
We started with the insertion of Ni(II) into porphyrin 1 (Scheme 1).
Using conventional heating, this metallation takes in excess of 3 h
when using Ni(OAc)2·4H2O in refluxing DMF or pyridine. Using
this Ni(II) source, pyridine as the solvent, heating using microwave
irradiation to 180 ◦C and holding at this temperature until a total
time of 15 min had elapsed, we obtained a quantitative conversion
of 1 to the desired metallated product 1Ni (Table 1, entry 1).
Reducing the reaction temperature, even just by 15 ◦C, had a
deleterious effect on product conversion.
These results provide the practitioner with the need to insert
Ni(II) into sensitive porphyrins the choice between heating, in a
pressurized tube, in pyridine to 180 ◦C for 15 min or several hours
at reflux temperatures. As the example below will demonstrate,
the longer reaction times may be of benefit over the use of higher
temperatures.
Next we moved from the relatively robust porpyrin 1 to
meso-tetraphenyl-2,3-dihydroxychlorin (2)15 (Scheme 2). Insertion
of Ni(II) into 2 in refluxing pyridine can take up to 3 days
conventionally.15 We applied our optimized conditions for the
preparation of 1Ni (180 ◦C, 15 min, 3 equiv Ni(II) acetate) towards
the preparation of 2Ni. However, in addition to approximately
70% of the desired diol chorin nickel complex 2Ni, we obtained
Scheme 1
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Scheme 3
of the chemistry and utility of porpholactones is still in its infancy
though a number of metal complexes have been described.5,19 In
some respects it behaves like a porphyrin, in others like a chlorin.
The formal replacement of a pyrrole moiety in 5 by an oxazolidone
moiety might also change the donor-characteristics of the imine-
type nitrogen, thus inhibiting metal complex formation. We were
able to prepare 5Ni quantitatively from 5 by microwave heating in
pyridine in a sealed tube at 180 ◦C for 15 min (Table 1, entry 4).
Hence, if there are any reactivity differences between 1 and 5, they
are not detected under the conditions of this experiment.
Insertion of Pd(II)
Preparation of the palladium(II) complexes 1Pd and 5Pd can be
accomplished using identical reaction conditions as for 1Ni or
5Ni, using Pd(acac)2 as the palladium source (Table 1, entries
5 and 6). This is a considerable improvement over the time-
consuming and harsher traditional method,2 or the improved but
capricious method.6 Further, pyridine as a solvent is more readily
removed from the reaction product by (rotary) evaporation or
aqueous acid extraction than benzonitrile. As expected, diol 2 is
not susceptible to clean metal insertion, a 50% yield of product
2Pd being obtained. In addition, small mounts (less than 10%) of
oxidation products 4 and 5Pd were identified amongst a number
of unidentified porphyrinic and non-porphyrinic products. In
the experiments in which extensive oxidative degradation of the
substrate was observed, metallic palladium tended to plate out at
the wall of the reaction vessel (for the consequences of this, see
below).
Scheme 2
a mixture of varying proportions of other products, analysis of
which was indicative of substantial dehydration of the dihydroxy-
chlorin functionality to enol 3 and oxidation to form dione 4 and
porpholactone 5, present largely in their metallated forms (3Ni,
4Ni, 5Ni) (Table 1, entry 3), in addition to unidentified pigments.
Products 3,16 4,17 and 518 are known, and all were previously
prepared from 2. The dehydration and diol-to-dione conversions
are readily understood, the mechanism of conversion to 5 not fully
−
understood. However, diol clevage of 2 using MnO4 in conjunc-
tion with a phase-transfer agent leads also to the formation of 5 in
high yields.18b It should also be noted that a fortuitous preparation
of 5, as its pentafluorophenyl-derivative, involved a Ag(I)-induced
oxidation of meso-tetra(pentafluorophenyl)porphyrin.
Insertion of Pt(II)
To complete the triad as well as increase the difficulty of the
insertion, we attempted to prepare 1Pt. Working in pyridine,
DMF, or pyridine–DMF mixtures as a solvent, we screened
Pt(acac)2, K2PtCl4, and PtI2 as platinum sources but found that we
could not generate 1Pt in any case (Table 1, entry 7). Increasing
the reaction temperature using pyridine as a solvent above 180 ◦C
could not be achieved without exceeding the upper pressure limits
of the sealed tube of 300 psi. Moving from pyridine or DMF to
benzonitrile allowed us to heat the reaction mixture to 250 ◦C in
the sealed tube. Using 3 equivalents of Pt(acac)2 we were able to
obtain a 75% conversion to 1Pt at 180 ◦C (Table 1, entry 8), and
We attribute all decomposition reactions of the diol moiety in
2 to the high reaction temperature and thus tried performing the
metallation in a sealed tube but at the temperature of refluxing
pyridine (115 ◦C). No product was obtained after a 15 min reaction
time. However, the integrity of the chlorin remained intact,
confirming our supposition. We then attempted the metallation
of the chlorin diol 2 with Ni(OAc)2 using microwave heating with
simultaneous cooling. Unfortunately this had no effect on the
reaction, conversion to product being essentially zero, and again
the starting material could be recovered quantitatively.
◦
We turned to meso-tetraphenyl-2-oxa-3-oxo-porphyrin (por-
pholactone, 5) as a substrate for metallation (Scheme 3). The study
only after heating at 250 C for 15 min, quantitative yields were
achieved (Table 1, entry 9).
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We revisited the effect of varying the metal salt on the
metallation process. When using M(II) salts that are prone to
oxidation, such as PdCl2 or PtI2, we observed faster plating out
of the metal than any insertion into the porphyrin. This is an
important observation since, in a microwave field, metal films
can be prone to arcing and superheating. If the reaction vessel
is at elevated pressures, arcing or superheating can result in local
melting of the glass walls and concomitant failure of the vessel
integrity. Thus, when using elevated temperatures and pressures
for the metallation of porphyrins, it is advisable to use more
oxidatively resilient complexes as the metal sources.
Even under the optimized conditions, diol 2 could not be
metallated to any detectable degree before the onset of extensive
decomposition of 2 into a multitude of products.
The Pt(II) complex of 5 (as its meso-pentafluorophenyl-
derivative) shows promise in pressure-sensitive paints.5 As for
1Pt, it was necessa◦ry to move to benzonitrile and the higher
temperature of 250 C in order to affect efficient metallation. It
was also necssary to extend the reaction time to 20 min in order
to obtain a quantitative conversion to 5Pt (Table 1, entry 10).
Synthesis
[meso-Tetraphenylporphyrin]nickel(II) (1Ni)—General proce-
dure. In a 10 mL thick-walled glass tube were placed the
porphyrin (0.020–0.040 mmol), metal salt (3 equiv.), and the
solvent (3 mL) of choice (Table 1). The vessel was sealed with
a septum and placed into the microwave cavity. The pressure
was controlled by a load cell connected to the vessel via a non-
invasive pressure measurement device above the septum surface.
The temperature of the contents of the vessel was monitored
using a calibrated infrared temperature control mounted under the
reaction vessel. All experiments were performed using a stirring
option whereby the contents of the vessel are stirred by means of
a rotating magnetic plate located below the floor of the microwave
cavity and a Teflon-coated magnetic stir bar in the vessel. Using an
initial microwave power of 300 W, the reaction mixture was heated
from r.t. to the target temperature (Table 1) where it was held by
automatically modulating the microwave power until the desired
total reaction time had elapsed. Upon completion the reaction was
cooled to 50 ◦C, the reaction vessel removed from the microwave
cavity, opened and the crude product decanted. Formation of
the products was confirmed using UV-vis spectrophotometry and
TLC. Product concentration and hence conversion was deter-
mined by comparison of select UV-vis bands of the product (and, if
still present, the starting material), using literature values.21 Select
runs were worked up using preparative chromatography, and the
products were compared against material prepared independently
using traditional methods and literature values.
Conclusions
The results presented here, and summarized in Table 1, show that,
by using controlled microwave heating, it is possible to insert
Group 10 metal ions rapidly and efficiently into heat-resistant
porphyrins. Reaction times are in the order of 15 min instead
of many hours required using conventional methods. In addition,
product conversions are higher and, for Ni(II) and Pd(II) insertions,
the use of pyridine facilitates the workup of the products. Caution
must be taken when choosing the metal salt since plating of metal
on the surface of the glass vessels can lead to dangerous conditions
in a microwave field. The limitation of this methodology was
that, when heat-sensitive chlorin 2 was used as a substrate, the
shorter reaction times did not off-set the effects of the required
higher temperatures. This lead to the formation of dehydration and
oxidation byproducts that are largely absent using the traditional
methods that require longer reaction times at lower temperatures.
The use of simultaneous cooling in conjunction with microwave
heating did not allow this problem to be circumvented. Our
attention is now turning to microwave-promoted demetallation
reactions.
[meso-Tetraphenyl-2-oxa-3-oxoporphyrinato]nickel(II)
(5Ni).
Prepared according to the general procedure and the conditions
listed in Table 1, entry 4. Rf = 0.89 (silica/20% petroleum ether
(bp 40–60 ◦C)–CH2Cl2); UV-vis (CH2Cl2) kmax (log e): 416 (5.15),
1
544 (3.87), 586 (4.35); H NMR (400 MHz, CDCl3, d): 8.59 (d,
J = 4 Hz, 1H), 8.54 (d, J = 4 Hz, 1H), 8.52 (d, J = 4 Hz, 1H),
8.49 (d, J = 4 Hz, 1H), 8.46 (d, J = 4 Hz, 1H), 8.38 (d, J = 4 Hz,
1H), 7.96–7.88 (m, 6H), 7.81–7.78 (m, 2H), 7.70–7.63 (m, 12H)
ppm; 13C NMR (100 MHz, CDCl3, d): 164.5, 149.3, 145.1, 144.3,
142.0, 141.0, 140.1, 140.0, 134.0, 133.7, 133.4, 133.2, 132.2, 131.9,
131.3, 130.8, 129.9, 128.2, 128.1, 128.0, 127.8, 127.5, 127.1 (2
overlapping signals), 125.0, 121.0, 120.5, 101.0 ppm; + ESI-MS
(30 V, 100% CH3CN) m/z = 688.0 (M+).
[meso-Tetraphenyl-2-oxa-3-oxoporphyrinato]palladium(II)
(5Pd). Prepared according to the general procedure and the
conditions listed in Table 1, entry 6. Rf = 0.94 (silica/20%
petroleum ether (bp 40–60 ◦C)–CH2Cl2); UV-vis (CH2Cl2) kmax
(log e): 417 (5.23), 537 (3.98), 578 (4.55) nm; 1H NMR (400 MHz,
CDCl3, d): 8.65–8.55 (m, 5H), 8.45 (d, J = 4 Hz, 1H), 8.10–8.06
(dd, J = 8, 2 Hz, 4H), 8.04 (dd, J = 8, 2 Hz, 2H), 7.92 (dd, J =
8, 2 Hz, 2H), 7.78–7.68 (m, 12H); 13C NMR (100 MHz, CDCl3,
d): 164.8, 147.7, 146.0, 143.6, 142.1, 140.8, 140.6, 140.1, 139.7,
137.9, 136.8, 133.9, 133.8, 133.6, 132.9, 132.2, 131.0, 130.8, 130.2,
130.0, 129.3, 128.3, 128.2, 128.1, 127.8, 127.5, 127.3, 127.0 (2
overlappings signals), 123.5, 121.8, 117.7, 103.2 ppm; +ESI-MS
(30 V cone voltage, 100% CH3CN) m/z = 736.2 (M+).
Experimental
General
All solvents and reagents were used as received. Porphyrins 1,20
2
15 and 518 were prepared using literature methods. UV-vis spectra
were recorded on a Cary 50 spectrophotometer. Analytic thin
layer chromatography (TLC) was carry out using silica gel (layer
thickness 0.2 mm) aluminium-backed TLC-cards with fluorescent
indicator (254 nm), whilst the preparative TLC plates were 500 lm
20 × 20 cm silica glass plates, both purchased from Scientific
Absorbents, Atlanata, GA (USA). Microwave reactions were
conducted using a modified monomode microwave unit (CEM
[meso-Tetraphenyl-2-oxa-3-oxoporphyrinato]platinum(II) (5Pt).
Prepared according to the general procedure and the conditions
listed in Table 1, entry 10. Rf 0.94 (silica/20% petroleum ether (bp
40–60 ◦C)–CH2Cl2); UV-vis (CH2Cl2) kmax (log e): 403 (5.26), 527
R
Discoverꢀ). The machine consists of a continuous FocusedTM
microwave power delivery system with operator selectable power
output from 0–300 W.
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(4.11), 566 (4.71) nm; 1H NMR (400 MHz, CDCl3, d): 8.65–8.55
(m, 5H), 8.48 (d, J = 4 Hz, 1H), 8.10–8.06 (dd, J = 8, 2 Hz, 4H),
8.03 (dd, J = 8, 2 Hz, 4H), 7.92 (dd, J = 8, 2 Hz, 2H), 7.77–7.68
(m, 12H); 13C NMR (100 MHz, CDCl3, d): 164.7, 148.2, 145.8,
142.9, 140.9, 140.5, 140.4, 139.0, 138.9, 138.3, 137.6, 136.5, 133.6
(2 overlapping signals), 133.4, 133.3, 132.1, 131.1, 130.1, 129.6,
129.3, 128.7, 128.4, 128.3 (2 overlapping signals), 128.2, 128.0,
127.8, 127.5, 127.1 (2 overlapping signals), 124.1, 122.1, 116.6,
102.9 ppm; +ESI-MS (30 V cone voltage, 100% CH3CN) m/z =
825.0 (M+).
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17 4:(a) M. J. Crossley, P. L. Burn, S. J. Langford, S. M. Pyke and A. G.
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4/4Ni by oxidation of 2: H. W. Daniell, S. C. Williams, H. A. Jenkins
and C. Bru¨ckner, Tetrahedron Lett., 2003, 44, 4045.
This material is based upon work supported by the National
Science Foundation under Grant No. 0517782. The University
of Connecticut is thanked for funding and CEM Corporation for
equipment support.
Notes and references
1 The Porphyrin Handbook, ed. K. M. Kadish, K. M. Smith and
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2 J. W. Buchler, in The Porphyrins, ed. D. Dolphin, Academic Press, New
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3 For representative examples, see: (a) M. J. Crossley, M. M. Harding
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Dreher and G. Herget, Liebigs Ann. Chem., 1988, 34; (c) L. Barloy, D.
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4 See recent examples: (a) S.-W. Lai, Y.-J. Hou, C.-M. Che, H.-L. Pang,
K.-Y. Wong, C. K. Chang and N. Zhu, Inorg. Chem., 2004, 43, 3724;
(b) R. P. Brin˜as, T. Troxler, R. M. Hochstrasser and S. A. Vinogradov,
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5 See, e.g.: (a) S. Grenoble, M. Gouterman, G. Khalil, J. Callis and L.
Dalton, J. Lumin., 2005, 113, 33; (b) B. Zelelow, G. E. Khalil, G.
Phelan, B. Carlson, M. Gouterman, J. B. Callis and L. R. Dalton,
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6 G. K. Lahiri, J. S. Summers and A. M. Stolzenberg, Inorg. Chem., 1991,
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7 Pd chlorins: (a) A. Singh, W.-Y. Huang, P. Scheiner and L. W. Johnson,
Spectrochim. Acta, Part A, 2007, 66, 86; (b) V. V. Sapunov and G. D.
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8 Successful example for conversion of a Pd(II) porphyrin: (a) J. Fouchet,
C. Jeandon, R. Ruppert and H. J. Callot, Org. Lett., 2005, 7, 5257;
(b) Formation of a bacteriochlorin is only possible for a free base
chlorin: C. Bru¨ckner and D. Dolphin, Tetrahedron Lett., 1995, 36,
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18 5: (a) M. J. Crossley and L. G. King, J. Chem. Soc., Chem. Commun.,
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Jenkins and C. Bru¨ckner, Org. Lett., 2003, 5, 19.
19 (a) A. Cetin and C. J. Ziegler, Dalton Trans., 2005, 25; (b) G. Khalil,
M. Gouterman, S. Ching, C. Costin, L. Coyle, S. Gouin, E. Green,
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L. M. Ball, J. Terner, D. Mandon, R. Weiss, J. Fischer, A. DeCian, E.
Bill, M. Muether, V. Schuenemann and A. X. Trautwein, Inorg. Chem.,
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9 (a) A number of books on microwave-promoted organic synthesis
have been published recently: Microwaves in Organic Synthesis, ed.
21 D. W. Thomas and A. E. Martell, J. Am. Chem. Soc., 1959, 81, 5111.
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