1.1 Porphyrins - Friedrich-Alexander-Universität Erlangen-Nürnberg

3 Discussion and Results
3.2.6.4 Synthesis of Seven-Membered Ring Analog 57
To synthesize 57 from porphyrin methyl methanoate 59, the problem of the extension of the
side chain by one carbon atom had to be solved first. Since the substrate is a carboxylic acid
derivative, it was considered to try an ARNDT-EISTERT reaction 134 which would lead directly
from 65 to Cu(II)-64 like it is shown in Scheme 44.
80
HO 2 C
N
NH HN
N
Arndt-Eistert
homologation
homologation
HO 2 C
65 Cu(II)-64
Scheme 44. Adapted ARNDT-EISTERT homologation. Applied steps: 1. Cu(OAc)2·H2O, 2. C2O2Cl2,
3. CH2N2, 4. Ag(I) catalyst, heat.
To protect the inner-ring positions of the porphyrin, copper(II) was inserted before the
carbon acid chloride has been generated using oxalyl chloride, C2O2Cl2. After complete
evaporation of solvent and chlorinating agent, the obtained red residue was reacted with
freshly prepared diazomethane, CH2N2, at lowered temperature (-10 °C). Upon evaporation,
water and dioxane were added together with the necessary silver(I) catalyst, Ag2O, which
was also freshly prepared. After reacting that mixture under reflux over night, a reddish
product was isolated and indentified as metallated methyl ester derivative Cu(II)-59 without
any detectable by-products. This approach has thus not been applicable to that kind of
system, so that the more complex way depicted in Scheme 43 had to be chosen.
This protocol managed the elongation of the side chain by nucleophilic attachment of
cyanide like it has already been described for 53 (see paragraph 3.2.2). That necessitated the
initial conversion of the carboxylic acid moiety into a good leaving group like our well
approved bromo substituted methyl group. Thus, porphyrin methyl methanoate 59 had to
be reduced to the corresponding alcohol, hydroxymethyl porphyrin 61, by reaction with
lithium aluminum hydride, LiAlH4, in THF. 135 For protective means, the zinc(II) complex was
prepared in advance while the metal was removed in the course of the applied work-up.
After FC the desired sub-ordinate target 61 was obtained in pure as purple powder (87 %
yield based on 59).
ARNDT-EISTERT
N
N
Cu
N
N