Methine bridge

Naturally occurring porphyrins are usually symmetrically substituted about the 15-methine bridge. These porphyrins can be synthesized by the condensation of two dipyrroiic intermediates. Typical dipyrroiic intermediates in current use arc the dipyrromethanes and the dipyrromethenes. Both methods will shortly be described. This again is a highly specialized... 

The formation of porphyrins from 1,19-dideoxybi)enes-/r can be achieved starting either from the 1-methyl derivatives or from l,19-dideoxybilene-Z>-l,19-dicarboxylic acid esters. In the first case the desired methine bridge of the porphyrin stems from the 1-methyl group whereas in the latter case orthoformates have to be added in the condensation step as a precursor for the methine unit. The 1-methyl- and also 1,19-dimethyl-l,19-dideoxybilene- > salts can be cy-clized to the corresponding porphyrins with copper(II) acetate in methanol.56 However, when the bilenes contain /i-acceptor substituents, the yields of porphyrins obtained by this method are very low.57... 

A completely different concept13 makes use of a highly reduced bilane 5 which is oxidatively cyclized to an isobacteriochlorin 6 with copper(II) acetate. The ring closure is initiated by ester cleavage with trifluoroacetic acid and decarboxylative formylation with trimethyl orthoformate to yield a dialdehyde. One of the aldehyde functions forms the desired methine bridge whereas the other is lost during cyclization. 

The synthesis of homoporphyrins in which one methine bridge of the parent porphyrin is extended to a two-carbon bridge is one of the earliest and simplest examples of an expanded porphyrin.3a b>4 The synthesis of homoporphyrin 4 is based on the ability of N-alkylated porphyrin 1 to undergo nickel-induced rearrangement to form an expanded macrocycle 2. De-metalation of 2 by means of concentrated hydrochloric acid yields the cyclically conjugated [20]porphyrin(2.1.1.1) 4 and a macrocyclic side product 3 in which the cyclic conjugation is interrupted. 

Pentaphyrin(l.l.l.l.l) is the direct homologous congener of the porphyrin system. Formally, but also synthetically, it can be derived when the porphyrin-forming building blocks are extended by a pyrrole unit and a methine bridge or its precursor. 

The synthesis of [261hexaphyrin(l. 1.1.1.1.1) which is formally derived from porphyrin by extension of two pyrrole subunits and the corresponding methine bridges follows a classical analogous McDonald [3 + 3] approach by condensation of two tripyrrane building blocks.2 2a d... 

Yagi S, Fujie Y, Hyodo Y, Nakazumi H (2002) Synthesis, structure, and complexation properties with transition metal cations of a novel methine-bridged bisquarylium dye. Dyes Pigm 52 245-252... 

More recent publications describe a variety of isoindoline pigments with different chemical structures. Such species comprise an isoindoline ring attached to two methine bridges. 

An example of an N/C conjugated system involving an exocyclic carbon is the enamine conjugation in cyanine dyes, in which, the j3-carbon atom constitutes the methine bridge ([164], n = 0), or part of the bridge ([164], n = 1). Protonation of these cations occurs in... 

The isomers that are shown in Scheme 2 are only those that have been proven to exist in thermal equilibrium, although, in principle, eight isomers cis and trans about the three methine bridge bonds are possible, as we shall discuss in depth throughout this chapter. The nomenclature used for these different isomers in this chapter is the same as that used by the majority of authors. For... 

Much discussion about the photo-formation of the merocyanines is based on the possibility of four stable merocyanine isomers that are trans about the 13-bond on the methine bridge. Commonly, these are referred to as TTC, CTT, CTC, and TTT [6-8,28,36] and they are shown in Scheme 4. Note that these are possible or hypothesized structures. Corner et al. even suggest that isomers cis about the central 3-methine bond may have stability and equilibrate with the more planar isomers [14,46-51]. Certainly, the four isomers cis about the cental (3-methine bond could have transient stability. 

All studies show the zwitterionic nature of the spiropyran merocyanine beyond any doubt and we can further see how the acid-base characteristics of the C9 oxygen and indoline nitrogen are delocalized as alternating charges on the methine bridge, leading to the labile properties of the bridge itself. 

The methine bridge in acridines can be derived from dihalomethanes. Originally sodium carbonate was used, with dichloro- or diiodo-methane, to prepare dibenzacridine (624) from 2-naphthylamine (02JCS280). Subsequently, better yields were obtained when dichloromethane, 1,1-dichloroethane or benzal chloride, 1-naphthylamine, and a solvent were heated, the products being dibenz(c,/z jacridines (624) (06JCS1387). 

---