Methine position

Porphyrins are formally derived from the porphin (1) nucleus by substitution of some or all peripheral positions with various side chains. In the classical system of nomenclature T, introduced by H. Fischer,Sc the peripheral /5-pyrrolic positions are numbered front 1 to 8 and the methine positions (also named meso positions) between the pyrrole rings are designated a, //, y, and 5. The rings are lettered clockwise A, B, C, and D. The classical nomenclature was in the past more and more displaced by a nomenclature which numbers all the carbon... 

Methyl and methine protons naturally phase at 180° relative to the methylene carbons and the spectra are usually plotted with methyls and methines positive. (Note that should you encounter a signal that you cannot confidently assign to either a methyl or methine carbon, the DEPT 90 sequence may well be of use as it differentiates these carbons - methines appear positive and methyls are edited out of the spectrum but this technique can be considered obsolete if you have access to any of the 2-D proton-carbon correlated experiments discussed in Section 9.2.)... 

Sommer (130, 130a) and Hall (131) have independently described the low-temperature H/D exchange of isobutane on zeolites. The traditional mechanism involves a five-coordinate carbonium ion intermediate yet no exchange occurred for the methine position, and this is inconsistent with a carbonium ion. This surprising result was explained by Sommer with a reaction sequence beginning with hydride abstraction by an unknown route... 

Oxidized porphyrin chromophores are also chemically and biologically important. Oxo-phlorins (7) possess a carbonyl group at one of the methine positions nominally, they are... 

In the IUPAC system, the four methine positions are conveniently numbered 5, 10, 15 and 20, and the eight remaining peripheral positions fall at 2, 3, 7, 8, 12, 13, 17 and 18. The nitrogen atoms are numbered 21 through 24. Owing to the continued use of trivial names, both in the IUPAC and classical systems of nomenclature, certain other features of porphyrin notation and isomerism need to be explained. If the eight peripheral substituents are made up of two sets of four (for example, four methyls and four ethyls), and if there is one of each on the individual pyrrole subunits (a situation which usually occurs in biologically derived porphyrins), then there are four possible so-called primary type isomers. These four isomers for the methyl/ethyl series, trivially named etioporphyrins, are shown in Scheme 1 the compounds are named etioporphyrin-I (14), etioporphyrin-II (15), etioporphyrin-III (16), and etioporphyrin-IV (17). 

Except for usually minor variations in reactivity, porphyrins tend to react towards electrophilic substitution about equally at all four of the methine positions, though most work has been carried out using symmetrically substituted porphyrin ligands such as those from octaethylporphyrin. However, chlorins have been shown (61JA4676) to react preferentially at the methine positions which flank the reduced ring (i.e. at positions 15 and 20). 

Porphyrins can also be sulfonated a t the peripheral positions, and copper(II) porphyrins react with thiocyanogen to give the methine-thiocyanatoporphyrin which can be hydrolyzed to the corresponding mercapto derivative. Reactions between metalloporphyrins and car- benes are known, but they tend to give mixtures of products owing to addition at methine positions as well as across peripheral double bonds. Nitrenes also react with metal-free porphyrins to give insertion products and methine-substituted derivatives. 

Porphyrins have been synthesized with the crown ether benzo-15-crown-5 attached at the methine position.620 Binding of K+, Ba2+ or NH4+, which form 1 2 complexes with crown-5 ligands, promotes dimerization of the porphyrins (with a separation of the porphyrin planes of —4.2 A). 

Many other H27 abstractions by O from a single carbon atom can be explained in a similar way. For example, 0T abstracts a proton from the methine position of cyanocyclopropane to give the (M — H)- ion, but abstracts H2 from one of the corners of the ring to generate the (M — H2)t ion as shown by deuterium labelling (Dawson and Nibbering, 1980). Abstraction of H2 from different carbon atoms would lead in this case either to the radical anion of 1-cyanocyclopropene or to that of 3-cyanocyclopropene. By comparison with the radical anion of acrylonitrile,... 

Species such as [33] or [112] in which there is little positive charge on C(l) would seem unlikely to undergo interconversion of the methylene and methine positions at a rate required to explain the observed C-scrambling. The puckered cyclobutyl cation [35] is a more likely candidate, although the calculations indicate that it is not a minimum energy form. 

The nonexistence of alkane-alkene equilibrium in superacid medium has been elegantly demonstrated by the behavior of isobutane in deuterated superacid medium (DSOsF-SbFs or DF-SbFs). Isobutane at low temperature undergoes hydrogen-deuterium exchange only at the methine position through the involvement of a three-center bound pentacoordinate carbonium ion (Scheme 17). 

The mechanism of this reaction is still somewhat unclear but numerous results suggest that the hydride in the alcohol product comes from the borane attached to the phosphorus atom since its replacement by boron trideuterated led to the fully deuteration of the methine position of the alcohol (when one equivalent of catalyst, and of borane, was employed). 

In two closely related papers [40, 41] CP/MAS was used to examine a series of styrene-divinylbenzene (St-DVB) and chloromethylated resins. In the first part of this study the authors were concerned with trying to determine the residual amount of unreacted vinyl groups present in St-DVB resins (see Section 3). In order to increase the sensitivity of the method the authors used C -labelled divinylbenzene (labelled in the methine position) and combined this with unlabelled styrene (1-20% by weight). The final resins were examined by CP/MAS and it was found that even for a very lightly erosslinked... 

The greater lability toward vanadium removal and porphyrin destruction for the vanadyl petroporphyrins over the synthetic vanadyl porphyrins is a fortunate circumstance. This difference can be rationalized based upon structural differences of the porphyrins involved. Phyllo-type petroporphyrins all contain a cyclopentane ring, fused to one of the pyrrole rings with the methine carbon one of the units of the carbo-cyclic structure. Etio- and rhodo-type petroporphyrins appear to have alkyl substitution at one or more methine positions, as based upon nmr spectral data. Should all petroporphyrins have a carbon substituent on one or more methine carbons, the carbonium ion formed by the reaction would tend to have more charge localization on those methine carbons. These ions are a more stable species by an order of magnitude over those porphyrins without methine substitution. 

The fact that vanadyl tetraphenylporphine was less reactive is not inconsistent with this rationalization although in this instance all four methine carbons have phenyl substituents. It is likely that the formation of the mono-substituted carbonium ion would be inhibited because of steric interactions at the methine position involved. However, a more important consideration is that the benzene ring attached to the methine carbon is not coplanar with the macrocyclic porphyrin ring. The extent of deviation from planarity is enough that a positive charge generated on the methine carbon would receive no resonance stabilization but considerable inductive destabilization as a consequence of the unsaturated substituent. 

We can assign the remaining peaks in the spectrum by noting, that only one positive peak remains in the DEPT-135 spectrum. This peak must correspond to the methine position at C3 (30 ppm). The two remaining negative peaks (at 37 and 40 ppm) are assigned to the methylene carbons at C4 and C2. Without additional information, it is not possible to make a more specific assignment of these two carbons (see problem 4). 

Nitropropane. To see what type of information a COSY spectram may provide, we shall consider several examples of increasing complexity. The first is the COSY spectrum of 2-nitropropane. In this simple molecule, we expect to observe coupling between the protons on the two methyl groups and the proton at the methine position. 

Schiff base formation between pyridoxal (140) and amino acids leads to complexes of type (141) which are in tautomeric equilibrium with (142). This tautomeric equilibrium leads to transamination, thus the same metal complexes can be obtained when either pyridoxal and alanine or pyridoxamine and pyruvic acid are allowed to react together in the presence of a metal ion. Hopgood" has studied the rates of transamination of 15 amino acids in the presence of zinc(II) and pyridoxal 5-phosphate (143). On mixing the reagents zinc(II)-aldimine complexes are rapidly formed (ca. 5 min) and these species subsequently transaminate in a slow second step. Ai" and Zn" systems have been particularly well studied.The role of the metal ion seems to involve both stabilization or trapping of the Schiff base, and in addition it also ensures the planarity of the conjugated ir-system. In the case of the aldimine tautomer, extensive H NMR studies have shown that formation of the ternary complex results in activation at the amino acid 2-carbon. At room temperature the reaction occurs without incorporation of into the aldehyde methine position indicating that the primary mechanism is carbanion formation rather than tautomerism. 

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