Meso carbon atom

The meso carbon atom should present a carbenium structure with a low TT electron density in the ground state, in the excited state this carbon possesses the carbeniate structure (C ) with a high tt electron density (119). An electron-donating group in such a position should stabilize the ground state and rise the excited state to the highest level hypsochromic shift results as a whole. 

The conclusion that the initial attack of benzyl radicals on acridine occurs at the meso-carbon atom receives further support from the... 

Porphyrazines (pz), or tetraazaporphyrins, are compounds that can be viewed as porphyrin variants in which the meso carbon atoms are replaced with nitrogen atoms, as Fig. 1 shows (1). This difference intrinsically gives porphyrazines discrete physiochemical properties from the porphyrins. In addition, despite their similar molecular architecture, porphyrazines are prepared by an entirely different synthetic route than porphyrins—by template cyclization of maleonitrile derivatives, as in Fig. 2, where the open circle with the A in it represents the peripheral substituent of the pz—rather than by the condensation of pyrrole and aldehyde derivatives (1). The pz synthetic route allows for the preparation of macrocycles with chemical and physical properties not readily accessible to porphyrins. In particular, procedures have been developed for the synthesis of porphyrazines with S, N, or O heteroatom peripheral functionalization of the macrocycle core (2-11). It is difficult to impossible to attach the equivalent heteroatoms to the periphery of porphyrins (12). In addition, the preparation and purification of porphyrazines that bear two different kinds of substituents is readily achievable through the directed cocyclization of two different dinitriles, Fig. 3 (4, 5, 13). 

Levy, H. R., Talalay, P., Vennesland, B. The steric course of enzymatic reactions at meso carbon atoms application of hydrogen isotopes. In progress in stereochemistry (ed. de la Mare and Klyne), Vol. 3, p. 299—349. (London Butterworths 1962. 

Such a distribution has a plausible physical basis, since the driving force for phenyl rotation into the porphyrin plane provided by the electronic excitation (the eg orbital has particularly large coefficients at the meso carbon atoms ( )) encounters steric resistance from the non-bonded interactions between the protons at the ortho positions of the phenyl groups and those on the outer pyrrole carbon atoms (20). Consequently the phenyl torsion potential in the excited states may be relatively flat. Nevertheless, the vibrational frequencies are expected to be sensitive to the torsion angle for orientation close to co-planar because of the effect of conjugation. 

Template reactions between malonaldehydes and diamines in the presence of copper(II), nickel(II) or cobalt(II) salts yield neutral macrocyclic complexes (equation 15).99-102 Both aliphatic102 and aromatic101 diamines can be used. In certain cases, non-macrocyclic intermediates can be isolated and subsequently converted into unsymmetrical macrocyclic complexes by reaction with a different diamine (Scheme ll).101 These methods are more versatile and more convenient than an earlier template reaction in which propynal replaces the malonaldehyde (equation 16).103 This latter method can also be used for the non-template synthesis of the macrocyclic ligand in relatively poor yield. A further variation on this reaction type allows the use of an enol ether (vinylogous ester), which provides more flexibility with respect to substituents (equation 17).104 The approach illustrated in equation (15), and Scheme 11 can be extended to include reactions of (3-diketones. The benzodiazepines, which result from reaction between 1,2-diaminobenzenes and (3-diketones, can also serve as precursors in the metal template reaction (Scheme 12).101 105 106 The macrocyclic complex product (46) in this sequence, being unsubstituted on the meso carbon atom, has been shown to undergo an electrochemical oxidative dimerization (equation 18).107... 

Fig. 5.1 The structure of iron protoporphyrin IX, the prosthetic heme group of the peroxidases except where it is modified by the formation of covalent bonds to the methyl or vinyl groups. The a, (3-, y-, and 5-meso-carbon atoms, defined with respect to the pattern of the ring substituents, are labeled...

In addition to the complexes already discussed in this section, we have investigated a series of (i73-cyclooctenyl)Ni(Br)L complexes in order to study the effect of the nature of the ligand upon the NMR parameters (75). Typical results are shown in Table XV. Complexation of a tertiary phosphite or phosphine to (rj3-cyclooctenylNiBr)2 is accompanied by a high field shift of the meso carbon atom (Q) and the terminal carbon atom trans to the ligand (C3) and by a low field shift of C (cis to L). The values for the chemical shifts of C2 and C3 follow the order... 

The preparation of a corrole isomer, termed isocorrole, has recently been reported by Vogel and coworkers.The general, unsubstituted form of this compound is shown in Figure 2.2.1 and has the structure 2.261. Isocorrole may be regarded as being derived from the porphyrin isomer porphycene (e.g., 2.260) by formal removal of one of its four meso carbon atoms. Considered in this light, isocorrole 2.261 bears the same relationship to porphycene as the parent corrole does to porphyrin. 

However, these new systems contain three saturated meso carbon atoms in their macrocyclic framework. Thus, any kind of direct comparison between these systems and the bridged [18]annulenes 2.335-2.344 is not justified. Unfortunately, access to fully conjugated systems from any of these macrocycles appears rather unlikely because they contain indole subunits instead of pyrroles. Indeed, perhaps for this very reason, no conjugated systems of this type have been reported in the literature. 

With the conformational information in hand, the fluorescence lifetimes of the dyad and triad species were determined in dichloromethane solution in order to extract rate constants for photoinitiated electron transfer (e.g., step 2 in Figure 4) via Eq. (1). The results are tabulated in Table 1. Also included in the table are the distances from the center of the porphyrin macrocycle to the center of the quinone aryl ring and the distances from the edge of the porphyrin macrocycle (the nearest porphyrin meso carbon atom) to the edge of the quinone aryl ring (the carbon atom bearing the methylene group) (r e)-... 

Pure individual porphyrins such as (4)-(7) can, however, be synthesized using dipyrroles, and these approaches will be briefly discussed later in this section. If two dipyrrole units (8) and (9) with an appropriate future meso-carbon are reacted together with the intention of preparing porphyrin (10), there is actually a maximum of three possible products, (10) (12) (Scheme 3). This is because the two dipyrroles can either react with themselves, or (as required) with each other. If the dipyrroles do not possess attached (future) meso-carbon atoms (e.g., (13) and (14)) and also bear an unsymmetrical arrangement of substituents (indicated by the A and B labels on each pyrrole— oxidation levels, i.e., dipyrromethene or dipyrromethane, not defined), even greater mixtures can occur—in this case, porphyrins (15)-(20) (Scheme 4). Such symmetry problems are common with all so-called [2 + 2] syntheses. However, if a porphyrin synthesis involving two dipyrroles is to be attempted, the symmetry problems can often be overcome if one of the two dipyrroles is symmetrical about its interpyrrolic (5-) carbon atom (e.g., synthesis of (6) from (21) and (22), Scheme 5). 

The second product identified by Meyer, oxanthrone acetate (5, better 10-acetoxy-9-anthrone), was obtained in moderate amount by oxidation of 9-acetoxy-anthracene (3) with lead tetraacetate in acetic acid. Oxidation of (3) in refluxing benzene resulted in 1,4-addition to give the triacetoxy compound (4). This substance when heated in acetic acid is converted largely into lO-acetoxy-9-anthrone (5) by loss of acetic anhydride and to a lesser extent into 9,10-diacetoxyanthracene (7) by loss of acetic acid. If (4) is an intermediate in the oxidation of (3) in acetic acid lo (5), the acetoxy group in the product (5) must be attached to a different meso carbon atom (5) than in (3), and this inference was shown to be correct by oxidation of 2-methyl-9-acetoxyanthracene and identification of the product as 2-methyl-I O-acetoxy-9-anthrone by synthesis. Both (5) and (7) on further oxidation with lead tetraacetate in acetic acid yield anthraquinone, probably via the products of acetoxylation of (5) and 1,4-addition to (7). 

Then there should be an exponential relation between the rate constant and the sum of the free valences of the atoms where reaction takes place.16 The kinetic data which are available now are not sufficient for a quantitative consideration of this problem, but the qualitative agreement is very satisfactory. Figure 11 shows that, for example, in anthracene the sum of the free valences of the meso carbon atoms is greater than the sum of the frc e valences of other pairs of para positions. In fact, addition of mrueic anhydride takes place at the meso position.11 13... 

A most unusual main group derivative is a phosphorous(V) derivative which is probably the least metallic of the metallo species. The structure of the dihydroxo derivative, [P(TPP)(0H)2]0H , has been determined. The average P-Np bond distance is 1.891(19) A and the average axial P-O(OH) is 1.59(7) A. In order to accommodate the very small P cation, the porphyrin core is exceedingly S ruffled with deviations of the meso carbon atoms of 0.80 from the mean plane. 

Figure 3. Schematic model of the sites of oxidation of cytochrome c and guaiacol by cytochrome c peroxidase. Guaiacol migrates down the access channel and reacts with the heme near the 5-meso carbon atom. Styrene also migrates down the channel to react with the ferryl oxygen.

In several enzyme systems that catalyze the desaturation of stearic acid, oleic acid (cis-A -octadecenoic acid) is the sole product and the reaction is therefore characterized by positional and geometrical stereospecificity. The desaturating system of Corynehacterium diphtheriae has been found by Schroepfer and Bloch (1965) to have the additional property of selectively removing one particular hydrogen atom from each pair of hydrogens at carbons 9 and 10 of stearic acid. The enzyme can thus distinguish between the two hydrogen atoms attached to a carbon atom of a poly-methylene chain, and the system is a notable example of the stereospecificity of enzymatic reactions at meso carbon atoms (Levy et al., 1962). The chemical identity... 

The introduction of all l groups at the porphyrin meso positions induces ruffled deformation [9,10,27,28]. A typical example is meso-tetra(te/t-butyl)porphyrin prepared by Smith and coworkers [27,51]. The meso carbon atoms deviate from the mean porphyrin plane upward and downward alternately. The average deviation of the meso carbons in the zinc complex Zn(T BuP) reaches as much as 0.899 A [28], Thus, the iron complexes of this porphyrin such as Fe(T BuP)X and [Fe(T BuP)L2] should be the best model to elucidate the effect of ruffled porphyrin ring on the electronic structure of the iron(lll) complexes. However, the synthesis of Fe(T BuP)X has been unsuccessful until now. For this reason, meso-tetraisopropylporphyrin complexes Fe(T PrP)X, where the meso te/t-butyl groups are replaced by the less bullqr isopropyl groups, have been examined [52,53]. 

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