Axial Bond Formation

The tendency of porphyrins to form aggregates, and the structural control of the assembly formed can be directed by the help of axial binding in the case of metalloporphyrins. In this section we will first examine examples in which the axial bond formed is a coordination bond for which dissociation is still possible. The formation of reversible bonds in self-assembled processes is extremely important because it permits the use of combinatorial type approaches, and leaves the possibility of correcting errors that may have occurred during the assembly process. Such synthetic design features usually afford thermodynamically stable scaffolds instead of a more or less random assembling of the chromophores. 

These structures, which extend up to approximately 240 units per linear coordination polymer, have been characterized by tapping mode AFM imaging on 

UV-visible titrations confirmed that in the experimental conditions of the photophysical experiments, for which concentrations of free base are in the low micromolar range, 92% of the zinc dimers were chelated around pyrazines in the assembled structures of 86, 87, and 88. Upon excitation at 564 nm, fluorescence occurred primarily from the central free base energy acceptor, due to energy transfer according to the Forster mechanism. Array 86 shows a 77-fold increase of the free base emission compared to the individual free base component (where R = PY2) with the same number of energy collecting zinc dimers located at a longer distance, array 87 shows only a 20-fold increase of the free base emission. 

This is comparable with array 88 in which eight zinc dimers are also arranged around the free base, for a 12% fluorescence increase. In this latter 

A tentative explanation for this unusual behavior is the inhibition of molecular motions at 77 K, which may be responsible for a favorable orientation of transition moment dipoles at room temperature. Indeed, a too well organized structure may impose lower values of that usually intervenes in the Forster equation as a statistical value that stems from a random chromophore orientation. 

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In the rearrangement of aUyl vinyl ethers in conformationally rigid cyclohexane system 127, the major product 128 is derived from the preferred axial bond formation in the TS [88]. 

In the absence of steric factors e.g. 5 ), the attack is antiparallel (A) (to the adjacent axial bond) and gives the axially substituted chair form (12). In the presence of steric hindrance to attack in the preferred fashion, approach is parallel (P), from the opposite side, and the true kinetic product is the axially substituted boat form (13). This normally undergoes an immediate conformational flip to the equatorial chair form (14) which is isolated as the kinetic product. The effect of such factors is exemplified in the behavior of 3-ketones. Thus, kinetically controlled bromination of 5a-cholestan-3-one (enol acetate) yields the 2a-epimer, (15), which is also the stable form. The presence of a 5a-substituent counteracts the steric effect of the 10-methyl group and results in the formation of the unstable 2l5-(axial)halo ketone... 

Rearrangement of sulfoxides 38a, b exhibited the interplay of several conformational factors. Both diastereomers afford predominant axial (trans) alcohol, but with opposite absolute configuration. The (R, R)-diastereomer strongly prefers the exo-transition state, whereas the (R, S)-isomer prefers the endo conformation. Hoffmann interprets these results in terms of an approximately 3-fold preference for the exo-transition state but a 6-fold preference for formation of an axial bond, these effects reinforcing each other in one isomer but opposing each other in the second. 

An example of an iron-catalyzed C-C bond formation reaction was reported in 2001 [89]. Treatment of propargyl sulfides 87 with trimethylsilyldiazomethane in the presence of 5 mol% FeCl2(dppb) gave substituted homoallenylsilanes 88 in good to moderate yields (Scheme 3.43). The silanes 88d and 88e, which bear two centers of chirality, were obtained as 1 1 mixtures of diastereomers. Slight diastereoselectivity (2 1) was seen for the formation 88f, which is an axially chiral allene with a sterogenic center. 

Fig. 4j6 Stepwise eomplexing of Cu(OH)4 by a tetradentate macrocyclic ligand. The first Cu(II)-N bond is formed by replacement of an axial solvent molecule (k ) followed by a Jahn-Teller inversion (Ar, ) which brings the coordinated nitrogen into an axial position. Second-bond formation follows a similar pattern (k2 and 2b)- Reproduced with permisson from J. A. Drumhiller, F. Montavon, J. M. Lehn and R. W. Taylor, Inorg. Chem. 25, 3751 (1986). (1986) American Chemical Society.

Mo2(H20)8+ is one of only three dimeric aqua ions with no bridging ligands (Hgf and Rh are the others). The structure 5 is eclipsed with 6-bond formation and quadruple metal-metal bonding. A rapid pre-equilibrium involving substitution of anion into the axial (end) waters. 

Formation of a second axial bond of the metal on the opposite... 

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