Carbon bridging

Bradshaw and Hui, in their review of sulfur-containing macrocycles, have included the analog of compound 7 (this chapter) in which each carbon bridge is replaced by a dimethylsilyl bridge. Nevertheless, silicon-containing crown relatives remain rare. 

Antidepressant activity is retained when the two carbon bridge in imipramine is replaced by a larger, more complex, function. Nucleophilic aromatic substitution on chloropyridine 31 by means of p-aminobenzophenone (32) gives the bicyclic intermediate 33. Reduction of the nitro group (34), followed by intramolecular Schiff base formation gives the required heterocyclic ring system 35. Alkylation of the anion from 35 with l-dimethylamino-3-chloropropane leads to tampramine 36 [8]. 

Isoporphycene is of special interest because this structural type should exist in two stereoisomeric forms (E)- and (Z)-15, respectively, which arc possible due to the thrcc-carbon bridge linking two of the pyrrole rings. 

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. 

The dipyrrylacelylenedicarbaldehyde 18, which already contains, along with the acetylene moiety, two of the two-carbon bridges of the final macrotetracycle, can be prepared by a Heck-type coupling of 5-iodopyrrole-2-carbaldehydes with acetylene.8a,b The main conjugation pathway with 22n electrons is already present in the acetylene-cumulate systems 19. The expected planarity of this chromophore has been confirmed by X-ray structure analysis.8a b... 

Hydrogenolysis of the acetylene-cumulenes 19 can be achieved by means of a partially poisoned palladium catalyst to yield the regular [22]porphyrin(2.2,2.2) 20 with cisjrans.cis,-trans configuration of the two-carbon bridges. 

One-electron oxidation of the vinylidene complex transforms it from an Fe=C axially symmetric Fe(ll) carbene to an Fe(lll) complex where the vinylidene carbon bridges between iron and a pyrrole nitrogen. Cobalt and nickel porphyrin carbene complexes adopt this latter structure, with the carbene fragment formally inserted into the metal-nitrogen bond. The difference between the two types of metalloporphyrin carbene, and the conversion of one type to the other by oxidation in the case of iron, has been considered in a theoretical study. The comparison is especially interesting for the iron(ll) and cobalt(lll) carbene complexes Fe(Por)CR2 and Co(Por)(CR2) which both contain metal centers yet adopt... 

Using the very bulky rhodium porphyrins Rh(TTEPP)- and Rh(TTiPP)- (which contain triethylphenyl and triisopropylphenyl groups), neither of which can dimerize. direct evidence for an alkene adduct and its subsequent dimerization to the four-carbon bridged product has been obtained. Reaction of Rh(TTEPP)- with ethene... 

Triply bridging carbonates between three zinc centers have been identified in nine different X-ray structures deposited in the CSD 458,461,465-467 For example, a binuclear ft-OH zinc complex with a tetradentate /V-donor ligand absorbs atmospheric carbon dioxide to a triply bridged carbonate.468 Examples are also known where the metal atoms are in varying coordination environments. The complex cation [Zn3(bipyridine)6(/U3-C03)(H20)2]4+ contains one penta- and two hexacoordinate zinc centers.469 A tetrapodal compartmental ligand forms a tetrameric complex with zinc that contains the carbonate bridging between three of the four zinc centers.470... 

Alternatively, the (2 + 4)-tropilidene endoperoxide, which is the major product in the singlet oxygenation of cycloheptatriene 30 a) affords on diimide reduction the desired bicyclic peroxide 20. The double bond in the two-carbon bridge is reduced selectively, but on exhaustive treatment with excess diimide, the fully saturated substance is obtained. A number of substituted derivatives have been prepared in this way30). 

Fig. 3 Chemistry of site-specific PEGylation developed by Brocchini et al. [5, 6]. After cleavage of the native disulfide bond between two cysteine thiols by reduction, the free cysteines are reacted with an a, (3-unsaturated PEG derivative to produce a PEG conjugate via a three-carbon bridge...

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