Ion pairing in tetraphenylporphyrin oxidation: a semiquantitative study

In 1,2-difluorobenzene (DFB), electrolyte conductivity measurements and cyclic voltammetric titration on the traditional benchmark tetraphenylporphyrin, H(2)tpp, permit the first estimate of ion pair association constants for singly- and doubly-oxidized free-base porphyrins. From ion titration cyclic voltammetry and digital simulation, measured association constants for H(2)tpp(+)X(-) were 65, 120, 210, 520 and 730 M(-1), for X(-)= PF(6)(-), ClO(4)(-), NTf(2)(-), BF(4)(-) and OTf(-), respectively, relative to the association constant for the H(2)tpp(+)TFPB(-) complex. By similar methods it was found that the association constants for the corresponding dication, H(2)tpp(2+), were at least 3.0 x 10(4) M(-1)(PF(6)(-)), 2.5 x 10(6) M(-1)(ClO(4)(-)), 5.2 x 10(5) M(-1)(NTf(2)(-)), 1.9 x 10(6) M(-1)(BF(4)(-)) and 2.7 x 10(6) M(-1)(OTf(-)). We demonstrate that differences in association constants allow the formal potential of the second oxidation of H(2)tpp to be shifted by more than 800 mV simply by varying the solvent and electrolyte. In addition, calculated electrostatic potential energy maps for porphyrin dications suggest that exposure of the core N-H groups is responsible for the change in ordering of anion affinities that occurs upon oxidation of H(2)tpp(+).     

Utility of the tandem Pauson-Khand reaction in the construction of tetracycles

The scope of the tandem Pauson-Khand reaction has been explored for the regiospecific construction of [5.5.5.5]- and [5.6.6.5]tetracyclic systems via the photolytic method of Livinghouse. The rapid regiospecific entry into the two dicyclopentapentanoid systems 17 and 29 was accomplished from the key diene-diynes 11 and 19b. A photochemically mediated catalytic tandem Pauson-Khand cyclization was employed to prepare the parent ring systems of dicyclopenta[a,e]pentalene (from 19b) and dicyclopenta[a,f]pentalene (from 11) in regiospecific fashion in a one-pot process. Under these conditions, conversion of acyclic diene-diyne 16 into tetracyclic system 17 was achieved in 74% yield, while a similar process was employed to convert 28 into tetracycle 29 in 90% yield. This is much improved over the previous conditions that employed NMO. Six carbon-carbon bonds were generated in this process constituting up to 98% yield for each carbon-carbon bond so formed. Furthermore, tetracyclic [5.6.6.5] systems such as dicyclopenta[b,g]decalins 37, 38, and 40 were prepared from similar diene-diyne precursors via the tandem Pauson-Khand cyclization. Importantly, acetal 36 provided the desired cis-fused [5.6.6.5] system 38a (via 40a/b) in stereospecific fashion. This reaction is unique in that it provides a cis-decalin ring system; moreover, the yield of each of the six carbon-carbon bonds formed in this process was at least 89%. The structure of cis diol 38a was confirmed by X-ray crystallography.     

An Alternative to the Sandmeyer Approach to Aryl Iodides

Iodoarenes are important synthons for a wide range of organic transformations. Here we report a general strategy to prepare singly iodinated electron‐rich aromatic compounds through the intermediacy of diaryliodonium salts. This process, which incorporates a phase separation that greatly simplifies product purification, is an attractive replacement for the Sandmeyer approach to iodoarenes that are otherwise difficult to access.