Pyrrole geometry

A dimer made up of two zinc porphyrins bearing a 7-azabicy-clo[2.2.1]heptadiene fused at the C2-C3 /3-positions was reported by Knapp (61). The compound was designed to dimerize with a pyrrole-over-pyrrole geometry similar to that found in the photosynthetic special pair. Dimerization at KT3 M was confirmed by VPO and JH NMR spectroscopy. Dilution to 10-5 m or addition of DMAP caused disaggregation of the complex. In the solid state, this compound assembles as a cyclic hexamer with the vicinal porphyrin planes almost perpendicular. 

Pyrrole, 3-hydroxy-geometry, 4, 158 synthesis, 4, 343 tautomerism, 4, 36, 198 Pyrrole, 3-([Pg.816]

Draw appropriate Lewis structures for pyrrole. How many K electrons does pyrrole have Is pyrrole aromatic Would you expect the three carbon-carbon bonds to be approximately the same length Explain. Examine the actual geometry of pyrrole. Are the bonds the same length ... 

The ligand group can be introduced either on the meso or on the /5-pyrrole position of the porphyrin ring, but the synthesis of the meso-functionalized derivatives is easier and has been more widely exploited. Balch (50-53) reported that the insertion of trivalent ions such as Fe(III) (32) and Mn(III) (33) into octaethyl porphyrins functionalized at one meso position with a hydroxy group (oxophlorins) leads to the formation of a dimeric head-to-tail complex in solution (Fig. 11a) (50,51). An X-ray crystal structure was obtained for the analogous In(III) complex (34), and this confirmed the head-to-tail geometry that the authors inferred for the other dimers in solution (53) (Fig. lib). The dimers are stable in chloroform but open on addition of protic acids or pyridine (52). The Fe(III) octaethyloxophlorin dimer (52) is easily oxidized by silver salts. The one-electron oxidation is more favorable than for the corresponding monomer or p-oxo dimer, presumably because of the close interaction of the 7r-systems in the self-assembled dimer. 

The fe-Ir1 complex (344) of the selena sapphyrin ligand has been synthesized and characterized by X-ray diffraction studies.551 The pairs of bound N atoms in (344) are bent towards the Ir1 centers, and the respective pyrrole rings are twisted from the macrocyclic plane. The coordination geometry around the Ir is close to square-planar, and the Ir Ir distance is 4.233 A. The Se center is not involved in bonding to the Ir centers. 

A corollary to the above argument is that enantioselectivities depend on alkene geometry. Indeed, isomeric enolsilanes provide enantiomeric products. Because obtaining enolsilanes such as 344 in high isomeric purity is difficult, enantioselectivities with these nucleophiles are reflective, Eqs. 214 and 215. Pyrrole-derived enolsilanes are accessible in very high isomeric purity (>99 1) thus providing a convenient solution to this problem. Their use in the catalytic amination reaction provides access to a-hydrazino acid derivatives in high enantioselectivity. 

The pz ring has a dished geometry, the pyrrole rings being inclined by between 8 and 17° with respect to their associated central N4 planes, away from the metal center (the comparable values in the porphyrin structure are between 12° and 19°). The N4 centroid-- centroid distance between the pz rings is significantly shorter at 2.43 A than in the porphyrin case (2.75 A). The molecules pack to form stepped stacks with a Ce -Ce separation of 9.62 A and a lateral offset of 6.07 A. This value contrasts with a total absence of... 

The distance between the two S atoms on each pyrrole unit is 3.58 A, this distance is intermediate between that observed for the tin capped porphyrazineoctathiolate, Ni[pz(A4)], A = di-ferf-butyl tin (62a, Section IV.B) (3, 23, 24) in which the tin is coordinated by the S-N-S unit in a tridentate geometry, of 3.74 A and that observed for the nickel capped pz octathiolate in which the nickel is coordinate to the dithiolene (63, Scheme 11) of 3.25 A, which can be attributed to the fact that both types of bonding are observed in this crown species. 

The initial removal of electrons (following the oxidation, p-doping process) leads to the formation of a positive charge localised in the polymer chain (radical cation), accompanied by a lattice distortion which is associated with a relaxation of the aromatic structural geometry of the polymer chain towards a quinoid form. This form extends over four pyrrolic rings ... 

Cycloisomerization represents another approach for the construction of cyclic compounds from acyclic substrates, with iridium complexes functioning as efficient catalysts. The reaction of enynes has been widely studied for example, Chatani et al. reported the transformation of 1,6-enynes into 1-vinylcyclopentenes using [lrCl(CO)3]n (Scheme 11.26) [39]. In contrast, when 1,6-enynes were submitted in the presence of [lrCl(cod)]2 and AcOH, cyclopentanes with two exo-olefin moieties were obtained (Scheme 11.27) [39]. Interestingly, however, when the Ir-DPPF complex was used, the geometry of olefinic moiety in the product was opposite (Scheme 11.28) [17]. The Ir-catalyzed cycloisomerization was efficiently utilized in a tandem reaction along with a Cu(l)-catalyzed three-component coupling, Diels-Alder reaction, and dehydrogenation for the synthesis of polycyclic pyrroles [40]. 

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