Dibenzyl 3,3′,4,4′-tetramethyl-5,5′-(ethynediyl)bis(pyrrole-2-carboxylate)

The title molecule, C30H28N2O4, has crystallographic twofold rotation symmetry, with the pyrrole planes forming a dihedral angle of 40.49 (4)°. The pyrrole N—H donor and adjacent ester carbonyl acceptor form R 2 2(10) hydrogen-bonded rings about inversion centers, leading to chains of hydrogen-bonded molecules along [001].

The title molecule, C 30 H 28 N 2 O 4 , has crystallographic twofold rotation symmetry, with the pyrrole planes forming a dihedral angle of 40.49 (4) . The pyrrole N-H donor and adjacent ester carbonyl acceptor form R 2 2 (10) hydrogen-bonded rings about inversion centers, leading to chains of hydrogen-bonded molecules along [001].

S1. Comment
Di(5-benzyloxycarbornyl-3,4-dimethyl-2-pyrrolyl)-ethyne (I) is an important intermediate in the synthesis of porphyrin analogues containing a two-carbon interpyrrolic bridge as in corrphycenes (Vogel, 1996). Compound (I) is also an interesting intermediate in the synthesis of dipyrroles with a two carbon bridge, which have shown selective binding properties to fluoride and other ions (Black et al. 1999). The title compound was prepared via an improved Sonogashira coupling reaction (Chinchilla & Najera, 2007), that took place between benzyl 5-iodo-3,4-dimethyl-1H-pyrrole-2carboxylate and trimethylsilanyl-ethyne, in the presence of palladium(0) and copper(I) catalysts at room temperature. The exact experimental details are described below.
The structure of the title compound (I), which lies on a crystallographic twofold axis, is shown in Fig 1. The triple-bond distance is 1.2088 (17) Å, and the alkyne bridge is not quite linear, with C4-C5-C5 i angle 175.61 (10)° (i = 1 -x, y, 1/2 -z). The pyrrole ring is essentially planar, its five atoms having a mean deviation 0.003 Å from their best plane, with maximum 0.0051 (7) Å for C4. The two pyrrole rings are not coplanar, but form a dihedral angle of 40.49 (4)°. The ester COO group lies nearly in the pyrrole plane, with N1-C1-C6-O2 torsion angle 3.67 (13)°.
The pyrrole N-H group and adjacent ester carbonyl form intermolecular hydrogen bonded rings about inversion centers, having graph-set notation (Etter, 1990) R 2 2 (10). Thus, each molecule engages in four hydrogen bonds with two other molecules, forming chains in the [0 0 1] direction, as shown in Fig. 2.

S2. Experimental
To a 100 ml round bottom flask was added benzyl 5-iodo-3,4-dimethyl-1H-pyrrole-2-carboxylate (2.93 g, 10 mmol) followed by Pd(PPh) 2 Cl 2 (0.7019 g, 0.1 mmol) and CuI (0.191 g 0.1 mmol). The flask was sealed and placed in a dry ice bath under N 2 . Trimethylsilanyl-ethyne (0.6996 ml, 5 mmol), DBU (8.973 ml, 60 mmol) and water (0.072 ml, 40 molar equiv.) were dissolved in 30 ml of acetonitrile and added to the reaction flask. After the mixture froze in the dry ice bath, the flask was evacuated and N 2 gas added. The resulting reaction mixture was allowed to warm slowly to room temperature and was stirred until complete disappearance of the starting material by TLC. The reaction mixture was worked up by adding ethyl acetate (150 ml), and washing the organic layer three times with saline. The organic phase was dried over anhydrous sodium bicarbonate and concentrated under reduced pressure. The crude mixture was purified by flash column chromatography using hexane/ethyl acetate (5:1) for elution. The dipyrrole-ethyne (I) was obtained (0.755 g) in 54% yield and recrystallized from dichloromethane to afford colorless crystals.

S3. Refinement
H atoms were placed in idealized positions with C-H distances 0.95 -0.99 Å and thereafter treated as riding. U iso for H was assigned as 1.2 times U eq of the attached C atoms (1.5 for methyl). A torsional parameter was refined for each methyl group.

Figure 1
Ellipsoids at the 50% level, with the asymmetric unit labeled. H atoms are represented with arbitrary radius. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.