1-Ethyl-2-phenyl-3-[2-(trimethylsilyl)ethynyl]-1H-indole

The title compound, C21H23NSi, was synthesized by Sonogashira-type reaction of 1-ethyl-3-iodo-2-phenyl-1H-indole with trimethylsilylacetylene. The indole ring system is nearly planar [maximum atomic deviation = 0.0244 (15) Å] and is oriented at a dihedral angle of 51.48 (4)° with respect to the phenyl ring. The supramolecular aggregation is completed by weak C—H⋯π interactions of the methylene and phenyl groups with the benzene and pyrrole rings of the indole ring system. The methyl groups of the trimethylsilyl unit are equally disordered over two sets of sites.


1-Ethyl-2-phenyl-3-[2-(trimethylsilyl)ethynyl]-1H-indole Iaroslav Baglai, Valérie Maraval, Carine Duhayon and Remi Chauvin Comment
Indole is one of the three heterocycles occurring in the 20 standard natural amino-acids (in (L)-trhyptophane). Its unique bicyclic aromatic structure makes it a key rigid structural basis of one of the largest classes of alkaloids, comprising more than 4000 natural products (Huang et al., 2004;Seferoğlu et al., 2007a,b;Ruiz et al., 2012). The synthesis and functionalization of indoles have been the object of research for over one and a half century (Shiri, 2012). Few examples of Sonogashira-type reactions on the indole core have been described (Hussain et al., 2011;Prateeptongkum et al., 2010).
In the molecule (Fig. 1), the indole ring is almost planar [maximum deviation for ring atoms = 0.0244 (15) Å], and the dihedral angle between the pyrrole and benzene ring of the indole system is equal to 1.83 (5)°. Methyl groups of the trimethylsilyl moieties are disordered into two positions with the corresponding occupancy of 0.5 for each part. The whole molecule is almost planar excluding methyl groups of trimethylsilyl and ethyl moieties as well as the benzene ring [maximum deviation for atoms = 0.0539 (16) Å]. The interplanar angle between indole and phenyl planes is equal to 51.48 (4)°. Inspite of the three aromatic rings of the molecule, the packing shown in Fig. 2 does not reveal any particular intermolecular π-stacking or columnar arrangement but weak C-H···π interactions (Table 1).

1-ethyl-3-iodo-2-phenyl-1H-indole (3).
To a solution of 1-ethyl-2-phenyl-1H-indole 2 (0.5 g, 2.26 mmol) in CHCl 3 (30 ml) at 0 °C was added N-iodosuccinimide (0.535 g, 2.37 mmol) as a small portions over 5 min. The mixture was then stirred at the same temperature for 3 h. After evaporating the solvent, the residue was extracted in dichloromethane and washed with H 2 O. The organic layer was separated and dried over MgSO 4 . The solvent was removed under reduced pressure, and the product was purified by silica gel chromatography using a mixture of acetone and pentane (2:98) as an eluent (yield 92%, 0.72 g).
R f (99/5 = C 5 H 12 /Ether) = 0.47. The title compound was dissolved in Et 2 O and CH 2 Cl 2 (1:1 mixture). After slow evaporation over two days, crystals of 1a suitable for X-ray diffraction analysis were obtained.

Refinement
The H atoms were located in a difference Fourier map and refined with riding constraints. Methyl groups are disordered over two positions in site occupancy ratio of 0.5:0.5.   Packing diagram of the title compound, viewed along the c axis.