Synthesis, characterization and X-ray structure of 3,4-lutidinyl-, 3-/4-picolyl- and pyridylselenium compounds

Bis(4,5-dimethyl-2-pyridyl)-, bis(5-methyl-2-pyridyl)- and bis(4-methyl-2-pyridyl) diselenide have been synthesized directly from 3,4-lutidine (1a), 3-picoline (1b) and 4-picoline (1c) respectively via BF3 aided lithiation reaction. The lithiation of 3,4-lutidinium-, 3- and 4-picolinium-BF3 adduct (2a/2b/2c) gives the corresponding carbanion (3a/3b/3c) which on subjecting to selenium insertion reaction followed by aerial oxidation affords the related diselenide in good yield. Reaction of BF3 complexed pyridylselenolate anion (4a/4b/4c) with diiodomethane gives the corresponding bis(2-pyridylseleno)methane. The dilithiation of 4-picolinium-/pyridinium-BF3 adduct (2c/2d) followed by reaction with 2.2 equiv. of selenium and iodomethane affords the related 2,6-bis(methylselenenyl)pyridine and 2-(methylselenenyl)pyridine in varying proportions. Preparation of tris(methylselenenyl) derivatives of 1a and 1c have been given in the present study. LiAlH4 has also been utilized to synthesize unsymmetrical monoselenides from the corresponding diselenides. Single crystal X-ray studies of bis(4,5-dimethyl-2-pyridyl) diselenide (5a), 3,4-dimethyl-2,6-bis(methylselenenyl)pyridine (9a), 4-methyl-2,6-bis(methylselenenyl)pyridine (9c), 2,6-bis(methylselenenyl)pyridine (9d) and 4-methyl-2,6-bis(methylselenenyl)-4-(methylselenenylmethyl)pyridine (12a) have also been carried out. The crystal data of these compounds reveals that the instances of Se⋯Se secondary interactions decreases with the increase in the number of methyl group attached to the pyridine ring.

Graphical abstractThe synthesis of bis(4,5-dimethyl-2-pyridyl)-, bis(5-methyl-2-pyridyl)- and bis(4-methyl-2-pyridyl) diselenide has been achieved directly from 3,4-lutidine(1a), 3- (1b) and 4-picoline (1c), respectively via BF3 aided lithiation reaction. BF3 aided pyridyl ring dilithiation and trilithiation of 1a, 1c and pyridine (1d) leads to the formation of compounds having two or three selenium atoms. The crystal structure of a molecule having two selenium atoms attached to a pyridine moiety and third selenium atom attached to a side-chain has been reported for the first time. LiAlH4 has also been utilized to synthesize unsymmetrical selenides from the corresponding diselenides.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Synthesis of bis(4,5-lutidinyl)- and bis(3-/4-picolyl) diselenide from lutidine and picoline. ► Synthesis of pyridylseleniums having two or three selenium atoms. ► Crystal structure of a molecule with three selenium atoms at different positions. ► Cleavage of Se–Se bond in the bis(4,5-lutidinyl)- and bis(4-picolyl) diselenides with LiAlH4.