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Zinc‐ and Tin‐Mediated C−C Coupling Reactions of Metalated (2‐Pyridylmethyl)(trialkylsilyl)amines − Mechanistic, NMR Spectroscopic, and Structural Studies
Author(s) -
Westerhausen Matthias,
Bollwein Tobias,
Makropoulos Nikolaos,
Schneiderbauer Stefan,
Suter Max,
Nöth Heinrich,
Mayer Peter,
Piotrowski Holger,
Polborn Kurt,
Pfitzner Arno
Publication year - 2002
Publication title -
european journal of inorganic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 1434-1948
DOI - 10.1002/1099-0682(20022)2002:2<389::aid-ejic389>3.0.co;2-9
Subject(s) - chemistry , metalation , amide , dimethylzinc , methyllithium , medicinal chemistry , zinc , tetrahydrofuran , intramolecular force , tin , coupling reaction , stereochemistry , organic chemistry , catalysis , solvent
The zincation of (2‐pyridylmethyl)(triisopropylsilyl)amine ( 1 ) gives dimeric methylzinc (2‐pyridylmethyl)(triisopropylsilyl)amide ( 2 ). Further addition of dimethylzinc to a toluene solution of 2 at raised temperatures yields the C−C coupling product [1,2‐dipyridyl‐1,2‐bis(triisopropylsilylamido)ethane]bis(methylzinc) ( 3 ). Heating of molten 2 , or UV irradiation of 2 , results in the formation of 3 and zinc bis[(2‐pyridylmethyl)(triisopropylsilyl)amide] ( 4 ). The reaction between the zinc dihalide complexes of 1 [ 5a (X = Cl) and 5b (X = Br)] and methyllithium yields the C−C coupling product 3 and the heteroleptic complex 2 , observed by NMR spectroscopy. During this reaction, zinc metal precipitates. The magnesiation of 1 with dibutylmagnesium gives magnesium bis[(2‐pyridylmethyl)(triisopropylsilyl)amide] ( 6 ) in a quantitative yield. Subsequent addition of dimethylmagnesium results in a dismutation reaction and the formation of heteroleptic methylmagnesium (2‐pyridylmethyl)(triisopropylsilyl)amide ( 7 ). Treatment of 1 with dimethylmagnesium also gives 7 . This complex slowly undergoes an intramolecular metalation during which dark red single crystals of (tetrahydrofuran)magnesium 2‐(triisopropylsilylamidomethylidene)‐1‐azacyclohexa‐3,5‐dien‐1‐ide ( 8 ) precipitate. In this compound the aromaticity of the pyridyl fragment is abolished. The magnesiation of ( tert ‐butyldimethylsilyl)(2‐pyridylmethyl)amine ( I ) proceeds quantitatively to give methylmagnesium ( tert ‐butyldimethylsilyl)(2‐pyridylmethyl)amide ( 9 ). This compound also undergoes an intramolecular metalation reaction, which results in the loss of the aromaticity of the pyridyl substituent and the formation of (tetrahydrofuran)magnesium 2‐( tert butyldimethylsilylamidomethylidene)‐1‐azacyclohexa‐3,5dien‐1‐ide ( 10 ). The metalation of 1 with tin(II) bis[bis(trimethylsilyl)amide] yields [bis(trimethylsilyl)amido]tin(II) (2‐pyridylmethyl)(triisopropylsilyl)amide ( 11 ). The elimination of tin metal occurs even at room temperature, and the C−C coupling product [1,2‐dipyridyl‐1,2‐bis(triisopropylsilylamido)ethane]tin(II) ( 12 ) is formed. The metalation of ( tert ‐butyldimethylsilyl)(2‐pyridylmethyl)amine with Sn[N(SiMe 3 ) 2 ] 2 gives [bis(trimethylsilyl)amido]tin(II) ( tert ‐butyldimethylsilyl)(2‐pyridylmethyl)amide ( 13 ). Within a few minutes, precipitation of tin metal takes place and the C−C coupled product [1,2‐bis( tert ‐butyldimethylsilylamido)‐1,2‐dipyridylethane]tin(II) ( 14 ) is produced. In order to examine the importance of the pyridyl ligand for the C−C coupling reactions, zinc bis[ N ‐( tert ‐butyldimethylsilyl)benzylamide] ( 15 ) was prepared by means of the metathesis reaction between lithium N ‐( tert ‐butyldimethylsilyl)benzylamide and zinc(II) halide. Treatment of 15 with dimethylzinc yields heteroleptic methylzinc N ‐( tert ‐butyldimethylsilyl)benzylamide ( 16 ). Refluxing of 16 with an excess of dimethylzinc in toluene does not give any C−C coupling reactions.