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New Stable and Persistent Acyclic Diaminocarbenes
Author(s) -
Schulz Tim,
Weismann Daniel,
Wallbaum Lars,
Guthardt Robin,
Thie Charlotte,
Leibold Michael,
Bruhn Clemens,
Siemeling Ulrich
Publication year - 2015
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201502315
Subject(s) - electrophile , chemistry , selenium , carbene , medicinal chemistry , crystallography , organic chemistry , catalysis
Abstract The portfolio of acyclic diaminocarbenes (ADACs) has been substantially expanded, owing to the synthesis of eleven new formamidinium salts, mostly of the type [( i Pr 2 N)CH(NRR′)][PF 6 ], for use as immediate carbene precursors. The corresponding ADACs ( i Pr 2 N)C(NRR′) were sufficiently stable for isolation in the case of NRR′=2‐methylpiperidino ( 13 ), 3‐methylpiperidino ( 14 ), 4‐methylpiperidino ( 15 ), morpholino ( 17 ) and N i PrPh ( 20 ), but had to be trapped in situ in the case of NRR′=2,2,6,6‐tetramethylpiperidino ( 12 ) and N i PrMe ( 19 ). The tetraaryl‐substituted ADACs (Ph 2 N) 2 C ( 22 ) and (Ph 2 N)C[N(C 6 F 5 ) 2 ] ( 24 ) also could only be generated and trapped in situ. Trapping with elemental selenium was particularly efficient, affording the corresponding selenourea derivative in all cases, whereas trapping with [{Rh(μ‐Cl)(cod)} 2 ] did not work for 12 and 24 . The 77 Se NMR chemical shifts, δ ( 77 Se), of the selenourea compounds derived from the new ADACs lie in the range 450–760 ppm, which indicates a much higher electrophilicity and π‐accepting capability of ADACs in comparison with NHCs, which typically exhibit δ ( 77 Se)<200 ppm. The extreme low‐field shift of 758 ppm observed for 12 Se can be rationalised by the results of DFT calculations, which revealed that ADAC 12 has a minimum energy conformation with the 2,2,6,6‐tetramethylpiperidino unit perpendicular to the N 2 C plane, which suppresses the π donation of this amino group and causes an unusually low LUMO energy and high electrophilicity.