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Rhodium‐Catalyzed Enantioselective Intramolecular [4+2] Cycloaddition using a Chiral Phosphine‐Phosphite Ligand: Importance of Microwave‐Assisted Catalyst Conditioning
Author(s) -
Falk Anna,
Fiebig Lukas,
Neudörfl JörgMartin,
Adler Andreas,
Schmalz HansGünther
Publication year - 2011
Publication title -
advanced synthesis and catalysis
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.541
H-Index - 155
eISSN - 1615-4169
pISSN - 1615-4150
DOI - 10.1002/adsc.201100658
Subject(s) - chemistry , enantioselective synthesis , ligand (biochemistry) , cycloaddition , rhodium , catalysis , intramolecular force , phosphine , moiety , chiral ligand , medicinal chemistry , asymmetric induction , stereocenter , combinatorial chemistry , organic chemistry , biochemistry , receptor
The use of modular α,α,α′,α′‐tetraaryl‐1,3‐dioxolane‐4,5‐dimethanol (TADDOL)‐ and 1,1′‐bi‐2‐naphthol (BINOL)‐derived phosphine‐phosphite ligands (L 2 *) in the asymmetric rhodium‐catalyzed intramolecular [4+2] cycloaddition (“neutral” Diels–Alder reaction) of ( E , E )‐1,6,8‐decatriene derivatives (including a 4‐oxa and a 4‐aza analogue) was investigated. Initial screening of a small ligand library led to the identification of a most promising, TADDOL‐derived ligand bearing a phenyl group adjacent to the phosphite moiety at the arene backbone. In the course of further optimization studies, the formation of a new, more selective catalyst species during the reaction time was observed. By irradiating the pre‐catalyst with microwaves prior to substrate addition high enantioselectivities (up to 93% ee ) were achieved. The new cyclization protocol was successfully applied to all three substrates investigated to give the bicyclic products in good yield and selectivity. 31 P NMR and ESI‐MS measurements indicated the formation of a [Rh(L 2 *) 2 ] + species as the more selective (pre‐) catalyst.