190907-18-7Relevant academic research and scientific papers
Tuning the Steric Properties of a Metathesis Catalyst for Copolymerization of Norbornene and Cyclooctene toward Complete Alternation
Torker, Sebastian,Mueller, Andre,Sigrist, Raphael,Chen, Peter
, p. 2735 - 2751 (2010/08/06)
We have recently published a mechanistic concept for the olefin metathesis reaction with ruthenium catalysts that explains the independent control of chemo- and stereoselectivity by substitution in two orthogonal planes. The basic structure from which we started the structural modifications for improved stereoselectivity had been optimized substantially for chemoselectivity, as compared to the prototype that had been published. We designed the catalyst according to a concept in which the factor that governs alternation is directly related to the size difference of the substituents on the bidentate phosphine/phenolate ligand 2a-e and in which the origin of chemoselectivity arises from diastereomeric site control. The most selective catalyst (11d) outperforms our prototype (3) in selectivity as well as reactivity. Synthetic problems, i.e., the undesired formation of 2:1-complexes (bidentate ligand-to-ruthenium ratio) (8) by reaction of the bidentate phosphine/phenolate ligands 2a and 2b with the first-generation Hoveyda-Grubbs catalyst 4, were solved by introduction of a larger carbene unit, which not only favors the formation of 1:1-complexes but also results in increased initiation rates. The work is supported by NMR data and X-ray crystallography, which give insights into the steric properties of the investigated system.
Nucleophilic displacement routes to P-chiral phosphines; the introduction of sterically encumbered groups
Brown, John M.,Laing, J. Christopher P.
, p. 435 - 444 (2007/10/03)
Methods for the asymmetric synthesis of P-chiral monophosphines carrying a bulky residue have been appraised. Reaction of 2-adamantylmagnesium bromide with P-chlorooxazaphospholidine 1 gives two diastereomeric products after oxidation with t-BuOOH; the SP isomer was characterised by X-ray and the RP isomer through a detailed NMR analysis. Although ring opening of either isomer 2 or 3 on reaction with o-anisylmagnesium bromide occurred satisfactorily, continuation of the desired reaction sequence through acid-catalysed methanolysis of the phosphinamide 4 and further with PhMgBr led to diminution of the enantiomeric purity in each step because of the more forcing conditions brought about by the steric demands of the 2-adamantyl residue. Similar difficulties were experienced when following the P-borane route of Juge and Genet, where the 2-adamantyl group was introduced in the initial step. More success was achieved by the borane route when the bulky residue was introduced last by reaction of RLi with the phosphinite complex 13. In this way the ferrocenyl, 1-adamantyl and t-butylphosphines carrying both phenyl and o-anisyl residues, were formed as borane complexes. Deboronation proceeded smoothly in the first two cases to give the tertiary phosphines in greater than 92% e.e.
