Tunable Phosphoramidite Ligands for Asymmetric Hydrovinylation: Ligands par excellence for Generation of All-Carbon Quaternary Centers

Abstract

Abstract1-Alkylstyrenes undergo efficient hydrovinylation (addition of ethylene) in the presence of a Nicatalyst prepared from [(allyl)NiBr] 2 , Na + [BAr 4 ] − (Ar = 3,5-bis-trifluromethylphenyl), and a phosphoramidite ligand giving products in excellent yields and enantioselectivities. In many cases phosphoramidites derived from achiral 2,2′-biphenol are almost as good as ligands derived from the more expensive enantiopure 2,2′-binaphthols. The hydrovinylation products, which carry two versatile latent functionalities, an aryl and a vinyl group, are potentially useful for the synthesis of several important natural products containing benzylic all-carbon quaternary centers.

Acyclic Quaternary Carbon Stereocenters via Enantioselective Transition Metal Catalysis

“…In a contemporaneous report, RajanBabu applied a nickel catalyst modified by the binaphthyl phosphoramidite ligand XXX to affect enantioselective hydrovinylation of α-ethylstyrenes at significantly lower catalyst loadings. 104–106 …”

Cobalt-catalysed asymmetric hydrovinylation of 1,3-dienes

“… 18 – 21 Among these, palladium, 15 nickel and cobalt, and to a limited extent, ruthenium catalysed reactions 22 – 29 have attracted the most attention in the stereoselective synthesis of small molecules. Through optimizations of ligands, promoters and reaction conditions, the Ni-catalysed asymmetric hydrovinylations (eqn (1)) of vinylarenes, 30 – 36 selected dienes, 37 – 39 and strained alkenes 40 , 41 have been accomplished with excellent regio- and stereoselectivities giving highly valuable intermediates. Prototypical examples of the products of these reactions ( 1–6 ) are shown in Fig.…”

Reactivity and Selectivity in Hydrovinylation of Strained Alkenes

“…Such metalation reactions involving Ni, Pd and Rh intermediates most often lead to ring opening reactions 24–31. In earlier work we had noticed that substrates carrying hetero-atoms, especially nitrogen, are slow to undergo hydrovinylation under moderate conditions 19. In light of this observation it is surprising that the azabicyclic alkene 7 undergoes hydrovinylation in very good yields, even though, not unexpectedly, the product is obtained as a mixture of two regio-isomers (Scheme 2, Table 2).…”