Oxazoline-Derived P,N Ligands

The oxazoline-derived P,N ligands can be classified into four groups according to structure phosphino-oxazolines phosphite- and phosphinite-oxazolines catalysts containing a P-N bond and structurally related non-oxazoline catalysts. 

The 1-t-butylphospholane sulfide intermediate to TangPhos was also used to prepare the P,N ligands 48 by reacting the lithium complex with C02 and then oxazoline formation with a range of chiral amino alcohols [69b, 74]. The Ir complexes of these ligands have been successfully used in the reduction of / -methylcinnamic esters (80-99% ee) and methylstilbene derivatives (75-95% ee), a particularly challenging class of unfunctionalized olefins [4 c]. 

Pfaltz and coworkers [43] developed P,N ligands 73 and 74 derived from pyridine and quinoline. They reasoned that oxazoline and pyridine/quinoline ligands, since they have different electronic effects, would induce different patterns of reactivity and selectivity in the asymmetric Mizoroki-Heck reaction. In the reaction of phenyl triflate (2) with... 

Among the potentially bidentate, P,N ligand derivatives of quite different structure can be found. The electronic and steric parameters of 2-(2 -diarylphosphino-phenyl)oxazoline-type ligand, PHOX) (81) may be varied both... 

Hashimoto et al. [38] reported the synthesis of the P,N oxazoline-containing ligand 59 derived from ds-2-amino-3,3-dimethyl-l-indanol and its application to the intermolecu-lar Mizoroki-Heck reaction. Their best reaction conditions for arylation of various cy-cloalkenes (Scheme 11.23) showed conversions of up to 91% and enantiomeric excesses of up to 98% for the reaction of cyclohexenyl triflate (35) with 2,3-dihydrofuran (1). Reactions using 4,7-dihydro-l,3-dioxepin (46) as the substrate proved less successful, with low conversions of 37% achieved, albeit with high enantiomeric excesses of up to 90%. In all cases, low amounts of isomerization were observed. These results are consistent with, if not better than, many analogous phosphinooxazoline ligands. 

Among the diverse N-functional moieties associated with P,N-chelating ligands, the heterocycles oxazoline and thiazoline as well as their substituted derivatives have enjoyed special attention. The P-donor group can be connected through a spacer to the 2-position of the oxazoline ring or to the 4-position, as in the basic subunits shown in Scheme 14.3. 

The N,P phosphine-oxazoline chelate (59) is chiral, and complexes can act as homogeneous catalysts for asymmetric synthesis the Ir(l) and Pd(II) complexes promote enantioselective olefin hydrogenation and allylic substitution respectively. An N,P analog of the N,N didentate ligand 2,2 -bipyridine is (60), the soft P donor helping to stabilize low-valent metals. Further, 2,2-bipyridine derivatives such as (61) can bind metals such as Ir and Ru as N,C chelates with one pyridine nitrogen rotated to the opposite side, away from the metal ion. 

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