Ph-pybox

Scheme 28 explains the stereochemical outcome from the tandem radical cyclization in the presence of the [Yb(Ph-pybox)(OTf)3] (pybox = 2,6-bis(2-oxazolin-2-yl)pyridine). The ytterbium complex 107 is shown in an octahedral geometry (with one triflate still bound to the metal) where re-face cyclization is favored due to the steric interactions of the substrate and the ligand s phenyl groups. The 6-endo cyclization takes place via a chair-like transition state to yield a tertiary radical 108 followed by a ring flip and... 

Complexes derived from the oxazolinylpyridine ligand Ph-Pybox and [Ir(COD)Cl]2 catalyze allylic substitution reactions of several oxygen and nitrogen nucleophiles... 

Hydroxylamine can act as either a N-nucleophile or O-nucleophile, depending on which of the reactive centers is protected. For all reactions Ph-Pybox has been used as ligand, and moderate to high levels of selectivity have been achieved. Hydroxamic acid derivatives and oximes have also been probed as O-nucleophiles [63]. 

The O-substitution products of oximes can be readily cleaved to give the corresponding alcohols. The (Ph-Pybox) Ir catalyst worked well at low temperatures with allyl phosphates as substrates (Scheme 9.36) [40, 63]. The selectivities depended heavily on the base used for activation of the oxime with, again, the best results being obtained with Ba(0H)2-H20. Only arylallyl phosphates have been reported as substrates. 

The asymmetric conjugate addition of thiophenol to ( >3-crotonoyloxazolidin-2-one, catalysed by the scandium(III) triflate complex of Ph-PYBOX, gave the corre- sponding adduct in 66% ee. Lanthanoid triflates gave lower enantioselectivities (<28% ee).132... 

Leigh and coworkers developed in parallel a Ni-catalyzed reductive homocoupling of alkyl halides 93 toward an efficient synthesis of rotaxanes (Fig. 22) [122]. In model studies 5-12 mol% of the Ni(II) precatalyst and terpy 8a or (/LA i-Ph-PyBOX 5g sufficed for the coupling of alkyl halides under normal conditions, under which 78-99% of dimers 94 were obtained. A macrocyclic Ni(PyBOX) complex, which has to be present stoichiometrically due to its function as the wheel, serves as the precatalyst in the rotaxane synthesis. Initial reduction of... 

Jorgensen et al. reported that C2-symmetric bis(oxazoline)-copper(II) complex 25 also acts as chiral Lewis acid catalyst for a reaction of allylic stannane with ethyl glyoxylate [37]. Meanwhile, p-Tol-BINAP-CuCl complex 26 was shown to be a promising chiral catalyst for a catalytic enantioselective allylation of ketones with allyltrimethoxysilane under the influence of the TBAT catalyst [38]. Evans and coworkers have developed (S,S)-Ph-pybox-Sc(OTf)3 complex 27 as a new chiral Lewis acid catalyst and shown that this scandium catalyst promotes enantioselective addition reactions of allenyltrimethylsilanes to ethyl glyoxylate [39]. But, when the silicon substituents become bulkier, nonracemic dihydrofurans are predominantly obtained as products of [3+2] cycloaddition. 

An important feature of the Evans system is the insight provided into catalyst structure. The X-ray crystal structure of the [Sn(Ph-pybox)](OTf)2 complex displays the central Sn(II) atom in square pyramidal geometry. The three amino donors occupy a meridional position with the triflate counter ions trans -diaxially bound. These are slightly distorted away from the stereochemically relevant, Sn-centered lone pair that resides in the meridional plane. Electrospray ionization studies reveal that the cationic complex is readily generated by dissociation of the triflate counter ions, underscoring the kinetic lability of [Sn(Bn-box)](OTf)2 and [Sn(Ph-pybox)](OTf)2 towards ligand exchange, the sine qua non of a catalytic process. 

An intermediate for the antiglaucoma drug dorzolamide was synthesized in 67% ee through asymmetric conjugate addition of thiophene-2-thiol to E)-3-crotonoyloxazolidin-2-one catalyzed by Sc(OTf)3 and Ph-pybox [126]. In addition, this catalytic system was proved to be successful in the Michael addition of thiophenol to the previous substrate (66% ee). 

In the Mukaiyama-Michael reaction of 3-acryloyl- or 3-crotonoyloxazolidinone with 2-trimethylsilyloxyfuran, the scandium-(4 S,5 S)-4-CH20TIPS-5-Ph-pybox... 

A comparative study on the influence of different lanthanide cations and the substituents at the 4 and 5 -positions in the pybox ligands in the Diels-Alder reaction of cyclopentadiene with 3-acryloyl-l,3-oxazolidin-2-one was realized [135], Of the catalysts derived from (4 P,5 P)-4-Me-5-Ph-pybox, the Sc(III)-based one is both strongly endo selective (endo/exo = 97/3) and highly enantioselective (97% ee (endo)). Later, it was found that the scandium-(4 S,5 S)-4-CH20TIPS-5-Ph-pybox complex also gave endo selective (endo/exo = 92/8-96/4) and highly enantioselective (93-99% ee (endo)) for the Diels-Alder reactions of cyclopentadiene with 3-acryloyl-or 3-crotonoyloxazolidinones. 

Evans and coworkers reported Ce(IV)-PyBOX catalyzed highly enantioselective nitrone cycloaddition by employing a,P-unsaturated 2-acyl imidazoles. Five mole percent of Ce(OTf)4/Ph-PyBOX promoted the reaction under mild conditions (0°C, EtOAc), giving products in up to 99% yield, 97% ee, and endojcxo =... 

Suga and coworkers reported utility of a Yb(OTf)3-PyBOX complex in 1,3-dipolar cycloaddition reactions of carbonyl ylide generated using Rh2(OAc)4 [71, 72]. As shown in Scheme 13.26, the Yb(OTf)3/Ph-PyBOX complex promoted the reaction of 2-benzopyryrium-4-olate with 3-acryloyl-2-oxazolidinone in endojexo = 12 88,... 

Asymmetric catalysis Initial attempts to develop non-racemic catalysts for the enantioselective hydrosilylation of alkenes have not been successful with C2-symmetric (5)-Ph-pybox and p-diketiminate calcium amide complexes catalyzing the addition of phenylsilane to styrene in the absence of solvent at 50 °C in, at best, 9 % e.e [69]. As with intramolecular hydroaminatiOTi catalysis, these results have been explained in terms of the loss of ligand control due to facile ligand redistribution reactions under the catalytic reaction conditions. 

Maruoka and co-workers reported the first catalytic asymmetric three-component 1,3-dipolar cycloaddition of terminal alkynes with acyclic azomethine imines generated in situ from the corresponding aldehydes and hydrazides, which was realized using CuOAc/Ph-pybox and axially chiral dicarboxylic acid cocatalysts (Scheme 27) [48]. This transformation has abroad tolerance with regard to the substrates, affording diverse chiral 3,4-disubstituted pyrazolines with high enantioselectivities. The role of the axially chiral dicarboxylic acid is to generate the protonated acyclic azomethine imine, which then reacts with chiral Cu-acetylide. 

Willis et al reported an enantioselective direct aldol reaction for the synthesis of protected aryl P-hydroxy-a-amino acids catalyzed by chiral Mg (I I) complexes (Scheme 4.12) [12]. A catalyst system composed of 10 mol% of Mg(Cl04)2,11 mol% of (S,S)-Ph-Pybox (35), and diisopropylethylamine generated an enolate of a chiral glycine equivalent derived from isothiocyanate-substituted oxazolidinone (33). The... 

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