Oxazoline-phosphine complexes

The arylation of secondary phosphines 201 with ortho-aiy iodides, catalyzed by generated in situ complex Pda (dba>3 x CHQ3, containing chiral ligand Et,Et-FerroTANE 207 and LiBr, led to the formatiOTi of corresponding tertiary phosphines with enantioselectivity of 90% cc [ 132,137]. The palladium complex 209 also showed high enantioselectivity in arylation of secondary phosphines [131,132]. Some examples of arylation reaction of secondary phosphines with low ee were described. The asymmetric arylation of phosphine boranes with anisyl iodide, catalyzed by chiral complex of oxazoline phosphine 208, led to the formation of enantiomerically enriched tertiary phosphines 206 with 45% ee [134]. The Pd complex 210 of (R )-t-Bu-JOSlPHOS ligand catalyzed arylation of PH(Me)(Ph)(BH3) by o-anisyl iodide with the formation of PAMP-BH3 with 10% ee (Table 3) [112]. 

In the last few years, an increasing interest in the elaboration of new NHC-Pt families of complexes has arisen. This includes Pt-oxazoline/NHC complexes, combining the heterocyclic ligand moiety as Al-substituent of NHC unit, or mixed Pt NHC/phosphine complexes developed by Rourke (Figure 10.4). The Cavell group described a l,3-diazepan-2-ylidene carbene and its coordination with Pt . On the other hand, Steinborn reported the synthesis and the characterization of Pt and Pt complexes with monodentate N,N-, N,0- or... 

Fig. 10.7 Ene-type isomerizations of 1,6-enynes promoted by a phosphine-oxazoline palladium complex [21]...

Asymmetric versions of the cyclopropanation reaction of electron-deficient olefins using chirally modified Fischer carbene complexes, prepared by exchange of CO ligands with chiral bisphosphites [21a] or phosphines [21b], have been tested. However, the asymmetric inductions are rather modest [21a] or not quantified (only the observation that the cyclopropane is optically active is reported) [21b]. Much better facial selectivities are reached in the cyclopropanation of enantiopure alkenyl oxazolines with aryl- or alkyl-substituted alkoxy-carbene complexes of chromium [22] (Scheme 5). 

Helmchen and co-worker investigated the use of phosphinooxazolines as ligands for copper(II) catalyzed Diels-Alder reactions (Scheme 19) (214). Optimal selectivities are found for a-naphthyl-substituted phosphinooxazoline (299). These catalysts require 2.5 h to induce complete conversion to cycloadduct, compared to 18 h using the triflate complex 269c under identical conditions. Helmchen invokes a square-planar metal geometry to explain the stereochemistry of the adducts, similar to the model proposed by Evans. He suggests that the bulky phosphine substituents are required to orient binding of the dienophile in such a way as to place the olefin directly below the terf-butyl substituent on the oxazoline. 

Burgess and co-workers (38) recently reported a similar approach to the evaluation of a phosphine-oxazoline ligand system. In this paper, a series of phosphine-oxazoline ligands (132) were synthesized individually. Palladium complexes of... 

The preparation of carbonyl-lr—NHC complexes (Scheme 3.1) and the study of their average CO-stretching frequencies [7], have provided some of the earliest experimental information on the electron-donor power of NHCs, quantified in terms of Tolman s electronic parameter [8]. The same method was later used to assess the electronic effects in a family of sterically demanding and rigid N-heterocyclic carbenes derived from bis-oxazolines [9]. The high electron-donor power of NHCs should favor oxidative addition involving the C—H bonds of their N-substituents, particularly because these substituents project towards the metal rather than away, as in phosphines. Indeed, NHCs have produced a number of unusual cyclometallation processes, some of which have led to electron-deficient... 

C27H23Cl2N03PRe, Rhenium(V), dichloro(2-(2 -hydroxyphenyl)-2-oxazoline)(triphenyl-phosphine) 0x0-, 34 58 C27H50P2, dcpp, l,3-bis(dicyclohexylphosphi-no)propane, complex with copper(I), 34 177... 

Mechanistic aspects of the hydrogenation of alkenes catalysed by iridium complexes with a new class of chiral phosphine-oxazoline ligands have been discussed and a selectivity model to help rationalize the results obtained has been presented.344... 

Naud, F., Malan, C., Spindler, F., Ruggeberg, C., Schmidt, A.T. and Blaser, H.-U. (2006) Ru-(phosphine-oxazoline) complexes as effective, industrially viable catalysts for the enantioselective hydrogenation of aryl ketones. Adv. Synth. Catal., 348, 47. 

Inspired by the chiral phosphine/oxazoline ligands developed by Helmchen and Pfaltz [131], Crudden and coworkers, have prepared a chiral NHC-oxazoline possessing a rigid backbone (Fig. 14) [ 132 ]. The rhodium complex 74 has been used in the catalytic hydroboration of olefins and the hydrosilylation of prochiral ketones with enantiomeric excesses that did not exceed 10%. 

A plethora of ligands has been reported in the literature but the most effective ones are 1,2-amino alcohols, monotosylated diamines and selected phosphine-oxazoline ligands. The active structures of the complexes reported are half-sand-... 

TTF in aqueous solution has been examined <07TL8430> and fabrication of thin crystals of [tetramethyltetraselenafulvalene]2+ PF6" within a confined electrode has been reported <07SM(157)492>. The synthesis, structures and electrochemistry of Cu and Co complexes of TTF-oxazolines 53 have been described <07ICA(360)233> and the crystal stmcture of the valine-derived TTF-oxazoline 54 has been determined while the corresponding phosphine 55 has proved to be an effective ligand for iridium-catalysed asymmetric hydrogenation of imines <07TA1877>. 

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. 

---