Asymmetric exchange of cyclopalladated ligands: A new route to optically active phosphapalladacycles

Abstract

An asymmetric version of the cyclopalladated ligand exchange reaction was developed. This procedure involves the use of prochiral phosphines in an aprotic medium. A benzylaminate palladacycle bearing the primary amino group and the bulky Bu t substituent at the C* stereo center serves as a chirality inductor.Key words: asymmetric induction, C-H bond activation, cyclopalladated ligand exchange, optically active phospha and azapalladacycles, X ray diffraction analysis.Although the idea of asymmetric C-H bond activa tion by transition metals is attractive, examples of this process are scarce. In a special case of cyclopalladation, two versions of asymmetric C-H bond activation under control of either external sources of chirality (bases) 1-7 or inner sphere sources are known. Bidentate N,N 8,9 and P,P donor 10,11 ligands were tested as the latter sources. The chirality transfer from chelated ligands proved to be highly efficient in the closure of C*O 9 and C*C palladacycles (see Refs 8, 10, and 11), whereas an attempt to generate planar chirality upon the formation of a CN palladacycle with the use of an optically active monodentate S* donor ligand (sulfoxide) in a metallating agent failed. 12 Chelation of phosphino P ylides giving rise to PC* palladacycles with diastereoselectivity of ~70% de (see Refs 13 and 14) is the only example of the use of CN palladacycles as chirality inductors. However, con figurational lability of such systems substantially limits their use.The aim of the present study was to develop a new method of asymmetric C-H bond activation based on cyclopalladated ligand exchange (CLE ) 15,16 (Scheme 1). Scheme 1Although the starting palladacycle in the CLE reac tion can serve not only as the metallating agent but also as the source of chirality, only achiral versions of this reac tion have been examined. The only attempt to perform asymmetric CLE has been unsuccessful. 17 High efficiency of cyclopalladated complexes in optical resolution 18,19 and enantioselective catalysis, 20-23 as reagents for the enantiomeric purity determination, 24,25 and as matrices for asymmetric synthesis 26-28 was documented. This stimulated the estimation of their potential as chirality inductors in CLE reactions. The preliminary results of our investigations in this field have been published re cently. 29 Results and DiscussionWe chose a series of benzylaminate CN dimers (1a,b and 2a-7a), which differ in the nature of the amino The asymmetric atom directly bound to the metal atom is marked with an asterisk.The more laconic term transcyclometallation has been pro posed 16 for the description of such reactions; however, this term does not reflects the essence of the process, in which ligands rather than metal atoms (transmetallation) are exchanged.The number of the complex corresponds to the superscript n in the code of the starting ligand HL n , and the letters stand for different derivatives of this palladacycle.

Synthesis of Planar Chiral Phosphapalladacycles by N-Acyl Amino Acid Mediated Enantioselective Palladation

“…31 P NMR (δ, 400 MHz, CDCl 3 ): 33.29. MS (m/z, EI): 754 (4%), 755 (6), 756 (25), 757 (64), 758 (100), 759 (74), 760 (64), 761 (60), 762 (38), 763 (28), 764 (10), 765 (2).…”

“…The majority of scalemic planar chiral palladacycles are obtained by resolution or by the use of a chiral auxiliary to control diastereoselective palladadation, the synthesis of 1 and 2 falling into this latter category. , In contrast, examples of enantioselective palladation are rare. One of us recently reported the application of 1a as a reagent for the transcyclopalladation of prochiral ferrocenes 3 , resulting in the highly enantioselective generation of phosphapalladacycles 4 (Scheme ). , In 1979 Sokolov and others described the reaction of (dimethylaminomethyl)ferrocene ( 5 ) with sodium tetrachloropalladate together with 1 equiv of ( S )- N -acetylleucine ( 7a ), which resulted in the isolation of palladacycle 6 in up to 79% ee (Scheme ) . There is currently a great deal of interest in transition-metal C−H activation chemistry and its application to arene substitution, including enantioselective functionalization .…”

Asymmetric Construction of a Ferrocenyl Phosphapalladacycle from Achiral Enones and a Demonstration of Its Catalytic Potential

“…The fixing of planar chirality onto the ferrocene framework demands appropriate chiral control, through either an existing internal stereogenic center or an external chiral auxiliary. Previous reports by Dunina adopted Pd(OAc) 2 and cyclopalladated ligand exchange (CLE) as primary means to consummate a ferrocenyl P–C-ligated palladium(II) complex. Although palladation attempts via Pd(OAc) 2 and CLE generated the targeted palladated product within 3 h, poor yields of 20% and 10% were obtained, respectively.…”