FULL PAPER DOI: 10.1002/ejic.201000308 Better Performance of Monodentate P-Stereogenic Phosphanes Compared to Bidentate Analogues in Pd-Catalyzed Asymmetric Allylic Alkylations Arnald Grabulosa, [a] Guillermo Muller,* [a] Rosa Ceder, [a] and Miguel Ángel Maestro [b] Keywords: Palladium / Allylic compounds / Asymmetric catalysis / Nucleophilic substitution The cationic allylpalladium complexes 3a–3f, 4a, 4e, 5e of type [Pd(η 3 -2-Me-C 3 H 4 )P 2 ]PF 6 were synthesized using a group of monodentate P-stereogenic phosphanes, P=PPhRR' (a–f) and diphosphanes (PhRPCH 2 ) 2 (1a, 1e) or PhRPCH 2 Si(Me) 2 CH 2 PPhR (2e). The analogous cationic com- plexes with the disubstituted allyl group (η 3 -1,3-Ph 2 -C 3 H 3 ) and monodentate phosphanes were not isolated as stable sol- ids; only [PdCl(η 3 -1,3-Ph 2 -C 3 H 3 )P] (6a, 6d) were obtained. Palladium allyl complexes were screened as precatalysts in the allylic substitution of rac-3-acetoxy-1,3-diphenyl-1-pro- pene (I) and (E)-3-acetoxy-1-phenyl-1-propene (III) with di- methyl malonate as the nucleophile. The various catalytic precursors showed a wide range of activity and selectivity. The bismonodentate phosphane complexes 3 are more active Introduction Palladium-catalyzed asymmetric allylic substitution has been thoroughly investigated in recent years. A large number of ligands containing mainly phosphorus-, nitro- gen- or sulfur-coordinating atoms have been prepared and tested in catalysis. [1,2] In particular, the use of P-stereogenic phosphanes has been widely studied, initially with limited success, like those obtained with (R,R)-DIPAMP. [3] Today, however, several P-stereogenic monodentate or bidentate phosphanes produce good results in the benchmark allylic alkylation of 1,3-diphenylallyl acetate (see examples in Table 1 and in ref. [2] ). The reaction is characterized by a well-established ge- neric catalytic cycle involving an oxidative addition step fol- lowed by a nucleophilic attack on an allylpalladium(II) in- termediate. It is normally assumed that the resting state of the process is the allylpalladium intermediate but recent fin- dings have improved our understanding of the reaction. These include the observation of allyl-bridged dinuclear [a] Departament de Química Inorgànica, Universitat de Barcelona, Martí i Franquès 1–11, 08028 Barcelona, Spain Fax: +34-934907725 E-mail: guillermo.muller@qi.ub.es [b] Área de Química Orgánica, Departamento de Química Fundamental, Facultade de Ciencias, Campus da Zapateira, Universidade da Coruña, 15071 A Coruña, Spain Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/ejic.201000308. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Eur. J. Inorg. Chem. 2010, 3372–3383 3372 than the bidentate analogues. With regard to the regioselec- tivity, precursors containing monodentate phosphanes favour the formation of the linear product in the allylic substitution of cinnamyl acetate (III) compared with those containing bi- dentate phosphanes. With substrate I, compounds with the diphosphanes 1a and 1e, containing a five-membered che- late ring, gave low enantioselectivities (less than 10 % ee), but those with the diphosphane 2e, forming a six-membered chelate ring or with two monodentate phosphanes, afforded products with moderate enantioselectivity under standard conditions (ee up to 74 %). The results show that the perform- ance of precursors containing monodentate phosphanes was superior to those containing bidentate ligands in both activity and selectivity. palladium(I) complexes formed by the interaction of the Pd 0 and allyl-Pd II species of the standard catalytic cycle, [4] careful kinetic studies of the ion-pair implications when dif- ferent stabilizing anions are present, [5] observations of the chloride effect in both systems, the consequences of strong regioretention in the allylic substitution [6] and the non- equivalence of the catalytic precursors prepared using mix- tures of [PdCl(μ-Cl)(allyl)] 2 plus ligand or ionic [Pd(allyl)- (ligand) 2 ]X compounds. [7] Impressive turnover numbers have been obtained using chiral diphosphite ligands in the allylic alkylation and amination of rac-1,3-diphenyl-3-acet- oxyprop-1-ene, [8a] and the origin of enantioselectivities has been explored using a library of phosphite-phosphoramid- ite ligands. [8b] Furthermore, the cationic nature of the palla- dium allyl intermediates allowed the use of mass spectro- metric screening (ESI-MS) to evaluate the enantioselectivity of a chiral catalyst or to measure the discrimination power of several chiral ligands in a single experiment. [9] Other met- als have also been used successfully in allylic substitution reactions: for example, high regioselectivity and enantio- selectivity have been obtained with Ir complexes containing a single phosphoramidite ligand. [10] The effect of monodentate phosphanes on the regioselec- tivity and enantioselectivity of symmetric and asymmetric allylic substrates has been studied in some detail. For exam- ple, some monodentate phosphoramidites show over 90% ee in the allylic alkylation of 1,3-diphenylallyl acetate. [19] In the regioselectivity for asymmetric substrates like (E)-3-