A [Pd (η³‐PhCHCHCHPh)(phosphino‐oxazoline)]⁺ complex with almost identical P‐ and N‐trans influences. ¹³C chemical shifts and allylic alkylation chemistry

Abstract

The Pd-catalysed enantioselective allylic alkylation of a 1,3-diphenylallyl substrate using a bulky phosphinooxazoline auxiliary leads to a relatively small enantiomeric excess of 66%. The ca 30% loss, relative to related P,N-auxiliaries, is rationalized by (a) the presence of additional isomers, (b) a dynamic equilibrium between two of these as shown by exchange spectroscopy and (c) the identification of one exchanging diastereomer in which there is almost no difference between the two terminal allyl 13 C chemical shifts, i.e. these P-and N-donors reveal an almost identical trans influence.

Dinuclear Allylpalladium Complexes ofC2-Symmetric Pyrazolate-Bridged Bis(oxazoline) Ligands (pyrbox's): Structures, Dynamic Behavior, and Application in Asymmetric Allylic Alkylation

“…Nucleophilic attack is generally expected to occur at the allyl C that is trans to the stronger trans-effect donor atom, which can usually be identified by the lower field 13 C NMR chemical shift of that allylic carbon atom. [41][42][43] In accordance with what has been concluded from the crystallographic data for 4a,c, however, there seems to be no substantial difference in ground-state trans influence (and likely also in kinetic trans effect) between the pyrazolate and oxazoline donors of the pyrbox ligands. 13 C NMR resonances are detected in the very narrow range 58.2-59.4 ppm for the methallyl CH 2 trans to the pyrazolate N and 58.8-59.9 ppm for the methallyl CH 2 trans to the oxazoline N. The locus of nucleophilic attack is therefore not evident from the spectroscopic and structural data, and it is quite likely that the lack of electronic difference of the pyrazolate and oxazoline donors, which propagates itself in a lack of electronic differentiation of the allylic termini, is a reason for the only modest enantiomeric excess of the present systems.…”

“…13 C NMR resonances are detected in the very narrow range 58.2-59.4 ppm for the methallyl CH 2 trans to the pyrazolate N and 58.8-59.9 ppm for the methallyl CH 2 trans to the oxazoline N. The locus of nucleophilic attack is therefore not evident from the spectroscopic and structural data, and it is quite likely that the lack of electronic difference of the pyrazolate and oxazoline donors, which propagates itself in a lack of electronic differentiation of the allylic termini, is a reason for the only modest enantiomeric excess of the present systems. 41 On a more subtle level, it is interesting to note that the allylic carbon trans to the pyrazolate N resonates at slightly lower field than the carbon trans to the oxazoline N in the case of 4a,b ( 13 C at 59.3/59.4 versus 58.8/58.8 ppm), while this order is reversed in 4c,d (58.3/58.2 versus 59.6/59.9 ppm). One has to bear in mind, though, that various factors, including the shielding effect of the oxazoline phenyl substituents in 4a,b, will contribute to these minor differences.…”

“…As an initial working model, the observed enantioselectivities may thus be rationalized by the scenario shown in Scheme . If one assumes that the syn/syn isomer of the 1,3-diphenylallyl moiety predominates, , due to steric repulsion of the bulky phenyl substituents within the bimetallic pocket the two 1,3-diphenylallyl groups in the intermediates can only adopt an antiparallel orientation, which corresponds to isomers A (exo/exo) and C (endo/endo) of complexes 4a − d (Scheme ). In the case of 1,3-diphenylallyl instead of methallyl, however, it is reasonable to suppose that isomer A is strongly disfavored because of interference with the oxazoline substituents.…”

Design of Cationic Mixed Phosphine/N-Heterocyclic Carbene Palladium(II) π-Allyl Complexes as Monoligated Phosphine Pd(0) Precatalysts: Synthesis, Structural Studies, Catalysis, and Reactivity

“…An interesting feature of Pd-allyl complexes is their tendency to undergo η 3 -η 1 -η 3 rearrangements (Scheme 1): This behavior can be observed directly by NOESY spectroscopy. 9,15,18,[24][25][26][27][28] Allyl decoordination occurs under steric or electronic control and may happen exclusively trans to either L 1 or L 2 . It is then followed by rotation around either the Pd-C or the C-C bond of the allyl.…”

“…Palladium(II) π-allyl complexes have attracted considerable attention over the years, − mainly due to their reactivity toward nucleophiles, which makes those complexes excellent catalysts for the so-called Tsuji−Trost allylic alkylation. − Transition metal allyl complexes can exist in two coordination modes, η 1 (anionic, 2 e donor ligand) and η 3 (mixed, 4 e ligand). An interesting feature of Pd-allyl complexes is their tendency to undergo η 3 −η 1 −η 3 rearrangements (Scheme ):…”