Reactivity of a Palladacyclic Complex: A Monodentate Carbonate Complex and the Remarkable Selectivity and Mechanism of a Neophyl Rearrangement

Abstract

The ligand N­(CH2-2-C5H4N)2(CH2CH2CH2OH), L1, reacted with [Pd­(CH2CMe2C6H4)­(COD)] to give a new fluxional “cycloneophyl” organopalladium complex [Pd­(CH2CMe2C6H4)­(κ2-L1)], 1, which on attempted recrystallization from THF gave the monodentate carbonate complex [Pd­(CO3)­(κ3-L1)], 2. Complex 2 was prepared in designed syntheses by reaction of [PdCl­(κ3-L1)]+ with silver carbonate or by reaction of [Pd­(OH)­(κ3-L1)]+ with CO2. Complex 1 reacted with aqueous CO2 to give the cationic neophylpalladium complex [Pd­(CH2CMe2C6H5)­(κ3-L1)]+(HCO3)−, 6. Complex 6 reacts with hydrogen peroxide to give complex 2 with release of a mixture of organic products, the major one being 2-phenyl-2-butanol, PB. The formation of PB involves a neophyl rearrangement with the unprecedented preference for methyl over phenyl migration. A mechanistic basis for this unexpected reaction is proposed, involving β-carbon elimination at a palladium­(IV) center.

Cycloneophylpalladium(IV) Complexes: Formation by Oxidative Addition and Selectivity of Their Reductive Elimination Reactions

“…They also carry a 3hydroxypropyl (L1) or 3-methoxypropyl substituent (L2), with the possibility that L1 might take part in hydrogenbonding interactions. 29 The complex [Pd(CH 2 CMe 2 C 6 H 4 )(κ 2 -N,N′-L1)] (1) was reported previously, and it was shown to be fluxional (Scheme 3). 29 The ligand binds through the central amine donor and one of the 2-pyridyl groups and might exist primarily as either isomer 1a or 1b, which interconvert rapidly through the proposed five-coordinate intermediate 1*.…”

“…29 The complex [Pd(CH 2 CMe 2 C 6 H 4 )(κ 2 -N,N′-L1)] (1) was reported previously, and it was shown to be fluxional (Scheme 3). 29 The ligand binds through the central amine donor and one of the 2-pyridyl groups and might exist primarily as either isomer 1a or 1b, which interconvert rapidly through the proposed five-coordinate intermediate 1*. The new complex 2 behaves similarly, as shown by the variabletemperature 1 H NMR spectra (Figure 1).…”

“…Previous studies of reductive elimination from cycloneophylpalladium(IV) complexes have shown a high degree of selectivity, though the reactions may occur by C−C coupling to give the cyclobutane derivative CB or by CH 2 −X or Ar−X coupling to give organopalladium(II) products (Schemes 1 and 2). [19][20][21][22][23][24][25][26][27][28][29]50 However, the reductive elimination reactions from the palladium(IV) complexes 3−6 were not very selective (Scheme 6 and Table 1). The decomposition of 3−6 in CDCl 3 solution gave the cyclobutane derivative CB by C−C coupling in yields ranging from 5 to 30% (Table 1).…”

“…51 The neophyl halides are stable to substitution under the mild conditions used in this work (most rearrangement studies have involved the more reactive triflates or tosylates); 51 thus, the neophyl rearrangement must occur while the organic group is still bound to palladium. 26,29,52 The reaction stoichiometry indicates that an additional 1 equiv of HX is required to form XP and XB. By using dry solvent, no XP or XB was formed and CB and MB yields increased (Table 1), indicating that adventitious water is the source of the additional proton equivalent.…”

“…It is now well established that Pd­(II)/Pd­(IV) cycles are important in catalysis by palladium complexes, especially in reactions involving strong oxidants such as halogens, dioxygen, and peroxides. , For example, the catalytic reaction of iodine with alkanes or arenes to form iodo-containing hydrocarbons is thought to involve alkane C–H bond activation to form an alkylpalladium­(II) complex, followed by oxidative addition of iodine and then reductive elimination from palladium­(IV). − These impressive advances have stimulated further efforts to understand the factors that affect reactivity and selectivity in oxidative addition at palladium­(II) and reductive elimination from palladium­(IV) complexes. − In studies of selectivity in reductive elimination, the cycloneophylpalladium­(IV) complexes have played an important role as a model to distinguish between reactivity at C­(sp 2 ) or C­(sp 3 ) centers. ,,,,− When the palladium­(IV) complexes ( A ) are sufficiently stable to be isolated, the mechanisms of reductive elimination have been studied and, in most cases, have been shown to involve five-coordinate intermediates such as B (Scheme ). − , External nucleophilic attack on such intermediates occurs selectively at the CH 2 group and can lead, for example, to C–O or C–N bond formation (Scheme a).…”