Diethylzinc‐Mediated Allylation of Carbonyl Compounds Catalyzed by [(NHC)(PR₃)PdX₂] and [(NHC)Pd(η³‐allyl)Cl] Complexes

Abstract

[(NHC)(PR 3 )PdX 2 ] complexes (NHC = N-heterocyclic carbene) are active precatalysts in the palladium-catalyzed allylation of carbonyl compounds with allylic acetates and diethylzinc. A comparative study examining the catalytic activity of a series of six of these complexes was carried out with allyl and cinnamyl acetates. [(IMesMe)(PPh 3 )PdI 2 ] was found to be the most versatile precatalyst (IMesMe = 1-mesityl-3-methylimidazol-2-ylidene) and the scope of the reaction was investigated with this complex. [(IMesMe)(PPh 3 )-PdI 2 ] catalyzes the allylation of aromatic (except 4-nitrobenzaldehyde) and aliphatic aldehydes (including enolizable aldehydes) with cinnamyl acetate to give the corresponding homoallylic alcohols in 57-98 % yields and diastereoselectivities ranging from 70:30 to 92:8. The allylation of acetone also takes place under the same conditions, leading to the expected adduct in 63 % yield. The reaction with cyclohexenyl acetate proceeds at room temperature to afford the homoallylic alcohols in 40-78 % yields with excellent diastereoselec-

Donor Strengths Determination of Pnictogen and Chalcogen Ligands by the Huynh Electronic Parameter and Its Correlation to Sigma Hammett Constants

“…This study was initiated with P(OMe) 3 , P(O i Pr) 3 , P(OPh) 3 and P(O‐2,4‐ t Bu‐Ph) 3 , which are diverse in terms of stereoelectronic properties. Instant color changes from orange to pale yellow were observed for most reaction mixtures indicative of a generally strong donating ability of these ligands and the fast formation of mixed NHC/phosphite complexes, which are surprisingly rare . Similar to their phosphine counterparts, electronically driven trans–cis isomerizations were observed.…”

“…Instant color changes from orange to pale yellow were observedf or most reactionm ixtures indicative of ag enerally strong donating ability of these ligandsa nd the fast formation of mixed NHC/ phosphite complexes, which are surprisingly rare. [24,25] Similar to their phosphine counterparts, electronically driven trans-cis isomerizationsw ere observed. The thermodynamically preferred cis-arrangement in these complexes is due to the socalled "transphobia effect" of phosphorus donors.…”

Well‐defined NHCPd complex‐mediated intermolecular direct annulations for synthesis of functionalized indoles (NHC = N‐hetero‐cyclic carbene)

“…It has been demonstrated recently [15,16] that, in addition to the dominant NHC ligand, other ancillary ligands surrounding the Pd metal center also play a crucial role in their catalytic performance. As a result, a variety of NHC-Pd complexes stabilized by different anionic ligands, including cinnamyl, 3, [17] and Cp, 4, [18] OAc, 5, [19] acac, 6 [20] and PPh 3 , 7, [21] were also prepared and their catalytic potential evaluated in direct annulation reactions of o-haloanilines and ketones (Scheme 2). We observed that, of the NHC-Pd complexes that we have examined thus far, all were able to catalyze conversion of acetophenone and o-bromoaniline into the desired 2-phenylindole to a certain extent with decreased catalyst loadings (5 mol%; entries 1-7, Table 1).…”

“…Well-defined NHC-Pd complexes (NHC)Pd(allyl)Cl, 1, [13b] and PEPPSI, 2, [14d] are commercially available and could also be prepared easily according to the literature method. (NHC)Pd(cinnamyl)Cl, 3, [17a] CpPd(NHC)Cl, 4, [18] Pd(OAc) 2 (NHC)(H 2 O), 5, [19] (NHC)Pd(acac)Cl, 6 [20b] and PdCl 2 (IPr)(PPh 3 ), 7, [21] were prepared following the literature procedures.…”

Catalyst‐ and Solvent‐Dependent Stereodivergence in the Intramolecular Et2Zn/Pd0‐Promoted Carbonyl Propargylation: Mechanistic Implications

“…For example, in addition to the reasonable intervention of allenylzinc intermediates (e.g., XXI ; Scheme ), product formation could also involve the direct nucleophilic addition of the allenylpalladium species XIX , a process which finds precedent in a number of reported additions of organopalladium complexes to aldehydes 37. Interestingly, in one of those cases, an allylmethylpalladium complex was found to add to an aldehyde, whereas the related allylpalladium chloride did not 37i. For Pd 0 /Et 2 Zn reactions run with L=P( n Bu) 3 in a non‐coordinating solvent (benzene), the presence of an ethyl group and two strongly bound P( n Bu) 3 ligands on Pd would be expected to similarly provide enhanced nucleophilicity for carbonyl addition, while at the same time preventing coordination of the carbonyl group.…”