Computational Mechanistic Study of Palladium(II)-Catalyzed Carboxyalkynylation of an Olefin Using an Iodine(III) Oxidant Reagent

The Pd­(II)-catalyzed chemical transformations using an iodine­(III) oxidant are mostly believed to proceed via a Pd­(IV)/Pd­(II) catalytic cycle. The present computational study, however, demonstrates that this statement is not always true, and, in some particular cases, an alternative mechanism could be operative. Herein, the reaction mechanism of the Pd­(II)-catalyzed carboxyalkynylation of an olefin using an alkynyl benziodoxolone reagent was elucidated with the aid of density functional theory calculations. The catalytic reaction was found to proceed via a mechanism in which a Pd­(II) vinylidene-like complex, not a Pd­(IV) complex, plays a leading role. The mechanistic understanding of the carboxyalkynylation reaction may have significant implications in a variety of processes catalyzed by transition metal complexes in the presence of alkynyl benziodoxolones

A Density Functional Theory (DFT) Mechanistic Study of Gold(I)-Catalyzed Alkynylation of the Indole and Pyrrole Substrates, Using a Hypervalent Iodine Reagent

Density functional theory (DFT) was utilized to probe the mechanism of AuCl-catalyzed alkynylation of the indole and pyrrole substrates using a hypervalent iodine reagent ([(<u>t</u>ri-<i><u>i</u>so-</i><u>p</u>ropy<u>s</u>ilyl)<u>e</u>thynyl]-1,2-<u>b</u>enziodo<u>x</u>ol-3­(1<i>H</i>)-one (TIPS-EBX)). An unprecedented reaction mechanism was shown to be operative. In this mechanism, the catalytic reaction starts with coordination of the alkyne moiety of the iodine­(III) reagent to the AuCl catalyst, followed by transfer of the alkynyl group from I^III to Au^I. The iodine­(III) center was found to be capable of activating the alkyne triple bond more efficiently than the gold­(I) center. The nucleophilic attack of the aromatic substrates on the I^III-activated alkyne gives a iodine­(III) gold­(I) vinyl complex. According to the calculations, this step was predicted to be the rate-determining step. Starting from the vinyl complex, the product is formed through the interaction of the occupied σ_Au–C-orbital with the vacant σ_I–C^*-orbital, followed by a very fast deprotonation reaction. This process that leads to the reduction of iodine­(III) to iodine­(I) occurs without protonation of the benzoate group of the iodine­(III) moiety and with a small activation energy of 6.6 kcal/mol. It was concluded that the presence of the Au–C σ-bond at the β-position converts the vinyl group to a potent reductant. The regioselectivity for the catalytic C–H alkynylation of arenes is dictated by the stability of the vinyl complex. It was found that the cationic gold complexes such as PMe₃Au⁺ are not effective catalysts for the alkynylation reaction, because they are strongly poisoned by coordination to the benzoate group of the iodine­(III) reagent

Stability of Heavier Group 14 Analogues of Vinylidene Complexes: A Theoretical Study

Density functional calculations were carried out to study the stability of the heavier group 14 analogues of vinylidene complexes M(Cl)2(CEH2)(PH3)2, M(Cl)2(ECH2)(PH3)2 (M = Ru, Os), Cp2M(CEH2)(Cl), and Cp2M(ECH2)(Cl) (M = Nb, Ta), where E = C, Si, Ge, Sn. The results of the calculations show that the d6 osmium complexes Os(Cl)2(CEH2)(PR3)2 are the most promising targets for synthesis

DFT Studies on the Mechanism of Allylative Dearomatization Catalyzed by Palladium

The reaction mechanism of the Pd-catalyzed benzyl/allyl coupling of benzyl chloride with allyltributylstannan, resulting in the dearomatization of the benzyl group, was studied using density functional theory calculations at the B3LYP level. The calculations indicate that the intermediate (η3-benzyl)(η1-allyl)Pd(PH3) is responsible for the formation of the kinetically favored dearomatic product. Reductive elimination of the dearomatic product from the intermediate occurs by coupling the C-3 terminus of the η1-allyl ligand and the para-carbon of the η3-benzyl ligand in (η3-benzyl)(η1-allyl)Pd(PH3). For comparison, various C−C coupling reaction pathways have also been examined

Concerted Oxidative Addition of Diaryliodine(III) Reagents to a Pincer-Palladium(II) Substrate: A Computational Analysis

Density functional theory applied in a mechanistic study of the oxidation of pincer complex PdII(mer-NCN)(K1–O2CPh) (NCN = 2,6-(dimethylaminomethyl)phenyl-N,C,N) by diphenyliodine(III) triflate, in the presence of the widely used bicarbonate base as an additive/reagent in organic synthesis, indicates that concerted oxidative addition by Ph2I(OCO2H) is preferred over a Ph+ transfer mechanism to initially form octahedral PhPdIV(mer-NCN)(K1–O2CPh){I(Ph)(···OCO2H)–I}. Interaction of bicarbonate with the iodine center has little effect on the dz2 orbital interaction with the σ* I–Ph orbital required for the concerted transition state but does destabilize the Ph+ transfer mechanism, which requires a later transition state with a much weaker interaction with bicarbonate

Understanding the Relative Easiness of Oxidative Addition of Aryl and Alkyl Halides to Palladium(0)

Density functional theory calculations were carried out to study the relative easiness of oxidative addition of aryl and alkyl halides to Pd(0). Kinetic but not thermodynamic factors were found to contribute to the better reactivity of aryl versus alkyl halides

Structure and Bonding of d⁸ Allyl Complexes M(η³-allyl)L₃ (M = Co, Rh, Ir; L = Phosphine or Carbonyl)

Density functional theory calculations were used to study structure and bonding of d8 five-coordinate allyl complexes M(η3-allyl)L3 (M = Co, Rh, Ir; L = phosphine or carbonyl). In these pseudo-square-pyramidal d8 complexes, we found that only the exo structures correspond to energy minima on the potential energy surface. The exo structures are able to maximize the metal(d)-to-allyl(π*) back-bonding interaction. The calculations predicted that the endo structures for the Ir and Co complexes are transition states for interconversion of two different exo structures. Complexes such as Ir(η3-allyl)(PMe3)3 having only phosphines as the ancillary ligands possess the strongest metal−allyl bonding interaction, while complexes such as Co(η3-allyl)(CO)3 having only carbonyls have the weakest interactions. We also studied the η3 → η3 → η3 and η3 → η1 → η3 rearrangements of the allyl ligand and found that both the rearrangement mechanisms are energetically feasible for the d8 complexes studied in this paper

Computational Study Illustrating NCN-Palladium(IV) Involvement in Generating Pd⁰ Species to Facilitate Pd⁰/Pd^II Heck-Type Catalysis with Diphenyliodine(III) Species

Density functional theory has been applied in a mechanistic study of the role of pincer complex PdII(NCN–N,C,N)­(O2CPh-O) ([NCN]- = [2,6-(Me2NCH2)2C6H3]−) (3) in Heck-type catalysis in the presence of diphenyliodine­(III) triflate as the oxidative arylating agent for CH2CHAr and bicarbonate as the base to afford PhCHCHAr (Ar = p-BrC6H4). The initially formed palladium­(IV) complex PhPd­(NCN–N,C,N)­(OBz···HOCO2–O,O) (9) (ΔG‡ 31.6 kcal/mol) undergoes Ph···Cipso reductive elimination to form PdII{NC­(Ph)­N–N,C,N}­(OBz···HOCO2–O,O) (11) (ΔG‡ 25.6 kcal/mol), which is reduced by bicarbonate to form palladium(0) species. Reduction to Pd0 occurs via deprotonation of one NMe2 group by bicarbonate to provide a “–CH2–N­(Me)–CH2–PdII″ moiety (ΔG‡ 23.6 kcal/mol) followed by nucleophilic attack on this moiety by bicarbonate to give a Pd0 product with a “–CH2–NMe­(CH2OCO2H)″ group (ΔG‡ 14.5 kcal/mol). The Pd0 complex undergoes exceptionally facile oxidative addition by Ph2I­(HCO3) (ΔG‡ = 5.1 kcal/mol). Modeling the Pd0 complex as [Pd­(benzene)­(O2CPh)]− provides a similar result (ΔG‡ = 5.6 kcal/mol), allowing entry to PhPdII species to be able to undergo migratory insertion for CH2CAr (ΔG‡ = 14.4 kcal/mol) and β-hydride elimination (ΔG‡ = 16.2 kcal/mol) processes of Pd0/PdII Heck-type catalysis. Activation barriers for reduction of PdIV to Pd0, and in the Heck-type process, are lower than the initial oxidation to form PdIV species, ensuring that only a small quantity of PdII(NCN)­(OBz) (3) is consumed, in accord with its presence on completion of catalysis. Computational studies of PdIV-mediated Heck-type catalysis revealed energetically unfavorable processes and a preference for the formation of CH2C­(Ar)­Ph rather than the experimentally reported PhCHCHAr. This study reveals the role of a pincer complex as a precatalyst, the oxidation of PdII to PdIV followed by reductive elimination, the role of bicarbonate in reducing PdII to Pd0, the extremely facile oxidative addition of a diaryliodine­(III) reagent to Pd0, and the selectivity differences in migratory insertion for PdII and PdIV centers

Two-Stage Catalysis in the Pd-Catalyzed Formation of 2,2,2-Trifluoroethyl-Substituted Acrylamides: Oxidative Alkylation of Pd^II by an I^III Reagent and Roles for Acetate, Triflate, and Triflic Acid

In the synthesis of 2,2,2-trifluoroethyl-substituted acrylamides, two-stage palladium-catalysis is indicated experimentally, including oxidative alkylation of PdII to PdIV by [IIIIMes­(CH2CF3)]+ (Besset et al., Chem. Commun., 2021, 57, 6241). For N-(quinolin-8-yl)-2-(phenyl)­acrylamide [LH2 = H2CC­(Ph)–C­(O)–NH∼N], studied by density functional theory herein, the first stage involves palladium acetate-promoted NH-deprotonation and concerted metalation-deprotonation CH-activation for Pd­(OAc)2(LH2), followed by the transfer of [CH2CF3]+ from IIII to give a PdIV intermediate that undergoes reductive elimination to form the acrylamide-CH2CF3 linkage. The second stage employs [Pd­(LH)­(NCMe)]+ as the catalyst, with steps including outer-sphere CH-activation by triflate and crucial roles for PdIV, acetonitrile solvent, and N-protonation of the product by triflic acid to form [LH2(CH2CF3)]+. In an apparently unique process, the first stage is faster than the second and produces the catalyst, but the second stage is catalytic to provide high yields of the product

Concerted Oxidative Addition of Diaryliodine(III) Reagents to a Pincer-Palladium(II) Substrate: A Computational Analysis

Density functional theory applied in a mechanistic study of the oxidation of pincer complex PdII(mer-NCN)(K1–O2CPh) (NCN = 2,6-(dimethylaminomethyl)phenyl-N,C,N) by diphenyliodine(III) triflate, in the presence of the widely used bicarbonate base as an additive/reagent in organic synthesis, indicates that concerted oxidative addition by Ph2I(OCO2H) is preferred over a Ph+ transfer mechanism to initially form octahedral PhPdIV(mer-NCN)(K1–O2CPh){I(Ph)(···OCO2H)–I}. Interaction of bicarbonate with the iodine center has little effect on the dz2 orbital interaction with the σ* I–Ph orbital required for the concerted transition state but does destabilize the Ph+ transfer mechanism, which requires a later transition state with a much weaker interaction with bicarbonate