Carbopalladation of allenic hydrocarbons

Efficient propyne/propadiene separation by microporous crystalline physiadsorbents

Selective separation of propyne/propadiene mixture to obtain pure propadiene (allene), an essential feedstock for organic synthesis, remains an unsolved challenge in the petrochemical industry, thanks mainly to their similar physicochemical properties. We herein introduce a convenient and energy-efficient physisorptive approach to achieve propyne/propadiene separation using microporous metal-organic frameworks (MOFs). Specifically, HKUST-1, one of the most widely studied high surface area MOFs that is available commercially, is found to exhibit benchmark performance (propadiene production up to 69.6 cm3/g, purity > 99.5%) as verified by dynamic breakthrough experiments. Experimental and modeling studies provide insight into the performance of HKUST-1 and indicate that it can be attributed to a synergy between thermodynamics and kinetics that arises from abundant open metal sites and cage-based molecular traps in HKUST-1.

Enantioselective Allylation Using Allene, a Petroleum Cracking Byproduct

Allene (C₃H₄) gas is produced and separated on million-metric-ton scale per year during petroleum refining but is rarely employed in organic synthesis. Meanwhile, the addition of an allyl group (C₃H₅) to ketones is among the most common and prototypical reactions in synthetic chemistry. Herein, we report that the combination of allene gas with inexpensive and environmentally benign hydrosilanes, such as PMHS, can serve as a replacement for stoichiometric quantities of allylmetal reagents, which are required in most enantioselective ketone allylation reactions. This process is catalyzed by copper salts and commercially available ligands, operates without specialized equipment or pressurization, and tolerates a broad range of functional groups. Furthermore, the exceptional chemoselectivity of this catalyst system enables industrially relevant C3 hydrocarbon mixtures of allene with methylacetylene and propylene to be applied directly.

Palladacycles: Effective Catalysts for a Multicomponent Reaction with Allylpalladium(II)-Intermediates

Palladium(II) complexes with an auxiliary bidentate ligand featuring one C-Pd bond and a Pd-N-donor bond (palladacycles) have been shown to afford improved yields of homoallylic amines from a three-component coupling of boronic acids, allenes and imines in comparison to the yields of homoallylic amines achieved with the originally reported catalyst (Pd(OAc)₂/P(t-Bu)₃), thus extending the scope of the reaction. ³¹P NMR monitoring studies indicate that distinct intermediates featuring Pd-P bonds originate in the reactions catalyzed by either Pd(OAc)₂/P(t-Bu)₃ or the pallada(II)cycle/P(t-Bu)₃ systems, suggesting that the role of the pallada(II)cycles is more complex than just precatalysts. The importance of an additional phosphine ligand in the reactions catalyzed the pallada(II)cycles was established, and its role in the catalytic cycle has been proposed. Insights into the nature of the reactive intermediates that limit the performance of the originally reported catalytic systems has been gained.

Sequential Pd- and Rh-Catalyzed Three-Component Cyclization with Allenylboronate Platform

[reaction: see text] A novel three-component cyclization using allenylboronate ester is described. A three-component assembly of allenylboronate ester, aryl iodides, and stabilized carbon nucleophiles took place in the presence of palladium catalyst, furnishing functionalized alkenylboronate esters with high regio- and stereoselectivity. A Rh-catalyzed cyclization of the resultant three-component products then afforded interesting carbocyclic frameworks efficiently.

Stereochemistry of the Asymmetric Carbopalladation of Allenes Followed by Nucleophilic Substitution Reactions with Carbo- and Aminonucleophiles

The stereochemistry of the asymmetric palladium-catalyzed reaction of allenes with iodobenzene followed by nucleophilic substitution reaction with sodium malonate and N-methylbenzylamine is described. On the basis of the absolute configuration of the product and the stereochemical result of a similar reaction of a chiral allene, the mechanism of the above asymmetric reactions is discussed.

Nickel-Catalyzed Highly Regio- and Stereoselective Three-Component Assembly of Allenes, Aryl Iodides, and Alkenylzirconium Reagents

Cyclohexylallene (1a) and alkenylzirconium reagent (n-PrCH=CHZr(Cp)2Cl) undergo three-component assembly with a variety of aromatic iodides (R1-I: R1 = 4-PhOMe, 4-PhCOMe, 4-PhMe, 2-thiophene, 2-PhOMe, 3-PhOMe, and 4-PhCO2Et) in the presence of NiCl2(PPh3)2 and zinc powder in THF at 50 degrees C to afford corresponding three-component assembly products in moderate to good yields with very high regio- and stereoselectivity. In addition to cyclohexylallene, also phenylallene, n-butylallene, cyclopentylallene, and cycloheptylallene underwent the three-component assembling reaction to furnish the corresponding 1,4-diene derivatives in good yields. Several alkenylzirconium reagents (R2CH=CHZr(Cp)2Cl: R2 = Ph (3b), t-Bu, TMS) were also employed for this three-component assembly. The nickel-catalyzed three-component assembly was further extended to vinyl iodides. Thus, 1a reacts with 3b and ethyl (Z)-3-iodoacrylate to produce a highly substituted triene in excellent yield. In all of these three-component assembly reactions, the E isomers were formed predominantly with E/Z ratios between 94/6 and 99/1. A possible mechanism for this highly regio- and stereoselective three-component assembly is proposed.

Highly Regio- and Stereoselective Acylboration, Acylsilation, and Acylstannation of Allenes Catalyzed by Phosphine-Free Palladium Complexes: An Efficient Route to a New Class of 2-Acylallylmetal Reagents

A new method for the synthesis of substituted 2-acylallylmetal reagents in a highly regio- and stereoselective fashion involving a three-component assembly of allenes, acyl chlorides, and bimetallic reagents (B-B, Si-Si, and Sn-Sn) catalyzed by phosphine-free palladium complexes is described. Treatment of various allenes (CR(2)R(3)=C=CH(2)) with acyl chlorides (R(1)COCl) and bispinacolatodiboron in the presence of PdCl(2)(CH(3)CN)(2) in toluene at 80 degrees C gave 2-acylallylboronates in moderate to good yields. The acylsilation of allenes with acid chlorides and hexamethyldisilane (5) proceeded successfully in the presence of Pd(dba)(2) in CH(3)CN affording the corresponding allylsilanes (CR(2)R(3)=C(COR(1))CH(2)SiMe(3)) in good to moderate yields. Several chloroformates (R(4)OCOCl) also react with 1,1-dimethylallene (2a) and 5 to afford allylsilanes (CR(2)R(3)=C(COOR(4))CH(2)SiMe(3)) in 66-70% yields. Acylstannation of allenes could also be achieved by slow addition of hexabutylditin (10) to the reaction mixture of acyl chloride (or chloroformate) and allene 2a in CH(3)CN in the presence of Pd(dba)(2) at 60 degrees C; the corresponding 2-substituted allylstannanes were isolated in moderate to good yields. The above catalytic reactions are completely regioselective and highly stereoselective. A mechanism is proposed to account for the catalytic reactions and the stereochemistry.