α- and γ-Regiocontrol and Enantiospecificity in the Copper-Catalyzed Substitution Reaction of Propargylic Phosphates with Grignard Reagents

Regioselective Propargylic Suzuki-Miyaura Coupling by SciPROP-Iron Catalyst

The iron-catalyzed Suzuki-Miyaura cross-coupling of secondary propargyl electrophiles with lithium organoborates has been established. A propyl-bridged bulky bisphosphine ligand, SciPROP-TB, cooperated with the bulky TIPS substituent at the alkyne terminal position to achieve the cross-coupling reaction with exclusive propargylic selectivity. The reaction features high functional group compatibility, regioselectivity, and yield with a broad substrate scope. The reaction of an optically active chiral propargyl bromide proceeds with complete racemization, supporting a mechanism involving propargyl radical formation.

Cationic indium catalysis as a powerful tool for generating α-alkyl propargyl cations for SN1 reactions

Dehydration is an abundant and promising process in chemical, biochemical, and industrial fields. Dehydration methods can contribute to building a modern and sustainable society with minimal environmental impact. Breakthrough advances in the dehydrative SN1 reaction can be achieved through the discovery of new cationic indium catalysts. Here we show that the breakthrough advances in the dehydrative SN1 reaction can be achieved using the cationic indium catalysts. The dehydrative carbon-carbon bond formation of α-alkyl propargyl alcohols afforded a wide variety of α-aryl- and heteroaryl-propargyl compounds. Mechanistic investigations into this process revealed that the InCl3/AgClO4/Bu4NPF6/1,1'-binaphthol catalytic system generated a powerful cationic indium catalyst that could promote the dehydration of alcohols. Labile α-alkyl propargyl cations were found to self-condense, and the catalyst system efficiently regenerated propargyl cations for reaction with nucleophiles. This propargylation reaction directly proceeded from the corresponding alcohols under mild and open-air conditions and tolerated a broad scope of functional groups. Furthermore, a wide variety of nucleophiles, including aromatic and heteroaromatic compounds, phenols, alcohols, and sulfonamides, reacted with the corresponding cations to afford the propargyl compounds in good to high yields. Finally, the synthetic utility of this reaction was demonstrated by the synthesis of colchicine and allocolchicine analogues. The dehydration process could help create new compounds that were previously impossible to synthesize and is more eco-friendly and efficient than conventional methods.

Lanthanum-Catalyzed Stereospecific Cross-Coupling of Propargylic Substrates with Grignard Reagents

Transition-metal-catalyzed cross-coupling of propargylic electrophiles and Grignard reagents provides densely functionalized products that are extremely useful synthetic intermediates. However, examples of conversion of propargylic derivatives to form propargyl compounds remain limited due to the challenging regioselectivity. We use LaCl3·2LiCl to catalyze propargylation of Grignard reagents in the absence of ligand in high regioselectivity and stereospecificity. The approach shows a wide substrate scope using alkyl or (hetero)aryl Grignard reagents and alkynyl electrophiles with different leaving groups. Our protocol was further applied for the formal synthesis of frondosin B. It is worth exploring methodologies utilizing the naturally abundant and relatively nontoxic lanthanum catalysts.

Copper-Catalysed Rearrangement of Cyclic Ethynylethylene Carbonates: Synthetic Applications and Mechanistic Studies

Transition-metal-catalysed reactions of cyclic ethynylethylene carbonates have been intensively studied because of their robustness in new bond formation and diversified molecule construction. Known reaction modes usually involve a substitution step occurring at either the propargylic or terminal alkyne positions. Here, we report an unprecedented reaction pattern in which cyclic ethynylethylene carbonates first undergo a rearrangement to release allenal intermediates, which subsequently react with diverse nucleophiles to furnish synthetically useful allylic and propargylic allenols, phosphorus ylides, and cyclopropylidene ketones through an addition process rather than a substitution pathway. The products enable various further transformations, and mechanistic studies and theoretical calculations reveal that the reaction does not proceed via a semipinacol type [1,2]-hydride shift, but through base-mediated deprotonation as the key step to induce the rearrangement.

Coupling Reactions on Secondary Allylic, Propargylic, and Alkyl Carbons Using Organoborates/Ni and RMgX/Cu Reagents

In the first part of this review, secondary carbon-carbon bond formation by using allylic coupling reactions with aryl and alkenyl borates is presented. Early investigations have revealed the suitability of a nickel catalyst and [RTB(OMe)3]Li (RT: transferable group). Due to their low reactivity, the borates were converted to more reactive congeners possessing an alkanediol ligand, such as 2,3-butanediol and 2,2-dimethyl-1,3-propanediol. Borates with such diol ligands were used to install aryl and alkenyl groups on the monoacetate of 4-cyclopentenyl-1,3-diol. Furthermore, alkenyl borates showed sufficient reactivity toward less reactive allylic alcohol derivatives with bromine atoms at the cis position, producing dieneyl alcohols. In the second part, copper-based and/or copper-catalyzed substitutions of secondary allylic picolinates, propargylic phosphonates, and alkyl (2-pyridine)sulfonates with RMgX are briefly summarized. The application of these reactions to the synthesis of biologically active compounds is also discussed.

Ni-Catalyzed Decarboxylative Silylation of Alkynyl Carbonates: Access to Chiral Allenes via Enantiospecific Conversions

A Ni-mediated decarboxylative silylation of alkynyl cyclic carbonates used as versatile propargylic surrogates is reported affording a wide range of highly substituted 2,3- and 3,4-allenol products in good yields. The formal cross-coupling between a tentative intermediate Ni(allenyl) and the silyl reagent was further extended to enantiospecific conversions providing access to chiral allene synthons. This protocol marks the first Ni-catalyzed propargylic silylation proceeding through an S_N2' manifold.

Stereoselective Synthesis of (–)-Heliannuol E by α-Selective ­Propargyl Substitution

This paper describes a stereoselective synthesis of (–)-heliannuol E through intramolecular cyclization of a phenol mesylate. The introduction of the aromatic group was achieved by α-selective propargyl substitution of a propargylic phosphate.

Synthesis of a TNF inhibitor, flurbiprofen and an i-Pr analogue in enantioenriched forms by copper-catalyzed propargylic substitution with Grignard reagents

The copper-catalyzed substitution reaction of diethyl phosphate derived from TMSCCCH(OH)CH₂CH₂OTBDPS with 3-c-C₅H₉-4-MeOC₆H₃MgBr, followed by several transformations, afforded a tumor necrosis factor inhibitor possessing a Ph-acetylene moiety. The inhibitor was also synthesized from phenylacetylene phosphate PhCCCH(OP(O)(OEt)₂)CH₂CH₂OTBDPS. Furthermore, the substitution of phosphates derived from TMSCCCH(OH)CH₃ and TMSCCCH(OH)-i-Pr with 3-F-4-PhC₆H₃MgBr gave the corresponding substitution products, which were transformed to flurbiprofen and its i-Pr analogue, respectively. The copper-catalyzed substitutions in these syntheses proceeded in a regio- and stereoselective manner.