A highly efficient preparative method of alpha-ylidene-beta-diketones via Au(III)-catalyzed acyl migration of propargylic esters

Gold-catalyzed regioselective oxidation of propargylic carboxylates: a reliable access to α-carboxy-α,β-unsaturated ketones/aldehydes

Gold-catalyzed intermolecular oxidation of carboxylates of primary or secondary propargylic alcohols are realized with excellent regioselectivity, which is ascribed to inductive polarization of the C-C triple bond by the electron-withdrawing carboxy group. The gold carbene intermediates thus generated undergo selective 1,2-acyloxy migration over a 1,2-C-H insertion, and the selectivities could be dramatically improved by the use of a P,S-bidentate ligand, which is proposed to enable the formation of tris-coordinated and hence less electrophilic gold carbene species. α-Carboxy α,β-unsaturated ketones/aldehydes can be obtained with fair to excellent yields.

Metal-catalyzed double migratory cascade reactions of propargylic esters and phosphates

Propargylic esters and phosphates are easily accessible substrates, which exhibit rich and tunable reactivities in the presence of transition metal catalysts. π-Acidic metals, mostly gold and platinum salts, activate these substrates for an initial 1,2- or 1,3-acyloxy and phosphatyloxy migration process to form reactive intermediates. These intermediates are able to undergo further cascade reactions leading to a variety of diverse structures. This tutorial review systematically introduces the double migratory reactions of propargylic esters and phosphates as a novel synthetic method, in which further cascade reaction of the reactive intermediate is accompanied by a second migration of a different group, thus offering a rapid route to a wide range of functionalized products. The serendipitous observations, as well as designed approaches involving the double migratory cascade reactions, will be discussed with emphasis placed on the mechanistic aspects and the synthetic utilities of the obtained products.

In situ generation of nucleophilic allenes by the gold-catalyzed rearrangement of propargylic esters for the highly diastereoselective formation of intermolecular C(sp3)-C(sp2) bonds

New perspectives, in particular for the synthesis of isochromane derivatives (see scheme), are provided by the title reaction. Excellent diastereoselectivites are achieved in this reaction which proceeds through a gold-catalyzed 1,3-acyloxy migration. In some cases exclusively the Z isomer is detected.

Rhodium-catalyzed acyloxy migration of propargylic esters in cycloadditions, inspiration from the recent "gold rush"

Transition metal-catalyzed acyloxy migration of propargylic esters offers versatile entries to allene and vinyl carbene intermediates for various fascinating subsequent transformations. Most π-acidic metals (e.g. gold and platinum) are capable of facilitating these acyloxy migration events. However, very few of these processes involve redox chemistry, which are well-known for most other transition metals such as rhodium. The coupling of acyloxy migration of propargylic esters with oxidative addition, migratory insertion, and reductive elimination may lead to ample new opportunities for the design of new reactions. This tutorial review summarizes recent developments in Rh-catalyzed 1,3- and 1,2-acyloxy migration of propargylic esters in a number of cycloaddition reactions. Related Au- and Pt-catalyzed cycloadditions involving acyloxy migration are also discussed.

Gold-catalyzed cycloisomerization of 1,7-enyne esters to structurally diverse cis-1,2,3,6-tetrahydropyridin-4-yl ketones

A synthetic method that relies on gold(I)-catalyzed cycloisomerization of 1,7-enyne esters to prepare highly functionalized cis-1,2,3,6-tetrahydropyridin-4-yl ketone derivatives in good to excellent yields and as a single regio-, diastereo-, and enantiomer is described. By taking advantage of the distinctive differences in the electronic and steric properties between an NHC (NHC = N-heterocyclic carbene) and phosphine ligand in the respective gold(I) complexes, a divergence in product selectivity was observed. In the presence of [PhCNAuIPr](+)SbF6(-) (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidine) as the catalyst, tandem 1,3-acyloxy migration/6-exo-trig cyclization/1,5-acyl migration of the substrate was found to selectively occur to give the δ-diketone-substituted 1,2,3,6-tetrahydropyridine adduct. In contrast, reactions with the gold(I) phosphine complex [MeCNAu(JohnPhos)](+)SbF6(-) (JohnPhos = (1,1'-biphenyl-2-yl)-di-tert-butylphosphine) as the catalyst was discovered to result in preferential 1,3-acyloxy migration/6-exo-trig cyclization/hydrolysis of the 1,7-enyne ester and formation of the cis-1,2,3,6-tetrahydropyridin-4-yl ketone derivative. The utility of this piperidine forming strategy as a synthetic tool that makes use of 1,7-enyne esters was exemplified by its application to the synthesis of an enantiopure analogue of the bioactive 2,3,4,4a,5,9b-hexahydroindeno[1,2-c]pyridine family of compounds.

Electrophilic activation of unsaturated systems: Applications to selective organic synthesis

Selected examples from previous work on iodonium-triggered approaches for the functionalization of unsaturated systems, which summarize innovative transformations, are presented. This section is mostly focused on C–C bond-forming processes from alkynes that are directly bonded to relevant heteroatoms, such as iodine, silicon, or sulfur. Besides, recent advances related to iodonium-promoted C–H functionalization reactions are briefly outlined. A second section shows representative examples of our current research activity on electrophilic reactions aimed at the activation of unsaturated systems, which now are built upon the potential offered by the so-called carbophilic catalysis. More specifically, a new catalytic cyclopentannulation sequence from N-tosylimines and propargyl tosylates and novel C–H functionalization processes from related tosylates are studied. Likewise, representative examples for the intermolecular Au(I)-catalyzed [2 + 2] cycloaddition reaction of sulfonylallenamides with activated alkenes are given, including the first enantioselective reaction of this type, which is also among the first examples of an intermolecular asymmetric gold-catalyzed reaction. The discussion of the reported iodonium and gold chemistry emphasizes a search for new intermolecular processes, although intramolecular reactions are also pursued and developed.

Rhodium-catalyzed carbonylation of cyclopropyl substituted propargyl esters: a tandem 1,3-acyloxy migration [5 + 1] cycloaddition

We have developed two different types of tandem reactions for the synthesis of highly functionalized cyclohexenones from cyclopropyl substituted propargyl esters. Both reactions were initiated by rhodium-catalyzed Saucy-Marbet 1,3-acyloxy migration. The resulting cyclopropyl substituted allenes derived from acyloxy migration then underwent [5 + 1] cycloaddition with CO. The acyloxy group not only eased the access to allene intermediates but also provided a handle for further selective functionalizations.

Gold-catalyzed cascade cyclizations of 1,6-diynyl carbonates to benzo[b]fluorenes involving arylation of oxocarbenium ion intermediates and decarboxylative etherification

Rearranged: The described gold-catalyzed cycloisomerizations give access to highly substituted benzo[b]fluorenes under mild reaction conditions (see scheme). Experimental results indicate that the in situ formed oxocarbenium ion intermediates, derived from gold-catalyzed 3,3-rearrangement and 6-endo-dig cyclization, undergo intramolecular arylation and subsequent decarboxylative etherification to furnish the final ether products.

Highly efficient gold(I)-catalyzed Overman rearrangement in water

A highly efficient gold(I)-catalyzed Overman rearrangement of allylic trichloroacetimidates to allylic trichloroacetamides in water is reported. With this environmentally benign and scalable protocol, a series of C3-alkyl substituted allylic trichloroacetamides were synthesized in good to high yields.

Triazole-Au(I) complex as chemoselective catalyst in promoting propargyl ester rearrangements

Triazole–Au (TA–Au) catalysts were employed in several transformations involving propargyl ester rearrangement. Good chemoselectivity was observed, which allowed the effective activation of the alkyne without affecting the reactivity of the allene ester intermediates. These results led to the investigation of the preparation of allene ester intermediates with TA–Au catalysts under anhydrous conditions. As expected, the desired 3,3-rearrangement products were obtained in excellent yields (generally >90% yields with 1% loading). Besides the typical ester migrating groups, carbonates and carbamates were also found to be suitable for this transformation, which provided a highly efficient, practical method for the preparation of substituted allenes.

Gold-catalyzed regioselective oxidation of terminal allenes: formation of α-methanesulfonyloxy methyl ketones

Synthetically useful α-methanesulfonyloxy methyl ketones are readily prepared in one-step from terminal allenes in fair to good yields. The chemistry relies on a gold-catalyzed intermolecular oxidation of the 1,2-diene unit using 3,5-dichloropyridine N-oxide as the oxidant. The reaction tolerates a range of functional groups and shows excellent regioselectivity.

Syntheses of α-pyrones using gold-catalyzed coupling reactions

Sequential alkyne activation of terminal alkynes and propiolic acids by gold(I) catalysts yields compounds having α-pyrone skeletons. Novel cascade reactions involving propiolic acids are reported that give rise to α-pyrones with different substitution patterns.

Enantioselective [4+2] cycloaddition of ketenes and 1-azadienes catalyzed by N-heterocyclic carbenes

Optically active highly functionalized 3,4-dihydropyridin-2-ones were synthesized by N-heterocyclic carbene-catalyzed [4+2] enantioselective cycloaddition of ketenes and 1-azadienes.