Catalytic Reactions of Terminal Alkynes Using Rhodium(I) Complexes Bearing 8-Quinolinolate Ligands

Abstract

8-Quinolinolate is a monoanionic, hard, strongly σ-donating ligand that can form strong chelates with various metals. However, studies of the catalytic activities of soft transition metal complexes such as greater than second row, low-valent, late-transition metals having 8-quinolinolate ligands had not been well explored until recently. In recent years, several research groups including our own have studied and developed various reactions of terminal alkynes using rhodium­(I) catalysts containing an 8-quinolinolate ligand. In this Perspective, we surveyed the transformations of terminal alkynes using 8-quinolinolato rhodium­(I) catalyst such as additions of alcohols, amines, and thiols as well as polymerization, trimerization, dimerization, and alkyne/alkene [2 + 2] cycloaddition. Mechanistic studies based on theoretical calculations of hydroalkoxylation of alkynes and alkyne/alkene [2 + 2] cycloaddition reactions have also been described.

Rhodium‐NHC‐Catalyzed gem‐Specific O‐Selective Hydropyridonation of Terminal Alkynes

“…Moreover, the particular stereoelectronic properties of the bulky powerful electron releasing IPr might play a role in the stabilization of metallacyclopropene species (See Figure S125 in Supporting Information for comparison with a PPh 3 analogue). In fact, these uncommon structures can be considered as essential intermediates in the gem ‐selective alkyne hydroalkoxylation in analogy to the role played by vinylidenes in the formation of E / Z isomers [25] …”

Rhodium‐NHC‐Catalyzed gem‐Specific O‐Selective Hydropyridonation of Terminal Alkynes

“…In fact, these uncommon structures can be considered as essential intermediates in the gem-selective alkyne hydroalkoxylation in analogy to the role played by vinylidenes in the formation of E/Z isomers. [25] The catalytic cycle ends with the associative release of the alkenyl ether (TSEF, 23.5 kcal mol À 1 ), which is the ratedetermining step (see Figure S126 in Supporting Information). At this point, the interplay between steric hindrance and the high trans effect imparted by the NHC might trigger the release of the catalytic product from 2.…”

DFT calculations bring insight to internal alkyne-to-vinylidene transformations at rhodium PNP- and PONOP-pincer complexes

“…Transition metal (TM) vinylidene complexes are key species in organometallic chemistry. [1][2][3][4][5][6][7][8][9] These compounds can be subjected to a variety of reactions such as hydrogenation, dehydrogenation and C-C bond formation processes. 10 The most general strategy to form TM-vinylidene complexes under facile and simple conditions is the reaction of TM complexes with alkynes.…”

“…A DFT study by Hall and co-workers featured these reaction systems and shows that oxidative cleavage of the rst HC^CR substrate (R ¼ Ph) followed by a C-H reductive elimination at the Rh centre releases propene and forms a Rhalkenyl intermediate. 35 Addition of a second HC^CR substrate to the Rh-alkenyl intermediate forms the bridging hydrogen intermediate 7 (Scheme 1), through which the H-migration proceeds with a 1,2-H shi mechanism to give [(PhC^C) Rh(P i Pr 3 ) 2 (C]C(Ph)(H)] (6).…”