Efficient rhodium-catalyzed installation of unsaturated ester functions onto porphyrins: site-specific Heck-type addition versus conjugate addition

A facile introduction of alpha,beta- or alpha,beta,gamma,delta-unsaturated ester functions onto porphyrins was achieved through rhodium-catalyzed addition of beta-borylporphyrins to acrylate or 2,4-pentadienoate esters. The reaction of meso-borylporphyrins with acrylates exclusively afforded saturated esters by 1,4-conjugate addition under the same reaction conditions. Thus, the reaction mode (Heck-type versus conjugate addition) is highly dependent on the reaction site (beta versus meso). This functionalization has a significant impact on the electronic properties of the pi system of porphyrins, which induces a substantial redshift and broadening in the absorption spectra by effective conjugation through the beta positions. The coplanar structure of the unsaturated ester moieties with respect to the porphyrin core has been unambiguously elucidated by X-ray crystallographic analysis.     

C-H activation and C=C double bond formation reactions in iridium ortho-methyl arylphosphane complexes

The Vaska-type iridium(I) complex [IrCl(CO){PPh(2)(2-MeC(6)H(4))}(2)] (1), characterized by an X-ray diffraction study, was obtained from iridium(III) chloride hydrate and PPh(2)(2,6-MeRC(6)H(3)) with R=H in DMF, whereas for R=Me, activation of two ortho-methyl groups resulted in the biscyclometalated iridium(III) compound [IrCl(CO){PPh(2)(2,6-CH(2)MeC(6)H(3))}(2)] (2). Conversely, for R=Me the iridium(I) compound [IrCl(CO){PPh(2)(2,6-Me(2)C(6)H(3))}(2)] (3) can be obtained by treatment of [IrCl(COE)(2)](2) (COE=cyclooctene) with carbon monoxide and the phosphane in acetonitrile. Compound 3 in CH(2)Cl(2) undergoes intramolecular C-H oxidative addition, affording the cyclometalated hydride iridium(III) species [IrHCl(CO){PPh(2)(2,6-CH(2)MeC(6)H(3))}{PPh(2)(2,6-Me(2)C(6)H(3))}] (4). Treatment of 2 with Na[BAr(f) (4)] (Ar(f)=3,5-C(6)H(3)(CF(3))(2)) gives the fluxional cationic 16-electron complex [Ir(CO){PPh(2)(2,6-CH(2)MeC(6)H(3))}(2)][BAr(f) (4)] (5), which reversibly reacts with dihydrogen to afford the delta-agostic complex [IrH(CO){PPh(2)(2,6-CH(2)MeC(6)H(3))}{PPh(2)(2,6-Me(2)C(6)H(3))}][BAr(f)(4)] (6), through cleavage of an Ir-C bond. This species can also be formed by treatment of 4 with Na[BAr(f)(4)] or of 2 with Na[BAr(f)(4)] through C-H oxidative addition of one ortho-methyl group, via a transient 14-electron iridium(I) complex. Heating of the coordinatively unsaturated biscyclometalated species 5 in toluene gives the trans-dihydride iridium(III) complex [IrH(2)(CO){PPh(2)(2,6-MeC(6)H(3)CH=CHC(6)H(3)Me-2,6)PPh(2)}][BAr(f) (4)] (7), containing a trans-stilbene-type terdentate ligand, as result of a dehydrogenative carbon-carbon double bond coupling reaction, possibly through an iridium carbene species.     

Palladium complexes of N-heterocyclic carbenes as catalysts for cross-coupling reactions--a synthetic chemist's perspective

Palladium-catalyzed C-C and C-N bond-forming reactions are among the most versatile and powerful synthetic methods. For the last 15 years, N-heterocyclic carbenes (NHCs) have enjoyed increasing popularity as ligands in Pd-mediated cross-coupling and related transformations because of their superior performance compared to the more traditional tertiary phosphanes. The strong sigma-electron-donating ability of NHCs renders oxidative insertion even in challenging substrates facile, while their steric bulk and particular topology is responsible for fast reductive elimination. The strong Pd-NHC bonds contribute to the high stability of the active species, even at low ligand/Pd ratios and high temperatures. With a number of commercially available, stable, user-friendly, and powerful NHC-Pd precatalysts, the goal of a universal cross-coupling catalyst is within reach. This Review discusses the basics of Pd-NHC chemistry to understand the peculiarities of these catalysts and then gives a critical discussion on their application in C-C and C-N cross-coupling as well as carbopalladation reactions.     

Unexpected hydrobromic acid-catalyzed C-C bond-forming reactions and facile synthesis of coumarins and benzofurans based on ketene dithioacetals

Hydrobromic acid was found to be a unique catalyst in C? C bond‐forming reactions with ketene dithioacetals. Distinctly different from other acids (including Lewis and Brønsted acids), the remarkable catalytic performance of hydrobromic acid in catalytic amounts was observed in the “acid”‐catalyzed reactions of readily available functionalized ketene dithioacetals 1 with various electrophiles. Under the catalysis of 0.1 equivalents of hydrobromic acid, the reaction of 1 with carbonyl compounds 2 a – l gave polyfunctionalized penta‐1,4‐dienes 3 or conjugated dienes 4 in good to excellent yields. The reaction tolerated a broad range of substituents on both the ketene dithioacetals 1 and the carbonyl compounds 2 . Application of this efficient C? C bond‐forming method generated coumarins 5 and benzofurans 7 under mild, metal‐free conditions by hydrobromic acid‐catalyzed reactions of 1 with salicylaldehydes 2 m – o and p‐quinones 6 a – d , respectively. A new reactive species, a sulfur‐stabilized carbonium ylide, formed depending on the nature of the counterion, and this was proposed as the key intermediate in the unique catalysis of hydrobromic acid.     

trans-1-Sulfonylamino-2-isoborneolsulfonylaminocyclohexane derivatives: excellent chiral ligands for the catalytic enantioselective addition of organozinc reagents to ketones

The catalytic enantioselective addition of different organozinc reagents (such as alkyl and aryl derivatives or in situ generated aryl, allyl alkenyl, and alkynyl derivatives obtained through different transmetallation processes) to simple ketones has been accomplished by using titanium tetraisopropoxide and chiral ligands derived from substituted trans-1-sulfonylamino-2-isoborneolsulfonylaminocyclohexane, producing the corresponding tertiary alcohols with enantiomeric excesses (ee) up to >99 %. A simple and efficient procedure for the synthesis of the chiral ligands used in these reactions is described.     

Investigations on the coupling of ethylene and alkynes in [IrTp Me2] compounds: water as an effective trapping agent

The reaction of the bis(ethylene) complex [Tp(Me(2) )Ir(C(2)H(4))(2)] (1) (Tp(Me(2) ): hydrotris(3,5-dimethylpyrazolyl)borate) with two equivalents of dimethyl acetylenedicarboxylate (DMAD) in CH(2)Cl(2) at 25 degrees C gives the hydride-alkenyl species [Tp(Me(2) )IrH{C(R)=C(R)C(R)=C(R)CH=CH(2)}] (2, R: CO(2)Me) in high yield. A careful study of this system has established the active role of a number of intermediates en route to producing 2. The first of these is the iridium(I) complex [Tp(Me(2) )Ir(C(2)H(4))(DMAD)] (4) formed by substitution of one of the ethylene ligands in 1 by a molecule of DMAD. Complex 4 reacts further with another equivalent of the alkyne to give the unsaturated metallacyclopentadiene [Tp(Me(2) )Ir{C(R)=C(R)C(R)=C(R)}], which can be trapped by added water to give adduct 7, or can react with the C(2)H(4) present in solution generating complex 2. This last step has been shown to proceed by insertion of ethylene into one of the Ir--C bonds of the metallacyclopentadiene and subsequent beta-H elimination. Complex 1 reacts sequentially with one equivalent of DMAD and one equivalent of methyl propiolate (MP) in the presence of water, with regioselective formation of the nonsymmetric iridacyclopentadiene [Tp(Me(2) )Ir{C(R)=C(R)C(H)=C(R)}(H(2)O)] (9). Complex 9 reacts with ethylene giving a hydride-alkenyl complex 10, related to 2, in which the C(2)H(4) has inserted regiospecifically into the Ir--C(R) bond that bears the CH functionality. Heating solutions of either 2 or 10 in CH(2)Cl(2) allows the formation of the allyl species 3 or 11, respectively, by simple stereoselective migration of the hydride ligand to the Calpha alkenyl carbon atom and concomitant bond reorganization of the resulting organic chain. All the compounds described herein have been characterized by microanalysis, IR and NMR spectroscopy, and for the case of 3, 7, 7CO, 8NCMe, 9, 9NCMe, and 10, also by single-crystal X-ray diffraction studies.