Pietro Diversi

Rhodium-catalyzed synthesis of pyridines from alkynes and nitriles

Abstract Complexes of general formula [RhCpL2], (I), are excellent long-lived catalyst precursors for the synthesis of pyridine derivatives by cocyclization of alkynes and nitriles, at temperatures between 100 and 170 °C. The activity of the catalyst precursors (I) depends upon the nature of both Cp and L ligands, being highest when Cp = C5H5 and L = C2H4, and is not depressed by the presence of various functional groups in the substrates. The concurrent formation of benzene derivatives from alkyne self-trimerization can be strongly reduced by using nitrile-to-alkyne molar ratios higher than 5. The cocyclization of terminal alkynes with nitriles leads to a mixture of 2,4,6- and 2,3,6-trisubstituted pyridinic isomers, the former being largely favoured when (I) (Cp = C5Me5; L = C2H4) is used as catalyst precursor.

Electronic and steric effects in the rhodium-complex catalysed co-cyclization of alkynes and nitriles to pyridine derivatives

A study of the properties exhibited by the catalyst precursors of formula [Rh(η5-C5H4R)(C2H4)2] (R  NMe2, tBu, Me, H, Cl, NO2, CF3, or COOMe) in the co-cyclization of a variety of 1-alkynes (R′CCH) and nitriles (R″CN) to pyridine derivatives is reported. Initial reaction rates as well as chemoselectivity and regioselectivity are markedly influenced by the electron donor—acceptor properties of the R groups on the cyclopentadienyl ring, as well as by the steric and electronic effects induced by the substituents (R′ and R″) attached to substrates.

Cobalt-catalyzed cyclo-cotrimerization of alkynes and heterocumulenes

Abstract The cyclo-cotrimerization of alkynes and heterocumulenes (carbodiimides, isocyanates and carbon dioxide) in the presence of [Co(η5-C5H5)-(C2H4)2], (I), has been studied. This complex exhibits excellent catalytic properties in the cocyclization of alkynes with diimides and isocyanates. Variable amounts of benzene derivatives are also formed in all cases. The yields of both benzene compounds and co-trimers are particularly good in the case of terminal alkynes. (I) is also active in the cocyclization of 1-hexyne with CO2 to give 4,6-dibutyl-2-pyrone in poor yields. The regioselectivity of the reactions involving RNCNR or RNCO is markedly dependent on the nature of the R group. Two isomeric cotrimers, i.e. the 4,6- and the 3,6-disubstituted derivatives, are formed when R is the cyclohexyl group. When R is the phenyl or the p-tolyl group, only one isomer is formed: the 4,6-disubstituted derivative in the case of carbodiimides, and the 3,6-disubstituted derivative in the case of isocyanates.

On the formation of (triphenylphosphine)(ethylene)pentamethylcyclopentadienylrhodium(I) in the reaction of diiodo(triphenylphosphine)pentamethylcyclopentadienylrhodium(III) with butane-1,4-bis(magnesium bromide). An example of facile CO cleavage of diethyl ether by an organomagnesium compound

Abstract A study is described of the reaction between [RhI 2 (PPh 3 )(η 5 -C 5 Me 5 )] (I) and the alkylating reagents, BrMg(CH 2 ) 4 MgBr (II) or Mg(CH 2 ) 4 , in diethyl ether, which gives a mixture of the ethylenerhodium complex of formula [Rh(C 2 H 4 )(PPh 3 )(η 5 -C 5 Me 5 )] (III) and the rhodacyclopentane derivative, [Rh(CH 2 ) 4 (PPh 3 )(η 5 -C 5 Me 5 )] (IV). In tetrahydrofuran these reactions give only IV. Pure IV is also obtained by treating [RhCl 2 (PPh 3 )(η 5 -C 5 Me 5 )] or I with 1,4-dilithiobutane. By use of deuterium-labelled alkylating reagents it has been shown that the formation of the ethylenerhodium complex IV is due to a facile diethyl ether CO cleavage by organomagnesium compounds. The mechanism of the formation of III is briefly discussed.

Synthesis of pyridines from alkynes and nitriles catalyzed by polymer-anchored rhodium complexes

Abstract Cyclopentadienylrhodium(I) complexes have been attached to polymers by first binding cyclopentadiene to polystyrene resins in a variety of crosslink densities and then converting the resulting species to the cyclopentadiene anion. On subsequent reaction of the resin-bound anions with rhodium compounds of general formula [RhXL2]n (X = chloro or acac, L = C2H4 or CO; L2 = 1,5-C8H12), various rhodium-containing resins were obtained which were then analyzed by X-ray fluorescence and IR spectroscopies. These resin-attached cyclopentadienylrhodium complexes were active in the cocyclization of alkynes and nitriles to pyridine derivatives, at temperatures between 90 and 170 °C. In all cases, variable amounts of benzene derivatives were also formed, owing to the parallel self-trimerization of alkyne. A detailed study was carried out of the activity, chemo- and regio-selectivities of these catalysts as a function of the reaction parameters. The activity was seen to depend markedly upon the nature of the rhodium ancillary ligands and the crosslink density of the support: the ethylene complexes anchored to lightly crosslinked resins were the most active, all other variables being the same. Analogies and differences between heterogeneously and homogeneously rhodium-catalyzed pyridine syntheses are discussed.

The reaction of allene with β-diketonatoiridium(I) complexes: synthesis, properties and crystal structures of bis(η3-allyllic) complexes of iridium(III) and of iridocyclopentane derivatives

Abstract Allene reacts with compounds of the type (β-diketonato)Ir(η-C 8 H 14 ) 2 | (I) to give iridium(III) derivatives of formula (β-diketonato)IrC 12 H 16 (II) in which an allene tetramer i.e. the 2,3,6,7-tetramethyleneoctane-1,8-diyl group, is bonded to the iridium atom by two η 3 -allylic groups. The molecular structures of these complexes were deduced by 1 H NMR studies and by single-crystal X-ray analysis of (hfacac)IrC 12 H 16 (IIb). The reactions of the complexes II with hydrogen and with CO are described. The reaction of (acac)Ir(η-C 8 H 14 ) 2 with allene, at -78°C, gives a thermally unstable compound of probable stoichiometry (acac)Ir(C 3 H 4 ) 4 (VI). Its low-temperature IR spectrum and its reaction with bromine indicate that VI contains two η 2 -bonded allene molecules and teh 3,4-dimethyleneiridocyclopentane moiety. VI reacts with pyridine with loss of an allene molecule to give an iridium(III) derivative of formula (acac)Ir(C 6 H 8 )(C 3 H 4 )py (IX). Complex IX was shown by single-crystal X-ray analysis to contain the 3,4-dimethyleneiridocyclopentane moiety and one η 2 -bonded allene molecule. The role of irido cycles as precursors of the bis(allylic) complexes II is discussed.

Tricarbonyl[8-acetyl 2-4:6-7-η-bicyclo[3.2.1]octadienylium]iron, a novel rearrangement in the acylation of η-C8H8Fe(CO)3

Abstract X-ray diffraction studies have established the molecular structure of the title compound as involving an unexpected [3.2.1]cyclooctadienylium ligand; the implications of this in terms of chemical reactivity are described.

Disproportionation of the cyclooctene ligand in the reaction of [IrCl(C8H14)2]2. With AgOCOCF3: formation of [Ir(OCOCF3)(C8H14)2]2 and [Ir(OCOCF3)(1,5-C8H12]2 and their conversion into cationic arene complexes

Abstract Reaction of [IrCl(C8H14)2]2 with AgOCOCF3 in CH2Cl2 affords a mixture of [Ir(OCOCF3)(C8H12)2]2 and [Ir(OCOCF3)(1,5-C8H12)]2. These iridium trifluoroacetates, when treated with CF3COOH and arenes are converted into cations of general formula [Ir(arene)L2]+ (L = cyclooctene, L2 = 1,5-C8H12).

Regiospecific hydride abstraction from metallacycles: conversion of metallacyclopentanes to cationic π-allylic complexes

The rhoda- and irida-cyclopentane complexes [[graphic omitted]H2)(η5-C5Me5)(PPh3)][M = Rh (1) or Ir (2)] react with the trityl cation [CPh3]+ to give the n3-1-methylallyl derivatives [M(η3-CH2CHCHMe)(η5-C5Me5)(PPh3)][BF4][M = Rh (3) or Ir (4)]. Deuterium-labelling studies show that in these cases as well as in the previously reported palladacyclopentane →(η3-1-methylallyl) palladium complex transformations, the trityl cation abstracts regiospecifically one of the β-hydrogen atoms of the metallacyclic moiety. The involvement of a σ-3-butenyl intermediate which rearranges to a η3-1-methylallyl derivative is confirmed by reacting the palladium and rhodium dihalides, [PdCl2(Ph2PCH2CH2PPh2)] and [Rh(η5-C5Me5)(PPh3)I2], with 3-butenylmagnesium bromide. In the case of palladium a σ-3-butenyl complex is obtained which, by reacting with AgBF4, gives the η3-1-methylallyl derivative [Pd(η3-CH2CHCHMe)(Ph2PCH2CH2PPh2)][BF4]. In the case of rhodium the PPh3 ligand is lost and the η3-1-methylallyl compound [Rh(η3-CH2CHCHMe)(η5-C5Me5)I] is obtained directly. By reacting [Rh(η5-C5Me5)(PPh3)I2] with 3-pentenylmagnesium bromide, the η3-1,3-dimethylallyl complex [Rh(η3-MeCHCHCHMe)(η5-C5Me5)(PPh3)][BF4] is obtained. Mechanistic implications are discussed along with the significance of the reactions studied in connection with the role of transition-metal metallacyclopentane derivatives in organometallic chemistry and in catalysis.

Electron transfer catalysis in the activation of CH bonds by ruthenium complexes

Ruthenium(II) dimethyl complexes. [Ru(Me)2(η6-C6Me6)(PR3)] (R = Ph 1a, R3 = MePh2 1b, R3 = Me2Ph 1c, R = Me 1d, R = Et 1e), react with CH bonds of benzene or toluene under severe conditions (85–105°C depending on the phosphine ligand) to give methane and the new methyl aryl derivatives [Ru(Me)(Ar)(η6-C6Me6)(PR3)]. The methyl tolyl complexes are formed as 21 mixture of meta and para isomers. In contrast the reaction of 1a-1e with arenes, in the presence of [FeCp2]PF6, proceeds rapidly at room temperature: the corresponding methyl aryl derivatives [Ru(Me)(Ar)(η6-C6Me6)(PR3)] (Ar = Ph, Tol) and/or the intramolecular reaction products are formed depending on the steric hindrance of the phosphine. The fact that electrochemistry and ESR spectroscopy show that on oxidation that ruthenium(II) complexes give stable ruthenium(III) congeners suggests that the catalytic reaction triggered by ferrocenium ions proceeds through a different redox pathway.