Igor T. Chizhevsky

Ligand effects in the hydrogenation of methacycline to doxycycline and epi-doxycycline catalysed by rhodium complexes. Molecular structure of the key catalyst [closo-3,3-(η2,3-C7H7CH2)-3,1,2-Rh C2B9H11]

Abstract The catalytic reduction of the exocyclic methylene group of methacycline (A) leads to the formation of two diastereoisomers, doxycycline (B, the α-epimer) and 6-epi-doxycycline (C, the β-epimer), with a selectivity which markedly depends on the nature of hydrocarbon and carborane ligands of closo-(π-cyclodienyl)rhodacarborane catalysts. Neutral norbornadienyl complexes with unsubstituted carborane ligands [closo-3,3-(η2,3-C7H7CH2)-3,1,2-RhC2B9H11] (1) and [closo-2,2-(η2,3-C7H7CH2)-2,1,7-RhC2B9H11] (7) are more active and afford higher selectivity in the formation of doxycycline than those having mono- or di-substituents at the carborane cage, [closo-3,3-(cyclodienyl)-1-R-2-R′-3,1,2-RhC2B9H9] (R = H, R′ = Me, PhCH2; R = R′ = Me; cyclodienyl = η2,3-C7H7CH2 or η-C10H13) as well as those from the closely related series of η5-cyclopentadienyl complexes [(η2,3-C7H7CH2)Rh(η5-C5Rn)]+PF6− (Rn = H5, Me5, or H2-1,2,4-Ph3). Mechanistic aspects of the hydrogenation reaction of methacycline are sketched. The results of the X-ray diffraction study of the best catalyst 1 are reported.

Carborane Complexes of Ruthenium(III): Studies on Thermal Reaction Chemistry and the Catalyst Design for Atom Transfer Radical Polymerization of Methyl Methacrylate

The heating of the 18-electron complex [3,3-(dppb)-3-H-3-Cl-closo-3,1,2-RuC(2)B(9)H(11)] (3) in benzene at 80 °C in the presence of a small amount of CCl(4) as initiator afforded paramagnetic 17-electron species [3,3-(dppb)-3-Cl-closo-3,1,2-RuC(2)B(9)H(11)] (4) along with minor amounts of two P-phenylene ortho-cycloboronated derivatives [3-Cl-3,3,8-{Ph(2)P(CH(2))(4)PPh-μ-(C(6)H(4)-ortho)}-closo-3,1,2-RuC(2)B(9)H(10)] (5) and [3,7-Cl(2)-3,3,8-{Ph(2)P(CH(2))(4)PPh-μ-(C(6)H(4)-ortho)}-closo-3,1,2-RuC(2)B(9)H(10)] (6) in total yield of ca. 80%. The heating of either 3 or 4 in toluene at 95 °C in the absence of CCl(4) led to the selective formation of 5, which was isolated in 64% and 46% yield, respectively. Thermolysis of 3 at higher temperatures (boiling toluene, 110 °C) gives novel paramagnetic species [3-Cl-3,3,7,8-{Ph(2)P(CH(2))(4)P-μ-(C(6)H(4)-ortho)(2)}-closo-3,1,2-RuC(2)B(9)H(9)] (7) featuring bis(ortho-cycloboronation) of both P-phenyl groups at the same phosphorus atom of the ruthenium-bound dppb ligand. All new paramagnetic complexes 4-7, as well as starting diamagnetic species 3, were characterized by single-crystal X-ray diffraction and, in addition, by EPR spectroscopic studies of odd-electron complexes. Ruthenacarboranes 3-5 and 7 all display high efficiency as catalysts for the atom transfer radical polymerization (ATRP) of methyl methacrylate (MMA). Complex 5 gave the best catalyst performance in terms of polydispersity; the PDI (M(w)/M(n)) of the polymer samples is as low as 1.15.

Crown Compounds for Anions: Sandwich and Half-Sandwich Complexes of Cyclic Trimeric Perfluoro-o-phenylenemercury with Polyhedralcloso-[B10H10]2− andcloso-[B12H12]2− Anions

It has been shown by IR and NMR spectroscopy that cyclic trimeric perfluoro-o-phenylenemercury (o-C6F4-Hg)3 (1) is capable of binding closo-[B10H10]2- and closo-[B12H12]2- anions to form complexes [[(o-C6F4Hg)3](B10-H10)]2- (2), [[(o-C6F4Hg)3]2(B10H10)]2-(3), [[(o-C6F4Hg)3](B12H12)]2- (4), and [[(o-C6F4Hg)3]2(B12H12)]2- (5). According to IR data, the bonding of the [B10H10]2- and [B12H12]2- ions to the macrocycle in these complexes is accomplished through the formation of B-H-Hg bridges. Complexes 2, 3, and 5 have been isolated in analytically pure form and have been characterized by spectroscopic means. X-ray diffraction studies of 3 and 5 have revealed that these compounds have unusual sandwich structures, in which the polyhedral di-anion is located between the planes of two molecules of 1 and is bonded to each of them through two types of B-H-Hg bridges. One type is the simultaneous coordination of a B-H group to all three Hg atoms of the macrocycle. The other type is the coordination of a B-H group to a single Hg atom of the cycle. According to X-ray diffraction data, complex 2 has an analogous but half-sandwich structure. The obtained complexes 2-5 are quite stable; their stability constants in THF/acetone (1:1) at 20 degrees C have been determined as 1.0 x 10(2)Lmol(-1), 2.6 x 10(3)L(2)mol(2), 0.7 x 10(2)Lmol(-1), and 0.98 x 10(3)L(2)mol(-2), respectively.

Stereoselective hydrogenation of methacycline to Doxycycline catalysed by rhodium-carborane complexes

Doxycycline (2), a tetracycline antibiotic extensively used in chemotherapy, was obtained stereoselectively from the hydrogenation of methacycline (1), catalysed by novel rhodium-carborane complexes.

Norbornadiene complexes of transition metals: III. Stereospecificity in the generation of cationic complexes [Rh(2,3,8 : 5,6-η-C7H7CHR)(η-C5H5)]+ (R = Me, Ph, [Rh(2,3,8 : 5,6-η-C7H7CH2)(η-C5H5)]+PF6t-

Abstract Treatment of (2-hydroxymethylnorbornadiene)cyclopentadienylrhodium (V) with sulfuric acid offers the stable cationic complex XI, which can be isolated as PF 6 − or BF 4 − . 1 H and 13 C NMR data indicate that in cation XI the bicyclic ligand is bound to the metal via η 2 -ethylene and η 3 -allyl bonds, including an exocyclic methylene carbon. The structure of complex XI is supported by a single-crystal X-ray diffraction study of its PF 6 − salt. Reaction of diastereomeric carbinols ψ- exo -VI and ψ- endo -Rh {η 4 -C 7 H 7 CH(OH)Me} (η 4 -C 5 H 5 ) (VII) proceeds with absolute stereospecificity to form syn -XII and anti -[Rh(η 5 -C 7 H 7 CHMe)(η 5a -C 5 H 5 )] + (XIII) isomers, respectively. Similarly, cation [Rh(η 5 -C 7 H 7 CHPh)(η 5 -C 5 H 5 )] + (XV), derived from ψ- endo -Rh {η 4 -C 7 H 7 CH(OH)Ph}(η 5 -C 5 H 5 ) (IX), has the syn configuration. However, the same reaction of the ferrocenyl-substituted carbinol ψ- endo -Rh {η 4 -C 7 H 7 CH(OH)Fc} (η 5 -C 5 H 5 ) (X) leads to mixture of two isomeric complexes, syn -(XVI) and anti -[Rh(η 5 -C 7 H 7 CHFc)(η 5 -C 5 H 5 )] + (XVII), in a ratio of 4 1 . The absolute stereospecificity and the relative facility with which the formation of cationic η 3 -allylic complexes from carbinols VI, VII and IX takes place are due to the nucleophilic participation of a rhodium atom in stabilizing the electron-deficient center during the reaction. The disturbance of stereospecificity during the conversion of carbinol X to a cationic complex assumes the formation of an α-ferrocenylcarbenium ion (XIX) as an intermediate (a product of kinetic control). Owing to the relative non-hindrance of rotation around the exocyclic carboncarbon bond C(2)—C(8) in the carbenium ion XIX, the formation of a non-equal mixture of complexes XVI and XVII, the products of thermodynamic control of the reaction, takes place.

Semi-sandwich platinum metals metallacarboranes derived from nido-C2B9H12−: Chemistry and structural studies

Abstract A variety of metallacarboranes, synthesized and structurally characterized by the authors are reviewed. These complexes are derived from platinum metals (Rh, Ru, Os) and isomeric nido -carborane anions [ nido -C 2 B 9 H 12 ] − and involve non-hydrocarbon (i.e. hydride, chloride, phosphine) as well as π -diene/dienyl ligands at the metal vertex.

Metallacarborane diene π-complexes. I. Synthesis of rhodacarboranes with a η2,η3-2-methylenenorbornadienyl ligand at the rhodium vertex

Abstract Novel 18-electron rhodacarboranes with a 2-methylenenorbornadienyl ligand, η 2 ,η 3 -coordinated to a rhodium atom of closo-3,3-(η 2 ,η 3 -C 7 H 7 CH 2 )-1,2-R 1 R 2 -3,1,2-RhC 2 B 9 H 9 (IIIa–IIId, R 1  R 2  H (a), R 1  R 2  Me (b), R 1  Me or Ph, R 2  H (c,d) and closo-2,2-(η 2 ,η 3 -C 7 H 7 CH 2 )-2,1,7-RhC 2 B 9 H 11 (VIII) have been prepared. The structures of IIIa–IIId, and VIII were deduced from their IR, 1 H NMR and 13 C NMR spectra. Two alternative synthetic routes to the above complexes have been suggested: (a) reactionis of 16-electron complexes of (η 4 -C 7 H 7 -2-CH 2 OH)- Rh(acac) or [(η 4 -C 7 H 7 -2-CH 2 OH)RhCl] 2 with appropriate dicarbollide anions with a subsequent acidificatiion by HPF 6 and (b) reactions of unsubstituted and C-mono-substituted closo-bis(triphenylphosphine)hydridorhodacarboranes with 2-(hydroxymethyl)norbornadiene on heating in benzene. The possible mechanisms of these reactions are discussed. An X-ray study of complex IIIb, R 1  R 2  Me, confirms that there is allylolefin type metal-ligand bonding.

Cationic hydroxyallylolefin rhodium complexes [Rh(η2, 3-C7H6-3-R-2-CR1OH)(η5-Cp)]+PF6− (R, R1 = H, Alk, Cp = C5H5, C5H4Me, C5Me5) with intramolecular O-H ⋯ Rh hydrogen bonds. Formation of {[CpRh(η2, 3-C7H6-3-R-2-CR1OH)]2-F2POO}+PF6− dimers as a result of partial hydrolysis of the PF6− anion

Abstract Protonation of the 2-acyl derivatives of Rh(η4-norbornadiene)(η5-Cp) (Cp = C5H5, C5H4Me, C5Me5) by HPF6 in diethyl ether yields the stable cationic O-protonated complexes [Rh(η2, 3-C7H6-3-R-2-CR1 OH)(η5-Cp)]+PF6− (R, R1 = H, Me), whose structure was established by IR, 1H and 13C NMR spectroscopy. The formation of the intramolecular hydrogen bond O-H ⋯ Rh was found on the basis of IR and 1H NMR spectra of the cations. CH2Cl2 solutions of the cationic complexes that contained traces of moisture caused the partial hydrolysis of the PF6− anions, to give the novel “dimeric” complexes, {[CpRh(η2, 3-C7H6-3-R-2-CR1 OH}2F2P(O)O}+PF6−. The F2P(O)O group of this complex is involved in the strong intermolecular hydrogen bond, O-H ⋯O-P(F2)-O ⋯ H-O. An X-ray diffraction study has been carried out on one of the “dimeric” cations (R = Me, R1 = H).

Norbornadiene complexes of transition metals : II. Stereochemistry of {2-(α-hydroxyethyl)norbornadiene} cyclopentadienylrhodium complexes

Correlations were established between the stereochemistry of 2-(α-carbinol)norbornadiene complexes of rhodium and their mass spectra.

First agostic closo-metallacarboranes with η3-cyclooctenyl type ligand: synthesis and structural characterization of closo-3-[η3-(endo-1,5-dimethylcycloocten-1-yl)]-1,2-μ-(1′,2′-xylylene)-3,1,2-IrC2B9H9 and its isomerization to closo-3-[η3-(exo-1-methylene-5-methylcyclooctene-1-yl)]-1,2-μ-[η2-(1′,2′-xylylene)]-3,1,2-IrC2B9H9

Abstract The reaction of a new iridium reagent [Ir(η4-1,5-Me2COD)Cl]2 (1) with [nido-7,8-μ-(1′,2′-CH2C6H4CH2)-7,8-C2B9H10]−K+ (2) in solution of C6H6–MeOH mixture or in C6H6 afforded either an agostic (CH⋯Ir) closo-3-[η3-(endo-1,5-Me2COD)]-1,2-μ-(1′,2′-CH2C6H4CH2)-3,1,2-IrC2B9H9 (3) along with closo-3-[η3-(endo-1,5-Me2COD)]-1,2-μ-(1′,2′-CH2C6H4CH2)-8-(EtO)-3,1,2-IrC2B9H8 (4) or the only complex 3 in high yield. Complex 3 in dichloromethane solution is quantitatively converted to isomeric η3-exo-allylic complex closo-3-[η3-(1-exo-CH2-5-MeC8H12)]-1,2-μ-[η2-(1′,2′-CH2C6H4CH2)]-3,1,2-IrC2B9H9 (5) for a few days. All new complexes 3, 4 and 5 were characterized by single-crystal X-ray diffraction studies, which confirmed the existence of an agostic CH⋯Ir interaction in 3 and revealed a weak η2-coordination of the metal atom with one of the aromatic bonds of ortho-xylylene cage substituent in 5. The normal and low-temperature 1H- and 13C/13C{1H}-NMR spectra as well as 2D COSY/HETCOR NMR data obtained for the studied complexes are discussed in details.