V. A. Nikanorov

π-cyclohexadienone derivatives of rhodium(I) acetylacetonate. Crystal and molecular structure of rhodium (η4-4-methyl-4-exo-trichloromethyl-2,5-cyclohexadiene-1-one)(pentane-O,O′-2,4-dionate)

Abstract By means of the ligand exchange reactions of Rh(acac)(C 2 H 4 ) 2 (VII) with para -semiquinoid cyclohexadienone ligands (4,4-dimethyl- (VI), 4-methyl-4-n-butyl-(IX), 4-methyl-4-dichloromethyl- (X), 4-methyl-4-trichloromethyl- (XIII) - cyclohexadiene-2,5-ones-1) the corresponding π-diene derivatives of Rh(acac)(η 4 -dienone) have been obtained and characterized by 1 H NMR and IR spectra. In reactions with the ligands IX and X, mixtures ( 1 1 ) of diastereomeric complexes with exo - and endo -configuration are formed. Reaction with the ligand (XIII) is stereospecific and results in the formation of a complex with only exo -configuration, whose crystal and molecular structure have been determined by an X-ray structural study. The crystals are monoclinic with space group P 2 1 / c , a 7.673(3), b 10.794(4), c 18.38(1) A, β 100.89(4)°, Z = 4. Stereospecificity of the ligand exchange reaction is evidently due to the preferable direction of the metal atom attack from the sterically less crowded exo -site of the semiquinoid ligand.

Redox-rotational phenomena in organometallic chemistry. Unique features of the crystal structure and stereodynamic behaviour of acetylacetonato- and cyclopentadienylrhodium complexes with potentially dipolar two-ring semiquinoid ligands in solution

Abstract Replacement of the acac group in the 16-electron (metal oxidation state Rh1) (acac)Rh(L) complex A (where L is 4-methyl-A-trichloromethyl-1-(4,4-dimethyl-2,6-dioxocyclohexylidene)-2,5-cyclohexadiene) with Cp, under the action of CpTl, gives the 18-electron CpRh(L) complex B, in which, according to 1H and 13C NMR spectral data, the η4-diene structure of the ligand L rearranges into the dipolar delocalized form of 1-(4,4-dimethyl-2,6-dioxocyclohexylide)-4-methyl-4-trichloromethylbenzolonium with η5-coordination by the cyclohexadienyl ring to the RhIIII atom, in a reaction we have termed a ligand redox-exchange. An X-ray diffraction has revealed that the dimedone ring of molecule B in the crystal is twisted around the central polarized bond by an angle of 28.9°, and 1H and 13C DNMR spectroscopy has shown that the dimedone fragments of the complexes A and B in solution undergo internal rotation about this band. The effects of solvent on this process, including the enhancing effect by the presence of H+, has been studied. The rotation in the case of complex B is faster than that in complex A (“redox-rotation”). The kinetic parameters of the stereodynamics of the process (including the accompanying dimedone ring inversion) have been measured (e.g. in acetone-d6 ΔH≠ 18.4 kJ/mol, δS≠ −125.8 J mol−1 K−1, ΔG≠298 56.0 kJ/mol). Treatment of B with CF5COOH gives a stable η5-cyclohexadienyl complex of RhIII, CpRh(LH)OCOCF3, with a protonated oxygen in the CO group of the ligand. A periodic system for virtually all the rotatable β-charged organometallic molecules has been proposed, which includes four structural types (and two classes) of models capable of undergoing either isovalent-odd or anisovalent-even internal rotations, depending on the evenness of the ligand fragment coordinated with the metal.

The first three aromatic tribromides of the [2.2]paracyclophane series

From the reaction mixtures in the uncatalyzed polybromination of [2.2]paracyclophane by the action of excess Br2 in CCl4, there have been found along with the known products — 4,15- and 4,16-dibromo[2.2]paracyclophanes — two new aromatic tribromides of this series, which have been isolated in pure form: 4,12,15- and 4,15,16-tribromo[2.2]paracyclophanes. Special experiments demonstrated that the mixtures of these tribromides are formed as a result of competitive monobromination of 4,15-dibromo[2.2]paracyclophane; the 4,15,16-tribromo[2.2]paracyclophane, together with still another newly isolated isomer of this series — 4,8,12-tribromo[2.2]paracyclophane — is formed as a result of competitive monobromination of 4,16-dibromo[2.2]paracyclophane. As an explanation of the features of the orienting effect of substituents in these competing reactions, a rule was proposed: On the conventional orientation (from the electronic point of view) of entry of the bromine atom into the substituted ring (para > ortho > meta), a steric limitation is imposed on its attack in the pseudo-gem-position, owing to the bulky bromine atom that is transannularly positioned above it in the neighboring aromatic ring. The structures of all of the tribromides were established on the basis of elemental analyses, mass spectrometry, and1H NMR spectrometry (including PMR using the homonuclear Overhauser effect). The data obtained in this work indicate that the 4,12,15-tribromo[2.2]paracyclophane and 4,15,16-tribromo[2.2]paracyclophane are predecessors of the two tetrabromides previously obtained by Cram — 4,7,12,15- and 4,5,15,16-tetrabromo[2.2]paracyclophanes; and the 4,8,12-tribromo[2.2]paracyclophane is a possible predecessor of 4,8,12,16-tetrabromo[2.2]paracyclophane, which is unknown up to the present time.

Structural features of the mixed η2:η4 coordination of two transition metal atoms in binuclear and trinuclear π-complexes with semiquinoid ligands. Crystal and molecular structure of (η2-ethylene)(4-methyl-4-exo-n-butyl-1-anti-η2-methylene-[(2,5-η4-cyclohexadiene)rhodium2,4-O,O′-pentanedionate])- rhodium-2,4-O,O′-pentanedionate

The crystal and molecular structure of (η2-ethyelene)(4-methyl-4-exo-n-butyl-1-anti-η2-methylene-[(2,5-η4-cyclohexadiene)rhodium-2,4-O,O′-pentanedionate]- rhodium-2,4-O,O-pentanedionate has been determined by an X-ray diffraction study. Comparison of the data obtained with those of a previous study of a mononuclear η4-coordinated rhodium(I) complex with the 4-methyl-4-trichloromethyl-2,5-cyclohexadiene-1-one ligand reveals that the most prominent feature of the mixed η2:η4 coordination in this series consists in the greater deviation (up to 30 and 40°, respectively) of the exo-unsaturated and the saturated geminal fragments of the semiquinoid ligand from the central plane of its cyclohexadiene ring. The shielding of the HA proton of the n-butyl group (-CHAHBC3H7) by the η2-coordinated rhodium atom (as well as by other fragments of the molecule) is different from that of the HB proton of the same group, which accounts for the unusual features in the 1H NMR spectra of this complex. There is a pronounced transannular contact between the rhodium atom and the hydrogen atom of the peripheral Cue5f8HA bond, practically at the face of the fragment being coordinated (the Rh…HA and Rh…CHA distances are 2.57 and 3.50 A, respectively, and the Rh…XAue5f8CHB angle is 160°). Use of molecular modelling based on the results of the X-ray diffraction study shows that for the recently synthesized trinuclear rhodium(I) complexes with two exo-methylene-2,5-cyclohexadiene ligands, the anti-trans configuration is the most likely.

REACTION OF 2,6-DIBENZYLIDENECYCLOHEXANONE WITH PHOSPHORUS PENTACHLORIDE :TRIPLE FUNCTIONALIZATION OF THE ISOSEMIQUINOID SYSTEM

The reaction of 2,6-dibenzylidenecyclohexanone with PCl5 occursvia the sequential stages of desoxychlorination and substitutional phosphorylation to form (after oxidation, methoxylation, and hydrolysis on the surface of the chromatographic SiO2 adsorbent) organophosphorus products of the 1-(a-chloro-, a-hydroxy-, or α-alkoxy)benzyl-2-chloro-3-(α-(dimethylphosphoryl)benzylidene)cyclobex-1-ene series.

An unusual example ofendo-stereoselectivity in the ligand exchange reactions of rhodium(i)diethylene derivatives withpara-semiquinoid ligands

The ligand exchange reactions of [(C2H4)2Rh(acac)] in benzene and [(C2H4)2RhCl]2 in CH2Cl2 with 4-methyl-4-trichloromethyl-2,5-cyclohexadiene-1-one occur with 100%exo-stereoselectivity. The similar process with 4-methyl-4-trichloromethyl-1-(4,4-dimethyl-4,6-dioxocyclohexylidene)-2,5-cyclohexadiene (trienedione) is strictlyexo-stereospecific only if [(C2H4)2Rh(acac)] in benzene is used, while in the case of [(C2H4)2RhCl]2 in CH2Cl2, it proceeds with anendo-stereoselectivity of 43%. An explanation for these facts has been suggested that assumes that the metal atom initially attacks the central double bond in the trienedione, which is removed from the area of main steric hindrance. The subsequent metallotropic rearrangement of the resulting ethylene-triene intermediate gives rise to the final η4-coordinated π-diene structures.

Redox troponization of (η5-cyclopentadienyl)(η4-4-menthyl-4-exo-trichloromethylcyclohexa-2,5-dien-1-one)rhodium as a novel approach to the synthesis of metal-coordinated nonbenzenoid aromatics of the tropone seriesas a novel approach to the synthesis of metal-coordinated nonbenzenoid aromatics of the tropone series

The possibility of redox troponization of a gem-polyhalomethylated semiquinoid system, π-coordinated to a metal atom, was shown in relation to the reaction of (η5-cyclopentadienyl)(η4-4-menthyl-4-exo-trichloromethylcyclohexa-2,5-dien-1-one)rhodium with Pd(PPh3)4. The reaction occurs with retention of the metal coordination affording a sevenmembered organometallic derivative of the nonbenzoid aromatic series, namely, (η5-cyclopentadienyl)(η4-4-chloro-5-menthylcyclohepta-2,4,6-trien-1-one)rhodium, whose structure was established by means of elemental analysis, NMR, and mass-spectral data.

η4-endo-(17α,21-dihydroxypregna-1,4-diene-3,11,20-trione)-(η5-cyclopentadienyl)rhodium(I)

Abstractη4-endo-(17α,21-dihydroxypregna-1,4-diene-3,11,20-trione)-(η5-cyclopentadienyl)rhodium(I) was synthesized by successive treatment of 17α,21-dihydroxypregna-1,4-diene-3,11,20-trione with [(C2H4)2RhCl]2 and CpT1 in a yield of 54%.1H NMR studies demonstrated that coordination of the organometallic moiety occurs from the side opposite to the 19-CH3 group. Upon coordination, ringA of the prednisone ligand is fixed to adopt a boat conformation, and the basicity of the 3-CO group increases substantially.

New inter- and intramolecular carbo- and heterocyclizations of 4-methyl-4-trichloromethylcyclohexa-2,5-dien-1-one and its η4-rhodiumcyclopentadienyl complex under the action ofn-buthyllithium: formation of functionally substituted tricyclic spirofuran and bicyclo[2.2.1]heptadiene systemscomplex under the action ofn-buthyllithium: formation of functionally substituted tricyclic spirofuran and bicyclo[2.2.1]heptadiene systems

Treatment with BunLi in THF (−60–0°C) causes 4-methyl-4-trichloromethylcyclohexa-2,5-dien-1-one and itsexo-η4-coordinated (by a RhCp-group) π-diene metal complex to undergo reactions of two new types: intermolecular autocondensation-heterocyclization (affording 3,3-dichloro-3a,4′-dimethyl-4′-trichloromethyl-2,3,3a,6,7,8-hexahydrospiro-[benzofuran-2,1′-cyclohexa-2′, 5′-dien]-6-one; 24%) and intramolecular carbocyclization (yielding (η4-7,7-dichloro-1-hydroxy-4-methylbicyclo[2.2.1]hepta-2,5-diene)(η5-cyc;opentadienyl)rhodium: 77%). Both processes are assumed to involve the formation of an unusualgem-CCl2Li substitutedpara-semiquinoid intermediate, the conformation of the six-membered diene ring of which (planar or a boat-like) governs its subsequent intra- or intermolecular stereospecific carbonyl condensations.

BIS-PARA-SEMIQUINOID TYPE DOUBLE CYCLOPALLADATION IN THE SERIES OF SIX-MEMBERED TWO-NITROGEN BRIDGED ANNULENE-DIHYDROANNULENE LIGANDS

The cyclometallation reaction known in the series of annulene heteroorganic ligands was extended to six-membered dihydroannulenes with a dinitrogen bridge. Double cyclopalladation of 4-methyl-4-trichloromethylcyclohexa-2,5-dien-1-one azine yielded the first representative of a new class of cross-conjugated diazadipalladatetracycles,viz., 5,10-bis(acetylacetonato)-2,7-dimethyl-2,7-bis(trichloromethyl)-2,7-dihydro-4b,9b-diaza-5,10-dipalladaindeno[2,1-a]indene, isolated as a diastereomer mixture of the achiralmeso-form (E-isomer) and a racemate (Z-isomer). This reaction offers a method for transition metal-mediated activation of non-reactive C−H bonds at position 2 of cyclohexa-2,5-dienylidene systems and a route toward the very rare chiral polyheteroelement system with rotational symmetry.