David H. Williamson

Organometallic compounds in organic synthesis—XI

Abstract The concept is discussed of superimposed lateral control of reactivity, stereochemistry and structures, by attachment of complexed metal atoms to olefinic systems. This differs from classical endogenous control of synthesis by classical anionoid and cationoid groups in the skeleton, and its application has many theoretical and practical advantages. It is illustrated by considerations of reactions of substituted tricarbonylcyclohexa-l,3-dieneiron derivatives; notably the derived dienyliron salts, which are defined as equivalents of specifically substituted aryl cations or as cyclohex-2-enone cations, dependent on the structures and reaction sequences. The effects are noted of classical substituents on the positions and on the rates of reactivity of some complexed dienyl cations ; both regio- and stereospecificities are dependent on the nature of the anion involved and the conditions. Probable mechanisms are discussed. Other effects of lateral control include those on the classical reactivities of attached groups (such as hydrolysis of CO2Me) and on adjacent groups, such as stereochemistry of reduction of the 3-carbonyl in the ergosterone complex. Some useful new C-C bond-forming reactions made possible by the approach are noted.

Further studies on metal-promoted vinylcyclopropane to cyclopentene rearrangements. Structure and thermolysis of rhodium complexes of exo-6-vinylbicyclo[3.1.0]hex-2-ene and the crystal structure of the 1,6–8-η4-5-allylcyclopent-2-enyl(hexafluoroacetylacetonato)rhodium(III) tetramer

The reactions of exo-6-vinylbicyclo[3.1.0]hex-2-ene with bis(ethylene)rhodium(I) complexes lead to displacement of ethylene and formation of ring-opened σπ-bis-allylrhodium derivatives. The hexafluoroacetylacetonate (11) is tetrameric in the solid state and its structure has been determined by X-ray methods. Crystals of [C13H11F6RhO2]4 are monoclinic, space group P21/c, a= 19.641(7), b= 14.353(7), c= 44.87(1)A, β= 98.03(3), Z= 8; 3 266 observed reflections [I/σ(I) > 3.0] were collected by diffractometer and refined to R 0.077.The four rhodium atoms form the corners of a puckered square and are each π-bonded to one hydrocarbon at C(2),C(3) and to another σ at C(1) and π at C(6)–(8). This complex rearranges in acetone solution at 338 K, mainly to the corresponding derivative of bicyclo[3.3.0]octa-2,6-diene; thermolysis of the corresponding dimeric acetylacetonate (12) has been studied in detail by 1H n.m.r. The reaction exhibits half-order kinetics over the conentration range 0.02–0.2M in deuteriobenzene, rearranging rapidly (t1/2[0.205M] 30.30 min) to a mixture of acetylacetonato(bicyclo[3.3.0]octa-2,6-diene)rhodium(I)(2) and acetylacetonato-(endo-6-vinylbicyclo[3.1.0]hex-2-ene)rhodium(I)(1) in 88 : 12 ratio. In deuteriodichloromethane, the ratio is 33 : 67. The cyclopentadienyl complex (14) is monomeric, and thermally stable below 373 K. Reaction of the rhodium chloride complex (13) with bicyclo[2.2.1]hepta-2,5-dienesilver hexafluorophosphate leads directly to a rearranged cation (17) at 273 K.The mechanisms of these transformations are discussed.

Reactive rhodium complexes derived from trans-6-vinylbicyclo[3.1.0.]hex-2-ene. Mechanistic studies and X-ray structure of η4-3,1′–3′(4-allylcyclopent-1-enyl)hexafluoroacetylacetonatorhodium tetramer

The title compound forms rhodium(I) complexes with attendant cyclopropane ring-opening, and the resulting acetylacetonates rearrange cleanly above room temperature.

Organometallic complexes in synthesis. Part V. Some tricarbonyliron derivatives of cyclohexadienecarboxylic acids

Reactions of iron pentacarbonyl with several methyl cyclohexadienecarboxylates lead to tricarbonyliron complexes. The most stable of these is the ‘conjugated’ isomer (2; R = Me), which can be obtained from the others by treatment with methanolic acid or in some cases with methanolic methoxide. The latter method is the first observed example of base-catalysed isomerisation in tricarbonylcyclohexadieneiron complexes. When deuteriated reagents are used the acidic procedure leads to incorporation of one deuterium atom only, in the 6-position. irrespective of the starting material. Reactions of the acids with diborane yield the carbinols, one of which [tricarbonyl-1 -(hydroxymethyl)cyclohexa-1,3-dieneiron (8)], on treatment with acid, produces the rearranged carbonium salt (9). Hydride abstraction from the isomeric esters gives a series of carbonium salts, which react with water at various rates to produce hydroxy-complexes and dimeric ethers. Complexing of some methyl methylcyclohexa-1,4-dienecarboxylates is reported. Aspects of theory connected with hydrogen migration and hydride abstraction are discussed.

Synthesis and thermolysis of rhodium and iridium complexes of endo-6-vinylbicyclo[3.1.0]hex-2-ene. A metal-promoted vinylcyclopropane to cyclopentene rearrangement

The reaction of endo-6-vinylbicyclo[3.1.0] hex-2-ene with bisethylenerhodium(I) acetylacetonate gives rise to a 1 : 1 complex by displacement of ethylene in which the intergrity of the divinylcyclopropane is maintained. Unlike the parent hydrocarbon, which rearranges at room temperature to bicyclo[3.2.1] octa-2,6-diene, the derived rhodium complex is stable to 80°, and then the major product of rearrangement is the corresponding complex of bicyclo[3.3.0] octa-2,6-diene, minor amounts of the bicyclo [3.2.1] octa-2,6-diene derivative also being formed. Related studies on the hexafluoroacetylacetonatorhodium (I) and acetylacetonatoiridium (I) derivatives of endo-6-vinylbicyclo [3.1.0] hex-2-ene are reported. Possible mechanisms for the thermal transformation are discussed.