B.M.Ratnayake Bandara

Alkylation of tricarbonylcyclohexadienyliron salts (1) with lithium alkyls

Abstract The salts 1 (Y=H, OMe, Me) react with RLi(R=Me, n-Bu, i-Pr, t-Bu) in CH 2 Cl 2 at low temperature to give 2 and 3 in excellent yields. Nucleophilic addition at the 1-position of 1 (Y=OMe) has been observed for the first time.

Optical resolution of tricarbonyl(1-carboxycyclohexa-1,3-diene) iron and the absolute configuration of the products

Resolution of the acid (1) (shown as the (+)-isomer) into its optically pure (+) and (−) isomers and reduction of the CO2H yields the 2-Me derivative (2) (shown as the (−)-isomer). The absolute configuration of (2) is defined by conversion of the salt (3) of known configuration into (2) and (4). This is the first resolution leading to preparation of pure complexes of known absolute configuration.

The first full resolution of 2-methoxytricarbonylcyclohexadienyliron hexafluorophosphate, an example of a synthetic organic equivalent in the series of chiral cyclohex-2-enone-4-cations.

Abstract A superior method, using Hunig base in CH2Cl2, converts complexed cations of type (1) into type (2) by the action of nucleophiles HXR′, and permits the optical resolution of the title salt via the ethers of (2,R=Ome,R′ = (−)-menthoxy).

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.

An alternative formation of tricarbonylcyclohexadienylium iron salts by acid-catalysed decarbonylation

The action of concentrated sulphuric acid on (1, R = H or CO2Me, R′ = H) generates the cation (2, R = H or CO2Me, R′ = H); the stereoisomer (5, R = H, R′ = H) is unaffected. This is an alternative procedure of some utility for the preparation of some cation salts, notably of (2, R = CO2Me, R′ = H).

Stereoselectivity in the formation of tricarbonyliron complexes of some dihydrobiphenyls

Abstract Ratios of isomeric products in complexation of some substituted cyclohexa-1,4- and 1,3-dienes, using Fe(CO) 5 or Fe 3 (CO) 12 are significantly affected by the nature of an allylic substituent: CO 2 Me tends to direct the metal to its occupied face, Ph less so and Me is inhibitory. An appropriately substituted cyclohexadiene gives a single stereoisomer.

The steric course of proton elimination in conversion of a tricarbonylcyclohexadieneiron carbinol into an endocyclic cation

Abstract Tricarbonyl-1-carbomethoxy-5α 2 H -cyclohexadieneiron ( 3 ) reacts with MeLi to tricarbonyl-1(1 1 -hydroxy-1 1 -methyl-ethyl)-5α 2 H -cyclohexadiene- iron ( 1 ) which yields with acid the salt tricarbonyl-1(1 1 -methyl-ethyl)-5 2 H - cyclohexadienyliron(+)-PF 6 (−) (4). Retention of 2 H demonstrates the stereochemistry of the elimination of the ring proton as β-( endo ).

Organometallic compounds in organic synthesis. Part 12. Methods of determination of the stereochemistry of tricarbonylcyclohexa-1,3-dieneiron complexes

The steric configuration (α- or β-) of a substituent attached to an sp3 carbon atom in tricarbonylcyclohexadieneiron complexes can be determined by 1H n.m.r. spectroscopy if both epimers are available. With only one epimer present, the most useful method involves examination of the vicinal coupling constants of hydrogens attached to sp3 carbon atoms. The β-H atoms have a vicinal coupling constant of 10–12 Hz and the corresponding α-H atoms have a value of ca. 8 Hz. The CH2 splitting patterns are indicative of stereochemistry when flanked by two vicinal protons. In some cases simplification of the spectrum by use of a paramagnetic shift reagent is necessary for evaluation. Usually δ(β-substituent) > δ(α-substituent). The CO2Me substituents show δ(β-CO2Me) > 3.63 > δ(α-CO2Me) and often, but not invariably, δ(β-H) > δ(α-H). Computer simulation confirms the expected pattern. Aromatic solvent induced shifts are not useful; 13C n.m.r. have only a limited usefulness and require both isomers; mass spectrometric fragmentation is intepretable in steric terms, but is not useful for definition. 1H N.m.r. spectra of the isomeric cations (45), (46), and the alkyl analogue (47) show indications of utility in connection with 6-H, but more examples are needed. The vicinal coupling of CH2 seems to be at present the only reliable method for the assignment of configuration in neutral diene complexes.

Organometallic compounds in organic synthesis. Part 13. Stereoselectivity of complexation of cyclohexadiene esters

Complexation of cyclohexadienes with Fe(CO)3 as the entering group are subject to classical steric hindrance by alkyl groups, but CO2R and related groups introduce a competing factor because they favour sterically the entering group, probably through an intermediate complex. Increase of π-electron availability in the initial 1,4-diene by the attachment of OMe reduces this influence, probably because it favours direct olefin complexation. Non-polar conditions increase the directing effect of CO2R, but complete stereospecificity cannot be achieved except through additional alkyl substitution. Nevertheless, many stereoisomeric pairs can be separated and the route leads to precursors of a number of sterically defined products which are otherwise not readily accessible.

Organometallic compounds in organic synthesis. Part 14. Tricarbonyliron as lateral control group in the selective alkaline hydrolysis of some cyclohexa-1,3-diene carboxylic esters

Alkaline hydrolyses of a number of methoxycarbonyl derivatives of cyclohexa-1,3-diene–Fe(CO)3 have been used as indices of steric and electronic effects on reactivities of the ester groups. A β-CO2Me or a 1-CO2Me group is resistant to hydrolysis for steric and electronic reasons, respectively. A number of substituted esters are examined and, with dicarboxylic esters complexation used successfully to enable half hydrolyses, which cannot readily be accomplished with the parent esters.