Cyclohexadienyl Complexes Research Articles

1,2-Disubstituted [(η6-Arene)Cr(CO)3] Complexes by Sequential Nucleophilic Addition/endo-Hydride Abstraction

The surprising answer to the question Are the structural characteristics of the product of the nucleophilic addition to complex 1 those of a cyclohexadienyl complex (2 a) or those of a diene aza-enolate complex (2 b)? is: both. This is indicated by the X-ray structure determination of the complex with R=naphthyl. An unprecedented endo-hydride abstraction concludes the sequence that provides a new route to 1,2-substituted planar chiral [(n6-arene)Cr(CO)3] compounds.

Oxidation of (η5-6-exo-cyclopentadienylcyclohexadienyl)- (η5-cyclopentadienyl) iron by trityl hexafluorophosphateby trityl hexafluorophosphate

Oxidation of the cyclohexadienyl complex Fe(η5-C5H5)(1-5-75-6-exo-C5H5-C6H6) (2) by (Ph3C)PF6 (CH2Cl2, from −30 to +20 °C) occurs as two concurrent processes: elimination of an H atom from the cyclohexadienyl ligand and replacement of an H atom in the cyclopentadienyl ring by a CPh3 fragment. A mixture of cationic complexes [Fe(η5-C5H5) (η6-Ph-C5H5]+ (1+) and [Fe(η5-C5H4CPh3) (η6-Ph-C5H5]+ (4+) (4+) with PF6− anions is obtained. Deprotonation of the mixture of 1+ and 4+ complexes under the action of ButOK inm-xylene followed by boiling of the reaction mixture gives phenylferrocene (7) as the product of η6:η6 haptotropic rearrangement.

Synthesis and Reactivity of [(η6‐Biphenylene)Mn(CO)3]+

AbstractBiphenylene readily coordinates through one of the phenyl rings to form [(η6‐biphenylene)Mn(CO)3]+ (2+), the X‐ray structure of which is reported. The nucleophiles H−, Me−, Me3CC(O)CH−2 and P(OEt)3 add to 2+ at a bridgehead carbon to afford high yields of cyclohexadienyl complexes (3); the X‐ray structure of the product of hydride addition is reported. P(OMe)3 adds to 2+ to give 3, which then undergoes a spontaneous Arbuzov elimination to the dimethyl phosphonate adduct. Nucleophilic additions to 2+ are unusual in that they occur at a substituted (bridgehead) carbon. A kinetic and thermodynamic study of the reaction with P(OEt)3 shows that the bridgehead carbons in 2+ are orders of magnitude more electrophilic than are the arene carbons in unstrained [(arene)Mn(CO)3]+ systems.

Regioselectivity in the nucleophilic functionalization of dibenzofuran, dibenzothiophene and xanthene complexes of Mn(CO) 3+

The cationic complexes[(η 6-DBF)Mn(CO) 3]BF 4, [(η 6-DBT)Mn(CO) 3]BF 4 and[(η 6-XAN)Mn(CO) 3]PF 6 (DBF = dibenzofuran, DBT = dibenzothiophene, XAN = xanthene) are prepared. These species react with carbon-based nucleophiles to form exo-substituted cyclohexadienyl complexes. Addition is found exclusively at carbons 1 and 4. The C(1)-functionalized regioisomer predominates, especially for the DBF and XAN complexes, for which 90 to > 95% selectivity is found with stabilized nucleophiles. Preference for C(1) is greatly reduced for unstabilized magnesium and lithium nucleophiles. The origin of this selectivity is discussed. Loss of the aromatic ring from the cationic complexes in the presence of CH 3CN is found to be extremely facile for the DBF and DBT complexes owing to increased ease of ring slippage. Second-order rate constants for the displacement reactions with CH 3CN are4.6 ± 0.6 × 10 −5M −1,s −1 (DBF complex, 25°C),4.4 ± 1.0 × 10 −5,M −1,s −1 (DBT complex, 25°C) and about 1.0 × 10 −5,M −1,s −1 (XAN complex, 60°C). A crystal structure is presented for the hydride adduct of the DBF complex,[(η 5-C 12H 9O)Mn(CO) 3]: space group P2 1/c, a = 15.301(2), b = 6.4422(12), c = 12.988(2)A˚, β = 90.888(13)°, V = 1280.1(3)A˚ 3, Z = 4, D calc = 1.599Mg m −3, R = 0.0410 and wR2 = 0.0945 on 2446 reflections with F 2 > 4.0 σ(F 2).

Metal-Mediated Cycloaddition Reactions of the η5-Cyclohexadienyl Ligand. Synthesis and Reactions of [5 + 2] Single and [5 + 2],homo[5 + 2] Double Cyclohexadienyl−Alkyne Adducts of Manganese

UV irradiation of the cyclohexadienyl complex [(η5-C6H7)Mn(CO)3] (1) and 1 or 2 equiv of the alkynes PhC⋮CR (R = H, Me, Ph) in toluene gave both the [5 + 2] adducts [{η2: 3-C8H7Ph(R)}Mn(CO)3] (2) a...

Photoreactions of Chromium Cyclodienyl Complexes with Alkynes: Double [5 + 2],homo[5 + 2] Cycloaddition Reactions

UV irradiation of the cycloheptadienyl complexes [(η5-C7H8R)Cr(CO)3SnPh3] (1a, R = H; 1b, R = C(Me)S(CH2)3S; 1c, R = CHPh2) or cyclohexadienyl complexes [(η5-C6H6R)Cr(CO)2(NO)] (7, R = C(Me)S(CH2)3S) and 2 equiv of 2-butyne or 3-hexyne in toluene or n-hexane gave new tricyclic complexes 2, 5, and 8. These complexes arise from sequential [5 + 2] and homo[5 + 2] cycloadditions of two alkyne molecules to the η5-dienyl manifold forming 11-alkylidenetricyclo[5.3.1.04,10]undeca-2,5-diene or tricyclo[5.2.1.04,9]deca-2,5-dien-10-yl ligands, respectively. Heating 2 in acetonitrile gave [Cr(CO)3(CH3CN)3] and the tricyclic organic compounds 3. UV irradiation of 1a and 1-phenyl-1-propyne or 1c and 2-butyne directly gave the organic compounds 4a,b and 5c, 6a,b, respectively, as mixtures of isomers. The adduct resulting from 1a and two molecules of 3-hexyne has been characterized by an X-ray diffraction study.

Electrochemical behaviour and reactivity of cyclohexadienyl iron complexes

The electrochemical behaviour of the monosubstituted cyclohexadienyl complexes [Fe( η 5-2-R-cyclohexadienyl)(CO) 3][PF 6] (R  H, Me, n-Bu, MeO, Ph, MeOC 6H 4, CF 3C 6H 4), [Fe( η 5-1-EtO-cyclohexadienyl)(CO) 3][PF 6], the disubstituted species [Fe( η 5-R,R′-cyclohexadienyl)(CO) 3][PF 6] (R R′  MeO or Me; R  Me, R′  MeO), and [Fe( η 5-R-cyclohexadienyl)(CO) 2(PPh 3)][PF 6] (R  H, Ph), was studied by cyclic voltammetry and controlled potential electrolysis. They undergo a single electron irreversible cathodic process at a potential which is dependent on the electronic properties of both the cyclohexadienyl substituent and the co-ligands at the metal. In particular, tricarbonyliron complexes with aromatic substituents are reduced at significantly higher potentials than those with aliphatic ones, whereas substitution of a carbonyl by a triphenylphosphine ligand lowers the cathodic potential. By cathodic controlled potential electrolysis, they undergo CC coupling and, in some cases (unsubstituted or alkoxy-substituted cyclohexadienyl complexes) also FeFe coupling (with loss of CO) to form the corresponding dimeric species. The formation of these products has also been detected, in a number of cases, in the time-scale of the cyclic voltammetry.

Stable Cyclohexadienyl Complexes of Ruthenium in a Piano Stool Geometry Containing a Tridentate Nitrogen Donor Ligand. First Structural Characterization of the (eta(5)-Cyanocyclohexadienyl)ruthenium(II) Complex and Spectroelectrochemical Correlation.

The reactions of KCN, NaBH4, or NaOH with [(η6-C6H6)RuL](PF6)2 {L = (2-pyridylethyl)(2-pyridylmethyl)methylamine} afford cyclohexadienyl complexes of type [(η5-C6H6(Y))RuL](PF6), (Y = CN- (1), H- (2), OH- (3)). The X-ray structure of 1 has been determined. A spectroelectrochemical trend is observed.

The versatile chemistry of arenemanganese carbonyl complexes

The manganese carbonyl fragment Mn(CO)3+ is easily co-ordinated to a very wide range of aromatic molecules, e.g. benzenes, hydroquinones, naphthalenes, indoles and benzothiophenes. The resulting air-stable [Mn(η6-arene)(CO)3+] cations are quite electrophilic, and this is the basis for much interesting and useful chemistry. Carbon-donor nucleophilies ranging from weak to strong add rapidly and regioselectively to the arene to afford thermally stable cyclohexadienyl complexes from which functionalized arenes and cyclohexadienes can be obtained after oxidative removal of the metal. Chemical reduction of [Mn(η6-polyarene)(CO)3]+ with or without another metal complex present constitutes a general route to homo- and hetero-nuclear syn- and anti-facial bimetallic complexes with naphthalene-type ligands. Chemical reduction of [Mn(arene)(CO)3]+(arene =η6-benzothiophene or η5-thiophene) leads to C–S bond cleavage and the formation of novel metallacyclic complexes of relevance to hydrodesulfurization chemistry. These and other aspects of arenemanganese carbonyl chemistry are discussed.

Anomolous deprotonation of tricarbony( η5-1-arlycyclohexadienyl)iron complexes

1-Aryl substituted cyclohexadienyl complexes provide the first example of deprotonation from C-6 of tricarbonyl(cyclohexadienyl)iron(1+) complexes, and in the case of ortho-substituted aryl substituents, a reversed regiocontrol of nucleophile addition.

The synthesis and electrophilic reactivity of manganese tricarbonyl complexes of the C-ring aromatic diterpenoid methyl O-methylpodocarpate

The aromatic ring in the diterpenoid methyl O-methylpodocarpate (podo) coordinates readily to the Mn(CO) 3 + moiety to afford in high yield a nearly 1:1 isometric distribution of [( η 6-podo)Mn(CO) 3] BF 4 ([ 2]BF 4), in which the metal is situated on the α or β face. The nucleophiles PhLi, PhMgBr, MeLi, MeMgCl, LiCH 2C(O)CMe 3 and NaBH 4 were found to add cleanly to the activated aromatic ring in 2 + to give neutral cyclohexadienyl complexes. Nucleophilic addition occurs predominantly meta to the OMe substituent in the case of the α-isomer of 2 +, and at both ortho and meta sites in the case of the β-isomer. The X-ray structure is reported for a typical α- meta and a typical β- ortho cyclohexadienyl product, namely β-( η 5-podo · Ph)Mn(CO) 3 and α-( η 5-podo · CH 2C(O)CMe 3)Mn(CO) 3. The high yield and regioselectivity of the nucleophilic additions suggests that the manganese-mediated functionalization of aromatic diterpenes and steroids will prove to provide a useful synthetic methodology.

Arenes to Substituted Cyclohexadienes: Nucleophile/Electrophile Additions across an Arene Double Bond

Alkyl-, vinyl- and aryllithium reagents were reacted with tricarbonylchromium-phenyloxazoline and -phenylmethanimine complexes 1 and 2. The resulting anionic cyclohexadienyl complexes were trapped with primary alkyl, allyl, benzyl, and propargyl bromides to give, after metal removal, 1,3-cyclohexadienes with a 1,5,6-substitution pattern. The results are consistent with exo-nucleophilic addition to the ortho-position of the arene, followed by addition of the electrophile to the metal center. With allyl, benzyl, and propargyl groups, direct reductive elimination then yielded trans-5,6-substituted products. With alkyl halides, reductive elimination was preceeded by CO insertion and acylcyclohexadienes were formed preferentially. In situ deprotonation/alkylation of the acylcyclohexadienes gave products in which three C-substituents had been added across an arene double bond with complete regio-and stereocontrol. The competition between the two pathways is discussed in terms of migratory aptitude to carbonylation and effects of the cyclohexadienyl ligand. Assigned product stereochemistry was confirmed by X-ray structures of two 1,5,6-substituted cyclohexadienes. With complex 2, an intermediate acetyl enamine cyclohexadiene (11a) was isolated and structurally characterized. Its reaction with O 2 gave the corresponding 3,4,5-substituted phenol 12 (X-ray)

Sterically controlled double nucleophilic addition reactions of (η6-arene)(η6-[2.2]paracyclophane)ruthenium(II) complexes and reactions to form highly fluxional agostic cyclohexenyls

Action of the hydride source Na[BH4] on the (η6-arene)(η6-[2.2]paracyclophane)ruthenium(II) complexes [Ru(η6-C16H16)(η6-arene)][BF4]2(arene = benzene 1a, p-cymene 1b, 1,2,4,5-tetramethylbenzene 1c, pentamethylbenzene 1d or hexamethylbenzene 1e) results exclusively in the addition of two hydride nucleophiles to the non-cyclophane arene ring, giving the neutral (1,3-diene)ruthenium(0) complexes [Ru(η6-C16H16)(η4-diene)](diene = C6Me6H24, C6Me5H36, C6 Me4H47 or MeC6H6CHMe28). Complex 4 is the 1,3-diene isomer of the previously reported 1,4-diene compound [Ru(η6-C16H16)(η4-3,6-C6Me6H2)]2(formed in the reduction of 1e by Red-Al {Na[AIH2(OCH2CH2OMe)2]}). In complexes 1a–1e, attack on the [2.2]paracyclophane ligand is not observed, implying that nucleophilic additions to these compounds are not charge controlled. This contrasts with previously reported related reactions which give bis(cyclohexadienyl) complexes. Attempts to prepare functionalised diene complexes derived from 1a and 1e were largely unsuccessful although the functionalised cyclohexadienyl compounds [Ru(η6-C16H16)-(η5-C6R6X)][BF4]9–11(R = H or Me; X = Me or OMe) are prepared. Deprotonation of the hexamethylbenzene complex 1e results in the formation of the exo-methylene species [Ru(η6-C16H16){η5-C6Me5(CH2)}][BF4]12 and [Ru(η6-C16H16){η4-C6Me4(CH2)2}]13. Complexes 4, 6 and 7 react with HBF4 to generate agostic cyclohexenyl compounds [Ru(η6-C16H16)(η3-C6MenH9–n)][BF4](n= 6 16, 5 20 or 4 21). However, reaction of 2 with HBF4 gives the exocyclic agostic complex [Ru(η6-C16H16){η3-(HCH2)(CH2)C6Me4H4}][BF4]18. In 16 the agostic cyclohexenyl ligand is bound via an endocyclic allylic functionality whereas in the isomer, 18, the metal is bound externally to the ring. Deprotonation of 18 with LiBun results in the abstraction of the agostic proton to generate [Ru(η6-C16H16){η4-(CH2)2C6Me4H4}]19, which is isomeric with 2 and 4 and contains a saturated C6Me4H4(CH2)2 ring bound to the metal centre by two exocyclic olefinic functionalities. The mechanisms for the formation of these compounds have been probed by deuteriation studies and their extensive dynamic behaviour investigated by variable-temperature 1H NMR spectroscopy.

Synthesis of cationic indenyl- and fluorenyl-arene complexes of ruthenium

Interaction of [Ru(η-C 6 H 6 )Cl 2 ] 2 with indenyl- or fluorenyllithium in THF gives, together with cationic benzene complexes [Ru(η 5 -C 9 H 7 )(η-C 6 H 6 )]+ and [Ru(ν 5 -C 13 H 9 )(η-C 6 H 6 )]+, the neutral cyclohexadienyl derivatives Ru(η 5 -C 9 H 6 -C 9 H 7 ) and Ru(η 5 -C 13 H 9 )(η 5 -C 6 H 6 -C 13 H 9 ), respectively. Interaction of the cyclohexadienyl complexes with Al 2 O 3 , Ph 3 C+, and CF 3 CO 2 H has been studied. Reaction of Ru(η 5 -C 13 H 9 )(η 5 -C 6 H 7 ) with CF 3 CO 2 H in the presence of an arene yields cationic cyclohexadienylarene complexes: [Ru(η 5 -C 13 H 9 )(η 6 -arene)]+ (arene=C 6 H 6 or 1,3,5-Me 3 C 6 H 3 ).

Synthesis and reactions of acyl(cyclohexadienyl)manganates

The cyclohexadienyl complexes [Mn(η5-C6H6R1)(CO)3]1(R1=exo-H, Me or C6H4Me-4) react with LiR2(R2= Me or Ph) to give acylmetalates [Li(OEt2)][Mn(η5-C6H6R1)(CO)2{C(O)R2}]2. Complex 2(R1= Me, R2= Ph) has been characterised by an X-ray crystallography study: orthorhombic, space group P212121, a= 9.113(2), b= 14.491(8), c= 32.803(9)A, R= 0.064 for 1458 independent reflections. Protonation of 2 with HBF4·Et2O induces an aryl- or alkyl-group transfer to the endo face of the cyclohexadienyl ring yielding the new agostic cyclohexenyl complexes [Mn(η3:CHC6H7R1R2)(CO)3]3. The cyclohexenyl ligands can be decomplexed from the metal via treatment of 3 with 1,2-bis-(diphenylphosphino)ethane (dppe) in tetrahydrofuran to afford mixtures of substituted cyclohexa-1,3-dienes and [MnH(CO)3(dppe)]. The acylmetalates 2 react with [NO][BF4] at –78 °C to give [Mn(η5-C6H6R1)(CO)(NO){C(O)R2}] which decompose at room temperature to form the trans-disubstituted acylcyclohexadienes C6H6R1[C(O)R2]. Reaction of 2 with electrophiles SiMe3Cl, [Me3O][BF4] or [Et3O][BF4] results in O alkylation and formation of stable carbene complexes [Mn(η5-C6H6R1)(CO)2{C(OE)R2}](E = SiMe3, Me or Et).

Addition of Carbon Nucleophiles to Tricarbonylchromium Complexes of 1,2‐Dihydrocyclobutabenzene, Indane, 1,2,3,4‐Tetrahydronaphtbalene and ortho‐Xylene

Abstract3‐Substituted 1,2‐dihydrocyclobutabenzenes (bicyclo[4. 2. 0]octa‐1,3,5‐triene) are readily accessible from [Cr(CO)3(1,2‐dihydrocyclobutabenzene)] (1) via a two‐step sequence which involves addition of a nucleophile and oxidation of the intermediate anionic cyclohexadienyl complex. Nucleophiles used include LiCMe2CN (A), LiCH2CN (B), LiC(Me)(OR)CN (C), (D), (E), LiCMe2CO2Me (F), and LiCH2CO2(t‐Bu) (G). [Cr(CO)3(Indane)] (2) also reacts highly regioselectively to give α‐substitution, whereas [Cr(CO)3(tetrahydronaphthalene)] (3) and [Cr(CO)3(o‐xylene)] (4) give mixtures of products. In several cases, the mixtures of the intermediate anionic cyclohexadienyl complexes can be equilibrated to give, after oxidation, β‐substituted derivatives of 1,2,3,4‐tetrahydronaphthalene and ortho‐xylene selectively. EHMO calculations were carried out, and they rationalize the observed α‐regioselectivity of nucleophilic addition under kinetic control, The X‐ray structures of 1 and 4 are reported and in both compounds the Cr(CO)3 group adopts in the solid state a staggered syn‐conformation with respect to the substituted aromatic C‐atonis.

ChemInform Abstract: Aspects of Aromatic Iron Sandwiches: Application to Organic and Organometallic Synthesis

Reaction of nucleophile' with (Fe give substituted cyclohexadiene Fe' complexes which, on treatment with Ph3C+ produce substituted cyclohexadienyl complexes which can the? undergo reaction with a second nucleophile. Reaction of (F~CP(C~(CH~)~) 1 with

Single and Double Nucleophilic Addition to Coordinated Pi-Hydrocarbons: Manganese-Mediated Functionalization of Arenes

A range of carbon-donor nucleophiles add to the arene ring in (arene)Mn(CO)2L+ cations to give neutral cyclohexadienyl complexes that liberate monofunctionalized arenes upon oxidative removal of the metal. Treatment of the cyclohexadienyl complexes with the nitrosonium salt NOPF6 affords cationic metal nitrosyl complexes that are attacked by a second nucleophile to give cis- and trans-difunctionalized 1,3-cyclohexadienes. When the metal center is chiral, this procedure provides a route to enantiomerically pure cyclohexadienes. 1. Introduction 2. Mechanism of Nucleophilic Addition 3. Single Nucleophilic Addition 4. Double Nucleophilic Addition 5. Conclusions

Aspects of aromatic iron sandwiches: application to organic and organometallic synthesis

Abstract

The reactions of nitrosoarenes with cationic cyclohexadienyl complexes of iron tricarbonyl: an ESR study

The reactions of nitrosoarenes with the (cyclohexadienyl)Fe(CO)3 cation have been investigated by using electron spin resonance spectroscopy. The radicals produced are nitroxides of the type (OC)3Fe(C6H7)(Ar)N—O• but, in some cases, disproportionation and loss of the metal carbonyl fragment leads to the corresponding C6H5(Ar)N—O• radical. With bulky nitrosoarenes, such as C6Me5NO, isomers are observed in which the aryl ring rotation is slow on the ESR time scale. The analogous reactions with the cyclohexadienyl cation derived from the B ring of (ergosteryl acetate)Fe(CO)3 lead to initial attack not at one of the termini of the delocalized system but rather at the central carbon, i.e., at C-7. Subsequent hydrogen migration leads to the (5,7-diene)Fe(CO)3 complex bearing the arylnitroxide at the 7-position. The mechanisms of these reactions are discussed. Keywords: nitrosoarenes, iron cations, ESR.