Cyclohexa-1,4-diene complexes

A new cyclohexa-1,4-diene complex of Pd(I) has also been recently found in the autoxidation of this diolefin 48). 

Because the reactions of related in -cyclohexadienyl complexes are synthetically valuable, the reactions of this ligand have been studied extensively. An outline of how this chemistry can be conducted on the Fe(CO)j fragment is shown in Equation 11.51. A variety of cyclohexadienes are readily available from Birch reduction of substituted aromatics. Coordination and abstraction of a hydride, typically by trityl cation, leads to cationic cyclohexadienyl complexes. These cyclohexadienyl complexes are reactive toward organolithium, -copper, -cadmium, and -zinc reagents, ketone enolates, nitroal-kyl anions, amines, phthalimide, and even nucleophilic aromatic compounds such as indole and trimethoxybenzene. Attack occurs exclusively from the face opposite the metal, and exclusively at a terminal position of the dienyl system. This combination of hydride abstraction and nucleophilic addition has been repeated to generate cyclohexa-diene complexes containing two cis vicinal substituents. The free cyclohexadiene is ttien released from the metal by oxidation with amine oxides. ... 

Caution may be required in oxidation reactions. Ceric salts, for example, can alter or destroy products. Ferric salts seem to be more active in 1 N hydrochloric acid solution (P. Schudel, personal communication), but the acidic conditions may affect products such as enol ethers. Temperatures also seem to be important in some cases. For example, a substituted cyclohexa-diene complex was unaffected by CuClj at —20°, but it was converted into the free conjugated diene (mainly) at 0° and into the unconjugated diene and substituted benzene derivative at +20° (A. J. Birch and C. Sell, unpublished). In other cases, e.g., arylmanganese derivatives, although interesting synthetic reactions occur, the metal is not so readily removed. This general area is clearly one for further research if useful application is to be made in organic synthesis. 

However, the usual course of events in the isomerization of cycloalkadienes is to achieve conjugation where coordination is impossible. Thus cyclohexa-1,4-diene is isomerized to cyclohexa-1,3-diene by [RuC PPhsjs]. Nevertheless, RhCb 3H2O isomerizes cycloocta-1,3-diene, the most stable isomer, to a chelated cycloocta-1,5-diene complex (equation 6). Since no intermediate cycloocta-1,4-diene can be observed in the reaction, it was presumed to proceed via an... 

Spectroscopic investigation (227) of the cyclooctatetraene complex (CgHg)PdCl2 indicates that all double bonds in the molecule are coordinated, but the structure of the complex has not been determined. When suspended in methanol, this complex is rapidly solvolyzed to produce (1, - dichlorobis(2 - methoxy - 3,5,7 - cyclooctatrienyl)dipalladium(II) (501). Similar complexes are formed by the reaction of 1,3-cyclohexa-diene, 1,3-cycloheptadiene, or 1,3- or 1,5-cyclooctadiene with Na2PdCl4 in methanol at room temperature. The last species reacts with HCl to give (l,5-cyclooctadiene)dichloropalladium(II) (501). Pyrolysis of the complex [(CgHi20CHa)PdCl]2 obtained from 1,5-cyclooctadiene yields 1-methoxy- and 2-methoxy-l,3-cyclooctadiene (535). 

The benzene cyclohexa-l,3-diene complexes (CgHg)(l,3-CgHg)M (M = Fe, Ru, Os) have been prepared by reacting the metal trichlorides with i-PrMgBr in the presence of 1,3-CgHa 143, 144). The iron and... 

Poly-jp- Phenylenediamine) - (2,3,5,6-Tetrachloro-p-benzoquinone)) Complex (Poly-(p- Phenylenediamine) - (2,5- Cyclohexa--diene Dimalononitrile)) Complex (Poly-(p- Phenylenediamine) - Ethenetetracarbonitrile) Complex... 

The protonation of the Ti -cyclohexa-1,3-diene complex in Equation 12.64 illustrates a mechanism for the reaction of an electrophile at a coordinated diene via initial protonation of the metal and shows the stereochemistry that results from this pathway. The initially formed cationic hydride complex transfers the hydride onto the endo face of the cyclo-hexadiene ring by insertion of the C=C double bond into the Rh-H bond. The resulting Rh-C single bond is then part of the ligand in the product of the migration. Migra-... 

The interpretation of the early results is not obvious and in his review of 1980 (Ref. [4], pp. 84-103), Henry favored the 1,2-addition mechanism for all substrates. With the formation of the trany-acetoxy ir-allyl complex from acetoxylation of 1,4-cyclohexa-diene, clear evidence for the 1,2-addition process was later obtained (Scheme 14). 

The uniqueness of this reaction is at least partially attributable to the unusual nature of the tr-bonded fluorocarbon-iron carbonyl complex used as the starting material. Fluorocarbons such as octafluoro-l,3-cyclohexa-diene containing more than one double bond are relatively rare. 

Protonation of neutral diene complexes [Fe( -diene)(CO)3] (diene = butadiene or cyclohexa-1,3-diene) gives hydrido-complexes containing (T, 7r-bonded ligands. The cyclohexadiene cation (33) undergoes the interconversion of Scheme 11 with AG (- 50 O 11 kcal moH. 

Reaction of (2-naphthyl)Fe(H)(dmpe)2 with ethylene and other olefins yields (dmpe)aFe(olefin) complexes, which exhibit varying degrees of fluxional behaviour. Reaction with butadiene yields the analogous (jj -butadiene)Fe(dmpe)a complex, but use of cyclohexa-1,3-diene results in dechelation of one phosphorus atom to yield ( -cyclohexadiene)Fe(dmpe)a. Anthracene also forms an unusual analogous i7 -diene complex, while diphenylacetylene reacts to give ( / -PhaCa)-Fe(dmpe)2. The complexes (olefin)Fe(CO)i of cyclo-octene and cyclononene... 

Reports on the synthesis of diene complexes using Fe2(CO)9 are more common. Reaction of 2-phenylsulfonyl-1,3-cyclohexadiene with 2equiv. of Fe2(GO)9 in refluxing ether formed the [l-(phenylsulfonyl)-l,3-cyclohexa-diene]iron(0)tricarbonyl isomer 26. The reaction is catalyzed by 1-aza-1,3-butadiene. In a separate study, 1-aza-1,3-butadienes were shown to effect a quantitative catalytic complexation of cyclohexadienes with Fe2(GO)9. Activities are greatly enhanced in the presence of aryl rings bonded to nitrogen. 

Isomeric clusters 16 and 17 react further with cyclohexa-1,3-diene to give benzene-diene complexes Rus(/is-C)-... 

Cobalt and Rhodium.— Reduction of [( -C Me2)2Rh] + with Zn-HCl affords [( -C6Mee)Rh( 7 -C6H2Me6)]+ and not [( -C6Me6)2Rh]+ as previously claimed the crystal structures of the [PFg]" salts of this ( -cyclohexa-l,3-diene) complex and of the 20-electron sandwich complex [( y -CeMe6)2Co]+ have been determined. ... 

The catalytic complexation of cyclohexa-1,3-diene 2b is much superior to the classical procedures by direct reaction of the diene with pentacarbonyliron... 

The complexation of 2b in the presence of 12.5 mol% of the azadiene catalyst 4 provides quantitatively tricarbonyl[r] -cyclohexa-l,3-diene]iron 5... 

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