Dienyl complexes preparation

Although tricarbonylbutadieneiron (1) was prepared by Reihlen et a/.1 in 1930, some considerable time passed before the corresponding cyclohexadiene complex (2 equation 1) was reported.2 Fischer and Fischer described the conversion of (2) to the cationic cyclohexadienyliron complex (3 equation 1) by reaction with triphenylmethyl tetrafluoroborate in dichloromethane.3 This particular complex is extremely easy to prepare and isolate as the hydride abstraction reaction proceeds the product (3) crystallizes out. Precipitation is completed by pouring the reaction mixture into wet diethyl ether, the small amount of water present serving to destroy any excess triphenylmethyl tetrafluoroborate by conversion to triphe-nylmethanol. Filtration, followed by washing the residue with ether, gives pure dienyl complex. 

A method for preparing specific alkyl-substituted dienyl complexes takes advantage of the propensity of (diene)Fe(CO)3 complexes to rearrange under acidic conditions, coupled with the acid-promoted dehydration illustrated earlier for the conversion of (9) to (8). Birch and Haas14 discovered that complexes derived from methylanisoles could be converted to methyl-substituted cyclohexadienyliron complexes, whose substitution pattern is defined by the relative positions of the methoxy and methyl substituents in the precursor. Several examples are given in Schemes 5 and 6 and equation (17). 

Related to this reaction is the acid treatment of the (l-hydFoxymethylcyclohexadiene)tricarbonyliron (45), prepared via DIBAL-H reduction of ester (26) or borane reduction of the corresponding carboxylic acid, which leads to the 1-methylcyclohexadienyliron complex (46 equation 18).11 Using these methods, it is therefore possible to prepare a range of alkyl-substituted dienyl complexes having a defined substitution pattern. 

Although >/5-pentadienyl complexes have been known for a long time, the first examples being [Fe(C0)3(f/5-hexadienyl)][C104] 183) or [Cr( js-pentadienyl)2] and [Cr(//5-pentadienyl)2(CO)] (184), increasing interest in these ligands has arisen in the last few years. Homoleptic and mixed f/5-dienyl complexes of Ti (185), V, Cr (186), Mo (187), Mn (188-190), and Fe (191) have been prepared by metathetical reactions. Photochemistry provides an easy access to Mn derivatives. Quite recent is the development of syntheses for >/3-dienyl complexes. Examples are known for Mn (192,193), Fe (795), Co (194), and Ni (195,196). 

The (-l-)-neomenthylcyclopentadienyllanthanide complexes Cp LnX2(THF)3 [X = Cl, Ln = Sm, Gd, Yb, Y, Lu X = I, Ln = Sm, Yb Cp = (-l-)-neomenthylcyclopenta-dienyl] were prepared by reaction of LnCls with appropriate alkali metal cyclopentadienyl derivatives. In the solid state, the monomeric Cp Sml2(THF)3 adopts a pseudo-octahedral geometry with the two iodine atoms taking trans positions. A similar structure is found for (Ind)GdCl2(THF)3. 

Table 2 summarizes the mixed arene-arene and arene-ligand complexes prepared. w-Arene complexes of polymers are also listed. Iron and Ru with benzene or toluene yield t-arene complexes that can be isolated at low temperature. The (CgHg)2Ru species is stable to 0°C, whereas the CgHjCHj-Fe analog is stable to — 50°C. Thus, these compounds can be isolated for use as intermediates in other reactions, e.g., the addition of cyclopentadiene to the (benzene) Fe complex, where on warming hydrogen transfers to benzene to yield ( -cyclopentadienyl) ( -cyclohexa-dienyl)Fe ... 

Elimination of the proton from 1, 4-diene or 1, 3-diene compounds is often used for the preparation of metal dienyl complexes. The following compounds are utilized as deprotonating reagents alkyllithium compounds, (trimethylsilylmethyl )potassium, alkali metal amides, etc. Also useful are alkali metals as sands in the presence of triethylamine, which prevents anionic polymerization of dienes. 

Spirocyclic compounds have been prepared by making use of the regioselectiv-ity of these reactions. Dienyl complexes were constructed which contained latent nucleophiles in the side chain which were released on treatment with a mild base. Similar reactions have also been used to introduce angular substituents into bicyclic systems. 

Nucleophilic attack by bicarbonate onto a dienyl complex 10.118 with an ester substituent also proceeded regioselectively. Starting from the resolved complex, methyl shikimate 10.122 could be prepared in optically active form using this reaction (Scheme 10.30). Protection of the alcohol and decomplexation gave the free diene 10.121, which underwent selective dihydroxylation trans to the TBS ether. Desilylation gave the natural product ester 10.122. 

Compound 95 has been shown to be a useful precursor for the synthesis of a range of open and closed dienyl complexes, in particular, the preparation of open and closed ruthenocene. A series of 1,5-, 2,4- and 3-substituted pentadienyl complexes of ruthenium have been prepared and characterised. A series of heteroatom-containing diene complexes of type 96 have been synthesised. 1,2-Diketones were shown, however, not to coordinate in this fashion, rather to undergo a one-electron transfer reaction giving enediolato-containing complexes. Complexes of type 97 have been synthesised on treatment of the triple decker complex [ (CpR)Co 2(p-r r -C6F[5Me)] with acetone, cyclohexanone, and acetophenone. The mechanism of formation was said to proceed through a... 

Other Reactions. Trityl fluoroborate has been used often to prepare cationic organometallic complexes, as in the conversion of dienyl complexes of iron, ruthenium, and osmium into their cationic derivatives. It alkylates pyridines on the nitrogen atom in a preparation of dihydropyridines and acts as a tritylating agent. It has also been used in attempts to form silyl cations and silyl fluorides from silanes. Finally, it has been reported to be a usefiil desiccant. ... 

Iron pentacarbonyl and l-methoxy-l,4-cyclohexadiene react as shown by Birch and oo-workera, but in dibutyl ether this solvent has been found superior. The tricarbonyl(methoxy-l,3-cyclohexadiene)iron isomers undergo hydride abstraction with triphenylmethyl tetrafluoro-borate to form the dienyl salt mixture of which the 1-methoxy isomer is hydrolyzed by water to the cyclohexadienone complex. The 2-methoxy isomer can be recovered by precipitation as the hexafluoro-phosphate salt. By this method the 3-methyl-substituted dienone complex has also been prepared from l-methoxy-3-methylbenzene. The use of the conjugated 1-methoxy-1,3-cyclohexadiene in Part B led to no increase in yield or rate and resulted chiefly in another product of higher molecular weight. An alternative procedure for the dienone is to react tricarbonyl(l,4-dimethoxycyclohexadiene)iron with sulfuric acid. ... 

Attempts to prepare the mono(cyclopenta-dienyl) derivatives are sometimes frustrated by a Schlenk-type equilibrium (see p. 132), but judicious choice of ligands, solvent etc. occasionally permits the isolation of such compounds, e.g. the centrosymmetric halogen-bridged dimer (t - -C5Me5)Ca(/i-l)(lhf)2 2] which cry.stallizes from toluene solution. The complex is isostruc-tural with the dimeric organosamarium(ll) analogue. - ... 

An alternative route for stabilization of quinone methides by metal coordination involves deprotonation of a ri5-coordinated oxo-dienyl ligand. This approach was introduced by Amouri and coworkers, who showed that treatment of the [Cp Ir(oxo-ri5-dienyl)]+ B1, 22 with a base (i-BuOK was the most effective) resulted in formation of stable Cp Ir(r 4-o-QM) complexes 23 (Scheme 3.14).25 Using the same approach, a series of r 4-o-QM complexes of rhodium was prepared (Scheme 3.14)26 Structural data of these complexes and a comparison of their reactivity indicated that the o-QM ligand is more stabilized by iridium than by rhodium. 

Both dienyl- and 7r-allylmetal complexes of Fe,492 Mo,493,494 and W374 have also been prepared and undergone similar reactions with oxygen nucleophiles. One pertinent example is the reaction of 7r-allylmolybdenum complexes with internal alcohols leading to THFs (Equation (128)) 493 additional examples of this chemistry have appeared in a... 

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