Cyclohexadienones, complex

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. ... 

An electrocyclic ring closure then leads to a cyclohexadienone complex 7, which upon migration of a proton, yields the chromium tricarbonyl-hydroquinone complex 3. 

Upon substitution of the 4-position (114.b, R3 = Me), the cyclohexadi-enone complex 118 was also isolated from the product mixture obtained with dimethyl acetylenedicarboxylate. Only the cyclohexadienone complex... 

These results led to the proposal of the following mechanism. Decomplex-ation of the central C2 fragment allows coordination of the alkyne (intermediate 119), which then inserts to form the metallacycle 120. Deinsertion (reductive eliminate of the cobalt moiety allows ring closure to give the cyclohexadienone complex 121, which upon decomplexation yields the desired phenol. The regiochemistry of the alkyne insertion determines the ratio of 116 117 (for simplicity, only the sequence leading to 116 has been shown). 

Electrocyclization of E is expected to give cyclohexadienone complex F a related molybdenum complex 6 (Scheme 4), in which two carbonyl ligands have been replaced by alkyne ligands, has been isolated from the reaction of a vinyl molybdenum carbene complex with 3-... 

Complexes (160) (see equation 35) undergo conversion to phenols upon thermolysis. Alternatively, treatment with AICI3 under a CO atmosphere gives an j)" -iron cyclohexadienone complex thermolysis or treatment with CuCl2 affords a phenol as well. Complexes (162), derived from nucleophihc addition to ( ) -pentadienyl)Fe(CO)3+ cations (see equation 37), can be converted to vinyl cyclopropanes by using CAN (equation 41). ... 

The mechanism proposed by Ddtz involves the insertion of a carbon monoxide into the vinyl carbene complex intermediate with the formation of the vinyl ketene complex (255). Electrocyclic ring closure of (255) leads to the cyclohexadienone complex (252), which is related to the final tenzannulation product by a tautomerizadon when R is hydrogen. The mechanism proposed by Casey differs from that of Ddtz in that the order of the steps involving carbon monoxide insertion and cyclization to the aryl or alkenyl substiment is reversed. < Specifically, the vinyl carbene complex intermediate (248) first undergoes cyclization to the metallacyclohexadiene (249), followed by cartion monoxide insertion to give the intermediate (251), and finally reductive elimination to give cyclohexadienone intermediate (252). At this time the circumstantial evidence favors the intermediacy of vinyl ketene intermediates since they can be trapped from these reactions and isolated where the metal is dispaced from the vinyl ketene functionality however, there is not any evidence which can rule out the alternative mechanism. 

Electrocyclic ring closure affords the cyclohexadienone complex F which tautomerizes to give the naphthol complex G. A cyclohexadienone complex V has been isolated from the reaction of a molybdenum alkenylcarbene complex. [50]... 

Polymerization Mechanism. The mechanism that accounts for the experimental observations of oxidative coupling of 2,6-disubstituted phenols involves an initial formation of aryloxy radicals from oxidation of the phenol with the oxidized form of the copper—amine complex or other catalytic agent. The aryloxy radicals couple to form cyclohexadienones, which undergo enolization and redistribution steps (32). The initial steps of the polymerization scheme for 2,6-dimethylphenol are as in equation 6. 

Fig. 7-2. Potential energy E as a function of the reaction coordinate for reactions of the P-nitrogen of arenediazonium ions with nucleophiles yielding (Z)- and (is)-azo compounds, a) Reactant-like transition states (e. g., reaction with OH) b) product-like transition states (e. g., diazo coupling reaction with phenoxide ions product = cyclohexadienone-type o-complex (see Sec. 12.8).

Structural analogues of the /]4-vinylketene E were isolated by Wulff, Rudler and Moser [15]. The enaminoketene complex 11 was obtained from an intramolecular reaction of the chromium pentacarbonyl carbene complex 10. The silyl vinylketene 13 was isolated from the reaction of the methoxy(phenyl)-carbene chromium complex 1 and a silyl-substituted phenylacetylene 12, and -in contrast to alkene carbene complex 7 - gave the benzannulation product 14 after heating to 165 °C in acetonitrile (Scheme 6). The last step of the benzannulation reaction is the tautomerisation of the /]4-cyclohexadienone F to afford the phenol product G. The existence of such an intermediate and its capacity to undergo a subsequent step was validated by Wulff, who synthesised an... 

A plausible pathway is that the aromatisation of the cyclohexadienone 92 by a proton shift is accelerated in the presence of Ac20 under formation of acetate 93. The simultaneously generated acetic acid then cleaves the acetate to form the free phenol 94 (Scheme 44). This effect was observed for the first time during studies towards the total synthesis of the lipid-alternating and anti-atherosclerotic furochromone khellin 99 [64].The furanyl carbene chromium complex 96 was supposed to react with alkoxyalkyne 95 in a benzannulation reaction to give the densely substituted benzofuran derivative 97 (Scheme 45). Upon warming the reaction mixture in tetrahydrofuran to 65 °C the reaction was completed in 4 h, but only a dimerisation product could be isolated. This... 

For all QM complexes, the ring signals in both and 13C JH NMR spectra indicate the lack of aromaticity. The carbonyl group appears in the 13C JH NMR spectrum in the range of 18418-20116ppm, which is the region observed for 2,5-cyclohexadienones and quinones.38... 

Nucleophilic additions to (cyclohexadienone)Fe(CO)3 complexes (218) occur in a dia-stereospecific fashion (Scheme 56)197. For example, the Reformatsky reaction of ketone (218a) affords a simple diasteromeric alcohol product19715. The reduction of (1-carbo-methoxycyclohexa-l,3-dien-5-one)Fe(CO)3 (218b) to give 219 has been utilized in the enantioselective synthesis of methyl shikimate. In a similar fashion, cycloadditions of (2-methoxy-5-methylenecyclohexa-l,3-diene)Fe(CO)3 (220) occur in a diastereospecific fashion198. 

If the Ddtz benzannulation reaction is conducted with ori/zo-disubstituted aryl-carbene complexes, the final aromatization step is blocked and cyclohexadienones can be isolated (Figure 2.34) [356,378,379]. 

Fig. 2.34. Formation of cyclohexadienones from 2,2-disubstituted vinylcarbene complexes.

Complex 2,4-Cyclohexadienones. Usnic acid (Formula 116) is epimerized to Formula 117 on irradiation in dioxane (48). This reaction presumably proceeds through the diene-ketene (Formula 118). 

Some metals show such a pronounced tendency to adopt rj4-butadi-ene coordination that this may override the driving force to aromaticity. Selected complexes of d8 M(CO)3 (M = Fe, Ru) fragments illustrating this point are included in Figure 6.29. Phenol is far more stable than its keto tautomer (cyclohexadienone), which may be stabilized, however, by... 

Addition of alkynes to [2-(amino)alkenyl]carbene complexes ( )-96a does not afford cyclohexadienones (as might have been anticipated in case of a DOtz-type reaction),3 but gives cyclopentadienes 101 instead (Scheme 35) 3r,9a,9h jt js imp0rtant to note that the assembly of the cyclopentadiene... 

In recent years the group of C. A. Merlic has reported photochemically induced cyclizations of dienyl carbene complexes of type 39 to produce phenol derivatives 40 [19]. In these very intelligently designed reactions, which are related to the Dotz reaction, the primary, photochemically generated intermediates of type 41 undergo a (formal) electrocyclic ring-closure to form linear, conjugated cyclohexadienones 42, which then tautomerize to the phenols (Scheme 12). 

Regioselective reaction of phenols. Reaction of phenols with chloroform in aqueous alkaline solution catalyzed by P-cyclodextrin results in virtually complete attack at the para-position by dichlorocarbene to give, after hydrolysis, 4-hydroxybenzaldehydes. If the para-position is substituted, 4-(dichloromethyl)-2,5-cyclohexadienones are obtained as the major product. The selectivity results from formation of a ternary complex from P-cyclodextrin, chloroform, and the phenol. ... 

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