Cyclohexadiene complexes with cobalt

Cycloheptene complexes with gold, 12 348 Cyclohexadiene complexes with cobalt, 12 286 with group VIB metals, 12 225-227 with group VIIB metals, 12 240, 241 with iron, 12 264 with palladium, 12 313 with ruthenium, 12 278 with silver, 12 340... 

Ring-member substitution, a very characteristic reaction of some 18-e borabenzene complexes (see Section VII,B), can also occur with 1,4-dibora-2,5-cyclohexadiene complexes. The cobalt complex 53 cleanly reacts with MeCOCl/ A1C13 to give the cation 54 (Scheme 7) (75). The Rh complex (C5Me5)Rh[MeB(CHCH)2BMe] reacts analogously (75). 

Strained cycloalkynes can be stabilized by coordination to one or more transition metal centers (198). The unusual vicinal defluorination reaction of perfluoro-l,3-cyclohexadiene with [Co2(CO)8] to give the /i-alkyne complex 45 (see Section III,E) prompted a study of the reactions of OFCOT with cobalt carbonyl precursors. 

A carbon-carbon double bond of this intermediate then reacts with two alkyne units via a cobalt-catalyzed [2 + 2 + 2] process leading to a cobalt-1,3-cyclohexadiene complex which undergoes subsequent reductive elimination (Scheme 14). The regioselective outcome of this reaction was explained in terms of steric congestion. 

Herberich, G.E., Raabe, E. Syntheses with l-phenylbora-2,5-cyclohexadiene. Complexes of Iron, Cobalt and Nickel with Partially Hydrogenated Borabenzene Ligands . J. Organomet. Chem. 1986, 309, 143-156. 

A mild aerobic palladium-catalyzed 1,4-diacetoxylation of conjugated dienes has been developed and is based on a multistep electron transfer46. The hydroquinone produced in each cycle of the palladium-catalyzed oxidation is reoxidized by air or molecular oxygen. The latter reoxidation requires a metal macrocycle as catalyst. In the aerobic process there are no side products formed except water, and the stoichiometry of the reaction is given in equation 19. Thus 1,3-cyclohexadiene is oxidized by molecular oxygen to diacetate 39 with the aid of the triple catalytic system Pd(II)—BQ—MLm where MLm is a metal macrocyclic complex such as cobalt tetraphenylporphyrin (Co(TPP)), cobalt salophen (Co(Salophen) or iron phthalocyanine (Fe(Pc)). The principle of this biomimetic aerobic oxidation is outlined in Scheme 8. 

Schiff base-cobalt-nitro complexes are too mild as oxidants to react as such with alkenes. However, the addition of Lewis acids (e.g. BF3 Et20, LiPF6) to these complexes activates the nitro ligand and produces a variety of both stoichiometric and catalytic oxidations. Stoichiometric transformations involve the oxidation of sulfides to sulfoxides and 1,3-cyclohexadiene to benzene.467 Alcohols such as benzyl alcohol and cycloheptanol are catalytically transformed into the corresponding carbonyl compounds.467,474... 

The hydroformylation of conjugated dienes with unmodified cobalt catalysts is slow, since the insertion reaction of the diene generates an tj3-cobalt complex by hydride addition at a terminal carbon (equation 10).5 The stable -cobalt complex does not undergo facile CO insertion. Low yields of a mixture of n- and iso-valeraldehyde are obtained. The use of phosphine-modified rhodium catalysts gives a complex mixture of Cs monoaldehydes (58%) and C6 dialdehydes (42%). A mixture of mono- and di-aldehydes are also obtained from 1,3- and 1,4-cyclohexadienes with a modified rhodium catalyst (equation ll).29 The 3-cyclohexenecarbaldehyde, an intermediate in the hydrocarbonylation of both 1,3- and 1,4-cyclo-hexadiene, is converted in 73% yield, to the same mixture of dialdehydes (cis.trans = 35 65) as is produced from either diene. 

Upon extended heating with stoichiometric CpCo(CO)2 all-intramolecular cycloadditions of 1-ene-6,12-diynes to form cobalt-complexed cyclohexadienes take place in good yields (Scheme 23). - High diastereoselectivity in the formation of the complex is sometimes observed in these processes, as is considerable tolerance for steric hindrance. - ... 

In addition, hydrogen transfers occur in the 1,3-cyclohexadiene system to yield the 7t-arene-Fe complexes, (tj -C6Hg)Fe [P(OCH3)3]2 and (t -C6H6)Fe(f " -l,3-C6H8). Cobalt atoms with butadiene, in the presence of a hydrogen donor, yield a (diene)2-Co-H... 

The reaction of the iron-carbene complex [[Bu C CC(OEt)=)Fe(CO)4] with 1,3-cyclohexadiene afforded223 (32), which was characterised by X-ray diffraction. The addition of the anionic derivatives of i-(diphenylmethane)-il-fluorene- and ii-(9,10-dihydroanthracene)-bis tricarbonylchromium to the hydrocarbon moiety (alkene, benzene, cyclohexadienyl, cycloheptadienyl and cycloheptatrienyl) in various cationic compounds of manganese, rhenium, iron, chromium, molybdenum, tungsten and cobalt was reported224 to provide a new route to hydrocarbon-bridged heteronuclear species such as (33). 

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