Cycloocta-1,3-diene, isomerization

Cyclooctadiene isomers (i.e., 1,5-cod or 1,3-cod) are selectively hydrogenated by [Ru(/74-cod)(/76-C8H1o)] (51) to produce exclusively cyclooctene in THF, under ambient temperature (20 °C) and 1 bar H2 pressure [64]. Again, cyclooctane is only detected when the diene substrate is completely transformed to the monoene. The rate of hydrogenation is higher in case of the conjugated 1,3-cycloocta-diene substrate, whereas isomerization of the non-conjugated 1,5-cyclooctadiene... 

So wurde gefunden, daB 1,5,9-Cyclododecatriene (67), 1,3-Cycloocta-dien (68) und Cyclotetradecadien-1,8 (69) bei der photosensibUisierten Anregung in Losung, cis- trans-Isomere ergeben. 

The complex has enjoyed relatively little use in organic synthesis. For iridium-catalyzed homogeneous hydrogenation of alkenes, Crabtree s iridium complex ((1,5-Cycloocta-diene)(tricyclohexylphosphine)(pyridine)iridium(I) Hexafluoro-phosphate) is generally preferred, although this readily prepared Ir complex is active. It is more reactive than its rhodium counterpart in the catalytic isomerization of butenyl- to allylsilanes. ... 

These results for 1,3-butadiene, 1,4-pentadiene and 1,5-hexadiene allow the courses of thermal isomerization in other cases to be predicted. Hydroboration of 1,5-cycloocta-diene yields quantitatively a 72 28 mixture of 9-borabicyclo[3.3.1]nonane (9-BBN) (IX) and 9-borabicyclo[4.2.1]nonane (X), both of which exist as dimers. In refluxing THF, the latter can be isomerized to 9-BBN within 1 h. ... 

Earlier examples of copper(I)-photocatalyzed cycloalkene cyclodimerizations have been summarized in Houben-Weyl, Vol. 4/5a, pp 280-292 as was the copper(I) chloride photocata-lyzed isomerization of cycloocta-1,5-diene to tricyclo[3.3.0.02 6]octane (see Houben-Weyl, Vol. 4/5 a, p 231). This same reaction has recently been used for the preparation of 4-oxatctra-cyclo[6.3.0.02,(,.07,1 L]undecanes.10... 

Ring closure to cyclobutanes via valence isomerization of cycloocta-l,3,5-trienes is also possible when carbonyl functions are present in the ethane fragment. Thus, reaction of cyclohepta-2,4,6-trienone (tropone) with diazopropane gives 8,8-dimethylcycloocta-2,4,6-trienone (16, R = H), which rearranges quantitatively to 8,8-dimethylbicyclo[4.2.0]octa-2,4-dien-7-one (17, R = H).67... 

Asymmetric isomerization of allylamines.1 The enantiomeric rhodium(I) complexes with the ligands cycloocta-1,5-diene (cod) and ( + )- or ( - )-binap, Rh ( + )-... 

Cycloocta-l,5-diens (Formel 41) in der Gasphase. Neben polymerem Material entstehen zu 2—3 % zwei isomere Substanzen, die als Bicyclo-[5,l,0]octen-(3) (Formel 42) und Tricyclo[3,3,0,02< ]octan (Formel 43) identifiziert werden konnten (280, 283). 

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... 

The rate of reaction of the other three dienes studied decreases with increasing ring size [27] in the case of cycloocta-1,3-diene, hydrovinylation is accompanied by isomerization or reaction with a second ethylene molecule, and the yield of 3-vinyl-cyclooct-l-ene never exceeds 50%. 

Further studies on the photoisomerization of ci5-cyclohexene and cycloocta-1,3-diene have been reported. Again the work has focused on enantiodiffer-entiation. In this case a series of optically active chiral sensitisers (3) have been used under conditions where solvent and temperature have been varied. Some of the o-disubstituted and tetra substituted amide sensitisers afford mixtures with enantioisomeric excesses of 14%. The influence of pressure and temperature on the asymmetric photochemistry of cyclooctene has been reported. A variety of chiral sensitisers were used. Some of these are shown in (4). Other work has shown that aromatic phosphates, phosphinates and phosphines (e.g. 5-8) can also sensitise the isomerism of cyclooctene. Moderate stationary-state ratios were obtained. 

When phosphane-free nickel complexes, such as bis(cycloocta-l,5-diene)nickel(0) or te-tracarbonylnickel, are employed in the codimerization reaction of acrylic esters, the codimer arising from [2-1-1] addition to the electron-deficient double bond is the main product. The exo-isomer is the only product in these cyclopropanation reactions. This is opposite to the carbene and carbenoid addition reactions to alkenes catalyzed by copper complexes (see previous section) where the thermodynamically less favored e Jo-isomers are formed. This finding indicates that the reaction proceeds via organonickel intermediates rather than carbenoids or carbenes. The introduction of alkyl substituents in the /I-position of the electron-deficient alkenes favors isomerization and/or homo-cyclodimerization of the cyclopropenes. Thus, with methyl crotonate and 3,3-diphenylcyclopropene only 16% of the corresponding ethenylcyc-lopropane was obtained. Methyl 3,3-dimethylacrylate does not react at all with 3,3-dimethyl-cyclopropene, so that the methylester of tra 5-chrysanthemic acid cannot be prepared in this way. This reactivity pattern can be rationalized in terms of a different tendency of the alkenes to coordinate to nickel(O). This tendency decreases in the order un-, mono- < di-< tri- < tet-... 

Cyclohexenes, " cycloheptene and cyclooctene" gave mixtures of endo- and exoisomers their ratio was strongly dependent on the reaction conditions. Cycloocta-1,5-diene by the photolytic method afforded the monoadduct 1 (major component), two isomeric bis-adducts 2 and 3 and rearranged product 4. ... 

When a mixture of benzene and perfluorobutyne was irradiated in a Vycor tube with 253.7 nm light, slow formation of at least eight compounds was detected by gas chromatography. The major compound was l,2-bis(trifluoromethyl)cycloocta-l,3,5,7-tetraene (10, 40%), accompanied by three isomeric bis(trifluoromethyl)tricyclo[3.3.0.0 ]octa-3,6-dienes (semibullvalenes) 11-13 in 25, 12 and 5% yield, respectively. ... 

When (2,2 -bipyridine)(cycloocta-l,5-diene)nickel was used as the catalyst various isomeric (2,2 -bipyridyl)nickelaspirocycloalkanes 4 and 5a-c were isolated.As expected, treatment of the dispiro complex 4 with methyl acrylate or maleic anhydride released dispiro[2.1.2.1]oc-tane (1) whereas the complexes 5 with one cyclopropane ring opened gave mainly 5-methyl-enespiroheptane 2. The formation of the 4-methylene isomer (from 5c) has not been observed in the dimerization reaction with other nickel(O) complexes. A few more nickel(O) complexes with an ability to catalyze the oligomerization of methylenecyclopropane have been de-scribed. ... 

Only a limited number of vinyl sulfones, e.g. phenyl ( )-2-phenylvinyl sulfone (14), undergo codimerization with MCR Homocyclodimerization of MCP is the most efficient side reaetion. Interestingly, yields and product distributions are solvent dependent. No reaction takes place with catalytic amounts of bis(t -cycloocta-l,5-diene)nickel(0)/triphenylphosphane. In this case the vinyl sulfones are strongly coordinated to the catalyst metal, thus preventing interaction with MCP. When the sulfones bear alkyl-substituted vinyl groups, isomerization to yield allyl sulfones usually proceeds faster than cycloaddition, at least in the case of palladium(O) catalysis. 

The reaction of substituted methyienecyciopropanes, such as (1-methylethylidene)- or (diphenylmethylene)cyclopropane, with compound 2 only leads to the formation of [3 -I- 2] cycloadducts. The best combined yield of three isomeric cyclocodimers (93%) is obtained with (1-methylethylidene)cyclopropane in a bis(> -cycloocta-l,5-diene)nickel(0)/tris(2-phenylphenyl) phosphite (1 1) catalyzed reaction at 120°C after 8 hours. ... 

Bicyclo[6.1.0]nona-1,6-diene (16) isomerized in dimethylformamide to 3-methylene-cycloocta-1,4-diene (17), a reaction which must involve a hydrogen migration. 

The first example of a bicyclic variant of the 1 - 3 isomerization involved bicyclo[5.1.0]oct-2-ene (10) which, on heating in the gas phase, rearranged to cycloocta-1,4-diene (11). The process has been studied between 180 and 300 °C and is reversible. Its kinetic data ( = 38.6 kcal moPlog A = 13.36 for the 10 - 11 isomerization and = 38.6 kcal mol, log A = 11.80 for the reverse reaction) as well as experiments with deuterated derivatives clearly indicate its concerted nature, the 1,5-hydrogen shift occurring in a transannular fashion. Above 325 °C the 10 11 equilibrium is irreversibly shifted towards the formation of cw-bicyclo[3.3.0]oct-2-... 

The metathesis of propene on WOs/Si02 is speeded up by pretreatment of the catalyst with HCl (Pennella 1974 Aliev 1977) but the product but-2-ene undergoes considerable isomerization to but-l-ene (Aliev 1978). The inclusion of 1% cycloocta-1,5-diene (COD) in the propene stream also increases the rate of metathesis and reduces the break-in time from 20 min to less than 5 min at 500°C. The latter effect disappears when the additive is removed, so it is not due to reduction of the catalyst. The effects of both HCl and COD have been attributed to a favourable modification in the metal d-orbital levels as the result of the presence of new ligands (Pennella 1973, 1974). Pretreatment with hexamethyldisilazane (HMDS) at 250°C also has a remarkable effect on the activity, increasing it as much as 140 times for the metathesis of propene at 427°C. The same treatment of silica alone completely eliminates its capacity to isomerize rra/j5-but-2-ene at 427°C, so it is concluded that the Bronsted acidic hydroxyl groups, poisoned by HMDS, are not likely to be the precursors for the active sites in propene metathesis over W03/Si02 (van Roosmalen 1980a, 1982). 

In order to eliminate the possibility for in situ carbene formation Raubenheimer et al. synthesized l-alkyl-2,3-dimethylimidazolium triflate ionic liquids and applied these as solvents in the rhodium catalyzed hydroformylation of l-hejEne and 1-dodecene [178]. Both, the classical Wilkinson type complex [RhCl(TPP)3] and the chiral, stereochemically pure complex (—)-(j7 -cycloocta-l,5-diene)-(2-menthyl-4,7-dimethylindenyl)rhodium(i) were applied. The Wilkinson catalyst showed low selectivity towards n-aldehydes whereas the chiral catalyst formed branched aldehydes predominantly. Hydrogenation was significant with up to 44% alkanes being formed and also a significant activity for olefin isomerization was observed. Additionally, hydroformylation was found to be slower in the ionic liquid than in toluene. Some of the findings were attributed by the authors to the lower gas solubility in the ionic liquid and the slower diffusion of the reactive gases H2 and CO into the ionic medium. 

Recently, the nickel-catalyzed isomerization of geraniol and prenol has been investigated in homogeneous and two-phase systems. The best results with respect to activity and selectivity have been obtained in homogeneous systems with a bis(cycloocta-l,5-diene) nickel(0)/l,4-bis(diphenylphosphanyl)butane/trilluoroacetic acid combination. Catalyst deactivation occurs in the course of the reaction owing to coordination of the aldehyde group formed to the nickel species or as a result of protonolysis of hydrido- or (7t-allyl)nickel complexes [1]. 

Cycloocta-1,5-diene is quantitatively isomerized to the 1,3-isomer on heating with Fe(CO)5 (, and it is possible that an unstable yellow oil obtained by reaction between Fe(CO)5 or Fe3(CO)i2 and the 1,5-diene, and formulated as w-l,5-CgHi2Fe(CO)3 (94, 114), may in fact be 7t-1,3-CgH 2Fe(CO)3. As pointed out by Pettit and Emerson (118), the instability of the complex could be due to the ring strain involved in making the double bonds of the 1,3-diene coplanar. 

Addition of dicarboethoxycarbene to cycloocta-1,3-diene yields a mixture of cis-and fran -cyclopropane adducts, probably by addition of the singlet carbene on the partially isomerized diene (due to the irradiation). Diastereoselective cyclopropan-ation of a ,/3-unsaturated acetals has been described using a camphor-derived chiral auxiliary. Intramolecular cyclopropenation of a diazo ester, tethered through a naphthalene, to an alkyne was catalysed by rhodium acetate and reported as a efficient method unfortunately, the use of chiral rhodium catalysts gave a less efficient reaction and did not provide high asymmetric induction. ... 

In metal complexes the ligands may occupy different positions around the central atom. Since the ligands in question are usually either next to one another (cis) or opposite each other (trans), this type of isomerism is often referred to as cis-trans isomerism. Cis-trans isomerism is very common for square planar and octahedral complexes. Consider the square planar complexes shown in structures [5-1H5-4]. Cis-trans isomerism arises from the relative position of the ethylene ligands. Therefore, [5-1] and [5-3] are cis forms and [5-2] and [5-4] are trans forms, respectively. Norbornadiene-palladium dichloride [5-5] and rhodium (Ti-cycloocta-1,5-diene) chloride [5-6] can exist only in the cis structure because of the chelate nature of the diene. 

---