Cyclodimerization of dienes

Catalytic cyclodimerization of dienes can also be performed selectively. 1,5-Cyclooctadiene, dimethylcydooctadienes, and 6-methyl-2,4,7-nonatriene can be obtained from butadiene, isoprene, and 1,3-pentadiene, respectively, upon treatment with a catalytic amount of (C5Me5)RuCl(diene) and AgOTf... 

Another important class of reactions is the cyclodimerization of dienes. Thus, cycloocta- 1,5-diene is accessible from buta-1,3-diene... 

Stereoselective 4 + 2 Cycloaddition Type Cyclizations in Cyclodimerizations of Dienes with Low-Valent Transition Metal Catalysts... 

Reports in the older literature suggested the existence of [Fe(NO)4]. However, this is likely to be a dimeric compound containing a bridging hyponitrite ligand (1). One electron reduction of the complex [Fe(NO)2Cl]2, chemically or electrochemically, leads to a species which acts as a catalyst for the cyclodimerization of dienes. This has been described as Fe(NO)2 , which probably exists in THF solution as [Fe(NO)2(THF) ]. Aqueous solutions of ferrous salts absorb nitric oxide to give the familiar colour of the brown ring test , which is due to the complex ion [Fe(H20)5N0] ". ... 

The use of zeolites is particularly advantageous for self-Diels-Alder reactions of gaseous dienes because it reduces the polymerization of the reactant. An example is the cyclodimerization of 1,3-butadiene to 4-vinylcyclohexene [20a] carried out at 250 °C with satisfactory conversion when non-acidic zeolites, such as large-pore zeolites Na-ZSM-20, Na- S and Na-Y, are used. 

The sterically hindered base 2,6-bis(tert-butyl)pyridine does not inhibit cyclization triaryl-amine retards this reaction photosensibilized one-electron oxidation of a diene leads to the same products, which are formed in the presence of ammoniumyl salt. As shown, in majority of cases, only the cation-radical chain mechanism of the diene-diene cyclization is feasible (Bauld et al. 1987). Meanwhile, cyclodimerizations of 2,4-dimethylpenta-l,3-diene (Gassman and Singleton 1984) and l,4-dimethylcyclohexa-l,3- or -1,4-diene (Davies et al. 1985) proceed through both mechanisms. 

For synthetic purposes, the conversion of diallyl ether (11, X = O) to 3-oxabicy-clo[3.2.0]heptane (12, X = O),11 of ethyl JV,7V-diallylcarbamate (11, X = NC02Et) to A7-car-boethoxy-3-azabicyclo[3.2.0]heptane (12, X = NC()2Et)12 and of 4-hydroxyhepta-l,6-diene (13) to 3-hydroxybicyclo[3.2.0]heptane (14)13 have become the most useful copper(I)-photocat-alyzed (intramolecular) cyclodimerizations of alkenes. 

Similarly, the complex 3 was formed from 3,3-dimethylcyclopropene and 2,2 -bipyridyl( 4-cy-cloocta-l,5-diene)nickel, which on treatment with maleic anhydride at 25 °C gave anti-3,3,6,6-tetramethyltricyclo[3.1.0.02,4]hexane (4) in >90% yield. Displacement of the hydrocarbon ligand from 3 with 3,3-dimethylcyclopropene proceeds at > 90"C. Since the complex 3 is regenerated in this step, 3 is the catalyst in the cyclodimerization of the cyclopropene.120... 

Cyclodimerization of isoprene to 1,5-dimethylcycloocta-l,5-diene and disproportion with a rhenium oxide catalyst and isobutene produce 2,6-dimethylhepta-l,5-diene. The diene is hydroformylated to citronellal, which after hydrogenation produces citronellol (137). 

For example, the dinitrosyliron(O) complex can be formed cathodically. This complex is able to catalyze the cyclodimerization of conjugated dienes (Scheme 8) 259,260) in case of 1,3-butadiene, 20000 turnovers are obtained per hour with... 

Scheme 8. Electro-generated dinitrosyliron(O) complex as catalyst for the cyclodimerization of conjugated dienes...

Homogeneous nickel complexes proved to be versatile catalysts in dimerization and trimerization of dienes to yield different oligomeric products.46-55 Depending on the actual catalyst structure, nickel catalyzes the dimerization of 1,3-butadiene to yield isomeric octatrienes, and the cyclodimerization and cyclotrimerization to give 1,5-cyclooctadiene and all-trans-l,5,9-cyclododecatriene, respectively46 56 [Eq. (13.13)]. Ziegler-type complexes may be used to form cis,trans,trans-1,5,9-cyclododecatriene37,57 58 [Eq. (13.14)], which is an industrial intermediate ... 

By using the hypersensitive molecular mechanistic probe 2-(2-methoxy-3-phenylcy-clopropyl)-5-methylhexa-2,4-diene in the 2 + 2-photocycloaddition of [60]fullerene, it was shown that the reaction proceeds via a biradical and not a dipolar intermediate.6 Zirconium-induced cyclodimerization of heteroaryl-substituted alkynes produces tetrasubstituted cyclobutenes with high regio- and stereo-selectivity.7 The ruthenium-... 

Catalyzed oligomerization and co-oligomerization of conjugated dienes have been performed with a wide range of transition-metal complexes. Catalytic cyclodimerizations of conjugated dienes have also been performed selectively [27]. Thus, a catalytic amount of CpRuCl(diene) and Ag(0S02CF3) led to the formation of 1,5-cyclooctadiene, dimethylcyclooctadienes, and 6-methyl-... 

In 1981, this cation radical Diels-Alder cyclodimerization of 1,3-cyclohexadiene was shown to be more cleanly (only 1 % of the cyclobutane dimers is produced), conveniently (in a synthetic organic context), and efficiently (70 % yield) carried out by chemical ionization of the diene, using 3+ (Scheme 15) [39]. 

The reaction was carried out in dichloromethane solvent at 0°C for 5 min, using 5 mol % of the initiator. The endofexo ratio (4.5 1) was similar to that found in the thermal Diels-Alder cyclodimerization of this same diene. This encouraging result and the convenience of the procedure led to the extensive study of the cation radical Diels-Alder reaction and its utility in synthetic organic chemistry [40]. It is of further note that the same cyclodimerization was subsequently carried out by the PET... 

In most cases the cation radical Diels-Alder reaction has proved to be highly re-giospecific with respect to the reaction of an unsymmetrical dienophile with an unsymmetrical diene. As examples, the cyclodimerizations of both 1-methyl-1,3-cyclohexadiene and 1-methoxy-1,3-cyclohexadiene are essentially regioexclusive (Scheme 18) [47-49]. 

However, the cyclodimerization of 1,3-cyclohexadiene and also the addition of the cis,cis isomer of 2,4-hexadiene to 1,3-cyclohexadiene are only modestly stereoselective. The addition of cis,rra i-2,4-hexadiene to 1,3-cyclohexadiene is highly stereoselective for the addition to the lra s-propenyl group, but only modestly stereoselective for the addition to the cw-propenyl group. Further, the addition of a dienophile having a pendant, unsubstituted vinyl double bond to this diene is also highly endo stereoselective. The installation of a cis group at the terminus of the dienophilic moiety consistently appears to reduce the endo stereoselectivity to a more modest level. It has been proposed that the cis substituent attenuates the secondary interaction involving the endo double bond in the transition state for cycloaddition [47, 48]. The effect has been termed the cfs-propenyl effect . The addition of the ira j-anethole cation radical to both 1,3-cyclohexadiene and 1,3-cyclopentadiene is, however, only moderately diastereospecific (ca 3 1) [49]. 

Of further interest is the observation that the same reaction is not observed at all under PET conditions, which should reliably furnish the same diene cation radical [50]. Further, the normally efficient cation radical cyclodimerization of 1,3-cyclohexadiene is completely inhibited in the presence of the hindered diene, so that... 

An intriguing competition arises in the context of cation radical cycloadditions (as in the context of Diels-Alder cycloadditions) which involve at least one conjugated diene component. Since both cyclobutanation and Diels-Alder addition are extremely facile reactions on the cation radical potential energy surface, it would not be surprising to find a mixture of cyclobutane (CB) and Diels-Alder (DA) addition to the diene component in such cases. Even in the cyclodimerization of 1,3-cyclohexadiene, syn and anti cyclobutadimers are observed as 1 % of the total dimeric product. Incidentally, the DA dimers have been shown not to arise indirectly via the CB dimers in this case [58]. The cross addition of tw 5-anethole to 1,3-cyclohexadiene also proceeds directly and essentially exclusively to the Diels-Alder adducts [endo > exo). Similarly, additions to 1,3-cyclopentadiene yield essentially only Diels-Alder adducts. However, additions to acyclic dienes, which typically exist predominantly in the s-trans conformation which is inherently unsuitable for Diels Alder cycloaddition, can yield either exclusively CB adducts, a mixture of CB and DA adducts or essentially exclusively DA adducts (Scheme 26) [59]. 

The DA dimer obtained in the aminium salt reaction differs from that obtained by the PET method and from that obtained using the aminium salt/hindered base method, but is the same as that obtained by the Bronsted acid catalyzed reaction of this diene. The addition of insoluble bases hke sodium carbonate is not sufficient to suppress the extremely facile, acid-catalyzed cyclodimerization of this particular diene, which yield a highly stabilized tetramethylallyl carbocation intermediate. Nevertheless, this results demonstrates the necessity for caution in assigning a cation radical mechanism to a cyclodimerization reaction observed under aminium salt conditions. 

The first catalytic cyclodimerization of 1,3-butadiene (BD) to 1,5-cycloocta-diene using modified Reppe catalysts was reported by Reed in 1954 [4], and only two years later Wilke reported on the titanium-catalyzed synthesis of cyclo-dodecatrienes from BD [5]. It remained for Wilke and his co-workers to show the tremendous versatility and scope of the nickel-catalyzed cyclooligomerizations of... 

The cyclooligomerization reaction is not confined to BD as the monomer. Activated or monosubstituted 1,3-dienes also react, but reaction rates are usually slow, and selectivity and turnover numbers (TONs) are low. Cyclotrimerization and cyclodimerization of substituted 1,3-dienes - either alone or in admixture with BD - give numerous isomers of substituted CDT, COD, VCH and divinyl-cyclobutane (DVCB). For example, isoprene [34], 1,3-pentadiene [35], 2,3-dimethylbutadiene [36], 1,3-hexadiene [37], and even 1-vinyl-1-cyclopentene [38] do react (eqs. (2)-(6)). 2,4-Hexadiene is inert. 

The cyclodimerization of 3,3-dimethylcyclopropene to form 3,3,6,6-tetramethyl-exo-tri-cyclo[3.1.0.0 ]hexane (4) can also be achieved with pentacarbonyliron in toluene at 95"C in 3 hours (no yield given) or in tetrahydrofuran at 60 °C with catalytic amounts of hexacarbonyl-molybdenum (77% yield, 93% conversion) or hexacarbonyltungsten (86.5% yield). With nonacarbonyldiiron a stoichiometric ring-opening reaction and formation of tricarbonyl(2-methyl-l-propenylketene)iron has been observed (see Section 2.C. for details). A 40% yield of the tricyclic dimer 4 is obtained under the catalytic action of ( / -cycloocta-l,5-diene)(t -cyclopentadienyl)cobalt(I). ... 

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-... 

Fe compounds have received much less significant attention than Ni or Co compounds as the diene polymerization catalyst. FeEt2(bpy)2 catalyzes cyclodimerization of 1,3-butadiene [79] and polymerization of vinyl monomers such as acyclic ester [80]. Recently, FeEt2(bpy)2/MAO was found to show high catalytic activity toward 1,2-polymerization of 1,3-butadiene and 3,4-polymerization of isoprene at -40 to +25 °C (Eq. 14) [81]. The crystalline polybutadiene prepared below 0 °C is composed of... 

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