Acyclic 1,3-dienes

Cyclopropanation reactions with alkyl 2-diazobut-3-enoates (entries 3-6) occur with a remarkably high diastereoselectivity. When 1,3-dienes (acyclic 1,3-dienes,cyclopen-tadiene, furan, " substituted pyrroles ) are cyclopropanated with these reagents, the CK-divinylcyclopropane derivatives are formed with high or complete diastereoselectivity, but they usually undergo a spontaneous Cope rearrangement under the reaction conditions. 

NMP [191-193] can be applied to a wide range of monomers such as styrenes, acrylates, acrylamides, acrylonitrile, and 1,3-dienes. Acyclic nitroxides such as 2,2,5-trimethyl-4-phenyl-3-azahexane-3-nitroxide (TIPNO) or Af-tert-butyl-iV-(l-diethyl phosphono-2,2-dimethylpropyl) nitroxide (DEPN) enables the reversible termination of the growing polymer chains. 

Compared to cyclic dienes, acyclic dienes are less commonly used in aryne Diels-Alder reactions, although one example has been reported by Sarandeses et al. in the synthesis of helicenes (Scheme 12.48) [89]. Functionalized [4]/[5]heli-cenes 166 have been synthesized in five steps starting from 164, using a synthetic approach based on a Diels-Alder reaction between the l,3-bis(trimethylsilyloxy)-1,3-dienes 165 and benzyne [90]. 

It is possible to prepare 1-acetoxy-4-chloro-2-alkenes from conjugated dienes with high selectivity. In the presence of stoichiometric amounts of LiOAc and LiCl, l-acetoxy-4-chloro-2-hutene (358) is obtained from butadiene[307], and cw-l-acetoxy-4-chloro-2-cyclohexene (360) is obtained from 1.3-cyclohexa-diene with 99% selectivity[308]. Neither the 1.4-dichloride nor 1.4-diacetate is formed. Good stereocontrol is also observed with acyclic diene.s[309]. The chloride and acetoxy groups have different reactivities. The Pd-catalyzed selective displacement of the chloride in 358 with diethylamine gives 359 without attacking allylic acetate, and the chloride in 360 is displaced with malonate with retention of the stereochemistry to give 361, while the uncatalyzed reaction affords the inversion product 362. 

Heteroannular and acyclic dienes usually display molar absorptivities in the 8000 to 20 000 range, whereas homoannular dienes are in the 5000 to 8000 range. 

Table 4. Effect of Cyclopentadiene—Acyclic Diene Codimers on DCPD-Based Thermal Polymerizations ...

The stereochemistry of both chlorination and bromination of several cyclic and acyclic dienes has been determined. The results show that bromination is often stereo-specifically anti for the 1,2-addition process, whereas syn addition is preferred for 1,4-addition. Comparable results for chlorination show much less stereospeciftcity. It appears that chlorination proceeds primarily through ion-pair intermediates, whereas in bromina-hon a stereospecific anfi-l,2-addition may compete with a process involving a carbocation mtermediate. The latter can presumably give syn or anti product. 

In a definitive study of butadiene s reaction with l,l-dichloro-2,2-difluoio-ethylene, Bartlett concluded that [2+4] adducts of acyclic dienes with fluorinated ethylenes are formed through a mixture of concerted and nonconcerted, diradical pathways [67] The degree of observed [2+4] cycloaddition of fluorinated ethylenes IS related to the relative amounts of transoid and cisoid conformers of the diene, with very considerable (i.e., 30%) Diels-Alder adduct being observed in competition with [2+2] reaction, for example, in the reaction of 1,1 -dichloro-2,2-difluoro-ethylene with cyclopentadiene [9, 68]... 

Strained pertrifluoromethyl-substituted valence tautomers of aromatic systems, such as tetrakis(trifiuoromethyl)Dewar thiophene [87] and hexalas(tnfluorQ-methyl)benzvalene [Diels-Alder reactions with various cyclic and acyclic dienes (equations 76 and 77). 

Because the fluorine substituents both inductively and hyperconjugatively withdraw electron density from the C(2)-C(3) tt bond, the LUMO is located there, and Diels-Alder reactions take place exclusively with this bond [25] 1,1 -Difluoro allene and fluoroallene reaet readily with a large selection of cyclic and acyclic dienes, and acyclic dienes, [2+2] cycloadditions compete with the Diels-Alder processes As shown in the example in equation 79, a significantly different regiochemistry is observed for the [2+4] cycloaddition compared with the [2+2]... 

The double bond may be present in an acyclic or alicyclic compound, including dienes. The reaction is activated by the presence of... 

The 5-methylene-2(5//)-furanone 216 was found to be a good dienophile in Diels-Alder reactions with acyclic dienes (R = H, 2-Me, 2,3-di-Me, 1-Me, 1,3-di-Me). The reaction took place specifically at the cxo-cyclic double bond to give the corresponding spiro adducts 217 in good yields (Scheme 59) (90JOC3060). 

Less reactive dienes such as cyclohexadiene can be employed efficiently, giving the adduct in 90% yield in 93% ee. Acyclic dienes such as piperylene, 2,4-hexadiene, and 1-phenylbutadiene also react with the acryloyloxazolidinone derivative to afford Diels-Alder cycloadducts in high optical yields (Scheme 1.38, Table 1.17). 

For the construction of oxygen-functionalized Diels-Alder products, Narasaka and coworkers employed the 3-borylpropenoic acid derivative in place of 3-(3-acet-oxypropenoyl)oxazolidinone, which is a poor dienophile in the chiral titanium-catalyzed reaction (Scheme 1.55, Table 1.24). 3-(3-Borylpropenoyl)oxazolidinones react smoothly with acyclic dienes to give the cycloadducts in high optical purity [43]. The boryl group was converted to an hydroxyl group stereospecifically by oxidation, and the alcohol obtained was used as the key intermediate in a total synthesis of (-i-)-paniculide A [44] (Scheme 1.56). 

Another metathesis polymerization procedure uses terminal dienes such as hexa-1,5-diene (16) (acyclic diene metathesis (ADMET)). Here again, the escape of the gaseous reaction product, i.e. ethylene, ensures the irreversible progress of the reaction ... 

Acyclic diene molecules are capable of undergoing intramolecular and intermolec-ular reactions in the presence of certain transition metal catalysts molybdenum alkylidene and ruthenium carbene complexes, for example [50, 51]. The intramolecular reaction, called ring-closing olefin metathesis (RCM), affords cyclic compounds, while the intermolecular reaction, called acyclic diene metathesis (ADMET) polymerization, provides oligomers and polymers. Alteration of the dilution of the reaction mixture can to some extent control the intrinsic competition between RCM and ADMET. 

Giirtler and Jautelat of Bayer AG have protected methods that use chloroalumi-nate(III) ionic liquids as solvents for both cyclization and polymerization reactions of acyclic dienes [52]. They employed the neutral ionic liquid [EMIM][G1-A1G13]... 

Figure 7.4-2 Acyclic diene metathesis polymerization (ADMET) reaction carried out in the...

X(A1C13) = 0.5) to immobilize a ruthenium carbene complex for biphasic ADMET polymerization of an acyclic diene ester (Figure 7.4-2). The reaction is an equilibrium processes, and so removal of ethylene drives the equilibrium towards the products. The reaction proceeds readily at ambient temperatures, producing mostly polymeric materials but also 10 % dimeric material. 

Conjugated dienes can be epoxidized to provide vinylepoxides. Cyclic substrates react with Katsuki s catalyst to give vinylepoxides with high ees and moderate yields [17], whereas Jacobsen s catalyst gives good yields but moderate enantiose-lectivities [18]. Acyclic substrates were found to isomerize upon epoxidation (Z, )-conjugated dienes reacted selectively at the (Z)-alkene to give trans-vinylepoxides (Scheme 9.4a) [19]. This feature was utilized in the formal synthesis of leuko-triene A4 methyl ester (Scheme 9.4b) [19]. 

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