Alkenes 1,4-cyclohexadienes

Alkenes in (alkene)dicarbonyl(T -cyclopentadienyl)iron(l+) cations react with carbon nucleophiles to form new C —C bonds (M. Rosenblum, 1974 A.J. Pearson, 1987). Tricarbon-yi(ri -cycIohexadienyI)iron(l-h) cations, prepared from the T] -l,3-cyclohexadiene complexes by hydride abstraction with tritylium cations, react similarly to give 5-substituted 1,3-cyclo-hexadienes, and neutral tricarbonyl(n -l,3-cyciohexadiene)iron complexes can be coupled with olefins by hydrogen transfer at > 140°C. These reactions proceed regio- and stereospecifically in the successive cyanide addition and spirocyclization at an optically pure N-allyl-N-phenyl-1,3-cyclohexadiene-l-carboxamide iron complex (A.J. Pearson, 1989). 

The cyclohexadiene derivative 130 was obtained by the co-cyclization of DMAD with strained alkenes such as norbornene catalyzed by 75[63], However, the linear 2 1 adduct 131 of an alkene and DMAD was obtained selectively using bis(maleic anhydride)(norbornene)palladium (124)[64] as a cat-alyst[65], A similar reaction of allyl alcohol with DMAD is catalyzed by the catalyst 123 to give the linear adducts 132 and 133[66], Reaction of a vinyl ether with DMAD gives the cyclopentene derivatives 134 and 135 as 2 I adducts, and a cyclooctadiene derivative, although the selectivity is not high[67]. 

We saw m Section 9 10 that the combination of a Group I metal and liquid ammonia is a powerful reducing system capable of reducing alkynes to trans alkenes In the pres ence of an alcohol this same combination reduces arenes to nonconjugated dienes Thus treatment of benzene with sodium and methanol or ethanol m liquid ammonia converts It to 1 4 cyclohexadiene... 

Pd-C, cyclohexene or 1,4-cyclohexadiene, 25°, 1.5-6 h, good yields. Some alkenes, benzyl ethers, BOM groups, and benzylamines are compatible with these conditions. 

We can get a quantitative idea of benzene s stability by measuring heats of hydrogenation (Section 6.6). Cyclohexene, an isolated alkene, has ff ydrog = -118 kj/mol (-28.2 kcal/mol), and 1,3-cyclohexadiene, a conjugated diene, has A/Chydrog = 230 kj/mol (-55.0 kcal/mol). As noted in Section 14.1, this value for 1,3-cyclohexadiene is a bit less than twice that for cyclohexene because conjugated dienes are more stable than isolated dienes. 

Nafion-H (144), a perfluorinated resin-sulfonic acid, is an efficient Bronsted-acid catalyst which has two advantages it requires only catalytic amounts since it forms reversible complexes, and it avoids the destruction and separation of the catalyst upon completion of the reaction [94], Thus in the presence of Nafion-H, 1,4-benzoquinone and isoprene give the Diels-Alder adduct in 80% yield at 25 °C, and 1,3-cyclohexadiene reacts with acrolein at 25 °C affording 88 % of cycloadduct after 40 h, while the uncatalyzed reactions give very low yields after boiling for 1 h or at 100 °C for 3.5 h respectively [95], Other examples are given in Table 4.24. In the acid-catalyzed reactions that use highly reactive dienes such as isoprene and 2,3-dimethylbutadiene, polymerization of alkenes usually occurs with Nafion-H, no polymerization was observed. 

These reactions are found to be promoted by electron-donating substituents in the diene, and by electron-withdrawing substituents in the alkene, the dienophile. Reactions are normally poor with simple, unsubstituted alkenes thus butadiene (63) reacts with ethene only at 200° under pressure, and even then to the extent of but 18 %, compared with 100% yield with maleic anhydride (79) in benzene at 15°. Other common dienophiles include cyclohexadiene-l,4-dione (p-benzoquinone, 83), propenal (acrolein, 84), tetracyanoethene (85), benzyne (86, cf. p. 175), and also suitably substituted alkynes, e.g. diethyl butyne-l,4-dioate ( acetylenedicarboxylic ester , 87) ... 

OH radicals react very fast (almost in a diffusion-controlled rate) with simple alkenes (k = 7.0 x 109 for 1-butene or cyclopentene and 8.8 x 109 M 1 s 1 for cyclohexene) and there is almost no change for 1,3- or 1,4-cyclohexadiene. Cycloheptatriene reacts very fast with all the three radicals formed in the radiolysis of water k = 6 x 109 with eaq, 8 x 109 with H atoms and 1 x 101CI M 1 s 1 with hydroxyl radicals13. 

Similarly, in some cases, dienes can be selectively hydrogenated into the corresponding alkenes, but they usually provide the corresponding alkane or a mixture of alkanes and alkenes [6, 45, 49, 59]. For example, the hydrogenation of 50 equiv. of 1,3- or 1,4-cyclohexadiene in the presence of [Cp2TiCl2]-iPrMgBr... 

Conjugated dienes (such as 1,3-cyclohexadiene, cyclopentadiene, 2,4-hexadienoic-sorbic-acid) and polyenes can be selectively hydrogenated to monoenes unactivated alkenes are totally unreactive [20]. Unfortunately, the possibilities for modification of the catalyst by ligand alteration or by the use of additives are very limited [50, 51]. 

Attempts to liberate l-methyl-l-aza-2,3-cyclohexadiene (329) from 3-bromo-l-methyl-l,2,5,6-tetrahydropyridine (326) by KOtBu in the presence of [18]crown-6 and furan or styrene did not lead to products that could have been ascribed to the intermediacy of 329 (Scheme 6.70) [156], Even if there is no doubt as to the allene nature of 329 on the basis of the calculations on the isopyridine 179 and 3d2-lH-quinoline (257), it is conceivable that the zwitterion 329-Za is only a few kcal mol-1 less stable than 329. This relationship could foster the reactivity of 329 towards the tert-butoxide ion to an extent that cycloadditions to activated alkenes would be too slow to compete. On the other hand, the ultimate product of the trapping of 329 by KOtBu could have been an N,0-acetal or a vinylogous N,0-acetal, which might not have survived the workup (see, for example, the sensitivity of the N,0-acetal 262 [14], Scheme 6.57). 

Oxidation of alkenes and dienes involving an allene substituent as a formal nucleophile is a conceptually new reaction. Allcnc-substitutcd 1,3-cyclohexadienes 34 undergo a pal-ladium(II)-catalyzed oxidation to give bicydic compounds 35 or 36 in good yields (Scheme 17.14) [14]. When y-alkenylallenes, e.g. 37, 41 and 43, were treated with 1 mol% palladium trifluoroacetate, a similar oxidative carbocyclization took place [15]. In both reactions the new stereocenters are formed with high stereoselectivity. 

Another example of a diene undergoing a [2 + 2] cycloaddition reaction with an alkene has been reported recently4. 2-Dimethylaluminumoxy-l,3-cyclohexadiene (7) reacted with phenyl vinyl sulfoxide (8) to afford a diastereomeric mixture of cis substituted cyclobutanols 9 (equation 3). The occurrence of a [2 + 2] cycloaddition as well as the high cis stereoselectivity observed were explained by a pre-organization of the reactants by complexation of the diene bound aluminum with the sulfoxide oxygen on the olefin. 

These dications react with alkenes to give 1,2-disulfonium salts, and with conjugated dienes to afford 1,4-adducts. Furthermore, while 1,4-disubstituted linear dienes yield complex mixtures of unidentified substances, 1,3-cyclohexadiene (96) produces a moderately stable salt 102 (equation 106). The formation of the kinetically controlled 1,2-addition product has never been observed. 

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