13-Cyclohexadiene, elimination product

Vinylcyclohexene formed a rather stable intermediate which persisted even after a day at room temperature. The structure is thought to be 16, arising from isomerization of the internal double bond. An alternative possibility, 17, is considered less likely because 1,3- and 1,4-cyclohexadiene gave much more reductive elimination product after a day. 

Some synthetic applications of 1,4-difunctionalization of various 1,4-dienes are cited here. Stereoselective 1,4-difunctionalization of the 1,3-cyclohexadiene 217 using Pd(OAc)2 and O2 in DMSO afforded nearly equal amounts of 1,4-adduct 218 and addition-elimination product 219 [97]. Pd(II)-promoted intramolecular reaction of the allene-substituted 1,3-cyclohexadiene 220 gave the triene 223 regio-and stereoselectively. The reaction may be explained by nucleophilic attack of the... 

Coordination of Ni(0) to the alkyne gives a n complex, which can be written in its Ni(II) resonance form. Coordination and insertion of another alkyne forms the new C6-C7 bond and gives a nickelacyclopenta-diene. Maleimide may react with the metallacycle by coordination, insertion, and reductive elimination to give a cyclohexadiene. Alternatively, [4+2] cycloaddition to the metallacycle followed by retro [4+1] cycloaddtion to expel Ni(0) gives the same cyclohexadiene. The cyclohexadiene can undergo Diels-Alder reaction with another equivalent of maleimide to give the observed product. 

Selenosulfonylation of olefins in the presence of boron trifluoride etherate produces chiefly or exclusively M products arising from a stereospecific anti addition, from which vinyl sulfones can be obtained by stereospecific oxidation-elimination with m-chloroper-benzoic acid134. When the reaction is carried out on conjugated dienes, with the exception of isoprene, M 1,2-addition products are generally formed selectively from which, through the above-reported oxidation-elimination procedure, 2-(phenylsulfonyl)-l,3-dienes may be prepared (equation 123)135. Interestingly, the selenosulfonylation of butadiene gives quantitatively the 1,4-adduct at room temperature, but selectively 1,2-adducts at 0°C. Furthermore, while the addition to cyclic 1,3-dienes, such as cyclohexadiene and cycloheptadiene, is completely anti stereospecific, the addition to 2,4-hexadienes is nonstereospecific and affords mixtures of erythro and threo isomers. For both (E,E)- and ( ,Z)-2,4-hexadienes, the threo isomer prevails if the reaction is carried out at room temperature. 

In this cyclodecarbonylation reaction, a ketene species is unlikely to be the reaction intermediate as added alcohols produce no esters. As shown in Scheme 6.26, the ruthenium acyl species 72 is likely to be the intermediate [25], which is prone to decarbonylationto give ruthena-cyclohexadiene 73 this species undergoes subsequent reductive elimination to form 2H-indene. Addition of proton or Ru to species 74 generated the benzylic cation 75, which after a 1,2-aryl shift gave the observed products. 

When the l-diazo-2-silyl moiety is incorporated into cycles then endocyclic silicon-carbon double bonds should be formed. Most interesting in this connection is the decomposition of l-diazo-2-sila-3,5-cyclohexadiene 331, because the initial Si=C product should be silabenzene 332170. The outcome of the nitrogen elimination depends on the conditions used. The products isolated are 333 (equation 81), 335 via bicyclic 334 (equation 82) and 338, formed via silafulvenes and 336 (equation 83). 

Further functionalizations are obtained via the electron transfer— radical cation fragmentation pathway a typical example is side-chain nitration by irradiation of methyaromatics with tetranitromethane. Aromatics form charge-transfer complexes with C(N02)4 irradiation leads to electron transfer and fragmentation of the C(N02)4 radical anion to yield the triad [Ar + C(NO)J N02], followed by combination between the arene radical cation and the trinitromethanide anion. Thus, cyclohexadienes are formed that generally eliminate and rearomatize at room temperature yielding ring-functionalized products [234] (Sch. 21). 

Reaction of the intermediate 2c, X = H, with electrophiles E+ can give interesting cydo-hexadienes 13 (Scheme 2, path c). Indeed, if addition of the nucleophile is irreversible, carbon monoxide can be incorporated into the product, resulting in dearomatization accompanied by the introduction of an acyl group (Scheme 4). For example, complex 9 reacts with an electrophile E+ to give the 18e complex 10a, which can insert CO. A reductive elimination then affords the // -cyclohexadiene 12a, which liberates the free cydohexadiene 13 [li]. 

A similar two-step azidoselenation via (i) MeSeBr or PhSeBr, (ii) NaNs/CFaCHzOH, and subsequent elimination (iii) (O3) of selenium has been reported unfortunately, selectivities in the second and third steps are poor.2 Phenylselenyl azide (made in situ from PhSeCl/NaNs/DMSO, room temperature) adds to alkenes (20 C, overnight, 86-98%) stereospecifically 2 regiospecifrcity is poor with terminal alkenes but good with highly polarized alkenes, tj cal products being (55) to (57) cyclohexadiene gives (58). 

A general route to thioaldehydes is the base-induced 1,2-elimination of sulfenate derivatives, described initially by Kirby et al. In this reaction, phthalimide derivatives, e.g. (25), react with Et3N to generate a thioaldehyde, which is subsequently trapped by dimethylbutadiene to yield product 26 [83CC423 85JCS(Pl)1541]. Other dienes used in this study include thebaine, cyclohexadiene and anthracene. 

Whereas stoichiometric additions of allylpalladium species to norbomene and 1,3-dienes are known (c/. Section 1.2.2.4), simple alkenes (e.g. styrene, cyclohexene, 1,4-cyclohexadiene and 1,5-cycloocta-diene) did not undergo this reaction. However, it can be assumed that the intramolecular ene process (L) — (M) (Scheme 36) is entropically favored and that a subsequent irreversible -elimination (M) —> (N) withdraws the ene product (M) from the equilibrium (L) (M). Further options are insertionfreduc-tive elimination processes (M) (O). The thereby regenerated Pd° species should continue the catalytic... 

A number of diverse stmctural types of cyclic imino dienophiles have been used in cycloadditions. For instance, dehydrohydantoins are useful partners in hetero Diels-Alder reactions. Two methods have been developed for in situ generation of these species. In one tqjproach, methoxyhydantoins such as (24) (equation S) are heated or are treated with acid to promote elimination of methanol, affording dienophile (25).22- This intermediate can be trapped regio- and stereo-selectively with 1,3-dienes. For example, with 1,3-cyclohexadiene only endo adduct (26) is formed. There is no ambiguity in this case concerning the dienophile configuration, and thus product (26) clearly derives from an endo transition state. 

The energy of the 1236 A light is greater than the ionization potential of benzene, and hence additional reaction paths are available. All the above products were observed for the photolysis of benzene at 1236 A, with all except hydrogen having somewhat lower quantum yields. No pressure effect was observed in the photolysis at 1236 A. For the photolysis with either lamp, the addition of 3 torr NO to 1 torr of benzene eliminated allene, cyclohexadienes, biphenyl and dihydrobiphenyl as products, while the other products remained unaffected. Hentz and Rzad conclude that the former are formed by free-radical reactions, while the latter are formed by molecular elimination. In view of the curious pressure effects observed by Shindo and Lipsky at 1849 A, conclusions regarding the effect of added gases must be made with caution. 

---