2.4- Hexadiene cycloaddition products

As shown in Table VII, [2 + 4] cycloaddition is the most common reaction pathway followed by Me2Si=C(SiMe3)2, but it is usually accompanied by significant quantities of the product of an ene reaction. As the diene becomes more sterically hindered in its s-cis conformation, as in cis/trans-2,4-hexadiene, the product of an ene reaction predominates. With butadiene, where minimal steric effects are to be expected, the exclusive product of the reaction was found to be the [2 + 4] cycloaddition... 

When N-sulfinylcarbamate 224 was reacted with ( , )-2,4-hexadiene 225 the formal [4 -I- 2] cycloaddition products (the reaction mechanism is a matter of debate [141-143]) 226 and epi-226 were formed in a 15 1 ratio, respectively. Treatment of this mixture with phenylmagnesium bromide followed by refluxing the resulting mixture with trimethyl phosphite in methanol af-... 

Two groups have recently examined enantioselective A -sulfinyl dienophile Diels-Alder reactions. In one case, Whitesell et al. found that a phenylmenthol derived N-sulfinyl carbamate adds to ( , )-2,4-hexadiene under Lewis acid catalysis to afford a single diastereomer (equation 52). If the Lewis acid was omitted, a complex mixture of cycloaddition products was obtained. Attack of the diene on the N-sulBnyl dienophile as shown in the equation would rationalize the observed results. The site of Lewis acid complexation, however, is unknown. 

Yang and collaborators have established the structures of the photoadducts obtained by ultra-violet light irradiation of benz-[a]anthracene, (80), benz[b]anthracene (also known as tetracene or naphthacene), (81), and dibenz[a,c]anthracene, (82), with 1,3-cyclo-hexadiene. The products result from a single mode of 4+4 cycloaddition. The fact that none of the alternative isomers which could be formed by addition are observed is rationalised in terms... 

At higher temperatures, the product rearranged to 5-methylenebicyclo[2.1.1]hex-ane which at higher temperatures rearranged to 3-methylene-1,5-hexadiene, probably by a 3,3-shift in the retro 2 -h 2 cycloaddition product of the bicyclohexane (Scheme 8.47). 

As a model substrate, 2,5-dimethyl-2,4-hexadiene 51 has been intensively investigated by several research groups. Depending on the solvent polarity, hve peroxidic products have been isolated in varying relative yields the endoperoxide 52 (from [4 + 2]-cycloaddition), the aUylic hydroperoxide 53 (from ene reaction), the 1,2-dioxetane 54 (from [2 + 2]-cycloaddition), the diene hydroperoxide 55 (either from a vinylogous ene reaction or a radical-induced rearrangement of 53), and the methoxy-substituted hydroperoxide 56 (a methanol trapping product) were observed. A maximum of 23% of the [4 -I- 2]-cycloaddition product 52 was detected in tetrachloromethane, whereas the dioxetane 54 dominated in polar acetonitrile. 

If the reaction is carried out at a pressure of 12 kbar, the mechanism changes, and cycloaddition with ci5,traro-2,4-hexadiene proceeds stereo specifically yielding the traws-selenapyrans as the major products along with the corresponding tetraarylethylenes. 

In a thorough study on photooxidation of 2,5-dimethyl-2,4-hexadiene (455) it was found that 1,2-dioxene 456, 1,2-dioxetane 457, hydroperoxy dienes 458 and 459 and, when methanol was used as solvent, also hydroperoxy(methoxy)octene 460 are formed (Scheme 124) . Product distribution was found to be highly solvent dependent. These results led investigators to postulate a mechanism involving the intermediacy of perepoxide 461 and zwitterion 462 (Scheme 124). Accordingly, the product of [4-1-21-cycloaddition 456, the product of [2 + 2]-cycloaddition 457, as well as the products 458 and 459 deriving from ene-addition would originate from polar intermediates 461 and... 

The third radical cation structure type for hexadiene systems is formed by radical cation addition without fragmentation. Two hexadiene derivatives were mentioned earlier in this review, allylcyclopropene (Sect. 4.4) [245] and dicyclopropenyl (Sect. 5.3) [369], The products formed upon electron transfer from either substrate can be rationalized via an intramolecular cycloaddition reaction which is arrested after the first step (e.g. -> 133). Recent ESR observations on the parent hexadiene system indicated the formation of a cyclohexane-1,4-diyl radical cation (141). The spectrum shows six nuclei with identical couplings of 11.9G, assigned to four axial p- and two a-protons (Fig. 29) [397-399]. The free electron spin is shared between two carbons, which may explain the blue color of the sample ( charge resonance). At temperatures above 90 K, cyclohexane-1,4-diyl radical cation is converted to that of cyclohexene thus, the ESR results do not support a radical cation Cope rearrangement. 

Wiberg and coworkers published relative rate constants and the products of reaction of silene 6 with a number of alkenes and dienes in ether solution at 100 °C6 106-108. These data are listed in Table 2 along with an indication of the type of product formed in each case. As is the norm in Diels-Alder additions by more conventional dienophiles, the rate of [2 + 4]-cycloaddition of 6 to dienes increases with sequential methyl substitution in the 2- and 3-positions of the diene, as is illustrated by the data for 2,3-dimethyl-1,3-butadiene (DMB), isoprene and 1,3-butadiene. The well-known effects of methyl substitution at the 1- and 4-positions of the diene in conventional Diels-Alder chemistry are also reflected with 6 as the dienophile. For example, lruns-1,3-pen tadiene reacts significantly faster than the f/.v-isorrier, an effect that has been attributed to steric destabilization of the transition state for [2 + 4]-cycloaddition. In fact, the reaction of c/s-l,3-pentadiene with 6 yields silacyclobutane adducts, while the trans-diene reacts by [2 + 4]-cycloaddition108. No detectable reaction occurs with 2,5-dimethyl-2,4-hexadiene. The reaction of 6 with isoprene occurs regioselectively to yield adducts 65a and 65b in the ratio 65a 65b = 8.5 (equation 50)106,107. 

The amount of 1,2,4,5-hexatetraene in solution was estimated by the submitters from H NMR spectra and GC analyses. The major product of the dimerization reaction is l,2-hexadien-5-yne. The submitters have shown that this hydrocarbon does not interfere with the cycloaddition in part B. 

The cycloaddition of ,7. )-2.4-hcxadienc and diazenedicarboxylates affords mainly the cis-3,6-dimethyl-l,2,3,6-tetrahydropyridazine 37 16,17 the configuration was assigned by H NMR8. However, after conversion of the reaction product to hexahydro-l,2,3,6-tetramethylpyridazine 4 different stereomeric ratios were determined by different authors17 18. The analogous cycloaddition of ( ,Z)-2,4-hexadiene with dimethyl diazenedicarboxylate takes place at more elevated temperatures or under sunlamp irradiation with comparable diastereoselectivity, the trans-adduct was mainly obtained 16 -19. The crude reaction product was converted to hexahy-dro-1.2,3,6-tetramcthylpyridazine17 18. 

A large number of 3-lactams have been prepared - by this method (equations 56-61). Cycloadditions of CSI with 1,5-hexadiene, allyl iodide and vinyl acetate - yield azetidinones which have been used as starting materials in the synthesis of carbapenems and penems. In some cases the cycloaddition must be conducted at low temperature to avoid open-chain products (equations 61 and 62). 

Cycloaddition reactions of benzyne with cyclic olejms, Benzyne reacts with cyclo-hexadiene to give, as the main products, hydrocarbons (l)-(4). The first (I) arises by 2 + 4 cycloaddition, (2) and (3) arise by ene cycloadditions, and (4) is a result of 2 + 2 cycloaddition. Addition of catalytic amounts of silver fluoroborate exerts a marked effect in this case (1) becomes almost the exclusive product. Silver ton, however. 

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