Butenes cycloaddition with

Direct photochemical excitation of unconjugated alkenes requires light with A < 230 nm. There have been relatively few studies of direct photolysis of alkenes in solution because of the experimental difficulties imposed by this wavelength restriction. A study of Z- and -2-butene diluted with neopentane demonstrated that Z E isomerization was competitive with the photochemically allowed [2tc + 2n] cycloaddition that occurs in pure liquid alkene. The cycloaddition reaction is completely stereospecific for each isomer, which requires that the excited intermediates involved in cycloaddition must retain a geometry which is characteristic of the reactant isomer. As the ratio of neopentane to butene is increased, the amount of cycloaddition decreases relative to that of Z E isomerization. This effect presumably is the result of the veiy short lifetime of the intermediate responsible for cycloaddition. When the alkene is diluted by inert hydrocarbon, the rate of encounter with a second alkene molecule is reduced, and the unimolecular isomerization becomes the dominant reaction. 

Di- and trisubstituted nitroalkenes tethered to dipolarophiles (unsaturated esters, nitriles) undergo tandem [4+2]/[3+2] cycloadditions with 2,3-dimethyl-2-butene or butyl vinyl ether in the presence of Lewis acids (Eq. 8.112). For the dimethylene tether, the E-configuration of the dipolarophile is preferred, and the products arise selectively from a syn-endo pathway.177... 

Ene-additions of alkenes and dienes to silene 6 are considerably slower than [2 + 4]-cycloadditions. cA-Substitution in the ene component of the reaction causes a small acceleration in rate relative to fraws-substitution, as illustrated in Table 2 by the relative rate constants for reaction of 6 with cis- and rraws-2-butene. Reaction with cis, trans-2,4-hexadiene produces only a single adduct (66 equation 51), corresponding to selective ene-reaction with the cA-methyl group in the diene. 

A cyanosulfine [242] and an oxosulfine [162] were trapped with 2,3-dimethyl-1,3-butadiene (Table 6, entries 2 and 3). Capozzi and his group have extended their phthalimido-sulfenyl chemistry to the synthesis of a-oxosulfines, and have observed a dichotomic behaviour towards cycloaddition. With 1,3-dienes, these sulfines act [243, 244] as dienophiles through their C=S bond (Table 6, entry 4) to afford dihydro-2H-thiapyran S-oxides. With alkenes (Table 7), such as 2,3-dimethyl-2-butene (entry 1) or vinyl-ethers (entry 2), they behave as dienes to give dihydro-1,4-oxathiin S-oxides [243-245]. 

R and R may be H, methyl, cyclopropyl, cyano, or ester groups. The phenylcarbene formed on irradiation of trans-l,2-diphenyloxirane has been trapped and identified in the form of a cyclopropane derivative in methanol in the presence of benzyl methyl ether and alkenes. Photolysis in the presence of 2,3-dimethyl-2-butene proceeds by cycloaddition with the formation of cyclopropane-carboxylic acid and oxetane derivatives (Eq. 368). ... 

Diphenylcyclobutanones. Contrary to earlier reports, diphenylketene undergoes cycloaddition with simple alkenes to give 2,2-diphenylcyclobutanones.1 However, a long reaction time is needed for satisfactory yields. cw-Olefins react much faster than trans-olefins. Thus the reaction with cis-butene-2 requires 3 days, but the reaction with frans-butene-2 requires 3 months. With low-boiling alkenes, the reaction is carried out in a sealed tube or in an autoclave. 

The use of the CFs-substituted oxazole as an aza-diene for a Diels-Alder reaction was briefly examined. Thus, prolonged heating of the mixture of 5-ethoxy-4-trifluoromethyloxazole and an excess of acrylic acid afforded the CFs-substituted pyridine derivative in reasonable yield [61], However, cycloaddition with other olefins, such as ethyl acrylate, N-phenylmaleimide, 2,5-dihydrofuran, maleic anhydride, and 2-buten-4-olide did not take place to any appreciable extent presumably because of the deactivation of the oxazole ring by a strongly electron-withdrawing CF3 group. 

Keten-olefin [2 + 2] cycloadditions have received considerable attention recently The isomer with the larger keten substituent (L) in the endo position (293) is found in addition of keten to cyclopentadiene. This is predicted from the orthogonal keten-olefin transition state [2, + 2 ] of Woodward and Holfman. This model also accounts for the reduced reactivity towards ketens of /rofts-butene compared with cw-butene. Methylbromo-and methylchloro-keten undergo cycloaddition with a wide variety of olefins with only small variation in the endoiexo ratio, i.e. the ratio of (295) (294)... 

The p-acetonaphthone-sensitized reaction of cyclopentadiene with cis- and trans-but-2-enes has been investigated in some detail. It has been shown that both butene isomers give the same ratio of total ci> to total trans-adducts, and both also give the same ratio of [2 -I- 2]- to [2 + 4]-addition. The evidence is interpreted in terms of initial formation of two diastereoisomeric triplet biradicals in unequal amounts (and different unequal amounts from cis- and trans-but-2-enes). Both biradicals achieve rotational equilibrium before spin inversion and final ring-closure take place (see Scheme 18). Dialkylvinylene carbonates (146)-(148) have been prepared and, like the parent compound, undergo [2 + 2]cycloadditions with olefins in high yields. ... 

Reactions of fluorinated dipolarophiles. Electron-deficient unsaturated species generally make better dipolarophiles, therefore, fluonnated alkenes become better dipolarophiles when vinylic fluonnes are replaced by perfluoroalkyl groups For example, perfluoro-2-butene is unreactive with diazomethane, but more highly substituted perfluoroalkenes, such as perfluoro-2-methyl-2-pentene, undergo cycloadditions in high yields [5] (equation 2) Note the regiospecificity that IS observed in this reaction... 

It is possible that some of these photochemical cycloadditions take place by a lA + A] mechanism, which is of course allowed by orbital symmetry when and if they do, one of the molecules must be in the excited singlet state (5i) and the other in the ground state.The nonphotosensitized dimerizations of cis- and trans-2-butene are stereospecific,making it likely that the [n2s + n2s] mechanism is operating in these reactions. However, in most cases it is a triplet excited state that reacts with the ground-state molecule in these cases the diradical (or in certain... 

Photochemical [2h-2] cycloadditions of olefins occur with retention of configuration according to the Woodward-Hoffmarm rule [6,7], These are excited-state reactions in the delocalization band of the mechanistic spectrum. A striking example of the symmetry-allowed reaction was observed when the neat cis- and tran -butenes were irradiated (delocahzation band in Scheme 3) [8],... 

It was found that 2-propenyloxymagnesium bromide reacts much more readily with nitrile oxides than other known dipolarophiles of electron-deficient, electron-rich, and strained types, including 3-buten-2-one, ethyl vinyl ether, and norbomene, respectively (147). Therefore, this BrMg-alkoxide is highly effective in various nitrile oxide cycloaddition reactions, including those of nitrile oxide/Lewis acid complexes. 

An old approach to preparing hydrocarbons such as 246 and 249 consists in the cycloaddition of ethenylidene carbenes to olefins [104]. In a recent application, such a carbene, 251, was generated from the dichloride 250 by n-butyllithium treatment trapping with 2,3-dimethyl-2-butene then led to the permethylated hydrocarbon 252 in 40% yield [103]. 

Phenylcarbene generated by different reaction routes always gives a similar cycloaddition pattern with butenes Xhese results indicate that an equilibrium is present between singlet and triplet phenylcarbene. 

Without ion-radical initiation, the yield of the resulted product reaches 50% for 24 h. Practically the same yield can be achieved for the same time in the presence of tris(4-bromophenyl)ammoniumyl hexachloroantimonate and for only 6 h on sonication (Nebois et al. 1996). Sonication accelerates the rate-determining formation of the diene cation-radical. Of course, hydroxynaphthoquinone is strong enough as an electron-acceptor with respect to 2-butenal Af,Af-dimethylhydrazone. Therefore, the question remains whether sonication is more or less the general method for the initiation of ion-radical cycloaddition. A possible role of sonication in optimization of ion-radical reactions was considered in Section 5.2.5. 

If AAs are heated in acetic anhydride in the presence of 1,2-dicyanocyclo-butene as a dipolarophile, 4,5-dihydroazepines are formed in good yields. For example, the reaction with Ala is actually considered a cycloaddition of the intermediate mesoionic derivative (Scheme 43) (80JHC1593). 

The addition of alkenes to A -methylenium amide cations is stereo-specific in the sense that it obeys the cis principle of Alder and Stein, cis-2-butene giving a cis product (105, R = R = Me) and the trans isomer giving trans product (105, R = R = Me). The cycloaddition of unsymmetrical alkenes is highly regiospecific. Knowing how the alkene will react with a large cation allows prediction of the product (105). Thus styrene, butadiene, and vinyl acetate all react so that its substituent appears at position 6 of the dihydrooxazinium salt (105, R = Ph, vinyl, or OAc). 

---