2.3- Dimethylbuta-l,3-diene

The Diels-Alder adduct isolated from the reaction with 2,3-dimethylbuta-l, 3-diene at elevated temperatures [200] is in fact the product of a two step reaction a 1,3-cycloaddition followed by a 3,2-sigmatropic rearrangement [199] (equaPon 49)... 

Nitro-2-phenyloxazole, obtained by thermal isomerization of the corresponding nitroi-soxazole, is found to undergo Diels-Alder reactions with 2,3-dimethylbuta-l,3-diene (see Eq. 

Thiopyrylium ions have been shown to behave as effective dienophiles. Shimizu and co-workers demonstrated that 2-benzothiopyrylium salts undergo [4+2+]-type cationic polar cycloadditions with various 1,3-dienes yield benzo-fused bicyclic sulfonium salts <1994J(P1)3129> (see Section 8.32.2.1.2 of CHEC-II(1996)). However, in this study the authors observed that cycloaddition of 2-benzothiopyrylium salt 40 with 2,3-dimethylbuta-l,3-diene 41a with prolonged reaction times caused a decrease in yield of the cycloadduct 22a (see entries 1 and 2 in Table 5 and Equation 10). In fact, stirring of the reaction mixture for 30 h did not afford the cycloadduct at all, see entry 3, but gave an undetermined complex mixture. 

Benzothiopyrylium salts have also been shown to undergo [4+2+]-polar cycloadditions with 1,3-dienes in dry 1,2-dichloroethane at room temperature <1996J(P1)2227>. Treatment of 43 with 2,3-dimethylbuta-l,3-diene 41a afforded benzo-fused bicyclic sulfonium salts 22, as shown in Equation (12) and Table 6. The 4-cyano-substituted benzothiopyrylium salt 43d proved to be the most reactive of these substrates (see entry 4). 

Recently, much attention has been paid to hetero Diels-Alder reactions as powerful tools for the construction of heterocyclic compounds. For example, cycloaddition of 2,3-dimethylbuta-l,3-diene 41a with 1,2-thiazinylium salt 95, in acetonitrile at room temperature, resulted in the exclusive formation of product 76a resulting from cycloaddition across the C—S1 bond (see entry 1 in Table 15 and Equation 26) <1999TL1505>. Similarly, isoprene 41b and... 

The transient (RPSn)221, which possibly represents a reactive intermediate during the synthesis of the hexameric Sn6P6 clusters 19c and 19d, has been trapped by the transformation of 20d in the presence of 2,3-dimethylbuta-l,3-diene, which solely gave 19d and the corresponding lff-l,l,-dichlorostannol (Eq. 14). 

The Diels-Alder additions of methyl y-fluorocrotonate to 2 3-dimethylbuta-l 3-diene and cycZopentadiene were effected by heating the reactants in sealed tubes at 110-120° for about 3 hr. The reduction of the unsaturated products, (XVIII) and (XX), was carried out at room temperature with palladium as catalyst. In both cases the theoretical quantity of hydrogen was absorbed, although the hydrogenation of the dimethyl derivative was much slower than that of the ewdomethylene compound. 

Treatment of phosphorus trichloride with isoprene in the presence of acetone leads to 1-chlorophospholen 1-oxides (33).33 Evidence has been presented that the diene reacts first with the trichloride to produce (34), which then reacts with acetone.33 l-Bromo-3,4-dimethyl-A3-phospholen (35) does not form a spiro-compound with 2,3-dimethylbuta-l,3-diene, although the bisphospholen derivative (36) has been isolated after hydrolytic work-up.34... 

The cycloaddition of dienes to imines to form tetrahydropyridines (equation 46) has been investigated extensively40. Ordinary imines are not sufficiently reactive to add to dienes they have to be activated by the presence of electron-withdrawing substituents. Thus the triester 70 adds to cyclopentadiene under atmospheric pressure to form 71 (equation 47). The reactions with other dienes (cyclohexadiene, isoprene or 2,3-dimethylbuta-l,3-diene) require high pressures41. 

Treatment of a mixture of a chloro oxime 177 (R = H, Ph, 4-BrCgH4 or 2-furyl) and a diene (cyclopentadiene or 2,3-dimethylbuta-l,3-diene) with solid sodium carbonate results in the formation of a dihydrooxazine, the intermediate nitrosoalkene 178 having reacted as a heterodiene (equation 98)94. In contrast, l,l-dichloro-2-nitrosoethene and cyclopentadiene yield the epoxyepimine 179 (equation 99)94. 

The product 303 from disulfur dichloride and 2,3-dimethylbuta-l,3-diene rearranges spontaneously to the tetrahydrothiophene 304 (equation 159)150. The reaction of liquid sulfur dioxide with conjugated dienes 305 (e.g. butadiene, isoprene) results in cyclic sulfones which dissociate into their components on heating (equation 160)151 152. 

Isoprene, 2,3-dimethylbuta-l,3-diene and other dienes (but not butadiene itself) readily furnish analogous 2,5-dihydroselenophene 1,1-dioxides, e.g. 306, on treatment with selenious acid in chloroform at room temperature (equation 161)153. 

Diolefins such as butadiene, cyclohexa-1,3-diene, 2,3-dimethylbuta-l,3-diene and bicyclo[2,2,l]hepta-l,5-diene react with dicobalt octacarbonyl to... 

Silacydopent-3-enes are most conveniently synthesized from a 1,3-diene/alkali metal intermediate and the dihalosilane. The reaction conditions have been summarized (B-82MI12000) and the roles of the metal and solvent reviewed (74MI12001). Magnesium in THF is normally satisfactory for cyclizing buta-1,3-diene with a variety of dihalosilanes, However, extensive polymerization occurs with isoprene and 2,3-dimethylbuta-l,3-diene, so HMPT is added to THF to increase ring closure. 

Hexafluoro-2,3-bis(trifluoromethyl)butane (6) with sodium hydroxide gives perfluoro-(2,3-dimethylbuta-l.3-diene) (7).47... 

When phenyl(trimethylsilyl)diazomethane (20) is pyrolyzed in the gas phase, typical reactions of carbene 21 can be observed (see Section III.E.4). However, copyrolysis with alcohols or carbonyl compounds generates again products which are derived from silene 2239,40 (equation 6). Thus, alkoxysilanes 23 are obtained in the presence of alcohols and alkenes 24 in the presence of an aldehyde or a ketone. 2,3-Dimethylbuta-l,3-diene traps both the carbene (see Section ni.E.4) and the silene. 

Ishikawa and coworkers investigated the relative ease of migration of a Me3Si group to vinyl and phenyl groups in precursor compounds containing both groups133. In the case of the irradiation of compound 237 in the presence of methanol, silene 238 was found to be the major reactive intermediate, while with acetone and 2,3-dimethylbuta-l,3-diene the number and nature of different ene products can only be explained by the existence of silatriene 239 (equation 59). 

Cycloaddition and ene reactions. Dienes >C=C—C=C< such as buta-1,3-diene, isoprene, 2,3-dimethylbuta-l,3-diene, fraws-piperylene, cyclopentadiene or anthracene react with 92 in Diels-Alder fashion to give [2 + 4] cycloadducts 410 (equation 128)62. Ene products 411 are formed additionally when the relative reaction rates for the [2 + 4] cycloaddition reaction and the ene reaction are comparable (e.g. for isoprene and 2,3-dimethyl-l,3-butadiene) Alkenes with allylic hydrogen (propene, 2-butene, isobutene) give ene products see equation 129. 

DMB CHD 2,3-dimethylbuta-l,3-diene MBD cyclohexadiene NBD norbornadiene. 2-methylbuta-1,3-diene BD buta-1,3-diene Cp cyclopentadiene ... 

Silenolates like 180 do react with 2,3-dimethylbuta-l,3-diene in a formal [4+2] fashion and, when the alkoxide 463 is trapped by triethylsilyl chloride, the l-silacyclohex-3-ene... 

The competitive silyl migration to either a vinyl or phenyl group in aryl- and vinyl-disilanes has been investigated by Ishikawa and coworkers133. With 2,3-dimethylbuta-l,3-diene, two modes of ene reaction are possible for 238 to give 543 and 544. Another ene... 

The formation of the cyclooctatrienes was quenched in the presence of low concentrations of 2,5-dimethylhexa-2,4-diene or 2,3-dimethylbuta-l,3-diene. The photocyclization of the trienes was, however, unaffected by the presence of the dienes. The authors conclude that the primary photoreaction of intramolecular ortho cycloaddition involves the 3 tt, tt state of the arene and the subsequent cyclization of the cycloocta-l,3,5-triene arises from the singlet state. 

Rhodium (Il)-catalysed 3 + 2- and 3 + 4-cycloaddition of diazocarbonyl compounds (7) with conjugated dienes (e.g. 2,3-dimethylbuta-l,3-diene) provides a simple and rapid route to dihydrofurans (8) and dihydrooxepins (9) in high yields. A stepwise mechanism involving delocalized zwitterions has been proposed for the formation of the cycloadducts (Scheme 2).3... 

The dehydration of ditertiary alcohols in the presence of hydrobromic acid may lead to dienes (e.g. pinacol to 2,3-dimethylbuta-l,3-diene, cognate preparation in Expt 5.12), although in this case some concomitant rearrangement to t-butyl methyl ketone (pinacolone, Expt 5.98) occurs under the acidic conditions employed. 

Cognate preparations. 2,3-Dimethylbuta-l,3-diene. In a 1-litre round-... 

---