Cycloadditions with allenes

Electron-deficient olefins such as acrylonitrile can participate in the cross [2 + 21-cycloaddition with allenes. 3-Methylenecydobutanecarbonitrile (17) was obtained in 60% yield by the reaction of allene with a large excess of acrylonitrile under autogenous pressure at 200 °C [16]. Initial bond formation takes place between the central carbon of allene and the terminal carbon of acrylonitrile to give a diradical species, which cydizes to form the cydoadduct [17]. 

Ketene is also a suitable partner for [2 + 2]-cycloaddition with allenes. Diphenylke-tene and dimethylketene react readily with tetramethylallene at room temperature to give 2-isopropylidenecyclobutanones in good yields [49]. 

Diphenylketene and more reactive haloketenes will undergo cycloaddition with allenes such as 1,1-dimethylallene giving regioisomeric mixtures of cyclobutanones. Yields of these reactions are only modest5-7 and distributions of isomers are solvent dependent, which is indicative of a nonconcerted process involving ionic intermediates.8... 

Another reactive ketene which undergoes efficient cycloaddition with allenes is tert-butyl-cyanoketene (TBCK). Again mixtures of isomeric cycloadducts are obtained when unsymmet-rically substituted or 1,3-disubstituted allenes are employed.10-12... 

Nonetheless, there are a small number of systems that do mediate such [2 -i- 2 -t- 2] cycloadditions. With allenes as the alkene , cycloaddition with both acetylene and terminal alkynes proceeds regio-selectively to give 3,5-dimethylenecyclohexenes using Ni catalysts, and mostly 3,6-dimethylenecyclo-hexenes using Ni° catalyst precursors (equation 19). Norbomadiene undergoes so-called homo-Diels-Alder cycloaddition with both alkenes and a ynes in the presence of nickel catalysts. Further elaboration of this chemistry with alkynes but not alkenes has been described using a Co/Al catalyst system (equation 20). Attempts to produce cyclohexenes via all-intramolecular [2 + 2 + 2] cycloaddition of l,13-dien-7-ynes or 1,1 l-dien-6-ynes have been unsuccessful. ... 

Furan also undergoes cycloadditions with allenes," " with benzyne" and with simpler dienophiles, like acrylonitrile and acrylate various Lewis acidic catalysts can assist" in some cases, zinc iodide is one such, hafnium tetrachloride another, and improved endo exo ratios are obtained in an ionic liquid as reaction solvent. Maleate and fumarate esters react if the addition is conducted under high pressure. This device can also be used to increase markedly the reactivity of 2-methoxyfuran and 2-acetoxyfuran towards dienophiles. At higher reaction temperatures alkynes and even electron-rich alkenes will add to furan. 3- or 5-Halo-furans react faster in these cycloadditions. ... 

Furan also undergoes cycloadditions with allenes and even with simpler dienophiles, like acrylonitrile and acrylate (specifically enhanced by the presence of... 

Earlier work in cycloadditions with allene suggested that intramolecular isotope effects with values of > 1.00 derive from non-concerted, probably biradical processes and that when k-ojkjy < 1.00 a concerted mechanism... 

Cycloaddition of norbornadiene with allene takes place to yield the cyclobutene derivative 10[5], Cyclodimerization of 1,2-cyclononadiene (11) affords a mixture of stereoisomers of the cyclobutane derivatives 12[6,7],... 

The reaction of nitrones with allenes produced three main products an azepine, a pyrrolidinone and an isoxazolidine (Scheme 155) (79JOC4213). The intramolecular cycloaddition of nitrones (529) produced different products depending on the length of n (Scheme 156) (78H(10)257). 

Application in organic synthesis of optically active isoxazolidones obtained by asymmetric cycloaddition of nitrones with allenes 97T403. 

Reaction of the nitrone 4-184 with allenic esters 4-185 as described by Ishar and coworkers led to the benzo[b]indolizines 4-186, together with small quantities of 4-187 (<5%) (Scheme 4.40) [63]. The first transformation is a 1,3-dipolar cycloaddition this is followed by four further steps, including a [4+2] cycloaddition of an intermediate 1-aza-l,3-butadiene. 

Reversible pyridine dissociation yields the non-Lewis base stabilized imido complex [Cp(NHAr) Ti=NAr] (54). This coordinatively unsaturated species undergoes a [2 + 2] cycloaddition reaction with allene to give an azatitanocyclobutane (55), which is then protonated to the /mamido complex. Elimination of enamine occurs followed by isomerization to the energetically more favorable imine. 

Vinyl Fischer carbenes can be used as three-carbon components in Ni(0)-mediated and Rh(l)-catalyzed [3 + 2 + 21-reactions with alkynes (Schemes 48 and 49)142 and with allenes (Schemes 50 and 51).143 All three of the proposed mechanisms for the [3 + 2 + 2]-cycloadditions involve an initial carbene transfer from chromium to nickel or rhodium (Schemes 49, 52, and 53). As is seen from the products of the two [3 + 2 + 2]-reactions with 1,1-dimethylallene, although the nickel and rhodium carbenes 147G and 147K appear similar, the initial insertion of the allene occurs with opposite regioselectivity. 

Beyond dimerization and oligomerization, [2 + 2]- and [4+ 2]-cycloadditions with conjugated dienes and styrenes and the addition of nucleophiles are typical reactions of strained cyclic allenes. These transformations have been studied most thoroughly with 1,2-cyclohexadiene (6) and its derivatives [1, 2]. Concerning the cycloadditions, a theoretical study had the surprising result that even the [4+ 2]-cycloadditions should proceed in two steps via a diradical intermediate [9]. In the case of nucleophiles, the sites of attack at several 1,2-cyclohexadiene derivatives having an... 

By chance, the existence of the borane complex 330 of 329 was discovered. The liberation of 330 occurred with the best efficiency with sodium bis(trimethylsilyl)-amide from the borane complex 327 of 326. When styrene or furan was used as the solvent, three diastereomeric [2 + 2]-cycloadducts 328 and [4 + 2]-cycloadducts 331, respectively, were obtained in 30and 20% yield (Scheme 6.70) [156]. With no lone pair on the nitrogen atom, 330 cannot be polarized towards a zwitterionic structure, which is why its allene subunit, apart from the inductive effect of the nitrogen atom, resembles that of 1,2-cydohexadiene (6) and hence undergoes cycloaddition with activated alkenes. It is noted that the carbacephalosporin derivative 323 (Scheme 6.69) also does not have a lone pair on the nitrogen atom next to the allene system because of the amide resonance. 

Allenic esters such as 349 [12] or allenyl ketones such as 351 [42] are able to undergo intramolecular [4+2]-cycloadditions with participation of both the inactivated or the activated C=C bond of the allene, respectively. The latter starting material is consumed at room temperature, yielding only one product. However, similar... 

Zecchi and co-workers also reported 1,3-dipolar cycloadditions with nitrogen-substituted allenes. As illustrated in Scheme 8.75, the expected isoxazoline derivatives 285 were obtained by [3 + 2] cycloaddition reaction of aminoallenes 246 and nitrile oxide 284 [141, 142], Bis-adducts 286 became the major products when 2equiv. of nitrile oxide 284 were applied with prolonged reaction times. 

Intramolecular [2 + 2]-cycloadditions of allenes and enones occur with fair to good site- and regioselectivities. Thus, cyclohexenone 40 bearing an allenic side-chain at the C3 position was irradiated and a single product 41 was isolated in 95% yield [45],... 

Table 12.6 Uncatalyzed and catalyzed cycloaddition reactions of furans with allenic esters...

The cycloaddition of allenic lactone 124 with cyclopentadiene was carried out under thermal conditions and also in the presence of a Lewis add catalyst [102]. In both cases, the exo-adduct was formed predominantly. 

The cycloaddition of allenes carrying an electron-withdrawing phosphorus substituent has also been studied [118]. Allenyl phosphine oxide 138 is prepared in a manner analogous to allenyl sulfoxide. The [4+ 2]-cycloaddition reaction of 138 with cyclopentadiene proceeded with a high endo selectivity. 

A benzene ring can act as the diene in the intramolecular [4 + 2]-cycloaddition with an activated allene. Thus, aryl allene carboxylates 147 gave tricyclic lactones 148 (Table 12.10) [124],... 

Although allene itself is reluctant to react with ordinary 1,3-dienes, it underwent successful [4+2]-cycloadditions with relatively reactive cyclopentadienes to afford 5-methylenebicyclo[2.2.1]hept-2-ene derivatives [145]. 

Like alkenes, methoxyallene undergo [5 + 2]-cycloaddition with the oxidopyrylium ion formed from the precursor 254 and triethylamine. The allenic terminal C=C bond adds from its sterically less encumbered face to afford the [5 + 2]-cycloadduct [186],... 

Transition metal-catalyzed [4+ 2]-cycloadditions ofdiene-allenes 247 can lead to different results. With a nickel catalyst Wender et al. isolated the anellated system of two six-membered rings 248 with a rhodium catalyst the anellation of a five- and a six-membered ring 249 was possible (Scheme 15.78) [149]. Both transformations proceed readily at low temperatures whereas the uncatalyzed thermal reaction requires 185 °C. Even an anellation of a six- and a seven-membered ring was achieved. 

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