Diels-Alder reaction allene/diene

Because the fluorine substituents both inductively and hyperconjugatively withdraw electron density from the C(2)-C(3) tt bond, the LUMO is located there, and Diels-Alder reactions take place exclusively with this bond [25] 1,1 -Difluoro allene and fluoroallene reaet readily with a large selection of cyclic and acyclic dienes, and acyclic dienes, [2+2] cycloadditions compete with the Diels-Alder processes As shown in the example in equation 79, a significantly different regiochemistry is observed for the [2+4] cycloaddition compared with the [2+2]... 

Treating diene-yne derivatives 50 with ferrate 40 does not lead to the expected ene-allenes, instead the [4 + 2]-cycloaddition products 51 are obtained in moderate yields (eq. 1 in Scheme 11). As metal-catalyzed Diels-Alder-reactions of unactivated aUcynes and dienophiles are assumed to proceed via metaUacyclic intermediates, this supports the mechanism for the Alder-ene-reaction discussed before. 

Hexamethyl[3]radialene (25) does not undergo Diels-Alder-reactions with the typical electron-poor dienophiles, probably because of the full substitution at the diene termini. With TCNE, however, a violet-blue charge-transfer complex is formed which disappears within 30 min at room temperature to form a 1 1 adduct (82% yield) to which structure 55 was assigned9. Similar observations were made with tris(2-adamantylidene)cyclopropane (34), but in this case cycloaddition product 56 (81% yield) was identified its allenic moiety is clearly indicated by IR and 13C NMR data12. 

If the alkenes and acetylenes that are subjected to the reaction mediated by 1 have a leaving group at an appropriate position, as already described in Eq. 9.16, the resulting titanacycles undergo an elimination (path A) as shown in Eq. 9.58 [36], As the resulting vinyltitaniums can be trapped by electrophiles such as aldehydes, this reaction can be viewed as an alternative to stoichiometric metallo-ene reactions via allylic lithium, magnesium, or zinc complexes (path B). Preparations of optically active N-heterocycles [103], which enabled the synthesis of (—)-a-kainic acid (Eq. 9.59) [104,105], of cross-conjugated trienes useful for the diene-transmissive Diels—Alder reaction [106], and of exocyclic bis(allene)s and cyclobutene derivatives [107] have all been reported based on this method. 

Some cases are known in which Diels-Alder reactions of electron-deficient allenes and dienes compete with [2 + 2]-cycloadditions (see also Section 7.3.7) [12, 151, 335, 336]. Recently, a phosphane-catalyzed [4 + 2]-annulation starting from allenic ester 337 and N-tosylaldimines 338 was published [337]. However, the formation of the tetrahydropyridines 339 isolated in excellent yields is explained by a multi-step mechanism and only resembles a Diels-Alder reaction. 

Cycloadditions and cyclization reactions are among the most important synthetic applications of donor-substituted allenes, since they result in the formation of a variety of carbocyclic and heterocyclic compounds. Early investigations of Diels-Alder reactions with alkoxyallenes demonstrated that harsh reaction conditions, e.g. high pressure, high temperature or Lewis acid promotion, are often required to afford the corresponding heterocycles in only poor to moderate yield [12b, 92-94]. Although a,/3-unsaturated carbonyl compounds have not been used extensively as heterodienes, considerable success has been achieved with activated enone 146 (Eq. 8.27) or with the electron-deficient tosylimine 148 (Eq. 8.28). Both dienes reacted under... 

In contrast to the above-mentioned cydoadditions, normal electron demand Diels-Alder reactions exclusively form products where the terminal C=C bond of the allene was attacked by the diene. For example, cydoaddition of N-allenylsulfeni-mide 281 with cydopentadiene (282) affords norbornene derivative 283 (Eq. 8.37) [148]. 

The key reaction in Jung et al. s proposed assembly of Plaunol B (81a) and C (81b) was an intermolecular Diels-Alder reaction between a diene and an allenic lactone that should give the exo-methylene group in the natural product (Scheme 19.16) [20], The phenyl-substituted lactone 83 was prepared as a model for the eventual furan lactone of the plaunols. Cydoaddition of 82 possessing a TBS enol ether and... 

Spino and colleagues134 studied the Diels-Alder reactions of vinylallenes aiming to synthesize six-membered rings with a tetrasubstituted exocyclic double bond, which were to be employed as precursors of quassinoids. Some representative results of their investigations have been summarized in Table 5 (equation 56). Due to the presence of two different substituents at the allene terminus of 200, facial differentiation occurred, which resulted in non-equivalent amounts of geometrical isomers 201 and 202. The major isomers obtained in each case were formed by endo attack of maleic anhydride 144 at the less hindered face of the diene. 

Rhodium catalysis has played a critical role in the development of this type of reaction. The rhodium-mediated [4 + 2] carbocyclization between dienes and unactivated olefins or alkynes is a notable early example of this concept [2]. Further investigations demonstrated the extension of this methodology to the reaction between a diene and an allene [3]. Expansion of the scope of this strategy, to both the intra- and intermolecular [5-1-2] homologs of the Diels-Alder reaction, was accomplished with a vinylcyclopropane and either an alkyne or an olefin to afford the carbocyclization adducts (Scheme 11.1) [4, 5]. 

Studies of strained allenes, such as 1,2-cyclohexadiene, show that they undergo facile Diels-Alder reactions with otherwise unreactive dienes. A comparison of the calculated transition structures and intermediates along the reaction paths of 1,2-cyclohexadiene with 1,3-butadiene as well as propadiene and 1,3-butadiene show... 

Thiophenes can function either as a diene or as a dienophile in an intramolecular Diels-Alder reaction (90CC405). The 7V-(2-thienyl)allene carboxamide (182) on heating at 130°C leads to (183) by a [4 + 2] cycloaddition in which the thiophene functions as a 4ir component, followed by the usual extrusion of sulfur. 

Ley et al. also applied this method to the synthesis of sesquiterpenes through a strategy involving a Diels-Alder reaction. Taking into account the effect of conditions and silver counterion on allene isomerization (see Scheme 33), they obtained the diene partner via isomerization of the acetoxyallene produced on treatment of a propargyl acetate with catalytic amount of silver hexafluoroantimonate (Scheme 3.36).57... 

Based upon the studies on the mechanism of the Cl sequence we rationalized that the elusive allenol intermediate 19 (Chap. 2.2) could participate in intramolecular trapping reactions as an allenyl ether. Furthermore, vinyl allenes are perfectly suited as dienes in Diels-Alder reactions. Considering both reactive functionalities, allenyl ethers and vinyl allenes, which are perfectly suited for domino processes, we designed an insertion sequence based upon cyclizing carbopalladation [76], where the vinyl aUene results from an isomerization of an alkynylation of a vinyl... 

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