Cyclobutanes allenes + alkenes

Intramolecular [2n + 27r] cycloadditions leading to cyclobutanes formally belong to valence isomerizations. Alkene/alkcne, ketene/alkene, and allene/alkene cycloadditions have received detailed attention. These rearrangements provide powerful methods for the synthetic arsenal. An example is the facile synthesis of bieyelo[3.2.0]heptenones 3 by intramolecular ketene/alkene cycloaddition after electrocyclic ring opening of cyclobutenoncs l.89... 

The cycloaddition of allenes to symmetrically disubstituted alkenes gives mixtures of cyclobutanes with stereochemical equilibration of the substituents. The reaction of 1,3-dimethylallenc with either diethyl fumarate or diethyl maleate produces a mixture of the /raw.v-bis(ethoxycar-bonyl)cyclobutanes.s The same nonstereoselectivity was observed for phenylallene and 1,1-dimelhylallene cycloadditions to maleic and fumaric acid diesters.9 10... 

Donor-acceptor cycloaddition between electrophilic allencs and nucleophilic alkenes, or nucleophilic allenes and electrophilic alkenes, proceeds more efficiently and with high regiose-lectivity. Thus, cycloaddition between 1-morpholinocyclohexene (20) and buta-2,3-dienenitrile occurs to give the bicyclic cyclobutane derivative 21.18 This corrected an earlier erroneous structural assignment of a [3 + 2] adduct for the same reaction.19... 

Nucleophilic allenes react readily with electron-deficient alkenes giving cyclobutanes. Thus, instant cycloaddition occurs between methoxyallene (26) and 2-[bis(trifluoromethyl)methylene]-propanedinitrile.21... 

As in the case of catalyzed cycloadditions of alkenes, reactions of allenes catalyzed by Lewis acids or via vinyl cations proceed more efficiently than their thermal counterparts. Even nonactivated allenes can be induced to react with nonactivated alkenes to give good yields of cyclobutanes. Such reactions could not be carried out under noncatalyzed conditions.1... 

The thermal reaction between two molecules of olefin to give cyclobutane derivatives (a 2 + 2 cycloaddition) can be carried out where the olefins are the same or different, but the reaction is not a general one for olefins.921 Dimerization of like olefins occurs with the following compounds F2C=CX2 (X = F or Cl) and certain other fluorinated alkenes (though not F2C=CH2), allenes (to give derivatives of 97),922 benzynes (to give biphenylene deriv-... 

Analogously, the thermal formation of fused strained tricycles 77 can be rationalized by a mechanism which includes an exocyclic diradical intermediate 80 through an initial carbon-carbon bond formation, involving the proximal allene carbon and the internal alkene carbon atom (path C, Scheme 28). The alternative pathway leading to tricyclic 2-azetidinones 77 is proposed in path D (Scheme 28). This proposal involves an endocyclic diradical intermediate 81 arising from the initial attack of the terminal olefinic carbon onto the central allene carbon. The final ring-closure step of the diradical intermediates account for the cyclobutane formation. 

But amazing contortions are possible. Photocycloaddition of the allene 31 unites just one of the allene bonds with the conjugated alkene to give the very strained cyclobutane 32. Diagram... 

The formation of trans-products is observed to a lesser extent in the reaction of 3-alkoxycarbonyl-substituted cyclohexenones, in the reaction with electron-deficient alkenes and in the reaction with olefinic reaction partners, such as alkynes and allenes, in which the four-membered ring is highly strained (Scheme 6.11). The ester 26 reacted with cyclopentene upon irradiation in toluene to only two diastereomeric products 27 [36]. The exo-product 27a (cis-anti-cis) prevailed over the endo-product 27b (cis-syn-cis) the formation of trans-products was not observed. The well-known [2 + 2]-photocycloaddition of cyclohexenone (24) to acrylonitrile was recently reinvestigated in connection with a comprehensive study [37]. The product distribution, with the two major products 28a and 28b being isolated in 90% purity, nicely illustrates the preferential formation of HH (head-to-head) cyclobutanes with electron-acceptor substituted olefins. The low simple diastereoselectivity can be interpreted by the fact that the cyano group is relatively small and does not exhibit a significant preference for being positioned in an exo-fashion. 

The favored reaction of fluorinated alkenes with dienes is normally [2 + 2] rather than [4 + 2] cycloaddition. There are, however, exceptions for example, 2-(A, A-diethylcarbamoyloxy)-l, 1-difluoroethene reacts with cyclopentadiene to give [4 + 2] cycloadducts as the major products, Fluoroalkenes also readily add to allenes to give cyclobutanes with exocyclic C = C bonds. Examples of such [2 + 2]-cycloaddition reactions are given in Table 7. 

Lewis acid catalyzed reactions of allenes with alkenes generally give cyclobutanes rather than ene adducts. AlCb catalyzed reactions of alkylallenes with alkenes give alkylidenecyclobutanes. Similarly, AICI3 catalyzed reactions of alkynes with alkenes give cyclobutenes. These reactions are believed to occur by stereospecific cycloaddition of the alkene with the vinyl cation formed by complexation of AICI3 to the allene or alkyne. 

This chapter reviews thermal [2 + 2] cycloadditions of alkenes to form cyclobutanes, extending the coverage provided by earlier summaries of the literature.It docs not deal with [2 + 2] cycloadditions that are achieved photochemically, or that utilize allenes, arynes or ketenes, or that are deliberately catalyzed, or that result in heterocyclic four-membered rings, even though these other varieties of [2 + 2] cycloaddition reactions are important and of substantial interest. 

Like ketenes, allenes generally undergo [2 + 2] cycloadditions with alkenes affording methylene cyclobutanes . In reactions with 1,3-butadienes, both Diels-Alda- adducts and [2 -F 2] cycloadducts are formed. Cyclopentadiene, however, has been reported to react with several allenes to give exclusively Diels-Alder adducts. From the sevraal possible mechanisms by which [2 -F 2] cycloaddition reactions of allenes could occur,... 

Allenes also react with captodative alkenes to form 3-methylene cyclobutanes regioselectively. For example, 116 reacts with allene to give 117 upon heating at 140°C for two days [62]. Captodative allenes can even react at room temperature with captodative olefins as an illustration, a-methoxycyanoallene 118 adds to 116 to form 119 in 41% yield [62]. 

A few other types of compounds can react with alkenes to give cyclobutanes, but these reactions are believed to involve diradical intermediates, rather than being concerted.The best substrates for such additions are fluoroalkenes and allenes. Either alkynes or alkenes can serve as the other reactive component. Dienes and other conjugated compounds are more reactive than are simple alkenes. Some examples of this class of reactions are given in Scheme 6.7. 

The classic de Mayo reaction involves the [2 + 2] photocycloaddition of an alkene to the hydrogen-bonded enol tautomer of a P-dicarbonyl compound as exemplified by the formation of 1,5-diketone 9 from pentane-2,4-dione and cyclohexene (vide supra). In addition to alkenes, allenes are also used as the olefinic component. For example, irradiation of a mixture of dimedone and allene results in the formation of 3,3-dimethyl-7-methylenecycloocta-l,5-dione 12 via the cyclobutane intermediate 11, together with the corresponding head-to-tail product 13, which spontaneously dimerizes to the hetero Diels-Alder adduct 14. Diketone 12 is a versatile building block for the preparation of substituted cyclooctadienones and 8-valerolactones. 

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