Cyclobutanes allenes

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],... 

Fluorinated allenes are especially reactive in cycloadditions because of their highly strained double bonds [118, 119] 1,1-Difluoro- and 1-fluoroallene readily undergo both [2+2] and [4+2] cycloadditions [118 124] (equations 50-52) Exten sive studies of stereochemistry and regioselectivity show that cyclobutane forma-... 

Penta-alkylphosphorane intermediates are also inferred from the products of the reaction of these ylides with silacyclobutane, from which hydrogen is eliminated.44 However, the cyclobutane ring is left intact when the reaction is carried out with more bulky ylides (Scheme 10).46 Silicon- and germanium-substituted allenes have been prepared by the reaction of monometallated ketens with stable ylides, e.g. (43).46... 

The nature of the substituents on the allene can have an impact on the outcome of a [2 + 2] cycloaddition reaction, as was illustrated by the Lewis acid catalyzed cycloadditions of l-thioaryl-3,3-dimethylallene (24) and 1 -methyl- 1-trimethylsilylallene to various 2-alkoxy-p-benzoquinones 25 (e.g. equation 8)17. The reactions were considered to proceed via carbocation intermediates formed by nucleophilic attack of the thioallene on the Lewis acid activated quinone. At lower temperatures, these carbocations closed to cyclobutanes 26, whereas at higher temperatures, the thermodynamically more stable benzofurans 27 were formed. 

Cyclic allenes have improved reactivity due to ring strain. The cycloaddition of 1,2,4-cyclohexatriene (28) with styrene (29), for example, afforded exclusively cyclobutane 31 (equation 9)18. Semi-empirical calculations (AMI) determined the diradical intermediate 30 to be at an energy minimum19. 

Tetramethylene-ethane (TME), or 2,2/-bis-allyl diradical 81, was suggested as an intermediate in the thermal dimerization of allene, as well as in the interconversions of 1,2-dimethylenecyclobutane 82, methylenespiropentane 83, bis-cyclopropylidene 84 and other bicyclic systems (equation 30)45. The isolation of two different isomeric dimethylene cyclobutanes 87 and 88 (in a ca 2 I ratio) after the thermal rearrangement of the deuteriated 1,2-dimethylene cyclobutane 85 suggests that the rearrangement proceeds via a perpendicular tetramethyleneethane diradical (2,2/-bisallyl) 86 (equation 31)45. 

Regioselective stannylcupration of terminal alkynes and allenes, followed by quenching of the cuprate intermediate with ethylene oxide, provides a facile synthesis of cyclobutene and alkylidine cyclobutane derivatives, respectively (Scheme 3.23) [15c]. A number of total syntheses have exploited regioselective stannylcup-... 

Contrary to the phosphine-catalyzed reaction of the allene buta-2,3-dienoate, which adds to CgQ in a ]3-i-2] cycloaddihon ]354], the allene amide 317 forms the cyclobutane annulated fuUerene derivahve in good yields (Scheme 4.63) [355]. The reaction was also performed with similar allene amides bearing a six- or seven-membered lactam. 

The exhaustive controlled-potential reduction of 6-chioro-l-phenylhex-l-yne at — 1.57 V in dimethylformamide containing tetrabutylammonium perchlorate gave a mixture of products. among which was ( >(2-phcnylvinyl)cyclobutane (9).11 It is probable that the mechanism involves initial isomerization of the acetylene to an allene 8 which is reduced at — 1.57 V to the radical anion. Protonation and further onc-clectron reduction then yield the allylic anion. An intramolecular nucleophilic substitution eventually gives the cyclobutane.11... 

The cyclobutane 5 obtained from 1,1 -difluoroallene comes about from dissymmetrical head-to-tail dimerization, while the head-to-head dimer, formed in equal amounts, preferentially reacts further to give oligomers and polymers.25 Both eyclobutanes 4 and 5 have been previously reported (see Houben-Weyl, Vol. 4/4, pp 156 157) from different allene precursors. 

Other electron-withdrawing groups on allenes, such as cyano, facilitate dimerization. In the case of 1,1-dialkylallenes 17, mixtures of stereoisomeric cyclobutanes 18 and 19 are obtained.31... 

Cyclic allenes undergo head-to-head cyclodimerization to give tricyclic 1,2-alkylidenccyclobu-tanes. Unsaturated cyclic allenes represent molecules with increased strain and often dimerize spontaneously. Whereas cyclonona-1,2-diene (28) requires heating of a neat sample to 138°C,38 the tetraene 31 has a half-life of 10 minutes at 0°C.39 The more strained cycloocta-l,2,4,6-te-traene (34) could not be isolated and only the dimeric cyclobutane 35 was formed on thermolysis of the tosylhydrazone precursor 33.40... 

Allene ketene cycloadditions are of greater synthetic utility than cither mixed allene dimerization or mixed ketene dimerization. In this class of reaction the ketene is the more reactive species and homodimerization of ketene can be minimized by use of excess allene. Such cycloadditions always result in 2-alkylidenecyclobutanones with the sp carbons of both moieties forming the initial bond. In substituted allenes and ketenes, mixtures of stereoisomers of 2-alkylidenecyclobutanones are obtained with very little stereoselectivity, the stereoisomers arise from cisUrcins isomerism in the cyclobutane ring and EjZ isomerism of the exocyclic double bond. In unsymmetrically substituted allenes some regiochemical preference for ketene cycloaddition is observed. Examples of dimethylketene allene cycloadditions are summarized in Table 1,2... 

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... 

Virtually all reactions involving cyclobutane formation via cycloaddition of a cumulene to another C —C double-bond system involves excitation of this latter moiety, e.g. an enone or a quinone, and not of the allene or ketene itself.1 Earlier examples of such reactions have been discussed in Houben-Weyl, Vol. 4/5 b, pp 926 931. 

Photochemical reactions of quinones with allenes have also been studied and in some cases cyclobutane formation occurs, although in competition with products derived from attack of the allene on the carbonyl oxygen. Thus, photocycloaddition of tetramethyl-l,4-benzoquinone with 1,1-dimethylallene affords the four-membered carbocycle 6 in good yield.12... 

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 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-... 

Did = E, 3C2,2ffrf,2S4. Allene has this symmetry, as do puckered cyclobutane and cyclooctatetraene. 

The formation of four-membered rings through 2 + 2 cycloaddition is a well-established reaction and the most generally effective synthetic approach to cyclobutanes. Most olefins cannot be induced to undergo this reaction thermally, a finding that is readily rationalized by the forbidden nature of the 2s + 2s addition and the steric difficulties associated with the allowed 2s + 2a pathway. There are nevertheless exceptions. Olefins substituted by two or more fluorine atoms undergo thermal 2 + 2 additions under relatively mild conditions,16 as do ketenes and allenes. 

Keywords allene, thermal cycloaddition, cyclobutane, anthrocyclobutene... 

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... 

Beltrame et al. (1964) suggested this route for the reaction of 1,1-di-phenyl-2-halopropene (190) with EtO- ion which gives the substitution product (191) and a cyclobutane (192) which is a dimer of the alleged intermediate allene (193). The reaction is characterized by an element effect (fcBr/ ci = 2-54-2-79 at 80-125°), and by rate coefficients which are... 

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