Allenes 6+2 cyclodimerization reactions

Cyclodimerization reactions can occur across either one of the cumulative double bonds giving rise to the formation of head-to-head or head-to-tail cyclodimers. The stable head-to-tail cyclodimers of ketenes and the head-to-head cyclodimers of isocyanates are good examples and only one type of cyclodimer is formed. In contrast, allenes often provide mixtures of cyclodimers. A super-click reaction is observed in the cyclodimerization of bis-allenes, which occurs at room temperature in the solid state upon irradiation to give the cyclodimers in quantitative yields. ... 

In general the thermal reaction of allene gives a complex mixture of dimers, trimers, and higher oligomers including small amounts of spiro compounds (140). A highly selective dimerization to 1,2-dimethylene-cyclobutane is achieved by thermal reaction of dilute solutions Theoretically the process [2s + 2a] of allenes and related cumulenes may be facilitated by participation of the orthogonal pir-orbital in the addition 142). However, the concertedness of the cyclodimerization is still in dispute. Selective cyclotrimerization and pentamerization of allene have not... 

The last observations automatically lead to the conclusion that non-activated alkenes also could undergo these reactions. Indeed it was found that ethylene, norbornene, norbomadiene198) and allene 199) react with methylenecyclopropane to give cycloadducts (Scheme 7). The reason for the limitation to these alkenes lies in the ability of methylenecyclopropane to compete successfully with alkenes in it-complexation to the metal. Thus cyclodimerization of methylenecyclopropane is much faster than codimerization with other alkenes, which give less stable ic-com-plexes with Pd(0). 

Although the endothermicity of the 1- 2 interconversion may be overcome both photochemi-cally and thermally (see below), it is only by photoisomerization that cyclopropenes have been prepared from allenes in isolable amounts. As shown in the table below, the allene-cyclopropene rearrangement has been employed predominantly for the preparation of highly substituted and bicyclic cyclopropenes, respectively. It appears that the ring strain of the starting cycloal-lenes is a prerequisite for the success of the interconversion. Furthermore, in order to suppress [2 + 2] cyclodimerization of the substrates a bulky substituent in the cyclic or acyclic precursor is necessary. Low reaction temperatures serve the same purpose. 

In this latter reaction mode, which is observed much more rarely than /3-dehydropal-ladation, a wide variety of ligands can be coupled to each other with the formation of new C—C, C—H, C—N, C—O, and C—Hal bonds. This section does not cover the numerous cascade couplings in which a number of successive intramolecular additions of 2 onto double bonds is eventually completed by /3-dehydropalladationt as well as the numerous [2 + 2 -l- 2] and [4 -I- 2] cyclotri- and cyclodimerizations of alkynes, enynes, and related compounds.The Pd(0)-catalyzed Cope rearrangement also will not be considered here, as it proceeds via bis(i7 -allyl)palladium(II) intermediates. The carbopalladation reactions of allenes, which have been reviewed recently, are covered in Sect. IV.7. (For new examples see also refs. [10]-[12]). On the other hand, the numerous Pd-catalyzed formal [3 + 2] cycloadditions of trimethylenemethane (TMM) complexes may be classified as carbopalladations of alkenes without subsequent dehydropalladation. As the subject of this section has partially been covered in several newly published reviews, " the attention here will be on the most recent and interesting communications. 

The cycloaddition reactions are subdivided into di-, tri- and oligomerization reactions, [2-1-1]-, [2-1-2]-, [3-1-2]- and [4- -2] cycloaddition reactions and other cycloaddition reactions. The insertion reactions into single bonds are also discussed. The cyclodimerization or cyclotrimerization reactions are special examples of the [2-1-2] and the [2-I-2-I-2] cycloaddition reactions, respectively. The cumulenes vary in their tendency to undergo these reactions. The highly reactive species, such as sulfines, sulfenes, thioketenes, carbon suboxide and some ketenes, are not stable in their monomeric form. Other cumulenes have an intermediate reactivity, i.e. they can be obtained in the monomeric state at room temperature and only heat or added catalysts cause di- or trimerization reactions. In this group, with decreasing order of reactivity, are allenes, phosphorus cumulenes, isocyanates, carbodiimides and isothiocyanates. 

The thermal cyclodimerization of allenes proceeds via a stepwise [2-1-2] cycloaddition reaction, the intermediate being a diradical. From allene, in a flow reactor, a mixture of the head-to-head dimer 25 and the head-to-tail dimer 26 are obtained. At 500 °C (9 % conversion) a mixture of 34% of 25 and 13% of 26 are formed. The formation of the cyclodimer 26 is unusual, because substituted allenes afford only the head-to-head dimers. 

Further experimental results (see also ref. 164) on the stereospecificity of [2 -1- 2] cycloadditions are available. Tetrafluoroethylene adds to both cis- and tra s-2-butene and to ethylene in a non-stereospecific manner, interpreted as evidence of biradical intermediates. Some stereospecificity is observed in allene-keten [2 + 2] cycloadditions and a one-step [,2, + J cycloaddition is indicated. Further deuterium-isotope studies indicate that the cyclodimerization of allenes by a [2 q- 2] addition is non-concerted, but their reactions in [3 + 2] and [4 + 2] cycloadditions are concerted. ... 

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