Conjugated Bisallenes

To prepare the parent bisallene 118, 116 is first converted into its Grignard reagent (known from spectroscopic studies to possess the allenic structure), from which, presumably, by the addition of cuprous chloride the organocopper intermediate 117 is generated. Addition of further 116 subsequently provides a mixture of 118 and propargylallene (l,2-hexadien-5-yne) (29) (see below) in a 2 3 isomer ratio [44], 

After transmetalation of 121 with zinc chloride in THF, the resulting 122 could either be coupled with the undeuterated bromoallene 123 to the monodeuterio derivative 124 or with 120 to the dideuterated hydrocarbon 125. 

Hydrocarbon 136 subsequently cyclizes to a bismethylencyclobutene again, 137, in which the triple bonds of the substituent are so close that they can engage in a [2+ 2] cycloaddition, leading to the doubly annelated hydrocarbon 138. For less highly unsaturated bisallenes, a similar behavior is observed and they allowed the determination of the stereochemistry of the first ring-closure step [51]. 

Although conjugated bisallenic hydrocarbons display a rich chemistry (see Section 5.5), more and more functionalized derivatives are being described in the chemical literature [52, 53], 

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The cross-conjugated bisallene 226 is obtained when the diacetylene 224 is treated with potassium tert-butoxide in THF at -78 °C for 30 min (Scheme 5.34) [76]. 

In semicydic allenic hydrocarbons, one of the terminal allene carbon atoms is part of an alicyclic ring system, as illustrated by the general structure 37 in Scheme 5.3. Numerous hydrocarbons of this type are known, some of them carrying more than one allene group, such as in the case of the conjugated bisallenes 127 and 129 (see Scheme 5.17), and many of them are described in the review literature [2] and will not be repeated here. However, since Chapter 6 on cycloallenes does not treat these derivatives, some new developments in this area will be briefly presented, limited to the two cases in which cydopropane rings form the end groups of the allene moiety, i.e. 246 and 249. 

Five-membered ring compounds are obtained from conjugated bisallenes under a variety of conditions. Whereas oxidation with m-chloroperbenzoic acid (MCPBA) provides ketones of type 323 [137], carbonylation by iron pentacarbonyl in THF at 50 °C leads to the symmetrical dialkylidene cyclopentenones 326 [45]. With phosphi-nidene complexes such as 325, biallenyls react in a stepwise process that eventually leads to adducts such as 327 [138], mimicking the behavior of singlet carbenes [139]. 

Basak, A., Das, S., Mallick, D. and Jemmis, D. (2009) Which one is preferred Myers-Saito cyclization of ene-yne-allene or Garratt-Braverman cyclization of conjugated bisallenic sulfone A theoretical and experimental study. Journal of the American Chemical Society, 131(43), 15695-15704. 

Huntsman first observed that 1,5-hexadiyne gave 3,4-dimethylenecyclobutene upon heating at 250°C with an activation enthalpy of 34.4 kcal/mol and an activation entropy of — 9.4 e.u. " A reasonable reaction pathway would invoke a 3,3-shift to bisallene followed by electrocyclization of the destabilized conjugated diene moiety (Scheme 7.15). 

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