Butadiene from cyclopropenes

Wiberg reported the Diels-Alder reaction of butadiene and cyclopropene [53] and Baldwin estimated from the reaction between cyclopropene and 1-deuteriobutadiene at 0°C that 99.4% of the formed cycloadduct was the endo isomer [54], There are many suggestions which attempt to explain endo selectivity in Diels-Alder reactions (Alder s rule [55]), but none are firmly established. According to Woodward and Hoffmann [56], the preference is the result of favorable Secondary Orbital Interactions (SOI) or secondary orbital overlap [57-59] between the diene and dienophile in the corresponding transition state structure. One can also find an explanation for the reaction preference in the difference between primary overlap [60], volumes of activation [61], and the polarity of the transition states [62]. Secondary orbital overlap between the diene and the dienophile does not lead to bonds in the adduct, but primary orbital overlaps do. 

The reasons for the ewrfo-selectivity of Diels-Alder reactions are only useful for the reactions of dienophiles bearing substituents with lone pairs without a Lewis basic site no secondary orbital interactions are possible. But even in reactions of pure hydrocarbons the ewrfo-selectivity is observed, requiring alternative explanations. For example, the ewrfo-preference of the reactions of cyclopropene with substituted butadienes have been rationalized on the basis of a special type of secondary orbital interactions70. Apart from secondary orbital interactions which are probably the most important reason for the selec-tivities of Diels-Alder reactions, recent literature also advocates other interpretations. 

This stoichiometric reaction constitutes a new contribution to vinylidene chemistry and a novel method to generate alkenylcarbene ligand from simple propargyl alkyl ethers rather than via activation of cyclopropenes [4] or by stoichiometric activation of butadiene [6[. When linked to a suitable metal-ligand moiety this carbene constitutes an alkene metathesis initiator. 

As for additions of allylic Grignard reagents, the relative reactivity order of the olefins appears to be 1-alkenes < styrene < 1,3-butadiene < ethylene and a,oi-or a,/3-disubstituted alkenes do not react94. However, strained alkenes such as cyclopropenes constitute an exception. Indeed, dicrotylzinc smoothly reacted with 3,3-dimethylcyclopropene and afforded the dicyclopropylzinc reagent 130 resulting from a syn addition process (equation 62)93. 

The interesting zwitterionic compound (39) with the cationic component a butadien-2-yl cation was obtained by reaction of l,4-di(t-butyl)butadiyne with 2 mol of di(r-butyl)aluminium hydride, with the structure being established by X-ray analysis.80 The reactions of the l,2-diferrocenyl-3-(methylthio)cyclopropenylium ion with carbanions derived from active methylene compounds were investigated.81 Products were derived by ring opening of the cyclopropene ring after the initial carbanion addition. The bis(ferrocenylethynyl)phenylmethylium cation (Fc-C=C-)2C+Ph (Fc = ferrocenyl) was prepared 82 This cation proved to be much less stable than its bis-ethenyl analogue (Fc-CH=CH-)2C+Ph. 

The steroidal cyclopropenes 4-6 provide isomeric butadienes by initial protonation and proton loss from the corresponding allyl cation. A detailed study indicates that regioselective cr-bond fission takes place because of steric hindrance to protonation by the bulky steroid ring from one side. 

Cyclopropene acetals (Fig. 10) can be prepared from dichloroacetone (ref. 57). The first step needs 3 equivalents of NaNH2 (the use of a smaller amount of base results in the formation of the acetal of the chlorocyclopropanone), the third equivalent of NaNH2 is consumed by the formation of the sodium salt which can be alkylated (70-77% yield). Cyclopropenes have also been synthesized from allyl chlorides in various solvents (refs. 58-61). Results are summarized in Fig. 10. Methyl cyclopropenes are generally accompanied by 1,3-butadiene. It is to be noted that the use of KNH2 or of a complex base (NaNH2 /t-BuONa) with medially 1 chloride leads to methylene cyclopropane (ref. 62). 

In 1945 Michael J. S. Dewar suggested that the tropylium ion (the cation derived from cycloheptatriene) should also be aromatic (Figure 9). This was confirmed in 1954 since then, the dianion of butadiene and the dication of cyclooctatetraene have also been shown to be aromatic. Like benzene, all four of these ions are planar rings with six tt electrons. According to Hiickel s rule the cyclopropene cation should also exhibit aromaticity, and it does. (In this case n = 0, and 4n + 2 = 2.) The planar anion of cyclononatetraene and the dianion of cyclooctatetraene should also be aromatic (n = 2, and 4n + 2 = 10), and both of them are. 

Further evidence for the intermediacy of cyclopropenes in the cycloheptatrienyli-dene-arylcarbene interconversion comes from the trapping of the cyclopropene by Diels-Alder reactions with cyclopentadiene, furan, butadiene, and tetracyclone (see also p. 97). 

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