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In the first year of this grant a side-project investigating the reactivity of cyclopropenyl acetates, which was related to the main investigation of methyleneaziridines provided a new and exciting avenue of investigation. Cyclopropenyl acetates are interesting building blocks for organic synthesis due to their inherent strain and the presence of binding sites for both p-philic and s-philic Lewis acids. It was initially found that in the presence of the s-philic Lewis acids BF₃.OEt₂ rearrangement of cyclopropene 1 to heteroatom substituted methylenecyclopropane 2 occurred (Scheme 1). Interestingly, the presence of a gem-dimethyl group on the cyclopropene ring led to the formation of Z-acetoxy-diene 3 with high stereoselectivity.  The formation of diene 3 was accompanied by trace amounts of allene 4, which lends support to the intermediacy of an allenyl cation 5. 

Further, screening of Lewis-acids led to the discovery that TiCl₄ causes cyclopropenyl acetates 1 to rearrange to (E)-chloro-dienes 6 (Scheme 2). Again, this rearrangement is thought to proceed via an allenyl cation 5, which is then intercepted by chloride from the least hindered face. Work this year has completed the substrate scope investigation for this reaction, which has been shown to be general for a range of aryl groups. Interestingly, for alkyl-derived substrates the E-selectivity drops significantly.  In addition, we were delighted to find that TiBr₄ can also mediate the reaction, allowing stereoselective preparation of E-bromodienes.

This year we have also worked collaboratively with a computational chemist to elucidate the exact mechanism of the halo-diene forming reaction. These studies were in-line with out initial mechanistic thoughts, but provided additional detail as well as an explanation for the surprising regioselectivity of the halo-diene forming reactions (Figure 1).

These theoretical investigations uncovered two important facts. Firstly, the complete stereoselectivity of the reaction is likely due to the bulky TiCl₄(OAc)^- rather than Cl^- attacking the least hindered face of allenyl cation 3_Ph. Secondly the selectivity for the reaction of diene 8_Ph_Cl (6 in Scheme 2) over allene 7_Ph_Cl is due to thermodynamic control. Attack of TiCl₄(OAc)^- at C-4 of allenyl cation 3_Ph is reversible whereas attack at C-2 is irreversible, resulting in diene 8_Ph_Cl being the only observed product.