Allenes sterically encumbered

Remarkably, the regioselectivity of the cuprate addition to acceptor-substituted enynes is also insensitive to the steric properties of the substrate. Thus, enynes with tert-butyl substituents at the triple bond (e.g. 68) underwent 1,6-additions even when the cuprate was also sterically demanding (Scheme 2.24) [47]. The method is therefore highly suitable for the preparation of sterically encumbered allenes of type 69. 

Scheme 2.24 Synthesis of sterically encumbered allenes by 1,6-cuprate addition.

The mechanism of this transformation has explored by Hashmi, and postulated to proceed via a Pd(II)/Pd(IV) cycle, wherein palladium mediates cyclization to form 14 (Scheme 6.23), followed by either proton migration and reductive elimination (forming simple furans) or allene coupling via either a carbene or insertion. This approach can be employed to form exclusively monomeric cydoisomerization products with more sterically encumbered substituted allenes [34]. 

Like alkenes, methoxyallene undergo [5 + 2]-cycloaddition with the oxidopyrylium ion formed from the precursor 254 and triethylamine. The allenic terminal C=C bond adds from its sterically less encumbered face to afford the [5 + 2]-cycloadduct [186],... 

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