Orthogonal orbitals allenes

It might, at first glance, come as a surprise that, despite the ban on 5-endo-trig cyclizations, 5-endo-dig cyclizations are allowed. A possible reason for this observation is that intramolecular nucleophilic or electrophilic attack at an alkyne can proceed with the substrate in a flat conformation, because two orthogonal jt (or jt) orbitals are available. Similarly, hydroxymethyl allenes cyclize on treatment with a base (Scheme 9.7), because the terminal Jt orbital of the allene and the hydroxymethyl group can readily achieve a synperiplanar orientation. Intramolecular... 

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 addition of Pd-X groups to 1,3-dienes is an excellent route to jj -allyl complexes (equation 55). For example, a 1,3-diene and palladium salt in the presence of a nucleophile, such as CH, RO, or RCOO , will produce a TT-allyl complex through addition of the nucleophile to a palladium-coordinated double bond (equation 56). Allenes commonly react with addition of nucleophiles at the central sp-hybridized carbon, producing 2-substituted jr-allyls. However, because of the orthogonality of the TT-orbitals, an )) -allyl cannot form directly. First an -allyl is generated, then C-C rotation gives the r-allyl. 

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