Allene dication

Cation-radicals containing three electrons in the field of four atoms or anion-radicals with five electrons retained by four atoms represent a special group of multicentered ion-radicals. Thus, nonclassical, cyclically delocalized 3e/4C cation-radicals and 2e/4C (dication)-radicals of substituted cyclobutadienes are known (Allen and Tidwell 2001). The 3e/4N cation-radical and the 5e/4N anion-radical have also been discovered (Exner et al. 1998, 1999, 2000). The reactions in Scheme 3.31 illustrate structures of the 5e/4N anion-radical as well as the corresponding dianion and acety-lated products of the latter. 

From a preparative point of view the access to new CL2 or CLL compounds and the study of their coordination ability is a challenge for the chemical community. In most cases the carbon(O) atom is stabilized by simple and well known Lewis bases but the uptake as well as the removal of this carbon(O) atom is not trivial. The latter reaction, the transformation of a single carbon atom to other substrates, has not been observed as yet. The access to a variety of stable bisylides or bent allenes often proceeds upon deprotonation of related (HCL2)" cations or (H2C L 2) dications by appropriate strong Lewis bases. 

The palladium catalysed addition of N-H or O-H bonds onto allenes has successfully been exploited in the preparation of oxazepines, diazepines, oxazocines and diazocines. The nucleophilic attack of the pendant alcohol or sulphonamide function on the allene moiety was followed by the incorporation of the alcohol, used as solvent, to give the desired cyclic products in good yield (5.15. and 5.16.). The bromoallene in these processes is the synthetic equivalent of an allylic dication.15... 

A highly regio- and stereoselective synthesis of oxepine derivatives via cyclization of bromoallenes bound with hydroxy group by a four-carbon atom tether in the presence of a palladium(O) catalyst and alcohol is developed (Equation 16). In this reaction, the bromoallenes act as an allyl dication equivalent, and the intramolecular nucleophilic attack takes place exclusively at the central carbon atom of the allene moiety <2004JA8744>. 

Subsequently, the (5,5 )-dimethylated allenyl dication 179 has been synthesized and structurally characterized. The C=C=C distances (1.310 and 1.319 A) indicate allene character, that is, the resonance structure 179b does not play a significant role. The four C-O distances in cation 179 are nearly equal (2.641-2.750 A), considerably longer than that of a typical C-O bond (1.43 A), and shorter than the sum of the van der Waals radii (3.25 A). In the calculated structure [optimized B3PW91/6-3G(d) level], the C-O distances are slightly longer and more symmetrical (2.705-2.735 A). The results imply that allenyl dication 179 contains hexacoordinate carbon. 

The Ni-catalyzed reaction solves the problem in ring opening of recalcitrant oxabicycles. Alkynyl dithioacetals that can be described as allene 1,3-dication synthons give allenes. ... 

X- ein ring opening of recalcitrant de-.- N- d as allene 1,3-dication synthons... 

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