Electrophilic Additions to Allenes

The orientation of addition of water or methanol to allenic alcohols (358) catalysed by mercuric salts depends upon the degree of substitution of the allene and on the mercuric salt used.  

The rearrangement products (359) and (360) are obtained on oxymercura-tion of cyclodeca-l,2,5,8-tetraene (361). A bridged intermediate (362) is suggested homoallyl-cyclopropyl rearrangement leads to (363), a metal-complexed carbene which is known to undergo hydride shifts as indicated. This mechanism is supported by deuterium labelling.  

Albrand, Bull. Soc. chim. France, 1972, 1946. 

---

Electrophilic additions to allenes represent an interesting reaction type which is related to additions to both alkenes and alkynes. An allene could, for example, conceivably be protonated at either a terminal s[p- carbon or the central sp carbon. 

Electrophilic addition to allenes can occur via either of two ways ... 

Although at first glance addition to the central carbon and formation of what seems like an allylic carbonium ion would clearly be preferred over terminal addition and a vinyl cation, a closer examination shows this not to be the case. Since the two double bonds in allenes are perpendicular to each other, addition of an electrophile to the central carbon results in an empty p orbital, which is perpendicular to the remaining rr system and hence not resonance stabilized (and probably inductively destabilized) until a 90° rotation occurs around the newly formed single bond. Hence, allylic stabilization may not be significant in the transition state. In fact, electrophilic additions to allene itself occur without exception at the terminal carbon (54). 

Less common addition reactions such as the bromination of trifhioromethyl-substi-tuted butatrienes [30] or the reaction of tetrafluoroallene with boron trifluoride have also been reported [283]. Especially the interaction of phosphorylated allenes with electrophiles was summarized in a review by Alabugin and Brel [8], whereas Smadja [284] published a more general overview about the electrophilic addition to allenic derivatives. 

Allenes can also act as the -participant in electrophilic heteroatom cyclizations. Reviews of electrophilic additions to allenes discuss early examples of this type of cyclization.ld le-202 Numerous examples of cyclizations of a-functionalized allenes, including carboxylic acids, phosphonates, sulfinates and alcohols, to form five-membered heterocycles (equation 84) are cited in these reviews. The silver nitrate-mediated conversion of ot-allenic alcohols to 2,5-dihydrofurans203 has recently been applied to trimethylsilyl-substituted systems.204... 

Electrophilic additions to allene derivatives were shown to occur, in a number of cases via attack of a positively charged electrophile to a 8... 

The relative stability of vinyl and saturated cations in solution can in principle be evaluated by following three approaches (a) from the competitive formation of vinyl and saturated cations in electrophilic addition to allenes (b) from the relative rates of electrophilic addition to alkynes and alkenes (c) from the relative rates of solvolysis of vinyl and saturated derivatives. 

Recent structural and spectroscopic investigations of organometallic complexes bonding two carbons of an allenic ligand to one rhodium 50 72> 87,95) or platinum atom 58,87,98,132) may have some pertinence to possible bridged intermediates proposed for various electrophilic additions to allenes, and the cr-iron-jr-iron complexes derived from allene and diiron... 

Electrophilic Additions to Allenes.—Protonation of tetramethoxyallene, the bis(dimethylacetal) of carbon suboxide, takes place at the central carbon atom the resulting cation reacts with nucleophiles either by addition or by demethylation (Scheme 16). The tetramethoxyallyl cation (356), produced by... 

---