Allenes electrophilic additions

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). 

In contrast to the behavior of tetramethylallene, allene 38 undergoes central protonation in electrophilic additions. In an acetic acid sulfuric acid... 

A summary of electrophilic additions to triple bonds and allenes involving a vinyl cation is given in Table IV. 

Summary of Electrophilic Additions to Acetylenes and Allenes Involving Vinyl Cations... 

All of the above mentioned examples of vinyl cation intermediates have involved electrophilic additions to triple bonds or allenes or participation in solvolyses of such multiple bonds. In a sense, these reactions derive from analogies in normal... 

Both allenes141 and alkynes142 require special consideration with regard to mechanisms of electrophilic addition. The attack by a proton on allene might conceivably lead to the allyl cation or the 2-propenyl cation. 

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. 

In this chapter, both intermolecular and intramolecular electrophilic [1] and nucleophilic additions [2, 3] to allenes will be discussed. For electrophilic addition, the regio- and stereoselectivity depend on the steric and electronic effects of the substituents on the allenes and the nature of the electrophiles. However, nucleophilic addition usually occurs at the central carbon atom with very limited exceptions. 

The electrophilic addition of alkyl-substituted allenes may afford terminal attack and center attack products, depending on the structures of allenes and electrophiles (Scheme 10.1). 

Sulfenyl chlorides and halogens react with 1,2-alkadienylphosphonic acids to afford phosphorus-containing heterocydes [72], However, the electrophilic addition of dialkyl 4-methyl-2,3,5-hexatrien-2-yl phosphonates with sulfenyl or selenyl chloride afforded 2-thienyl methylphosphonates or the seleno analogues [73, 74]. The conjugate addition of sulfenyl or selenyl chloride with the 2,4-diene moiety in the starting allene leads to the formation of the five-membered skeleton (Scheme 10.69). 

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... 

Fluorinated carbocations play an important role as intermediates in electrophilic reactions of fluoroolefins and other unsaturated compounds. For example, F-allyl cation 1 was proposed as a reactive intermediate in reactions of HFP with fluoroolefins catalyzed by Lewis acids [7]. The difference in stability of the corresponding allylic cations was suggested as the explanation for regio-specific electrophilic conjugated addition to CF2=CC1CF=CF2 [11]. Allylic polyfluorinated carbocations were proposed as intermediates in the reactions of terminal allenes with HF [53] and BF3 [54], ring-opening reactions of cyclopropanes [55], Carbocations are also an important part of the classic mechanism of electrophilic addition to olefins (see Eq. 2). This section deals with the questions of existence and stability of poly- and perfluorinated carbocations. 

Treatment of 1-bromoallenyl ethyl ester 869 with bromine leads to 3,4,5-tribromo-6,6-dimethyl-3,6-dihydropyran-2-one 871. The reaction proceeds through initial electrophilic addition of bromine to the central allene carbon atom and cyclization of the resulting carbenium bromide furnishing the intermediate 870. Further reaction with bromine followed by loss of FIBr affords the 3,6-dihydropyran-2-one 871 (Scheme 240) <1996LA171>. 

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... 

Silver(I), Cu(II) and Hg(II) have been employed in the dimerization of 1 and discussed elsewhere as has the role of the first of these in the addition of alkenes, alkynes, allenes and conjugated dienes to cyclopropabenzene. It is more than probable that product formation with Ag(I) is dictated by electrophilic addition of the metal and subsequent interaction of the organosilver benzylic cation with the hydrocarbon reagent to give ring-opened or ring-expanded products. 

Selenium electrophiles add to conjugated dienes, only forming 1,2-adducts. Although Maricovnikov addition is seen, only a few examples have been reported. With allenes the additions are re ospeciric, with the i enylseleno group usually adding to the rp-carbon. Unfortunately, all four stereoisomers of halide attadc are seen with unsymmetrically substituted allenes. ... 

A number of useful enantioselective syntheses can be performed by attaching a chiral auxihary group to the selenium atom of an appropriate reagent. Examples of such chiral auxiliaries include (49-53). Most of the asymmetric selenium reactions reported to date have involved inter- or intramolecular electrophilic additions to alkenes (i.e. enantioselective variations of processes such as shown in equations (23) and (15), respectively) but others include the desymmefrization of epoxides by ringopening with chiral selenolates, asymmetric selenoxide eliminations to afford chiral allenes or cyclohexenes, and the enantioselective formation of allylic alcohols by [2,3]sigmafropic rearrangement of allylic selenoxides or related species. 

---