Electrophilic Addition to Allene Derivatives

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

However, in many cases, in particular for allene derivatives bearing substituent which can stabilize a positive charge at a terminal carbon atom, the electrophilic attack is favourable at the central carbon atom to give an allylic-type ion (equation 12). However, it is to be emphasized 

In poorly nucleophilic media such as FSOsH-SbF6, allylic carbonium ions can easily be generated by protonation of the central carbon of 1,3-dimethylallene and of tetramethylallene. These ions have been directly observed (Pittman, 1969) and identified from the XH n.m.r. and u.v. band positions and splittings and kept unchanged for 1 week at -70°. 

The alternative modes of addition under equations (11) and (12) can be easily distinguished on the basis of the primary product distribution. However, bridged species other than open ions are frequently involved as intermediates and ring opening may occur by direct nucleophilic displacement processes (Poutsma and Ibarbia, 1971). 

Protonation of the central carbon atom (equation 14) probably occurs in the case of alkoxyallene derivatives (56) which, by reaction in dilute sulphuric acid at 60°, yield the corresponding a,jS-unsaturated aldehyde 57 (van Boom et al., 1965 Richey and Richey 1970). 

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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. 

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... 

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... 

The discovery of carbene and carbenoid additions to olefins was the major breakthrough that initiated the tapping of this structural resource for synthetic purposes. Even so, designed applications of cyclopropane chemistry in total syntheses remain limited. Most revolve around electrophilic type reactions such as acid induced ring opening or solvolysis of cyclopropyl carbinyl alcohol derivatives. One notable application apart from these electrophilic reactions is the excellent synthesis of allenes from dibromocyclopropanes 2). 

Hence the positional selectivity is different from that of the furan additions to 417 (Scheme 6.90). Assuming diradical intermediates for these reactions [9], the different types of products are not caused by the nature of the allene double bonds of 417 and 450 but by the properties of the allyl radical subunits in the six-membered rings of the intermediates. Also N-tert-butoxycarbonylpyrrole intercepted 450 in a [4 + 2]-cycloaddition and brought about 455 in 29% yield. Pyrrole itself and N-methylpyr-role furnished their substituted derivatives of type 456 in 69 and 79% yield [155, 171b]. Possibly, these processes are electrophilic aromatic substitutions with 450 acting as electrophile, as has been suggested for the conversion of 417 into 442 by pyrrole (Scheme 6.90). 

Ethynyl carbinols (propargylic alcohols) such as 134 (Scheme 2.58) represent another important group of oxidation level 3 compounds. Their preparation involves nucleophilic addition of acetylides to the carbonyl group, a reaction that is nearly universal in its scope. Elimination of water from 134 followed by hydration of the triple bond is used as a convenient protocol for the preparation of various conjugated enones 135. Easily prepared O-acylated derivatives are extremely useful electrophiles in reactions with organocuprates, which proceed with propargyl-allenyl rearrangements to furnish allene derivatives 136. 

Trimethylchlorosilane, dimethyldisulfide, butyl bromide, butyl iodide, paraform, acetic and pivalic aldehydes, benzaldehyde, acetone, and cyclohexanone are used as electrophilic reagents relative to N-allenylpyrrole metalated with butyUithium. The conditions of the second stage of the reaction, electrophilic substitution, or addition (for aldehydes and ketones) depend upon the electrophile anployed. In all the cases, a-allenic derivatives are formed as a rule (Table 2.20). 

Addition of arylhydroxylamines to electrophilic allenes such as methyl propadienoate or l-methancsulfonyl-l,2-propadiene is another route to 0-vinyl derivatives[2]. The addition step is carried out by forming the salt of the hydroxylamine using NaH and the addition is catalysed with LiO CCFj. The intermediate adducts are cyclized by warming in formic acid. Yields are typically 80% or better. 

In contrast to the rich chemistry of alkoxy- and aryloxyallenes, synthetic applications of nitrogen-substituted allenes are much less developed. Lithiation at the C-l position followed by addition of electrophiles can also be applied to nitrogen-containing allenes [10]. Some representative examples with dimethyl sulfide and carbonyl compounds are depicted in Scheme 8.73 [147, 157]. a-Hydroxy-substituted (benzotriazo-le) allenes 272 are accessible in a one-pot procedure described by Katritzky and Verin, who generated allenyl anion 271 and trapped it with carbonyl compounds to furnish products 272 [147]. The subsequent cyclization of 272 leading to dihydro-furan derivative 273 was achieved under similar conditions to those already mentioned for oxygen-substituted allenes. 

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