Allenic alkyllithiums

The lithiation of allene can also be carried out with ethyllithium or butyl-lithium in diethyl ether (prepared from the alkyl bromides), using THF as a cosolvent. The salt suspension which is initially present when the solution of alkyllithium is cooled to -50°C or lower has disappeared almost completely when the reaction between allene and alkyllithium is finished. 

Likewise, the sulfoxide-metal exchange reaction of /3-acetoxy sulfoxides 164 (/3-mesyloxy sulfoxides can also be used), which are prepared from alkenyl aryl sulfoxides 163 and aromatic aldehydes, with a Grignard or alkyllithium reagent at low temperatures gives the allenes 162 in good to excellent yield (Scheme 5.24) [65],... 

Terminal propargylic mesylates are converted to alkylallenylzinc compounds by reaction with lithiotrialkylzincate reagents (Scheme 9.32) [117]. The latter are formed in situ from dialkylzinc and alkyllithium species. Deuterolysis of the allenylzinc intermediates gave rise to deuterated allenes (Eq. 9.138). 

Most, perhaps all, of the reactions that simple alkenes undergo are also available to allenes. By virtue of their strain and of the small steric requirement of the sp-hybrid-ized carbon atom, the reactions of allenes usually take place more easily than the corresponding reactions of olefins. Because the allenes can also be chiral, they offer opportunities for control of the reaction products that are not available to simple alkenes. Finally, some reaction pathways are unique to allenes. For example, deprotonation of allenes with alkyllithium reagents to form allenyl anions is a facile process that has no counterpart in simple alkenes. These concepts will be illustrated by the discussion of cyclization reactions of allenes that follows. 

Formally, in the two steps of the DMS-process (dibromocyclopropanation and reaction with alkyllithium) a carbon atom is inserted between the two centers of a double bond. The reaction may be extended to the preparation of still higher cumulated bond systems as well as to numerous other — including functionalized — allenic systems which cannot or only with much effort be prepared by other routes. The examples shown here serve illustrative purposes only, for more extensive coverage of the literature the reader is referred to the various reviews and monographs which have appeared recently [66, 69, 71, 72]. 

If the double bond is connected directly to the three-membered ring, i.e. if the substrate is a dihalo-vinylcyclopropane derivative, a vinylcyclopropylidene to cyclopentadiene isomerization, the so-called Skattebol rearrangement, takes place upon treatment with alkyllithium. Again, this ring-forming step competes with a ring-destruction process leading to allenic hydrocarbons (vinylallenes,... 

ALLENES Alkyllithium reagents. Chromium(HI) chloride-Lithium aluminum hydride. Formaldehyde. N,N-Mcthylphcnylamino(tri-n-butylphos-phonium) iodide. Tricthyl orthoaeetate. Trimethylsilylm ethyEmagncsium chloride. 

ALLENIC ALCOHOLS Alkyllithium reagents. Alumina. Lithium aluminum hydride. 

Conjugated vinyl sulfines 68, produced by treatment of the allene 67 with an alkyllithium and then with sulfur dioxide, undergo cycloaromatization to give thiophenes 69 in one pot (Scheme 17) <1995RTC51>. 

Alkyllithiums also induce C—P cleavage. In the reaction of (31) with MeLi, P—C cleavage occurs to give Ph2PMe and the corresponding allenyllithium, which is converted into tris(diphenylphosphine)-allene (32) upon hydrolysis. ... 

Although similar chemistry was observed with iodide 34 (R = Me, Scheme 7-26) which possesses a methyl substituent at the internal allenic carbon, the alkyl substitution at the remote end of the allene did not lead to the cyclic product [25]. The same methodology was applied to systems in which the reactive functions are separated by longer chains, as y-allenic alkyllithium 35. The latter cyclizes at low temperature to produce, after reaction with an electrophile, a mixture of cyclic products [27] (Scheme 7-28). 

The rhodium-cataiyzed intramoiecuiar [5+2] cycioaddition of an allene and vinylcyclopropane was the key step in the asymmetric total synthesis of the trinorguaiane sesquiterpene (+)-dictamnol by P.A. Wender and co-workers.The cyclization precursor allene-cyclopropane was assembled starting from commercially available cyclopropane-carbaldehyde. Using the HWE oiefination, the Weinreb s amide moiety was installed and subsequently reacted with a primary alkyllithium that was generated via lithium-halogen exchange. 

Moore, W. R., Ward, H. R. Reactions of gem-dibromocyclopropanes with alkyllithium reagents. Formation of allenes, spiropentanes, and a derivative of bicyclopropylidene. J. Org. Chem. 1960, 25, 2073. 

Allenes can be directly deprotonated using alkyllithiums, and the intermediate lithio allenes can then be alkylated efficiently by primary alkyl bromides or iodides." For example, good yields of homologated allenes can be realized under the conditions specified in Scheme 23 (the brief reaction time appears... 

The use of excess alkyllithium can result in the rearrangement and alkylation of terminal allenes to produce alkynes. This is particularly a problem with phenyl-substituted allenes, e.g. 34. ... 

With the exception of tetrasubstituted 1,1-dihalocyclopropanes, the dehalogenation by alkyllithium represents a general and convenient method for the preparation of allenes. With tetraal-kylcyclopropanes the major reaction is intramolecular C-H insertion of cyclopropylidene and formation of bicyclobutane, although low to modest yields of tetraalkylallenes can be achieved using this method (Table 4, entries 1 -4). 

The alkyllithium dehalogenation of dihalocyclopropanes can be used for derivatives having heteroatomic substituents (O, N, S) and is even compatible with alcohol functions and the preparation of allene alcohols. In the latter case two or more equivalents of the alkyllithium reagent must be used (see Table 5). Improved yields of allene alcohols can be obtained by protecting the alcohol function as the trimethylsilyl derivatives. Competing processes which can occur include C-H and O-H insertion, reductive dehalogenation and intramolecular insertion of cyclopropylidene at unsaturated sites of the substituent. 

Dihalocarbene adducts from bicyclic and polycyclic exomethylene hydrocarbons upon de-halogenation with alkyllithiums give the corresponding exocyclic allenes. Many of these allenes are chiral by virtue of their chiral hydrocarbon precursors (/ -pinene, camphene, etc.). Examples are the dehalogenation of 60, ° 62, 64, 66, 68, and 70. ... 

The reaction of a 1,1-dibromocyclopropane with an alkyllithium generally leads to 1,1-elimination producing a cyclopropylidene, which normally rearranges to an allene, but may be trapped by other carbenic reactions. When there is a silyl substituent on C2, the carbene is apparently trapped by a 1,2-silicon shift giving a silyl-substituted cyclopropene. ... 

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