Allenyl anion

Table 2 illustrates 1,3-dipoles with a double bond and with internal octet stabilization, commonly referred to as the propargyl-allenyl anion type. These are all reactive dipoles and a large number of five-membered heterocycles can be constructed from these readily available dipoles, especially when the dipolarophile is varied to include heterocumulenes, etc. 

Table 2 1,3-Dipoles with a Double Bond and Internal Octet Stabilization Propargyl-allenyl Anion Type...

The 1,3-dipoles consist of elements from main groups IV, V, and VI. The parent 1,3-dipoles consist of elements from the second row and the central atom of the dipole is limited to N or O [10]. Thus, a limited number of structures can be formed by permutations of N, C, and O. If higher row elements are excluded twelve allyl anion type and six propargyl/allenyl anion type 1,3-dipoles can be obtained. However, metal-catalyzed asymmetric 1,3-dipolar cycloaddition reactions have only been explored for the five types of dipole shown in Scheme 6.2. 

A related method was reported by Katritzky et al. [25], who prepared 1-alkoxy-l-(l,2,4-triazol-l-yl)allenes from the corresponding triazole-substituted alkynes, e.g. the reaction of 18 to 19 in Eq. 8.2. In this case the generated allenyl anion was trapped with methyl iodide. 

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. 

Another modification of the deprotonation/isomerization sequence starts with easily accessible 1-thio-substituted 1-propynes 303. Their deprotonation at the y-position generates allenyl anions that could be trapped regioselectively by different electrophiles R2X (Scheme 8.81) [167-169]. The resulting C-l-substituted allenyl sulfides 304 were obtained in high yields. 

Scheme 9.6 Calculated structures for the allenyl anion and allenyl and hydroxyallenyllithium.

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. 

Most of the substrates for these isomerizations have a tetrahedral carbon with at least one hydrogen substituent between the carbonyl group and the alkyne. Due to the comparable high acidity of this C-H bond neighboring the carbonyl group, already a weak base such as a carbonate, a tertiary amine or aluminum oxide can deprotonate this position and a subsequent protonation at the other end of the pro-pargyl/allenyl anion delivers the allene. 

The structure of the propargyl-allenyl anion is still uncertain but by analogy to the carbonium ion above it may be that shown in Eq. (2). 

Sixteen-Electron. Those for which the dipolar canonical form has a double bond on the sextet atom and the other resonance structure has a triple bond. Examples are azides (R—N3), diazoalkanes (R2C=N=N), and nitriloxides (R—C= N—O). These have also been labeled as propargyl/allenyl anion type [270]. 

Propargyl dianion (QF I ). This anion can be prepared by dilithiation of allene with BuLi in 1 1 ether/hexane. Use of THF (- 50°) or BuLi/TMEDA results in a mixture of propargylide and allenyl anions. The anion couples readily with alkyl and allyl halides to give terminal alkynes. The intermediate lithium acetylide can also react with various electrophiles.3 Example ... 

Reduction of propargylic compounds with Sml2 is possible in the presence of Pd catalysts. Propargylic acetates 233 are converted mainly to the allene 236 by Pd-catalysed reaction with Sml2 in the presence of a proton source [51]. In this reaction, the allenylpalladium 234 is reduced with Sm(II) to the allenyl anion 235, which is protonated to give allene 236. The alkyne 237 is a byproduct. 2,3-Naphthoquinodi-methane (240) as a reactive intermediate, can be generated by applying this reaction. 

Table 3 1,3-Dipoles with a double bond and internal octet stabilization propargyl-allenyl anion type...

Dilithiodibenzylacetytenebis(tmeda) (49) (68) is monomeric. Each lithium atom is bonded to a benzylic carbon and has further interactions with a phenyl C(l) and the acetylenic v system. The structure can be understood as being derived from a perpendicular 2-butyne-l,4-diyl dianion, "CH2—C=C—CH2, in which each negative charge is stabilized in its own orthogonal tt system. The lithiums bridge each of these propargyl (or allenyl) anion systems in a 1,3 manner. 

Alkyllithium reagents also yielded the corresponding alkylated sulfur-stabilized allenyl anions. Thus, treatment of 286 with -BuLi gave the lithio derivative 287 which was reacted with the aldehyde 288. Upon acidification with trifluoroacetic acid (TEA), the furan-containing diester 289 was obtained in 67% yield (Scheme 36) <2002CC2824>. 

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