Acyclic allenes

In summary, a broad range of synthetic methods are available today to allow the synthesis of practically any required acyclic allenic hydrocarbon. We can therefore turn to the preparation of specific target molecules and we preferentially select these from the hydrocarbons collected in Schemes 5.1-5.3. Before doing that, however, some recent approaches to alkyl-substituted allenes are considered. 

As far as acyclic allene hydrocarbon intermediates are concerned, these begin with a C3H2 species, 254, about which there has been considerable discussion since it has also been detected in radioastronomy studies to belong to the (growing) number of organic compounds present in interstellar and circumstellar space [105]. 

Reactions of complexes of 1,2-cycloheptadienes have received only cursory attention. 1,2-cycloheptadiene is readily displaced from bisftriphenyl-phosphine)platinum(O)118 [Eq. (54)], but no reagent has been found that will displace the allene from iron.119 Reaction of the iron complex with alcohol in the presence of base (e.g., 312 — 322) is typical of Fp+ complexes of acyclic allenes.131132 The thermal chemistry of 312 is unusual in its decomposition to 324 (Scheme 41). This is probably attributable to the presence of the triflate counterion, since the corresponding fluoroborate salt is stable when warmed to 40°C for 16 h.119 A mechanism to 324 via carbene complex 321 appears likely. 

The zirconium complex 315 reacts with diphenylacetylene to give the coupling product 326 [Eq. (55)].116 The regiochemistry of the reaction parallels that of an acyclic allene complex.133 Complex 315 also reacts with stilbene, but in this case, the seven-membered ring is displaced from the metal. The fate of the seven-membered ring is unknown. 

The reactions of simple, acyclic allenes with borane-THF are usually complicated. However, cyclic al-lenes give cleaner reactions, which can be controlled to give monohydroboration products, involving predominant attack of boron on the central carbon atom, though the intermediate vinylboranes have not generally been isolated. 

Reduction of allenes. Cyclic and acyclic allenes are reduced stereospecifically by diimide (generated from hydrazine, H2Oa, CuS04) to cis-alkenes.6... 

Marine organisms are a rich source of exotic polyalkynes many of which possess structures not found in terrestrial species in addition to conventional acyclic allene-alkyne hydrocarbons and their simple derivatives. For example, red algae (in particular of the Laurencia and Phacelocarpus genera) accumulate compounds such as 23-26 " in which bromine derived by enzymic oxidation of Br from the environment is incorporated... 

Diethyl azodicarboxylate will participate in the ene reaction with both cyclo-nona-l,2-dienes and acyclic allenes to give the corresponding 2-ylbicarbamate-1,3-dienes under mild conditions and in good yields. A study of the reactivity of diene (37) with activated olefins indicates that the percentage of ene adduct (38) over the Diels-Alder product (39) increases with a more highly substituted olefin. ... 

Figure 7.16 Extreme bending of acyclic allenes by electronic effects.

Unlike non-strained acyclic allenes, cyclonona-1,2-dienes undergo cycloaddition reactions with halogenated ketenes, e.g. 1-methylcyclonona-l,2-diene reacts with chloromethylketen to give a mixture of adducts with (168) predominating. The cycloaddition of dimethylketen to optically active bicyclo[7,l,0]deca-4,5-diene gave U -(169).2° ... 

The enoate 139 has been prepared from a chiral acyclic allene by a Heck-type reaction (Chapter 14), and was used to make the thymidine analogue 140. ... 

In a second example, again from the Brummond group, acyclic allene 87 was converted into pentacycle 90 in just one pot (Scheme 12.18) [22]. This impressive sequence began with a rhodium-catalyzed ene-type cycloisomerization, followed by an in situ rhodium-catalyzed intramolecular [4-1-2], concluding with an endo-selective intermolecular DA reaction with NMM to the bottom face of 89. 

Because the fluorine substituents both inductively and hyperconjugatively withdraw electron density from the C(2)-C(3) tt bond, the LUMO is located there, and Diels-Alder reactions take place exclusively with this bond [25] 1,1 -Difluoro allene and fluoroallene reaet readily with a large selection of cyclic and acyclic dienes, and acyclic dienes, [2+2] cycloadditions compete with the Diels-Alder processes As shown in the example in equation 79, a significantly different regiochemistry is observed for the [2+4] cycloaddition compared with the [2+2]... 

The addition of secondary amines to acetylenes is most applicable to the synthesis of conjugated acyclic enamines (50,171,172). Particularly the addition to acetylenic esters and sulfones has been investigated (173-177) and it appears that an initial trans addition is followed by isomerization to more stable products where the amine and functional group are in a trans orientation (178). Enamines have also been obtained by addition of secondary amines to allenes (179). 

In acyclic propargylic systems, organocopper and -cuprate reagents react similarly to afford allenes. An overall anti-Sfl process is involved, probably via a copper(III) intermediate. Typical examples are shown in Table 3. 

The facile isomerization of alkyl and vinyl zirconocenes complicates the stereocontrol in the transformations of internal acyclic aikenes and allenes, and so further studies are certainly needed to circumvent this problem. 

Some remarks concerning the scope of the cobalt chelate catalysts 207 seem appropriate. Terminal double bonds in conjugation with vinyl, aryl and alkoxy-carbonyl groups are cyclopropanated selectively. No such reaction occurs with alkyl-substituted and cyclic olefins, cyclic and sterically hindered acyclic 1,3-dienes, vinyl ethers, allenes and phenylacetylene95). The cyclopropanation of electron-poor alkenes such as acrylonitrile and ethyl acrylate (optical yield in the presence of 207a r 33%) with ethyl diazoacetate deserve notice, as these components usually... 

The versatility of this triafulvene reaction type is demonstrated by the interaction of ally pyridinium betaines 441 and l,2-diphenyl-4,4-diacetyl triafulvene272, which gives rise to fulvenes 565, benzene derivatives 566, or acyclic systems 567 these products are likely to result from an allenic precursor 563 and its isomer 564 originating from a 1,5-H-shift. 

TABLE 1. Enthalpies of formation of bis-allenes, the related bis-olefins and bis-acetylenes and their acyclic analogs... 

The reaction with silyl enol ethers 3f and 3g gave only the [3 + 2] cycloadducts in comparison with effective formation of acyclic adduct 15 in the reaction with ketene silyl acetals 3a and 3e at lower reaction temperature. This can be explained by the reactivity of cationic intermediates 19 the intermediates from 3f and 3g are more reactive owing to lower stabilization by the oxy group than those from 3a and 3e, and react with the internal allene more efficiently to give the cycloadduct(s). Cyclic product 17a could be obtained at higher temperature via the reaction of 3a (entry 2). 

Sulfur-containing acyclic and cyclic compounds have been prepared from allenyl sulfides in numerous transformations such as substitutions, additions, cydoaddi-tions and other cyclization reactions. Like the other donor-substituted allenes, allenyl sulfides are suitable substrates for regioselective lithiation and substitutions as exemplified in Scheme 8.86 [168, 169,175]. 

The (trimethylsilyl)cyclopentene annulation proceeds most efficiently with the use of 1-substituted (trimethylsilyl)allenes. Even the fully methyl-substituted (tri-methylsilyl)allene reacted with both cyclic and acyclic enones to provide the corresponding cyclopentenes in good yields. 

Interestingly, asymmetry originating in an axis rather than in a stereogen-ic center can also result in enantioselective hydrolysis. Thus, pig liver car-boxylesterase has been shown to discriminate between the enantiomers of racemic allenic esters [23], The preferred substrates (7.8) had the ( -configuration when R and R were relatively small or acyclic, but the ( -configuration when one substituent was cyclic. For example, a very high enantioselec-tivity was seen for R = Ph and R and R" = Me or Et. 

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