Yne-allenes

Sote 1. In the final stage of the distillation the remaining liquid is subjected to a relatively higli temperature. This causes dimerization of the greater part of the yne-allene RCeC-CH=C=CH2, which is formed as a by product. 

Finally, thermally induced isomerizations which generate carbon-centered biradical organic molecules have been shown to serve as alternative for conventional chemical and photochemical methods [71]. A straightforward procedure to accomplish such biradicals was described by Myers using a thermal conversion of yne-allenes [72]. According to this scheme, Wang and coworkers [73] heated 3-178 in 1,4-cyclohexadiene to 75 °C and obtained 3-181 in 22% yield via the biradicals 3-179 and 3-180 (Scheme 3.48). 

It should be mentioned that the same group has also developed a related method in which benzoene-yne-allenes were cydized via biradicals in a domino-like manner [131],... 

Product stabilization is much more pronounced when the enediyne or ene-yne-allene starting materials are not already part of an aromatic system, since forming an aromatic system constitutes a much higher degree of stabilization than expanding an aromatic system (Fig. 24). Conjugation of the radical center provides additional stabilization to the 71-radical formed by the Myers-Saito and Schmittel cyclizations. 

The cross-coupling reactions of allenes with components containing sp-carbon atoms are useful synthetic transformations since they provide yne-allenes and enyne-allenes, respectively. Due to the synthetic potential of these classes of carbon-rich unsaturated compounds, the scope and limitations were systematically investigated [1, 16-18]. The first synthetic application was reported in 1981, describing the preparation of alkynyl-substituted allenes by coupling of alkynylzinc chlorides with allenyl halides (Scheme 14.8) [11]. 

A convenient route for the preparation of yne-allenes was recently described by Saalfrank et al. [19]. Products 29a/b were formed by Stille cross-coupling of allenyl bromides 27a/b with alkynylstannanes such as 28 (Scheme 14.9). Allenyl phospho-nates such as 30 were also suitable substrates in palladium-catalyzed couplings with propargylstannane 31 (Eq. 14.1). Bisstannylated acetylene 33 as alkyne component furnished the expected yne-bisallene 34 in reasonable yield, but without any diaster-eoselectivity (meso-34 (R,R)-/(S,S)-34 =50 50) (Eq. 14.2). 

Treatment of bromoallene 27a and phenylethynyltrimethylsilane (35) with a reagent cocktail as given in Eq. 14.3 afforded yne-allene 36 in moderate yield [19]. The key intermediate in the formation of the tetrasubstituted allene 36 is phenylace-tylene, which is generated in situ by spontaneous desilylation of 35 by potassium hydroxide. 

To a stirred solution of 1.01 g (2-bromophenylethynyl)trimethylsilane (4 mmol) in 20 mL dry THF was added 3.25 mL 1.6 M n-BuLi in hexane (5.2 mmol) at -95°C. After the mixture was stirred for 10 min, 0.54 g trimethylborate (5.2 mmol) was added. The solution was allowed to warm to —5°C, and 3.71 g Ag20 in 16 mL water, 0.23 g tetrakis(triphenylphosphine)-palladium (0.2 mmol), 0.49 g triphenylarsine (1.6 mmol), and 0.99 g bromoallene (4 mmol) were added successively. The reaction was stirred at 25°C for 24 h. AgaO was removed by filtration, and the crude product was diluted in 40 mL Et20. The organic layer was washed with 50 mL distilled water, 50 mL saturated NaHCOs, and 50 mL brine and dried over MgS04. After filtration, the solvent was removed under reduced pressure, and the residue was purified by column chromatography with hexane/Et20 (10 1) to afford 0.39 g naphthalene (29%) and 0.44 g yne-allene (41%) as pale yellow oils. 

Rhodium catalysts were applied in this type of transformation as well. Notably, by the addition of enantioenriched C2-symmetric spiro-bipyridine ligands, the reaction proceeded in an enantioselective manner (Scheme 2.13a) [22], More recently, cyano-yne-allenes were found to be suitable substrates as well [23]. 2,6-Naphthyridine derivatives can be produced selectively here (Scheme 2.13b). 

For example, a yne-allene 141 is smoothly converted into a benzofulvene 142 via diradical 143 in good yield (Scheme 6.33a) [71]. Similar regioselective biradical cyclizations from aza-analogous enyneallenes 144 to carbazoles 145 have also... 

In 2007, a gold(I)-catalyzed cascade reaction of diynes and electron-rich aromatics and heteroaromatics for the synthesis of complex a-pyrones was successfully realized by Schreiber and Luo (Scheme 12.13) [16]. This reaction involves the [3,3]-sigmatropic rearrangement of the diynes 25 to an yne-allene intermediate 26, which is supported by the observed racemization of enan-tioenriched substrates. Gold(I)-induced a 6-endo-dig cyclization of intermediate 26 would give the oxocarbenium intermediate 27, which could be trapped by electron-rich aromatics and heteroaromatics to afford complex a-pyrones 28. 

Basak, A., Das, S., Mallick, D. and Jemmis, D. (2009) Which one is preferred Myers-Saito cyclization of ene-yne-allene or Garratt-Braverman cyclization of conjugated bisallenic sulfone A theoretical and experimental study. Journal of the American Chemical Society, 131(43), 15695-15704. 

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