Acetylene-Allene Interconversion

Acetylene-AUene Interconversion.—3,3-Sigmatropic rearrangements of acetylenes result in conversion into allenes. The energetics of the propargyl-Cope conversion, e,g. (230) to (231), are in agreement with a concerted 3,3-sigmatropic rearrangement.  

Thermal rearrangement of a range of enynols (232) gives y-allenic aldehydes or ketones (233), together with their secondary rearrangement products.  

Dienol-benzene rearrangements of various propargylcyclohexadienols, e.g. (234) and (235), prepared by lithium aluminium hydride reduction of the corresponding cyclohexadienones, give (236), (237), and (238) from [Ij, 2J-, [3 3J-, and [3g, 4g]-sigmatropic rearrangements of the intermediate methyl-propargylphenonium ions.  

Propargylic sulphonium salts (239) on treatment with base rearrange to allenic thioethers (240) a competitive reaction is isomerization of the acetylene to the allene sulphonium salt (241) followed by an analogous sigmatropic rearrangement to the acetylenic thioether (242). There is no isomerization of (240) to (242) under the reaction conditions.  

Studies of thio-Claisen rearrangements have been extended to 1-alkynyl-allenylsulphides (243), which are obtained by irrepressible isomerization of the corresponding thiodiacetylenes (244). ITie intermediate thioketens are 

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Other dimerizations initiated by acetylene-allene interconversions have also been reported, and dimerization has also been applied to other cumulenes. The structure of the solid-state photodimer of tetraphenylbutatriene has now been reassigned, the new bis-allene structure (251) proposed being substantiated by ozonolysis to the known diketone (252), and by X-ray analysis. The solid-state photochemical reaction thus contrasts with the thermal dimerization, which gives the radialene (253). ... 

See also the previous section on acetylene-allene interconversion. 

The various products obtained from acetylenic diols 54 in the presence of acids suggest the formation and interconversion of acetylene-allene-diene cationoid intermediates (equation 18)33. The allene intermediates can be sometimes isolated and they were reported as participants in the acid-catalyzed reactions of alkynylpyrylium salts 55, a driving force of which is an aromatization of the pyrane ring (equations 19-21)34,35. 

II. REARRANGEMENTS INVOLVING ACETYLENES A. Alkyne-Allene Interconversions... 

Cyclic alkynes (C9, Ci0, or Cn) also rearrange in the presence of bases to an equilibrium mixture containing cyclic allene [43]. The bases used were NaNH2-liq. NH3 at -33.4°C, KOH-C2H5OH at 131°-134°C (sealed tubes), KO-f-Bu-f-BuOH at 79.4°-l 20.0°C (sealed tubes) [44]. A solution of sodamide in liquid ammonia gave the most rapid ( to 3 hr) allene-acetylene interconversions of all systems examined. 

The process may be reversed by treating 2-alkynes with sodium to get 1-alkynes after acidification of the product. The presence of an alkyl substituent at C(3) of a 1-alkyne prevents its isomerization to the corresponding 2-alkyne. Instead, an allene is formed [Eq. (4.27)]. This observation led to the suggestion of the involvement of allene intermediates to interpret the shift of the triple bond in the interconversion of acetylenes [Eq. (4.28)] ... 

Figure 5. The irreversible inactivation of (3-hydroxydecanoyl thioester dehydrase by A(34) decynoyl N-acetyl cysteamine. The enzyme catalyzes the reversible interconversion of hydroxy-aecanoyl thioesters with their a,(3-trans and B,y-cis counterparts (Scheme 1). The acetylenic analog is converted by the enzyme into the highly reactive conjugated allene, which alkylates a histidine residue in the active center (Scheme 2) (23).

The observed results can be understood if one keeps in mind that the precursor ions HC=C—CH2—OH+ and HC=C—CH(CH3)—OH+ can undergo exothermic rearrangement to form more stable allenic ions (see Section I and Scheme 3). For example, (HC=C—CH2—OH+ ) = 1046 kJ mol" and A °f (H2C=C=CH—OH+ ) = 879 kJmoF thus, all propynol ions with sufficient energy to isomerize should readily do so near threshold. The allenic precursor ions could then be the source of 21+ and 22+ (Scheme 16). At higher internal energies, the acetylene ion allene ion interconversion becomes less competitive compared to direct cleavages, so that now the relative fraction of 20+ type products increases. 

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