Allenes isomerism

The first investigations in the 1960s [11,12] established the base-induced isomerization of alkyne precursors as the most practical and general route for the synthesis of alkoxy-and aryloxyallenes. In the meantime, a number of monosubstituted allenes 8 bearing an achiral or a chiral group R is smoothly accessible by this efficient procedure (Scheme 8.5) [1, 2,13-19]. Beside the most commonly used base potassium tert-butoxide, other bases, e.g. n-butyllithium, are also applicable for this isomerization. Recently, the yields of alkyne-allene isomerizations could be significantly increased, in particular with aryloxy-substituted allenes, by using microwave irradiation (Eq. 8.1) [20]. 

Such a behavior cannot always be guaranteed the heterocycle 32 with a terminal alkyne and an allene isomerizes both fuctional groups, the benzylic alkyne to an allene and the benzylic allene to a 1,3-diene, both now in conjugation with the het-eroarene [46] (Scheme 1.12). 

Occasionally the cyclopropylidene to allene isomerization cannot take place for structural reasons. If, for example, the expected allene would be very highly strained, as is the case for certain cyclic allenes, then the reaction is forced to follow an alternative path. A case in point is provided by 1-alkyl-7,7-dibromonorcaranes which undergo a carbene insertion reaction when treated with methyl lithium to provide bicyclobutanes rather than allene derivatives. 

Interestingly, no allenic products are formed during this reaction. The two oxygen atoms seem to prevent a competing cyclopropylidene to allene isomerization, at least at the reported reaction temperature of —75 °C [126],... 

Kanematsu and co-workers devised a simple pathway to isobenzofurans and dihydroisobenzofurans starting with substituted furans of type 62. Treatment with strong base results in an alkyne-allene isomerization. Subsequent intramolecular cycloaddition, ring opening (probably oxygen lone-pair assisted), and acidic workup give 63. 

Ley et al. also applied this method to the synthesis of sesquiterpenes through a strategy involving a Diels-Alder reaction. Taking into account the effect of conditions and silver counterion on allene isomerization (see Scheme 33), they obtained the diene partner via isomerization of the acetoxyallene produced on treatment of a propargyl acetate with catalytic amount of silver hexafluoroantimonate (Scheme 3.36).57... 

The double phosphinylation of propargylic alcohols with diphenylphos-phine oxide to form 2,3-bis(diphenylphosphinyl)-1-propenes is catalyzed by a thiolate-bridged diruthenium complex (Scheme 28) [69]. It has been shown that the reaction proceeds via three ruthenium-catalyzed transformations propargylation of the phosphine oxide, alkyne to allene isomerization, and addition of phosphine oxide to the allene structure. 

Mechanistically, this new insertion-CI-Diels-Alder hetero domino sequence can be rationalized as follows (Scheme 64) After the oxidative addition of the aryl halide 115 or 118 to the in situ generated Pd(0) species the arylpalladium halide 120 intramolecularly coordinates and inserts into the tethered triple bond via a syn-carbopaUadation to furnish cyclized vinylpalladium species 121 with a p-acceptor substitution in a stereospecific fashion. Transmetallation of the in situ generated copper acetylide 122 gives rise to the diorganylpalladium complex 123 that readily undergoes a reductive elimination and liberates the electron poor vinylpropargylallyl ether 124. The triethylamine catalyzed propargyl-allene isomerization furnishes the... 

The method has been extended to the preparation of difunctional compounds. Dihalides Including the mixed variety are formed in 90 to 98% yields. Primary unsaturated bromides of the type RCH=CHCH,Br have been formed from the corresponding alcohols by the action of phosphorus tribromide and pyridine at a low temperature without any apparent rearrangement. However, the corresponding secondary-carbinol system, RCHOHCH=CH is very susceptible to allylic isomerization. The formation of a, -acetylenic bromides from acetylenic alcohols and phosphorus tribromide is common (40-70%). An acetylenic-allenic isomerization has been observed, viz.,... 

The product ratio, particularly in reference to products III, IV, and V, remained the same regardless of the quenching procedure water, 5% sulfuric acid, saturated ammonium chloride, and so on. This means that products IV and V are not the consequence of allene isomerization during work-up. Further, dienes VI and VII are apparent isomerization products of III, although without the actual control experiment at hand, this assertion cannot be proved. 

Muller and coworkers have recently developed a coupling-isomerization reaction, initially identified as a side reaction which occurred under standard Sonogashira conditions [79]. As demonstrated below, the coupling reaction is followed by a shuffling of oxidation states via an alkyne-allene isomerization [80]. The product, a,P-unsaturated ketone 146, is reminiscent of a product which would be obtained from a Heck reaction. The utility of this reaction was further demonstrated when diamine 147 was added to the reaction pot. Following a conjugate addition reaction and imine formation, compound 148 resulted from the three-component, one-pot reaction sequence enabled by the coupling-isomerization reaction. 

Reactions at the Propargylic (a) Carbon 4.43.1 Alkyne/Allene Isomerization... 

Acetylene - allene isomerization of butynols 10a, 10c and lOd must be conducted under a strict control of reaction conditions to minimize side reactions. Thus, employing 1 M instead of 0.1 M NaOH led to formation of a different set of products - oxacyclo-pentenes 16a, 16c and 16d isolated in 17 - 38% yields (Scheme 1). It is likely that allenols 11a, 11c and lid are intermediates in this transformation. Thus, in a stronger base (1 M NaOH), ionization of the primary alcohol can provide a mwe effective driving force for cyclization. The reaction is an interesting example of how an q>en-chain derivative can be transformed in a simple fashion to a nucleoside-like compound. The molecular models indicate that a direct cyclization of 11a, 11c or lid is strongly disfavored. More probably, oxiranes 17a, 17c and 17d are formed first, and the formation of an oxacyclopentene skeleton of 16a, 16c and 16d is then the result of a [1,3] sigmatropic shift.32... 

Another route to cyclobutenes is [2+2] cyclization of 1,3-butadienes [440]. Normally the focus of this reaction goes back to the synthesis of 1,3-dienes, but in the case of bulky substituents, formation of cyclobutenes 4.987 are preferred. Such bulky substituents can be introduced by means of acetylene-allene isomerization of trivalent phosphorous propargyl esters, as shown in Scheme 4.11. 

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