Acetylene-allene transformation

The Acetylene-Allene Rearrangement. In Section 4.2.2 we discussed the base-catalyzed acetylene-allene transformation. In a study by Kitagawa et al. a useful... 

In Section 9.2, intermolecular reactions of titanium—acetylene complexes with acetylenes, allenes, alkenes, and allylic compounds were discussed. This section describes the intramolecular coupling of bis-unsaturated compounds, including dienes, enynes, and diynes, as formulated in Eq. 9.49. As the titanium alkoxide is very inexpensive, the reactions in Eq. 9.49 represent one of the most economical methods for accomplishing the formation of metallacycles of this type [1,2]. Moreover, the titanium alkoxide based method enables several new synthetic transformations that are not viable by conventional metallocene-mediated methods. 

The most probable sequence of reactions is this superbase effected the transformation of 1,2-dibromopropane into the isomer pair methyl acetylene-allene (142), which further adds ketoxime to form the O-... 

In the following steps (eq 7), we observed two HCl-catalyzed processes an acetylenic-allene rearrangement (to 10) and an allenic-acetylene rearrangement (to 11). While both 10 and 11 were easily identified by their characteristic NMR signals and could be distilled as a mixture without decomposition, only the final acetylene derivative (11) could be isolated in pure form after fractional distillation. Propargyl bromide reacted similarly with the three-coordinate phosphine, but in this case all of the chemical transformations occurred very rapidly and only the final acetylene phosphoranimine (11) could be isolated. 

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... 

We presume that the dilithiopropyne causes partial isomerization into the allenic derivative. The treatment with KOH is to complete this transformation. When the latter treatment was not carried out, considerable amounts of the acetylenic diacids were sometimes found in the crude product. 

Cutting and Parsons described the transformation of acetylenic alcohols 314 into allenyl phenyl thioethers 316 by a two-step procedure (Scheme 8.85) [174], Deprotonation of alkynes 314 with n-butyllithium is followed by addition of phenylsulfenyl chloride, forming sulfenyloxy intermediates which subsequently rearrange to allenic sulfoxides 315. Treatment of allenes 315 with methyllithium results in loss of the sulfoxide moiety to form allenyl sulfides 316 in reasonable yields. 

Primary propargylic formates decarboxylate in the presence of Pd(acac)2 and Bu3P at room temperature to give mainly allenic products (Eq. 9.115) [91]. Initial formation of a propargylic palladium complex, which rearranges to the more stable allenylpalladium species, accounts for this transformation. Under similar conditions, a terminal allenyl formate afforded a 99 1 mixture of allene and acetylene product (Eq. 9.116) [91]. However, a mixture of enyne elimination products was formed when a secondary propargylic carbonate was treated with a palladium catalyst (Eq. 9.117). 

Some acetylenic (with a non-terminal triple bond) or allenic compounds, RCH=C CH2, can be transformed into alkali metal derivatives of terminal acetylenes by treatment with a very strong base. Treatment of an acetylenic compound with the grouping CHjCsC- or CH3C=CCH=CH- with one equivalent of an alkali amide (preferably the soluble potassium... 

R. Epsztein, "The Formation and Transformations of Allenic-o-Acetylenic Carbanions", in E. Buncel, T. Durst (ed.), "Comprehensive Carbanion Chemistry", pan B, Elsevier, 1984, p. 107. 

A significant part of the examples of transition metal catalyzed formation of five membered heterocycles utilizes a carbon-heteroatom bond forming reaction as the concluding step. The palladium or copper promoted addition of amines or alcohols onto unsaturated bonds (acetylene, olefin, allene or allyl moieties) is a prime example. This chapter summarises all those catalytic transformations, where the five membered ring is formed in the intramolecular connection of a carbon atom and a heteroatom, except for annulation reactions, involving the formation of a carbon-heteroatom bond, which are discussed in Chapter 3.4. 

The mechanism of the cycloaddition process was partly clarified on the basis of experiments with acetylenic dithio derivatives 102, which were found to be thermally transformable to allenic isomers 103.149 The latter gave expected 2/7-thiopyrans 104 in 41 to 78% yields under triethylamine catalysis, whereas isomeric thiophenes 105 were formed in the presence of protic acids (Scheme 5). The activation energies for some of the processes were also measured.149... 

Different catalysts bring about different types of isomerization of hydrocarbons. Acids are the best known and most important catalysts bringing about isomerization through a carbocationic process. Brpnsted and Lewis acids, acidic solids, and superacids are used in different applications. Base-catalyzed isomerizations of hydrocarbons are less frequent, with mainly alkenes undergoing such transformations. Acetylenes and allenes are also interconverted in base-catalyzed reactions. Metals with dehydrogenating-hydrogenating activity usually supported on oxides are also used to bring about isomerizations. Zeolites with shape-selective characteristics... 

Recoil UC atoms have been produced by nuclear transformations and allowed to react with ethylene.15 Both Q1/)) and C(3P) atoms are formed, and both add to the double bond and insert into the vinylic C—H bond. The resulting hot singlet adducts relax primarily to allene and methylacetylene, whereas the hot triplet adducts decompose to acetylene or are stabilized as carbenes, which mainly add to more ethylene to yield various C5 products. 

Besides simple enones and enals, less reactive Michael acceptors like /3,/3-disubstituted enones, as well as a,/3-unsaturated esters, thioesters, and nitriles, can also be transformed into the 1,4-addition products by this procedure.44,44a,46,46a The conjugate addition of a-aminoalkylcuprates to allenic or acetylenic Michael acceptors has been utilized extensively in the synthesis of heterocyclic products.46-49 For instance, addition of the cuprate, formed from cyclic carbamate 53 by deprotonation and transmetallation, to alkyl-substituted allenic esters proceeded with high stereoselectivity to afford the adducts 54 with good yield (Scheme 12).46,46a 47 Treatment with phenol and chlorotrimethylsilane effected a smooth Boc deprotection and lactam formation. In contrast, the corresponding reaction with acetylenic esters46,46a or ketones48 invariably produced an E Z-mixture of addition products 56. This poor stereoselectivity could be circumvented by the use of (E)- or (Z)-3-iodo-2-enoates instead of acetylenic esters,49 but turned out to be irrelevant for the subsequent deprotection/cyclization to the pyrroles 57 since this step took place with concomitant E/Z-isomerization. 

Pyridazines were obtained also by photolysis of 1-phenyl-l-vinyl azide in the presence of iron pentacarbonyl (3,6-di-phenylpyridazine was obtained in 1.1% yield) (78HCA589) or by thermal decomposition of an allenic hydrazonate (81JA7011). Acetylenic hydrazides can be transformed into pyridazines [84BSF(2)129], and thermal cyclization of dialkali metal salts of cu-hydroxyketone tosylhydrazones afforded pyridazines in moderate yield (85TL655). Propionyl phenylhydrazine, after reaction with 4-bromobutyronitrile, converts into a pyridazine (87SC1253). 

The homologous (5-allenyl iodide 36 was transformed by t-BiiLi at — 78 C to the corresponding organolithium 37Li. Warming 37Li to room temperature for Ih prior to hydrolysis gave a 27 20 50 3 mixture of allene 38, acetylene 39, vinylcyclopentane 40, and cyclohexene 41 in 86% yield [27] (Scheme 7-29). 

The reagent reacts with ethynylcarbinol acetates to give alkylallenes.7 Sin-acetylenes are not transformed by (CH3)2CuLi into allenes. 

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