Dimers allene

When the palladiozincation was performed in the absence of an aldehyde substrate, two products could be isolated. The major product was the ethyl-substituted allene resulting from reaction of the putative allenylpalladium intermediate with diethylzinc (equation 22). Interestingly, this product is formed in only trace amounts when an aldehyde substrate is present. The second adduct is a dimeric allene, which can be envisioned as arising from coupling of the allenylzinc and allenylpalladium intermediates. 

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. 

Two monomeric and dimeric 2-substituied 7r-allylic complexes (548 and 549) are obtained by treatment of allene with PdCl2(PhCN)2. They are formed by the nucleophilic attack at the central carbon of allene[493, 494],... 

Oxidative cleavage of the complex 549 with CuCri affords 2,3-bis(chloro-methyl)-1,3-butadiene (550) and regenerates PdCri. Thus the preparation of this interesting dimerization product 550 can be carried out with a catalytic amount of PdCl2 and two equivalents of CuCb in MeCN[495], Similarly, treatment of allene with PdBr2 affords the dimeric complex 551. Treatment of this complex with 2 equiv, of bromine yields the dibromide 552. The tetra-bromide 553 is obtained by the reaction of an excess of bromine[496]. Similarly,... 

Carbonylation of the complex 548 proceeds in ethanol gives ethyl 3-chloro-3-butenoate (554), The lactone 555 and the two esters 556 and 557 are obtained by carbonylation of the dimeric complex 549. The oxidative carbonylation of allene in ethanol with PdCl2 gives ethyl itacoante (558), although the yield is low[498]. 

Like butadiene, allene undergoes dimerization and addition of nucleophiles to give 1-substituted 3-methyl-2-methylene-3-butenyl compounds. Dimerization-hydration of allene is catalyzed by Pd(0) in the presence of CO2 to give 3-methyl-2-methylene-3-buten-l-ol (1). An addition reaction with. MleOH proceeds without CO2 to give 2-methyl-4-methoxy-3-inethylene-1-butene (2)[1]. Similarly, piperidine reacts with allene to give the dimeric amine 3, and the reaction of malonate affords 4 in good yields. Pd(0) coordinated by maleic anhydride (MA) IS used as a catalyst[2]. 

Tlie thioketone 5-oxide 85, generated from allene 86 and SOCk in situ, decomposes to give the alkene 87. A mechanism, involving the transient formation of the 1,2-dithietane 88 (by dimerization of 85 followed by rearrangement), was proposed (85AGE855). 

Allen et al mainly on the evaluation of data reported by previous workers, advanced structure (5) for 2-methylpyrrole dimer. These data were (a) the monobasic nature of the dimer and (b) the conversion by aqueous acid into an indole which they showed by elimination (i.e., by the synthesis of 2,5-, 2,6-, and 2,7-dimethylindoIes) to be 2,4-dimethyIindole (this compound has since been synthesized by Marion and Oldfield ). Structure (5) for the dimer was confirmed by Edwards and Smith by conversion, by way of the methiodide (6), into the pyrrolidinopyrrole (7), the structure of which was proved by synthesis l,5-dimethylpyrrolid-2-one was condensed with 2-methyI-pyrrole by means of POCI3 to give the cation (8), isomeric with the... 

This compound was prepared by Allen et who found it to be monobasic and to form a simple methiodide. Attempted Hofmann degradation of this methiodide failed to give any information. The dimer was found to be perfectly stable to hot aqueous sulfuric acid. With the foregoing data, and by analogy with the alkyl pyrrole dimers, they proposed structure (9) for the dimer. Later work confirmed this structure by conversion via the methiodide into base (10) which was synthesized by way of the metho salt (11), isomeric with... 

The thermal [2 + 2] cycloaddition is limited to certain activated alkenes. For instance tetrafluoroethylene, tetrachloroethylene, allenes e.g. 17, ketenes and ena-mines can form cyclic dimers or react with other alkenes ... 

A derivative of cyclopentyne has been trapped in a matrix. Although cycloheptyne and cyclohexyne have not been isolated at room temperatures, Pt(0) complexes of these compounds have been prepared and are stable." The smallest cyclic allene" so far isolated is l-/err-butyl-l,2-cyclooctadiene 107." The parent 1,2-cyclooctadiene has not been isolated. It has been shown to exist transiently, but rapidly dimerizes." " The presence of the rert-butyl group apparently prevents this. The transient existence of 1,2-cycloheptadiene has also been shown," and both 1,2-cyclooctadiene and 1,2-cycloheptadiene have been isolated in platinum complexes." 1,2-Cyclohexadiene has been trapped at low temperatures, and its structure has been proved by spectral smdies." Cyclic allenes in general are less strained than their acetylenic isomers." The cyclic cumulene 1,2,3-cyclononatriene has also been synthesized and is reasonably stable in solution at room temperature in the absence of air." ... 

The addition of HC1, HBr, and HI to allene has been studied in some detail.145 In each case a 2-halopropene is formed, corresponding to protonation at a terminal carbon. The initial product can undergo a second addition, giving rise to 2,2-dihalopropanes. The regiochemistry reflects the donor effect of the halogen. Dimers are also formed, but we have not considered them. 

Attempts to employ allenes in palladium-catalyzed oxidations have so far given dimeric products via jr al lyI complexes of type 7i62.63. The fact that only very little 1,2-addition product is formed via nucleophilic attack on jral ly I complex 69 indicates that the kinetic chloropalladation intermediate is 70. Although formation of 70 is reversible, it is trapped by the excess of allene present in the catalytic reaction to give dimeric products. The only reported example of a selective intermolecular 1,2-addition to allenes is the carbonylation given in equation 31, which is a stoichiometric oxidation64. 

To date, the reductive cyclization of allenic alkenes remains undeveloped. However, the reductive cyclization of activated alkene partners in the form of 1,3-dienes and conjugated enones has been achieved using late transition metal catalysts. Indeed, the hydrosilylative dimerization of 1,3-dienes reported in 1969 appears to be the first reductive... 

Several examples are known of the transition metal-catalyzed synthesis of 1,2,3-buta-trienes, which possess one more cumulated C=C double bond than allenes. Most of the reported examples of the butatriene synthesis involve dimerization of terminal alkynes and conjugated enynes are typical side products of the reactions. 

The conformationally locked [46] bisallene 127 was first prepared by subjecting the allene dimer 126 to the DMS allene synthesis (Scheme 5.17) [47]. 

Beginning with allene (1) itself, its thermal dimerization to 1,2-bismethylenecy-clobutanes (126) constitutes one of the oldest allene reactions known. Although the yield is only moderate (in the region of 30%, depending on the pyrolysis conditions), the dimerization generates a very useful diene from a readily available starting material (Scheme 5.43) [117]. 

The comparatively low yield of the initial dimerization of allene is also caused by further addition of 1 to 126 and other allene oligomers produced subsequently in the pyrolysis. A reinvestigation of the reaction has revealed that not only are new tetramers such as 287 and 288 formed in the reaction, but also numerous hexamers such as 289-292, the latter certainly not giving an indication that it originates from 1 [119]. Since some of these products still contain conjugated diene subunits - see, e.g., 291 - further growth appears likely tert-butylallene behaves similarly [120]. 

The Pd °-catalyzed dimerization of 2 leads to the Diels-Alder adduct 314 as the main product (81%), for whose formation only one allene group has participated in the dimerization process, in addition to the dimer 315 (14%), in which no allene group has survived in addition, several other dimers are produced in trace amounts [131]. 

Beyond dimerization and oligomerization, [2 + 2]- and [4+ 2]-cycloadditions with conjugated dienes and styrenes and the addition of nucleophiles are typical reactions of strained cyclic allenes. These transformations have been studied most thoroughly with 1,2-cyclohexadiene (6) and its derivatives [1, 2]. Concerning the cycloadditions, a theoretical study had the surprising result that even the [4+ 2]-cycloadditions should proceed in two steps via a diradical intermediate [9]. In the case of nucleophiles, the sites of attack at several 1,2-cyclohexadiene derivatives having an... 

Elliott et al. [155] proposed a concerted cycloaddition also for the reaction of 417 with 1,1-dimethylallene to yield 426. However, even the 1,2-bismethylenecyclobu-tane entity of 426 provides support for a diradical intermediate, since it is characteristic of the dimers of many allenes (see, for example, Schemes 6.10 and 6.25). It is generally accepted that these [2+ 2]-cycloadditions proceed via diradicals as intermediates. 

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