Cumulenes intermediate

The need for a base additive in this reaction implies the intermediacy of acetylide complexes (Scheme 9.10). As in the Rh(III)-catalyzed reaction, vinylidene acetylide S4 undergoes a-insertion to give the vinyl-iridium intermediate 55. A [l,3]-propargyl/ allenyl metallatropic shift can give rise to the cumulene intermediate 56. The individual steps of Miyaura s proposed mechanism have been established in stoichiometric experiments. In the case of ( )-selective head-to-head dimerization, vinylidene intermediates are not invoked. The authors argue that electron-rich phosphine ligands affect stereoselectivity by favoring alkyne C—H oxidative addition, a step often involved in vinylidene formation. 

According to the proposed mechanism, the electrophilic arylpaUadium iodide 326, formed by an oxidative addition of Ar-1 to a Pd(0) species, was proposed to activate the central carbon-carbon double bond in the cyclic cumulene intermediate 317b to form the complex 327. A subsequent attack by the Zr-C(sp ) nucleophilic center produces the aUcenylpalladium intermediate 325, which upon reductive elimination furnishes the corresponding alkenylzirconium species 324. Hydrolysis of the latter finally produces the 3-methylenecyclopentene 319. To support the mechanistic hypothesis, deuterolysis was carried out to provide the deuterated compound 319-D in 60% yield with a high level of deuterium incorporation [87]. 

Chung and colleagues also employed gold(I) catalysts in their study focused on the rearrangement of cycloalkynols to a,p-unsaturated ketones. For example, 65 is smoothly produced from 64 under the conditions shown. Although the exact mechanism of the transformation was unknown, it was surmised that dehydration of the alcohol and subsequent addition of water to a cumulene intermediate may be involved. 

Burrell, R.C., Daoust, K.J., Bradley, A.Z., DiRico, K.J. and Johnson, R.P. (1996) Strained cyclic cumulene intermediates in Diels-Alder cycloadditions of enynes and diynes. Journal of the American Chemical Society, 118(17), 4218-4219. 

CONVERSION OF ACETYLENES AND CUMULENES INTO ANIONIC AND ORGANOHETALLIC INTERMEDIATES... 

One of the most versatile methods for the synthesis of derivatives with the acetylene or cumulene system consists In deprotonatlon of the starting acetylene or cumulene and subsequent reaction of the anionic or organometal1ic intermediate with an electrophilic reagent. 

Another, simple form of elemental carbon would be chains formed from carbon atoms. As a prototype model a single>stranded chain is most suitable. If branching were to be considered, all intermediate forms up to and including the diamond and graphite like clusters would be included. For non branched chains, the two variants to choose from are a system of alternating singly and triply bonded carbon atoms (poly-ynes), and a system with all double bonds (cumulenes). Cumulene structures are assumed to be the preferred ones for odd membered chains, whereas the even ones may have some poly-yne character. Recent studies on linear Cg show that a cumulene-like structure is preferred, both at the SCF level and when correlation is accounted for(50). 

Since cumulenes and alkynes are often easily interconvertible, many syntheses discussed above have allenic counterparts, especially base-catalyzed cyclizations of allenic alcohols.77 And, of course, several of the alkyne-based syntheses may well have allenic intermediates. There are, however, a few syntheses based specifically upon allene chemistry. In an important one, due to Stirling and his collaborators,78 an allenic sulfonium salt reacts with an enolate anion. Scheme 12 sketches the main features yields as high as 86% are recorded. Methoxyallene is easily metallated by butyllithium and so converted into an allenic epoxide that can be isomerized by fe/T-butoxide into a furan (Scheme 13) or an exocyclic equivalent similar to 15 clearly this method is particularly suited to the preparation of 3-methoxyfuran... 

Knoke and de Meijere [60] recently developed a highly flexible domino Heck-Diels-Alder reaction of a symmetrically substituted cumulene 125, which also involves cross-couplings of an allene at the central position. Both aryl and hetaryl halides react efficiently with l,3-dicyclopropyl-l,2-propadiene (125) and furnish 1,3,5-hexatriene derivatives 126 as intermediates, which are usually trapped by acceptor-substituted olefins in a subsequent cycloaddition, providing adducts 127a/b in moderate to good overall yields (Scheme 14.30). 

Otherwise, the reactions of indenyl-ruthenium(II) allenylidenes [RuCty -CgHy) =C=C=C(R )Ph (PPh3)2][PF6] (R = H, Ph) with ynamines R C CNEtj (R = Me, SiMea) have been reported to yield the alkenyl(amino)allenylidene complexes 41 via insertion of the ynamine into the Cp=Cy allenylidene bond (Scheme 10) [52, 53], This insertion process involves an initial nucleophilic addition of the ynamine at Cy atom of the cumulene, which leads to the cationic alkynyl intermediate complexes 39. Further ring closing, involving the Cp atom, generates the [2+2]... 

Analogous -substitutions take place when cumulenic ethers anti Grignard compounds are allowed. to interact in the presence of catalytic quantities of cqpper(I) salts. Organocopper compounds are the presumed intermediates [163,164]. 

Enyne ethers HC=CCH=CHOR are useful synthetic intermediates. They can be prepared by base-catalysed addition of alcohols to diacetylene. The required conditions are rather forcing and not very attractive for laboratory scale preparations. A much more convenient way to prepare the enyne ethers (in these cases more than 80 rel.% of the -isomer is obtained) consists in treatment of the easily accessible 1,4-dialkoxy-2-alkynes with two equivalents of alkali amide in liquid ammonia. The first step in this elimination is the (transient) formation of an "anion RO-fiH-C CCH OR, which eliminates ROH (143). The resulting cumulenic ether ROCH=C=C=CH2 is immediately converted into the metallaied enyne ether. 

Metal-catalyzed cyclodimerizations of cumulenes can often result in different regiochemistry compared to their thermal counterpart. For example the nickel-catalyzed dimerization of the cumulene 22 gives exclusively the 4-radialene 23 whereas the thermal reaction gives the symmetrical 4-radialene 24.29 Metal-catalyzed cyclodimerizations of alkenes as seen in this and other examples often proceed under mild conditions. However, as the result of the intervention of metal-complexcd intermediates, regio- and stereochemistry may differ from their thermal counterparts. 

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