Allenes rearrangements involving

Petrov and coworkers [41] showed that the reaction of dibromides of alkenephos-phonic acids with acetylenic alcohols involved an acetylene-allene rearrangement. The products so formed hydrolyzed easily to the corresponding phosphinic acids. The latter on heterocyclization afforded 2,5-dihydro-l,2-oxaphosphole-2-oxide derivatives (Scheme 11). 

A typical intramolecular rearrangement involves 1,4-addition across the conjugated system of alkenynes to give allenes (Eq. 36). 

The various types of acetylene-allene rearrangements [la, 38-41] have been described by Jacobs [38] to involve the following processes ... 

Displacement rearrangements involving propargyl derivatives allenes from a-acetoxyalkynes [136]. 

For a review of rearrangements involving allenes, see Huntsman, in Palai The Chemistry of Ketenes, Allenes, and Related Compounds, pi. 2 Wiley New York, 1980, pp. 521-667. 

The title reactions are discussed in a series of reviews . However, the most complete and detailed description of rearrangements involving allenes was presented in Huntsman s comprehensive survey, wherein the cumulated systems were considered as either the starting materials, the intermediates or the reaction products. More recently, very detailed reviews devoted to vinyl cations and containing numerous examples of rearrangements of cumulenes and other polyenes were published . Therefore, this section will cover only relatively recent publications. 

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

SCHEME 1. Meyer-Schuster rearrangement involving an allenic cation intermediate. 

Huntsman, W.D., Rearrangements involving allenes, in The Chemistry of Ketenes, Allenes and Related Compounds, Patai, S., Ed., Wiley, New York, 1980, part 2, chap. 15, p. 521. 

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). 

Abstract The photoinduced reactions of metal carbene complexes, particularly Group 6 Fischer carbenes, are comprehensively presented in this chapter with a complete listing of published examples. A majority of these processes involve CO insertion to produce species that have ketene-like reactivity. Cyclo addition reactions presented include reaction with imines to form /1-lactams, with alkenes to form cyclobutanones, with aldehydes to form /1-lactones, and with azoarenes to form diazetidinones. Photoinduced benzannulation processes are included. Reactions involving nucleophilic attack to form esters, amino acids, peptides, allenes, acylated arenes, and aza-Cope rearrangement products are detailed. A number of photoinduced reactions of carbenes do not involve CO insertion. These include reactions with sulfur ylides and sulfilimines, cyclopropanation, 1,3-dipolar cycloadditions, and acyl migrations. 

The two-step process, depicted by path b, involves initial addition of the carbene carbon to an adjacent it bond to form bicyclo[4.1,0]hepta-2,4,6-triene (2a). This process has precedent in the analogous rearrangement of vinylcar-bene to cyclopropene (Scheme 6),lc18 and is supported by Gaspar s work on 1-cyclohexenylcarbene.17 In the second step of the mechanism in Scheme 5, subsequent six-electron electrocyclic ring opening of 2a yields the cyclic allene 3a. 

The acidity of the propargylic proton of the starting compound 18 allows the equilibration with the allene 19 induced by bases such as tertiary amines or alcoholates (Scheme 7.4). Such prototropic rearrangements furnish the title compounds 19 with at least one proton at the terminal carbon atom, often in good yields. The EWG group involves carboxylic acids [33], esters [34], ketones [35, 36], isonitriles [37], sul-fones [38], sulfoxides [39, 40] and phosphonates [41], The oxidation of easily accessi-... 

The reactivity of allenyl ketones is also manifested in the Hg(II)-catalyzed ipso substitution that converts 54 to spirodione 55 (Eq. 13.17) [19]. The reaction presumably involves activation of the allene by Hg(II), followed by intramolecular electrophilic attack on the aromatic ring. Hydrolytic cleavage of the metal from the intermediate product of the reaction, followed by rearrangement leads to the observed spirocyclic dione. 

A similar process involving an all-carbon cyclic system has also been investigated [59]. Other examples involving the prototropic rearrangement of enediynes having an imino or a keto substituent at the propargylic position to form the corresponding enyne-allenes have also been observed [60, 61]. 

The synthetic strategy involving the use of a [3,3]-sigmatropic rearrangement of pro-pargylic vinyl ethers for the preparation of enyne-allenes has also been investigated. 

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