Cyclization with allenes

Chromium alkenyl Fischer carbenes have been shown to undergo a 3 + 2-cyclization with allenes under Rh(I) catalysis and a CO atmosphere, yielding 2-alkylidenecyclo-pentanone (54) after acidic hydrolysis.46 Reactions with electron-rich allenes are carried out with a neutral rhodium complex whereas electron-poor allenes require a... 

Reduced Zr complexes also cyclize with allenes, except that here the regiochemistry is different. Allene itself reacts with [( -C5Me5)2Zr(N2)]2N2 to afford XIX nearly quantitatively. ... 

Scheme 2.33 Pd-catalyzed cyclization with allene and amines.

The reaction of A-sulfonyl propargylamides with a base catalyst gives 5-sulfonylmethyl oxazoles via 1,4-sulfonyl migration and a two-step mechanism in the cyclization with allenes as intermediates (Scheme 77). °... 

Recently, a further unique domino methodology has been reported by Lu and coworkers (Scheme 2.74) [173]. Herein, a triphenyl phosphine-catalyzed umpolung addition/cyclization of allenes and alkynes containing an electron-withdrawing group 2-316-2-318 followed by reaction with a double nucleophile 2-319 is assumed to account for the production of a broad palette of various heterocycles 2-321 and 2-323 via 2-320 and 2-322, respectively. Dihydrofurans, piperazines, morpholines and diazepanes were obtained during the process. 

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... 

A single example of the reductive cyclization of allenic carbonyl compounds is reported, which employs a rhodium-based catalyst in conjunction with Et3SiH as terminal reductant.113 This protocol promotes hydrosilylation-cyclization to form both five- and six-membered rings with exceptional levels of yy -diastereocontrol. As revealed... 

The [2 + 2] photocycloaddition reaction of enones with allenes was first reported in 1966. A diradical intermediate is formed from a triplet enone via an exciplex. The triplet diradical cyclizes to the product after spin inversion to the singlet state [31,32]. 

The intramolecular dipolar cycloaddition of a nitrone with an unactivated allene was also studied [76], Treatment of 5,6-heptadien-2-one with N-methylhydroxyl-amine in refluxing ethanol yielded allenyl nitrone 78, which cyclized with the terminal allenic C=C bond to give an unsaturated bicyclic isoxazolidine. On the other hand, the site selectivity decreased with an allenic ketone having a trimethylene tether. 

An a-allenic sulfonamide undergoes Pd-catalyzed carbonylative cyclization with iodobenzene, affording a mixture of isomeric heterocycles (Scheme 16.12) [17]. The coupling reaction of an allene with a PhCOPdl species takes place at the allenyl central catrbon to form a 2-acyl-Jt-allylpalladium complex, which is attacked by an internal sulfonamide group in an endo mode, affording a mixture of isomeric heterocycles (Scheme 16.13). 

It was found later that the controlling factor inducing preferential cyclization with the terminal Jt-bond of the allene is not the substitution pattern but the catalyst used. [Rh(CO)2Cl]2 is probably the best catalyst for the effective formation of bicy-clo[4.3.0]nonane ring systems (Scheme 16.47) [48, 49]. 

A bis-silylated l,2-octadien-7-yne undergoes very clean cyclization with a slight excess of (7/2-propene)Ti(OiPr)2, prepared in situ from Ti(OiPr)4 and iPrMgCl, at -50 °C for 2 h to give a 1,4-diene as a single isomer after hydrolytic workup (Scheme 16.71) [78]- Deuteration of the reaction mixture afforded exclusively the bis-deuter-ated product, confirming the presence of an intermediate titanacycle. This reaction seems to be general for other allenes with diverse substitution patterns. 

The enyne-allene 12 having a methyl substituent at the allenic terminus was likewise prepared from the corresponding enediynyl propargylic alcohol 11 (Scheme 20.4). The presence of a methyl group accelerates the rate of cyclization by approximately sixfold and 12 cyclizes with a half-life of -3.6 min at 78 °C. The formation of a more stable secondary benzylic radical is apparently responsible for the rate enhancement. 

Brummond [28] was the first to illustrate that cross-conjugated trienes could be obtained via an allenic Alder-ene reaction catalyzed by [Rh(CO)2Cl]2 (Eq. 14). Selective formation of the cross-conjugated triene was enabled by a selective cycloisomerization reaction occurring with the distal double bond of the aUene. Typically directing groups on the allene, differential substitution of the aUene termini, or intramolecularization are required for constitutional group selectivity. However, rhodium(f), unlike other transition metals examined, facihtated selective cyclization with the distal double bond of the allene in nearly aU the cases examined. 

The cyclization onto allenic moiety takes place with high degree of regioselection, with exclusive xo-attack at internal double bond and formation of five-membered ring (Equation (40)). ... 

Silver(I) catalyzed cyclizations of allenic alcohols (202) lead to 2,5-dihydrofurans (203) (79S743), whilst another mild method for the synthesis of tetrahydrofurans is the intramolecular oxymercuration-demercuration process. Geraniol, when treated with mer-cury(II) acetate and subsequently with sodium borohydride, gave a tetrahydrofuran. 

Allenic alcohols with a variety of allene substitution patterns form 2-alkenyltetrahydropyrans by a 6-exo ring closure upon cyclization with silver or mercury salts (equation 91 ).209 The organosilver intermediate undergoes protiodemetallation under the reaction conditions, while the vinylmercury... 

Cyclizations with nitrogen nucleophiles involving alkynes and allenes have received little attention until recently. The cyclizations of several a-aminoallenes to 3-pyrrolines with silver tetrafluoroborate was reported by Claesson and coworkers (equation 133).264 A similar cyclization to form A -carba-penems has been reported (equation 134).265 Diastereomeric allenes (R1 R2) were shown to cyclize with complete stereocontrol. Cyclization with palladium chloride in the presence of allyl bromide or electrophilic alkenes allowed for the intermediate vinylpalladium species to be trapped by the electrophile.2651 A related product was obtained by cyclization of an alkynic substrate (equation 13S).265 Other examples of 5-endo cyclization of p-aminoalkynes50 include the formation of indoles by cyclization of 2-alkynylanilines with mercury salts200 or palladium chloride,266a,266b,266c formation of 1-pyrrolines with catalytic palladium chloride (equation 136)198 and formation of pyrroles by cyclization of hydroxy-substituted p-aminoalkynes.198,2666... 

Intramolecular cycloaddition can be carried out on a variety of alkynyl azomethine ylides analogous to reactions of the alkenyl azomethine ylides. There is also one report of cyclization with an allene. 

Azulenes.1 In the presence of this polymer as an acid scavenger, tropylium tetrafluoroborate undergoes [3 +2]cyclization with simple allenylsilanes to form azulenes. Trimethoxymethylsilane is the preferred scavenger in the case of higher substituted allenes. 

Larock and Zenner explored the reaction of allenes with A -(o-iodophenyl)-/Moluenesulfonamides. 3-Alkylideneindolines are formed. Enantioselectivity can be achieved in the presence of a chiral ligand The reactions are believed to occur by addition of an arylPd(Il) species to the allene to generate a rf-allylPd intermediate which cyclizes. With terminal allenes (R1 = H) the reaction is regiospecific and gives 3-methyleneindolines. <95JOC482>... 

Grigg has reported extensive studies of cascade cyclizations leading to various indole derivatives. For example, the propargylaniline 199 reacted with allene and piperidine in the presence of a palladium catalyst to afford the... 

An electrophilic cyclization of allene 98 with phenylselenyl chloride led to the formation of selenophene 99 along with dihydroselenophene 100 (Equation 14) <2004PS(179)1681>. 

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