Allenynes cycloisomerizations

Malacria and co-workers76 were the first to report the transition metal-catalyzed intramolecular cycloisomerization of allenynes in 1996. The cobalt-mediated process was presumed to proceed via a 7r-allyl intermediate (111, Scheme 22) following C-H activation. Alkyne insertion and reductive elimination give cross-conjugated triene 112 cobalt-catalyzed olefin isomerization of the Alder-ene product is presumed to be the mechanism by which 113 is formed. While exploring the cobalt(i)-catalyzed synthesis of steroidal skeletons, Malacria and co-workers77 observed the formation of Alder-ene product 115 from cis-114 (Equation (74)) in contrast, trans-114 underwent [2 + 2 + 2]-cyclization under identical conditions to form 116 (Equation (75)). 

PtCl2 was shown to catalyze a similar Alder-ene transformation, as in the cycloisomerization of allenyne 117 to triene 118 (Equation (76)).78 In the same study, it was noticed that tetrasubstituted allenes cyclized to bicyclic compounds, such as 120 (Scheme 23), under identical PtCl2 conditions, presumably due to A(1,3) strain in intermediate 119. 

Brummond and Shibata independently reported the Rh(i)-catalyzed cycloisomerization of allenynes to cross-conjugated trienes. The rhodium conditions were shown to have broad functional group tolerance. Brummond et al 9 observed rate and selectivity enhancements when they switched to an iridium catalyst (Equation (77)). The rate acceleration observed in the Alder-ene cyclization of aminoester containing allenyne 121 (Equation (78)) was attributed to the Thorpe-Ingold effect.80... 

Wilkinson s catalyst also allows the intramolecular cycloisomerization of allenynes 243 to interesting cross-conjugated trienes 244 (Scheme 15.76) [146], Similar compounds are observed as side-products in Pauson-Khand reactions of allenynes [147]. 

A computational study of the free energy surfaces of the PtCb-mediated cycloisomerization of allenynes has allowed the characterization of key steps and allowed comment on the structural, energetic, and electronic implications of the substitution at the allenyne upon the course of these catalysed reactions.139... 

The ether-tethered allenyne 248 undergoes a rhodium(l)-catalyzed intramolecular allenic Alder ene reaction to afford the ( )-3,6-dihydropyran 249 as the major product (Equation 111) <2002JA15186>. Likewise, ether tethered enynes can undergo rhodium(i)-catalyzed cycloisomerizations to afford 3,6-dihydropyrans <2005JA10180>. 

Review articles have featured advances in the cycloisomerization of l,n-allenynes and Rn-allenenes, " reactions for synthesis of quaternary centres bearing a nitrogen substituent, " the Ramberg-Backlund reaction transforming a-halosulfones into alkenes, " and ring expansion of l,2-benzisoxazol-3-ones with a Vilsmeier reagent system, POCI3 and dimethylformamide (Scheme 75). " ... 

Cycloisomerization of Aiienenes and Alleneynes. Allenes are a special class of compounds that possess cumulated double-bond systems. For a long period of time, the development of the chemistry of allenes was impaired under the false notion that such cumulated double-bond systems are highly unstable (90-92). Unlike the chemistry of alkene and alkyne that have been comprehensively developed, allenes only started to emerge in recent decades as versatile precursors in organic synthesis. The Brummond Laboratory was the first to illustrate that cross-conjugated trienes 83 could be prepared by Rh-catalyzed allenic Alder-ene reaction of allenyne 82 (Scheme 42) (93). 

Five years ago, Houk, Toste, and coworkers [31] suggested a fascinating new reaction mechanism. The cycloisomerization of 1,5-allenynes 23 by the [(Ph3PAu)30]Bp4 catalyst delivers cross-conjugated trienes 24 (Scheme 4.8). 

Brummond s group [23] and independentlythe group of Shibata [24] discovered that rhodium(I) complexes are excellent catalysts in the formal Alder-ene cycloisomerization of allenynes to give cross-conjugated trienes under mild conditions... 

Scheme 12.8 One-pot Alder-ene-Diels-/ lder-Diels-Alder reactions initiated by Rh-catalyzed cycloisomerization of an allenyne.

Mercury (II) triflate catalyzes the cycloisomerization of the allenynes 23 (X = N-Ts) to the vinyl allenes 24 in high yields. ... 

Mukai et al. have found scission of a cyclopentane ring of allenyne 34 in the rhodium-catalyzed cycloisomerization (Scheme 7.11) [14]. When 34 was treated with a rhodium catalyst, the bicyclo [7.4.0]tridecatriene 37 was formed. Mechanistically, initial coordination of 34 to rhodium(I) would occur between an allenic distal double bond and an alkyne to form the intermediary ir-coordinating complex, which undergoes oxidative cyclization to form the rhodabicyclo[4.3.0]nonadiene intermediate 35. Subsequent P-carbon elimination, presumably assisted by release of the ring strain of the cyclopentane (6.3 kcal mol ), results in the formation of the 10-membered rhodacycle 36. Reductive elimination ensues to give the final product 37. 

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