Transition Metal Catalyzed Cycloisomerizations of 1,n-Allenynes and -Allenenes

[5 + 2] Cycloaddition Reactions in Organic and Natural Product Synthesis

New catalytic ring-expansion reactions of strained ring (hetero- and carbocyclic) substrates reported in the last six years (2006–2012) are presented. As evident from the diversity of new approaches, this is a vibrant area of research. Metals ranging from magnesium to gold have been used as catalysts. Some of these reactions allow access to enantioenriched products by employing catalysts decorated with chiral organic motifs (primarily C-2 symmetric).

Recent Advances in the Metal-Catalyzed Ring Expansions of Three- and Four-Membered Rings

Progress in the synthesis and transformations of alkylidenecyclopropanes and alkylidenecyclobutanes.

The rapid generation of molecular complexity in a relatively easy manner has made the [5+2] cycloaddition approach a highly useful tool in the synthesis of a wide number of complex natural products and important biologically active products containing seven-membered rings. Besides the common and highly efficient [5+2] cycloadditions of oxidopyrylium and oxidopyridinium ions with π-systems which offer an easy access to a wide range of novel heterocyclic seven-membered ring molecules with a oxygen or nitrogen bridge, and the less employed [5+2] photocycloadditions, the metal-catalyzed [5+2] cycloadditions have attracted a lot of attention and have become one of the most popular ways of constructing seven-membered ring products. All these cycloadducts can be synthetically manipulated easily toward a number of interesting molecular structures with high potential applications. Abbreviations: Ac: acetyl; Ar: aryl; BarF: tetrakis[3,5-bis(trifluoromethyl)phenyl]borate; BINAP: 2,2′-bis(diphenylphosphanyl)-1,1′-binaphthyl; Bn: benzyl; Boc: tert-butoxycarbonyl; Bu: butyl; Bz: benzoyl; Cbz: benzyloxycarbonyl; cod: cyclooctadiene; Cp: cyclopentadienyl; DBU: 1,8-diazabicyclo[5.4.0]undec-7-ene; DCE: 1,2-dichloroethane; de: diastereomeric excess; DFT: density functional theory; DMF: dimethylformamide; DMSO: dimethyl sulfoxide; dppb: 1,4-bis(diphenylphosphino)butane; dppe: 1,2-bis(diphenylphosphine)ethane; dppp: 1,3-bis(diphenylphosphino)propane; ee: enantiomeric excess; Et: ethyl; HFIP: hexafluoroisopropyl alcohol; L: ligand; M: metal; Me: methyl; MOM: methoxymethyl; Ms: mesyl; Ph: phenyl; PMB: para-methoxybenzoyl; Pr: propyl; Segphos: 5,5′-bis(diphenylphosphino)-4,4′-bi-1,3-benzodioxole; TBDPS: tert-butyldiphenylsilyl; TBS: tert-butyldimethylsilyl; TEA: triethylamine; Tf: trifluoromethanesulfonyl; TFE: trifluoroethanol; THP: tetrahydropyran; TIPS: triisopropylsilyl; TMS: trimethylsilyl; Tol: tolyl; Tp: hydridotris(pyrazolyl)borate; Ts: 4-toluenesulfonyl (tosyl); VCP: vinylcyclopropane.

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Recent Developments in the [5+2] Cycloaddition

Highly strained cyclopropane derivatives have served as useful and powerful C3 building blocks [1] for the construction of various ring systems, and the metal-catalyzed cleavage of the activated carbon-carbon s bond of the cyclopropane ring 3] must be one of the most attractive methods from a synthetic point of view. Of particular interest is the Rhcatalyzed [5+2] cycloaddition of vinylcyclopropanes with carbon–carbon p-components for the efficient formation of seven-membered compounds. In 1995, Wender et al. reported the first example of the Rh-catalyzed [5+2] cycloaddition reaction of vinylcyclopropanes with alkynes to provide bicyclo[5.3.0]decadienes. This method was successfully applied to the [5+2] cycloaddition reactions of allenes as well as alkenes 4a] as an alternative carbon–carbon p-counterpart. In sharp contrast to the extensive investigation of vinylcyclopropanes, 3–5] very few examples of the ringclosing reaction of allenylcyclopropane, which is regarded as an alkylidene homologue of vinylcyclopropane, have been reported. Two representative examples are shown in Scheme 1; one involves the Rh-catalyzed cycloisomeriza-

Rhodium(I)‐Catalyzed Intramolecular [5+2] Cycloaddition Reactions of Alkynes and Allenylcyclopropanes: Construction of Bicyclo[5.4.0]undecatrienes and Bicyclo[5.5.0]dodecatrienes

RhI‐Catalyzed [6+2] Cycloaddition of Alkyne–Allenylcyclobutanes: A New Entry for the Synthesis of Bicyclo[6.m.0] Skeletons

A productive channel for pent-up energy: The [{RhCl(CO)2 }2 ]-catalyzed ring-opening of both E and Z 1-cyclopropylocta-1,2,6-trienes exclusively produced cis-4,5-dimethylbicyclo[4.3.0]nona-1(9),2-dienes with three contiguous stereogenic centers. When this transformation was applied to the 1,2,7-triene double-bond isomers, the same products were formed in a completely stereoselective manner.

Katsuya Sugikubo

Papers

Rhodium(I)‐Catalyzed Intramolecular [5+2] Cycloaddition Reactions of Alkynes and Allenylcyclopropanes: Construction of Bicyclo[5.4.0]undecatrienes and Bicyclo[5.5.0]dodecatrienes

RhI‐Catalyzed [6+2] Cycloaddition of Alkyne–Allenylcyclobutanes: A New Entry for the Synthesis of Bicyclo[6.m.0] Skeletons

Rhodium(I)-catalyzed cycloisomerization of alkene-substituted allenylcyclopropanes: stereoselective formation of bicyclo[4.3.0]nonadienes.

Rhodium(I)‐Catalyzed Cycloisomerization of Allene–Allenylcyclopropanes

Rhodium(I)‐Catalyzed Cycloisomerization of Alkene‐Substituted Allenylcyclopropanes: Stereoselective Formation of Bicyclo[4.3.0]nonadienes.