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

The allene moiety represents an excellent partner for the [2+2] cycloaddition with alkenes and alkynes, affording the cyclobutane and cyclobutene skeletons in a single step. This strategy has been widely studied under thermal, photochemical and microwave induced conditions. More recently, the use of transition metal catalysis has been introduced as an alternative relying on the activation of the allenic component. On the other hand, the intramolecular version has attracted much attention as a strategy for the synthesis of polycyclic compounds in a regio- and stereoselective fashion. This critical review focuses on the most recently developed [2+2] cycloadditions on allenes along with remarkable early works accounting for the mechanism, the regio- and diastereoselectivity of the cycloadducts formed (103 references).

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Exploiting [2+2] cycloaddition chemistry: achievements with allenes

[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

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

The simple refluxing of allenynes, having a phenylsulfonyl functionality on the allenyl group, in xylene (or mesitylene) without microwave irradiation resulted in the efficient formation of bicyclo[5.2.0]nona-1,7-dienes and bicyclo[4.2.0]octa-1,6-dienes in high yields. This method was shown to be successfully applicable to the first construction of bicyclo[6.2.0]deca-1,8-dienes. Construction of the corresponding oxa- and azabicyclo[m.2.0] frameworks could also be attained. This thermal ring-closing reaction involves the formal [2+2] cycloaddition in which the distal double bond of an allenyl moiety exclusively served as one of the π-components regardless of the position of the phenysulfonyl functionality on the allenyl moiety.

Thermal [2+2] cycloaddition of allenynes: easy construction of bicyclo[6.2.0]deca-1,8-dienes, bicyclo[5.2.0]nona-1,7-dienes, and bicyclo[4.2.0]octa-1,6-dienes.

Thermal [2 + 2] Cycloaddition of Allenynes: Easy Construction of Bicyclo[6.2.0]deca-1,8-dienes, Bicyclo[5.2.0]nona-1,7-dienes, and Bicyclo[4.2.0]octa-1,6-dienes.

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.

Yusuke Miyashita

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

Thermal [2+2] cycloaddition of allenynes: easy construction of bicyclo[6.2.0]deca-1,8-dienes, bicyclo[5.2.0]nona-1,7-dienes, and bicyclo[4.2.0]octa-1,6-dienes.

Thermal [2 + 2] Cycloaddition of Allenynes: Easy Construction of Bicyclo[6.2.0]deca-1,8-dienes, Bicyclo[5.2.0]nona-1,7-dienes, and Bicyclo[4.2.0]octa-1,6-dienes.

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.