Gold(I) complexes featuring electron acceptor ligands such as phosphites and phosphoramidites catalyze the [4C+2C] intramolecular cycloaddition of allenedienes. The reaction is chemo- and stereoselective, and provides trans-fused bicyclic cycloadducts in good yields. Moreover, using novel chiral phosphoramidite-based gold catalysts it is possible to perform the reaction with excellent enantioselectivity. Experimental and theoretical data dismiss a cationic mechanism involving intermediate II and suggest that the formation of the [4C+2C] cycloadducts might arise from a 1,2-alkyl migration (ring contraction) in a cycloheptenyl Au-carbene intermediate (IV), itself arising from a [4C+3C] concerted cycloaddition of the allenediene. Therefore, these [4C+2C] allenediene cycloadditions and the previously reported [4C+3C] counterparts most likely share such cycloaddition step, differing in the final 1,2-migration step.

Efficient at room temperature: The Au complex generated in situ from [(IPr)AuCl] and AgSbF(6) promotes the [4C+3C] intramolecular cycloaddition of allenes and dienes at room temperature, and in a particularly efficient and versatile manner. A DFT study on dimethylallenyl precursors agreed with the formation and cycloaddition of a metal-allyl cation intermediate, and points to the 1,2-hydride shift as the key rate-limiting step.

Crossing the seven C's: The three carbon atoms of several allenyl fragments can be incorporated into seven‐membered carbocycles by means of a Pt‐catalyzed intramolecular [4C+3C] cycloaddition with dienes (see scheme). The transformation provides a straightforward and atom‐economical entry to a variety of cycloheptane‐containing polycycles from readily available acyclic precursors.

The reactivity of the dihydrides MH 2 Cl 2 (P i Pr 3 ) 2 (M = Os (1), Ru (2)) toward allenes has been studied. Complex 1 reacts with 2 equiv of 3-methyl-1,2-butadiene and 1-methyl-1-(trimethylsilyl)allene to give 1 equiv of olefin and the π-allene derivatives OsCl 2 (η 2 -CH 2 dCdCRMe)(P i Pr 3 ) 2 (R = Me (3), Me 3 Si (4)). The X-ray structure of 4 proves the coordination to the metal center of the carbon-carbon double bond of the allene with the lowest steric hindrance. In toluene, complexes 3 and 4 are unstable and evolve into the hydride-alkenylcarbyne derivatives OsHCl 2 (tCCHdCRMe)(P i Pr 3 ) 2 (R = Me (5), Me 3 Si ( 6)). DFT calculations on the model compound OsCl 2 (η 2 -CH 2 dCdCMe 2 )(PMe 3 ) 2 (3t) suggest that the π-allene to hydride-alkenylcarbyne transformation involves the migration of both hydrogen atoms of the CH 2 group of the allene. The first of them occurs between the terminal and central carbon atoms and takes place throught the metal center. The second one is a 1,2-hydrogen shift from the allene terminal carbon to osmium. The reactions of the ruthenium complex 2 with the previously mentioned allenes give olefins and RuCl 2 (η 2 -CH 2 dCdCRMe)(P i Pr 3 ) 2 (R = Me (7), Me 3 Si ( 8)), which in dichloromethane and in the presence of allene afford the disubstituted vinylidene complexes RuCl 2 (dCdCRMe)(P i Pr 3 ) 2 (R = Me (9), Me 3 Si ( 10)). The structure of 10 in the solid state has been determined by X-ray diffraction analysis. DFT calculations show that the formation of 9 and 10 can be rationalized in terms of the initial isomerization of 7 and 8 to alkenylcarbene species, which subsequently undergo metathesis reactions with a second allene molecule.

We report a ruthenium-catalyzed (2 + 2) intramolecular cycloaddition of allenes and alkenes. We have found that the use of the ruthenium complex RuH(2)Cl(2)(P(i)Pr(3))(2), which has previously gone unnoticed in catalytic applications, is crucial for the observed reactivity. The reaction proceeds under mild conditions and is fully diastereoselective, providing a practical entry to a variety of bicyclo[3.2.0]heptane skeletons featuring cyclobutane rings.

We report a Pd-catalyzed intramolecular [3C + 2C + 2C] cycloaddition between alkylidenecyclopropanes, alkynes and alkenes. The method provides synthetically relevant 5-7-5 tricyclic structures, with good chemoselectivity and complete diastereoselectivity.

No abstract

Allenes are useful and versatile twocarbon partners in palladium catalyzed [3C + 2C] intramolecular cycloadditions with alkylidenecyclopropanes enabling the preparation of dienyl bicycloA C H T U N G T R E N N U N G [3.3.0]octane adducts with good yields and high diastereoseletivity. Keywords:alkylidenecyclopropanes; allenes; bicycloA C H T U N G T R E N N U N G [3.3.0]octanes; cycloaddition; palladium Alkylidenecyclopropanes are strained but readily accessible structures which can participate as threecarbon components in metal-catalyzed cycloadditions to afford different types of p-systems.[1] Our group has recently shown that alk-5-ynylidenecyclopropanes undergo a mild [3C + 2C] intramolecular cycloaddition upon treatment with appropriate palladium or ruthenium catalysts to provide a variety of fused bicycloA C H T U N G T R E N N U N G [3.3.0]octenes.[2] More recently we have also demonstrated that it is possible to use alkenes in place of alkynes as two-carbon components in the cycloaddition. Although attaining good efficiencies in these cycloadditions requires a relatively high catalyst loading (6 % of Pd 2 dba 3 , and 20 % of the ligand), the reactions provide a rapid and direct entry to bicyclopentanoid rings equipped with up to three stereogenic centers.[3] The cycloadditions can be performed with either activated (electron-deficient) or non-activated alkenes, but it is less efficient when the terminal position of the alkene is substituted by an alkyl group due to competing b-hydride elimination processes on putative palladacyclic intermediates. Thus, treatment of the cyclopropylideneethylamine 1 with Pd 2 dba 3 (6 mol %) and ligand L1 (20 mol %) in refluxing dioxane provided the cycloadduct 2 in only 57 % yield because of concomitant formation of the dienyl cycloisomerization side product 3 [25 % yield, Eq.(1)].[4] Thereby, while the cycloaddition seems still to be the major reaction pathway, there are competitive processes that result in an important decrease in the efficiency of the transformation. We envisaged that using allenes instead of alkenes might provide a good solution to the above problem and perhaps lead to more efficient [3 + 2] cycloaddition processes owing to the presumably higher reactivity of the allene unit. Allenes are being increasingly used in metal-catalyzed annulation reactions due to their unique reactivity and the special manipulation possibilities that they offer to the resulting products owing to their retaining one double bond of the allene.[5] Despite these potential advantages, we are not aware of studies on the behaviour of this type of p-system in transition metal-catalyzed [3C + 2C] cycloadditions with methylene-or alkylidenecyclopropanes.[6] Herein, we describe the first examples of palladium-catalyzed intramolecular cycloadditions between allenes and alkylidenecyclopropanes, examples that confirm the special utility of allenes as twocarbon partners in this type of annulation.Allene 4a, which can be easily assembled from 1-vinylcyclopropyltosylate and diethyl ...