Sergei I. Kozhushkov

Ruthenium-catalyzed direct oxidative alkenylation of arenes through twofold C–H bond functionalization

Significant progress has been accomplished in direct olefinations through twofold C–H bond functionalization of arenes and heteroarenes employing readily accessible, selective and relatively inexpensive ruthenium catalysts. Particularly, ruthenium(II) complexes have allowed challenging direct double C–H/C–H bond alkenylations of arenes with ample scope. These catalysts set the stage for step-economical C–H/C–H bond functionalization with electron-rich as well as electron-deficient arenes and heteroarenes, and, thereby, provide versatile access to diversely decorated styrenes.

Direct C–H bond arylations and alkenylations with phenol-derived fluorine-free electrophiles

Significant progress has been accomplished in direct C–H bond arylations of arenes and heteroarenes with readily accessible, inexpensive phenol derivatives. Thus, ruthenium biscarboxylate complexes and inexpensive cobalt compounds allowed for challenging C–H bond derivatizations of arenes. Further, palladium, nickel and cobalt catalysts set the stage for step-economical C–H/C–O bond functionalization with electron-rich as well as electron-deficient heteroarenes.

Bicyclopropylidene — A Unique Tetrasubstituted Alkene and a Versatile C6-Building Block

Strain in an organic molecule often correlates with increased reactivity, at least for certain types of reactions. Thus, the elucidation of the chemistry of highly strained alkenes like the unusual tetrasubstituted alkene, bicyclopropylidene (1), which is a particularly strained derivative of methylenecyclopropane (2), has proved to be fruitful both with respect to synthetic applications as a C6 building block, as well as understanding certain reaction principles. The different, steadily improved methods which have been developed for the preparation of this unusual alkene in the last thirty years as well as rather recent methods for the synthesis of functionally substituted, and spirocyclopropanated derivatives as well as bis(bicyclopropylidenyls) are presented. The special aspects of molecular geometry in bicyclopropylidenes as well as strain and its influence on chemical properties are discussed.The rich chemistry of bicyclopropylidene beginning with its well-known thermal rearrangement and dimerization, its dihalocarbene additions all the way to its recently developed organometallic chemistry, especially its reactions under the catalysis of palladium and other transition metals, is covered. Finally, some synthetically useful chemical transformations of bicyclopropylidene derivatives, e.g., synthetic approaches to the cyclopropanated analogs of natural products, are presented.

Heterogeneous catalytic approaches in C–H activation reactions

Despite the undisputed advances and progress in metal-catalyzed C–H functionalizations, this atom-economical approach had thus far largely been developed with the aid of various metal catalysts that were operative in a homogeneous fashion. While thereby major progress was accomplished, these catalytic systems featured notable disadvantages, such as low catalyst recyclability. This review summarizes the development of user-friendly, recyclable and easily separable heterogeneous catalysts for C–H activation. This strategy was characterized by a remarkably broad substrate scope, considerable levels of chemo- and site-selectivities and proved applicable to C–C as well as C–heteroatom formation processes. Thus, recyclable catalysts were established for arylations, hydroarylations, alkenylations, acylations, nitrogenations, oxygenations, or halogenations, among others. The rapid recent progress in selective heterogeneous C–H functionalizations during the last decade until December 2015 is reviewed.

Ruthenium-catalyzed hydroarylations of methylenecyclopropanes: mild C-H bond functionalizations with conservation of cyclopropane rings.

Ring conservation was observed in the first catalytic intermolecular hydroarylation of methylenecyclopropanes via C-H bond functionalization, a remarkable reactivity mode for a transformation proceeding through (cyclopropylcarbinyl)metal intermediates.

An Evolving Multifunctional Molecular Building Block: Bicyclopropylidene

The elucidation of the chemistry of the highly strained and unusually tetrasubstituted alkene bicyclopropylidene (2), has proved to be fruitful, both with respect to synthetic applications as a multifunctional C6 building block and for the understanding of certain reaction principles. The different, and steadily improved, methods developed over the last thirty years for the preparation of this unusual alkene, and the more recent methods for the synthesis of functionally substituted as well as spirocyclopropanated derivatives, are presented. The rich chemistry of bicyclopropylidene, beginning with its well-known thermal rearrangement and dimerization, its [2+n] cycloadditions with various carbenes, alkenes, 1,3-dipoles, and dienes all the way to its recently developed organometallic chemistry, especially its reactions under the catalysis of palladium and other transition metals, is covered. Some of the peculiar physical properties of bicyclopropylidene (2) which explain its unique reactivity, are also discussed. Finally, some synthetically useful chemical transformations of bicyclopropylidene derivatives, for example, synthetic approaches to certain cyclopropanated analogs of natural products, are presented.

The First Enantiomerically Pure Triangulane (M)‐Trispiro[2.0.0.2.1.1]nonane Is a σ‐[4]Helicene

A remarkably high specific rotation, even at 589 nm, is shown by (M)-1, the first enantiomerically pure unbranched [4]triangulane, although it has no chromophore that would lead to any significant absorption above 200 nm. This outstanding rotatory power is in line with a helical arrangement of its σ bonds, as confirmed by high-level computations. Thus, it is appropriate to call (M)-1 a “σ-[4]helicene”, the first σ-bond analogue of the aromatic [n]helicenes.

A new highly efficient three-component domino Heck-Diels-Alder reaction with bicyclopropylidene: rapid access to spiro[2.5]oct-4-ene derivatives.

Bicyclopropylidene (1) was found to surpass even methyl acrylate (17 a) in its rate of undergoing carbopalladation with aryl- or alkenylpalladium species, leading to substituted allylidenecyclopropanes 5, 7 and 10, mostly in high yields (37-78 %). These dienes and cross-conjugated trienes react in a Diels-Alder mode with dienophiles to give spiro[2.5]octenes 18 a-Ph, 18 b-Ph and 18 a-Vin, respectively, in good yields (89, 69 and 65 %). The overall transformation can be achieved as a one-pot three-component reaction with a variety of dienophiles to furnish the domino Heck-Diels-Alder products 18 regioselectively in most cases in good to very high yields (49-100 %). The reaction of 1 with iodobenzene (2-Ph) and 17 a gave 18 a-Ph in virtually quantitative yield-also on a gram scale-using only 1 mol % of catalyst, and even bromobenzene (22) gave 18 a-Ph in 59 % yield. Bicyclopropylidene (1), in the presence of palladium acetate/triphenylphosphane underwent rearrangement to allylidenecyclopropane (5-H), which in turn dimerized (73 %) in the absence of other reaction partners, or could be trapped by diethyl fumarate (17 c) to give the Diels-Alder adduct 18 c-H in 45 % yield. The coupling of oligoiodobenzenes with 1 and subsequent cycloaddition could be extended to a multicomponent reaction. In this way, 1,4-diiodobenzene (37), 1 and an alkyl acrylate gave the products 38 of a twofold Heck-Diels-Alder reaction in up to 87 % yield, 1,3,5-triiodobenzene (39) reacted in up to 72 % yield and ultimately 1,2,4,5-tetraiodobenzene (41) gave the fourfold domino Heck-Diels-Alder product 42 in 47 % isolated yield, in a single operation in which 12 new carbon-carbon bonds were formed.

Intramolecular Diels-Alder reactions of furans with a merely strain-activated tetrasubstituted alkene: Bicyclopropylidene

Bicyclopropylidene derivatives 1, 6, 8 with furan moieties attached on tethers of various length and nature all undergo clean intramolecular Diels-Alder reactions with complete endo-diastereoselectivities when heated to 70 – 130°C, under 10 kbar pressure. The reaction rates are at least as high or higher than those of analogously tethered methylenecyclopropane-furan derivatives 15, 17, 20, in spite of the fact that the reactive double bond in bicyclopropylidene is tetrasubstituted and not activated by any electron withdrawing group.

Palladium(0)- and Nickel(0)-Catalyzed [3 + 2] Co-Cyclization Reactions of Bicyclopropylidene with Alkenes

Bicyclopropylidene (1) readily undergoes a palladium(0)-catalyzed [3 + 2] co-cyclization with electron deficient alkenes (methyl acrylate, methyl trans-crotonate, methyl cinnamate and diethyl fumarate) as well as with some strained alkenes (norbornene, norbornadiene) by distal ring cleavage of one of the three-membered rings of 1. All these co-cyclizations are regioselective with respect to 1 as well as regio- and stereoselective with respect to the alkenes to give the corresponding 4-methylenespiro[2.4]heptane-trans-6-carboxylates 2a−5 with the electron deficient alkenes and the cycloadducts 6 and 7 with the strained alkenes in acceptable to good yields (56 to 83%). In contrast to palladium(0) catalysts nickel(0) complexes catalyze both distal ring opening of 1 and oxidative coupling of the two double bonds when 1 is reacted with e.g. diethyl fumarate. The result is a mixture of the methylenecyclopentane derivative 5 with the [2 + 2] cycloadduct 8 and the cotrimer 9.