Metal-catalysed cleavage of carbon-carbon bonds

Reactions cleaving carbon-carbon bonds with the assistance of transition metals as the catalyst have provided various molecular transformations that are otherwise difficult to execute, opening a scenic avenue along organic synthesis. Construction of structural motifs like medium-sized carbocycles and chiral quaternary carbon centres have been set within an access of such paradoxical approaches in the past decade.     

Silylzincation of carbon-carbon multiple bonds revisited

The first investigation of the copper-catalyzed silylzincation of alkynes as well as a diene and styrene using bis(triorganosilyl) zinc reagents led to the development of an efficient procedure and the disclosure of an unexpected bissilylation and unforeseen regioselectivity.     

Asymmetric hydroamination of non-activated carbon-carbon multiple bonds

The catalysed enantioselective formation of carbon-nitrogen bonds by the hydroamination reaction is reviewed. All examples deal with substrates containing non-activated carbon-carbon multiple bonds which are transformed either via intramolecular or intermolecular reactions. Structurally different complexes already provided nitrogen containing compounds/heterocycles with high enantioselectivities.     

Stable alkanes containing very long carbon-carbon bonds

The metal-induced coupling of tertiary diamondoid bromides gave highly sterically congested hydrocarbon (hetero)dimers with exceptionally long central C-C bonds of up to 1.71 ? in 2-(1-diamantyl)[121]tetramantane. Yet, these dimers are thermally very stable even at temperatures above 200 °C, which is not in line with common C-C bond length versus bond strengths correlations. We suggest that the extraordinary stabilization arises from numerous intramolecular van der Waals attractions between the neighboring H-terminated diamond-like surfaces. The C-C bond rotational dynamics of 1-(1-adamantyl)diamantane, 1-(1-diamantyl)diamantane, 2-(1-adamantyl)triamantane, 2-(1-diamantyl)triamantane, and 2-(1-diamantyl)[121]tetramantane were studied through variable-temperature (1)H- and (13)C NMR spectroscopies. The shapes of the inward (endo) CH surfaces determine the dynamic behavior, changing the central C-C bond rotation barriers from 7 to 33 kcal mol(-1). We probe the ability of popular density functional theory (DFT) approaches (including BLYP, B3LYP, B98, B3LYP-Dn, B97D, B3PW91, BHandHLYP, B3P86, PBE1PBE, wB97XD, and M06-2X) with 6-31G(d,p) and cc-pVDZ basis sets to describe such an unusual bonding situation. Only functionals accounting for dispersion are able to reproduce the experimental geometries, while most DFT functionals are able to reproduce the experimental rotational barriers due to error cancellations. Computations on larger diamondoids reveal that the interplay between the shapes and the sizes of the CH surfaces may even allow the preparation of open-shell alkyl radical dimers (and possibly polymers) that are strongly held together exclusively by dispersion forces.     

Metal-catalyzed additions of H-P(O) bonds to carbon-carbon unsaturated bonds

In this short review, we briefly summarize our studies on metal-catalyzed additions of H-P(O) bonds to carbon-carbon unsaturated compounds (alkynes, alkenes, allenes, dienes and related). These new reactions provide regio- and stereoselective ways for the preparation of a variety of organophosphorus compounds. Mechanistic aspects related to the catalytic processes are discussed.     

Stannylation of unsaturated carbon-carbon bonds by tin tetrachloride

The reactions of tin tetrachloride and four terminal alkynes (PhCCH, ^tBuCCH, ⁿBuCCH, HOCH₂CCH), norbornene, and norbornadiene in dichloromethane or chloroform solution lead to the formation of stannylation products, which were characterized by ¹H, ¹³C and ¹¹⁹Sn NMR spectroscopy. Virtually complete α-regioselectivity was obtained in reaction of all four alkynes without any effect of the relative steric bulk of the substituent R at the triple bond of alkyne RC_βC_αH. The reaction of norbornene and norbornadiene with SnCl₄ is stereoselective, giving an exo stannylation product.     

Catalytic reactions involving the cleavage of carbon-cyano and carbon-carbon triple bonds

The catalytic reactions that involve the cleavage of C-CN bonds and carbon-carbon triple bonds are described in this tutorial review. Regarding the cleavage of a C-CN bond, the catalytic reaction can proceed by two different mechanisms: oxidative addition and deinsertion of silyl isocyanide. A carbon-carbon triple bond can be cleaved in the absence of an organic promoter via the formation of unique organometallic species, such as allenylidene and cyclopropyl carbenoid complexes.     

碳碳不饱和键氟化加成反应的研究进展

选择性地向有机物中引入氟原子越来越受到重视，近年来通过碳碳不饱和键的 氟化加成来引入氟原子发展迅速。综述了碳碳不饱和键氟化加成反应的研究进展。     

Bond properties of metal-ligand bonds

An empirical relation has been developed to compute the bond order of ionic systems. The computed values have been compared with those calculated by using the experimental values of bond polarizability derivatives and are found to be in fair agreement. This shows that the proposed empirical relation for computing bond orders is quite suitable for ions.

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Bindungseigenschaften von Metall-Ligand-Bindungen

Zusammenfassung Es wurde eine empirische Beziehung entwickelt, die die Berechnung von Bindungsordnungen für ionische Systeme erlaubt. Ein Vergleich der berechneten Werte mit solchen, die aus experimentellen Daten für Differentiale von Bindungspolarisierbarkeiten erhalten wurden, zeigte die Brauchbarkeit der Methode für Ionen.

     

Metal-metal and carbon-carbon bonds as potential components of molecular batteries

The reductive coupling of [M(salophen)] derivatives, where M is an early transition metal and salophen is N,N'-o-phenylenebis(salicylideneaminato) dianion, led to the formation of dimers linked through C-C and M-M bonds. Both of these bonds can potentially function as electron reservoirs: each bond can be used as a reversible source of a pair of electrons under the condition that it is not chemically transformed by the incoming substrate which functions as an electron acceptor. To explore this potential function as well as the competition in the redox processes between C-C and M-M bonds within the same molecular framework, we investigated the reduction of [(tBu4-salophen)NbCl3] (1) and [(tBu4-salophen)MoCl2] (7) as model compounds. In the former case, the reduction led to [(Nb-Nb)(tBu4-*salophen2*)] (2) which contains both a Nb-Nb bond (2.6528(7) A) and two C-C bonds across two imino groups of the ligand. Complex 2 can be reduced further to a transient compound 5 that contains an Nb=Nb bond. In the second case, the reduction of 7 by two electrons led to [(Mo[triplebond]Mo)(tBu4-salophen)2] (8), which does not contain any C-C linkages between the two salophen units. Complexes 2 and 5 are able to transfer one pair and two pairs of electrons, respectively, to give compounds 3, 4, and 6, with the consequent cleavage of the Nb-Nb and Nb=Nb bonds. In the present case, it is surprising that the C-C bonds do not participate in the reduction of the substrates. A careful theoretical treatment anticipates, both in the case of 1 and 7, the preferential formation of metal-metal bonds upon reduction. This is indeed the case for 7, but not for 1, where the formation of C-C bonds competes with that of M-M bonds, the latter being the first ones, however, to be involved in electron-transfer reactions. The theoretical approach allowed us to investigate the possibility of intramolecular electron transfer from C-C bonds to M-M bonds and vice versa.     

Reduction of carbon-carbon double bonds and hydrogenolysis by sodium hypophosphite

Sodium hypophosphite plus palladium-charcoal is a mild, economical, selective system for the reduction of carbon-carbon double bonds and hydrogenolysis of benzyl ethers and benzyl carbonates.     

Reduction of carbon-carbon double bonds using organocatalytically generated diimide

An efficient method has been developed for the reduction of carbon-carbon double bonds with diimide, catalytically generated in situ from hydrazine hydrate. The employed catalyst is prepared in one step from riboflavin (vitamin B(2)). Reactions are carried out in air and are a valuable alternative when metal-catalyzed hydrogenations are problematic.     

Bond functions and the topological properties of the bonds

The effect of bond functions on the topological properties of the bonds is studied by performing Hartree-Fock calculations for O₂, Cl₂, F₂, CN^–, N₂ and LiCl molecules. The results show that bond functions increse the degree of covalency of the bonds and the electronic density at the (3, –1) bond critical point. This effect is quite noticeable for intermediate interactions molecules (F₂ and CN^–) where a is obtained instead of a positive value. The optimal position of the bond functions seems to be at or near the (3, –1) bond critical point.     

Addition of ArSSAr to carbon-carbon multiple bonds using electrochemistry

ArS(ArSSAr)⁺ (arylbis(arylthio)sulfonium ions), which were generated and accumulated by the electrochemical oxidation of diaryl disulfides (ArSSAr) in CH₂Cl₂ at −78 °C, reacted with alkenes to give the corresponding diarylthio-substituted compounds in a stereospecific manner in good yields, when the reaction was quenched with a soft nucleophile, such as allylsilanes, ketene silyl acetals, and triethylamine. A mechanism involving the initial formation of an episulfonium ion followed by ring-opening by the attack of ArSSAr has been suggested. The reactions of ArS(ArSSAr)⁺ with alkynes also took place to give 1,2-diorganothio-substitued alkenes stereoselectively under similar conditions.     

Catalysis of hydrosilylation of carbon-carbon multiple bonds: Recent progress

Recent progress in the catalytic hydrosilylation of organic and organosilicon compounds containing carbon-carbon multiple bonds is reviewed. Related to this, dehydrogenative silylation is also discussed. During the last decade new hydrosilylation catalysts, predominantly homogenous and heterogenous transition methal complexes, have been developed. These catalysts offer not only increased efficiency and turnover rate but also improved regioselectivity and stereoselectivity; moreover, there has been development in the mechanistic rationale behind these improvements. Application and extension of these basic chemical advances are found in many areas including polyorganosiloxane curing, hydrosilylation polymerization, polysiloxane functionalization, and silicon-containing dendrimer development.     

Allylstannylation of carbon-carbon and carbon-oxygen unsaturated bonds via a radical chain process

In the presence of a radical initiator, allyltributylstannanes bearing an electron-withdrawing group at the beta-position smoothly reacted with electron-deficient terminal alkenes to give allylstannylated products in good yields. The stannyl group was introduced into the terminal carbon with high regioselectivity. The allylstannylation of homochiral 8-phenylmenthyl acrylate proceeded with moderate to good diastereoselectivity. Terminal and electron-deficient internal alkynes as well efficiently underwent the radical-initiated allylstannylation in an anti addition mode. The reaction of terminal alkynes showed the same regioselectivity as that of terminal alkenes. The present radical reaction was applicable to allylation of aromatic aldehydes and ketones.     

Biomimetic heterocyclisation of aryl olefins one-step formation of two carbon-carbon bonds

Cyclisation of ethoxylactam 5 in formic acid at room temperature gives rise to stereospecific formation of 9. Similar reaction of the olefin 6 possessing unnatural geometry also proceeds stereospecific and in quantitative yield to the tetracyclic product 10. The acetylene 7 equally affords 11. Upon Me-substitution of the olefinic double bond the cyclisation of 8 has to be carried out in trifluoroacetic acid-dichloromethane in order to obtain a 84% yield of 12 again formed with a high degree of stereoselectivity. The results are discussed on the basis of a mechanism involving synchronous bond formation.