Titanocene(II)-promoted multicomponent reactions utilizing alkynyl sulfones, alkenyl sulfones, and carbonyl compounds: a novel method for the synthesis of vinylallenes

Titanocene(II)-promoted cross coupling of alkynyl- and (Z)-alkenyl sulfones affords alpha-(phenylsulfonyl)alkenyltitanium species. Further treatment of these species with the titanocene(II) reagent generates titanium vinylvinylidene complexes, which react with carbonyl compounds in one pot to produce substituted vinylallenes with complete stereoselectivity. By using alpha,beta-unsaturated ketones, 1,3,4,6-tetraenes are also obtained stereoselectively.     

Highly diastereoselective palladium-catalyzed cyclizations of 3,4-allenylic hydrazines and organic halides -- highly stereoselective synthesis of optically active pyrazolidine derivatives and the prediction of the stereoselectivity

Pyrazolidines containing two chiral centers, an interesting class of heterocyclic compounds possessing a range of biological activities, have been prepared highly diastereoselectively (up to 95:5) through asymmetric Pd(OAc)(2)-catalyzed cyclizations between the easy available optically active allenylic hydrazines and organic halides in THF in the presence of (R,R)-Bn-Box (L2) as the ligand. It was observed 1) that in most cases (3R,5S)-pyrazolidines were obtained in good yields with very high enantiopurities (>99%) and high diastereoselectivities (up to 95:5) in the presence of (R,R)-Bn-Box (L2), 2) that aryl halides containing electron-donating or -withdrawing groups, heteroaryl, and 1-alkenyl iodides are all suitable substrates for this diastereoselective cyclization, 3) that the absolute configurations of the newly formed chiral centers in the pyrazolidines depend on the structure of substrate 1, and 4) that the enantio- and diastereopurities of the trans-pyrazolidines are co-controlled by the chiralities of the chiral catalysts and the substrates. A model for prediction of the enantiopurities of the products and the diastereoselectivities of the reactions based on an HPLC study of the starting hydrazines and the products was established.     

Distinguishing carbones from allenes by complexation to AuCl

Quantum chemical calculations have been performed for the dicoordinated carbon compounds C(PPh(3))(2), C(NHC(Me))(2), R(2) C=C=CR(2) (R = H, F, NMe(2)), C(3)O(2), C(CN)(2)(-) and N-methyl-substituted N-heterocyclic carbene (NHC(Me)). The geometries of the complexes in which the dicoordinated carbon molecules bind as ligands to one and two AuCl moieties have been optimized and the strength and nature of the metal-ligand interactions in the mono- and diaurated complexes were investigated by means of energy decomposition analysis. The goal of the study is to elucidate the differences in the chemical behavior between carbones, allenes and carbenes. The results show that carbones bind one and two AuCl species in η(1) fashion, whereas allenes bind them in η(2) fashion. Compounds with latent divalent carbon(0) character can coordinate in more than one way, with the dominant mode indicating the degree of carbone or allene character. The calculated structures of the mono- and diaurated tetraaminoallenes (TAAs) reveal that TAAs exhibit a chameleon-like behavior: The bonding situation in the equilibrium structure is best described as allene [(R(2)N)(2)]C=C=C[(NR(2))(2)] in which the central carbon atom is a tetravalent C(IV) species, but the reactivity suggests that TAAs should be considered as divalent C(0) compounds C{C[(NR(2))(2)]}(2), that is, as "hidden" carbones. Carbon suboxide binds one AuCl preferentially in the η(1) mode, whereas the equilibrium structures of the η(1)- and η(2)-bonded diaurated complex are energetically nearly degenerate. The doubly negatively charged isoelectronic carbone C(CN)(2)(2-) binds one and two AuCl very strongly in characteristic η(1) fashion. The N-heterocyclic carbene complex, [NHC(Me)(AuCl)], possesses a high bond dissociation energy (BDE) for the splitting off of AuCl. The diaurated NHC adduct, [NHC(Me)(AuCl)(2)], has two η(1)-bonded AuCl moieties that exhibit aurophilic attraction, which yield a moderate bond strength that might be large enough for synthesizing the complex. The BDE for the second AuCl in [NHC(Me)(AuCl)(2)] is clearly smaller than the values for the second AuCl in doubly aurated carbone complexes.     

Gold catalysis: 1,3-oxazines by cyclisation of allene amides

A series of allene amides was prepared and their gold-catalysed cyclisation was investigated. The formation of six-membered rings, 1,3-oxazines, was observed. Dihydropyrroles originating from intramolecular hydroamination of the distal C=C double bonds of the allenes were minor side products. Mechanistic studies by in situ (31)P NMR spectroscopy showed only one additional species during the conversion in each case; a computational study of the different allyl gold(I) species involved allowed this to be assigned as the σ-allyl gold species bearing the gold catalyst at the sterically less hindered methylene end. The regiospecific deuterodeauration of this intermediate confirmed a S(E) '-type mechanism for this last step of the catalytic cycle.     

Allenes as three-carbon units in catalytic cycloadditions: new opportunities with transition-metal catalysts

Allenes are very versatile synthetic units that are used in many types of catalytic cycloaddition reactions. Most examples reported so far involve their use as 2C-atom components, whereas their participations as 3C-atom components have been much less frequent. In this concept article, we present an overview of this latter strategy, emphasizing on those more recent contributions involving the use of Pt(II) and Au(I) catalysts, which have uncovered new opportunities in this area.