Carbonate moieties, bridging

The characterization and crystal structure of the dimer [Pt2( -dppm)3] (dppm = bis(diphenyl-phosphino)methane), first reported as a deep red complex in 1978, was described by Manojlovic-Muir et al. in 1986.11 The structure, the first of its type, is made up of two parallel and almost eclipsed trigonal-planar platinum moieties bridged by three diphosphine ligands. The Pf Pt separation is 3.0225(3) A, too long to be considered a bond.11 [Pt2(//-dppm)3] catalyzes the hydrogenation/reduction of carbon dioxide with dimethylamine to give dimethylformamide12 (Equation (1)) and the reverse reaction.13... 

As outlined previously, aggregation of organocopper compounds is a consequence of the fact that the carbon moieties in these compounds are capable of bridging between two copper atoms. It is therefore to be expected that other anionic ligands capable of bridging between metal centers - halides, for example - might easily become incorporated into such aggregates. 

Unsymmetrically en -substituted 1,8-diborylnaphthalenes have also been prepared and investigated as bidentate Lewis acids. The synthesis of such derivatives is non-trivial because it necessitates the sequential introduction of boryl moieties at the / en-positions of the naphthalene backbone. Thus far, this strategy has only been applied successfully on few occasions. The reaction of 1,8-dilithionaphthalene-tmeda with one equivalent of dimesitylboronfluoride results in the formation of dimesityl-1,8-naphthalenediylborate 26 as a monoborylated naphthalene product (Scheme ll).32 This derivative is the only example of an anionic 1,8-boron-bridged naphthalene derivative. However, it is important to note that Siebert has reported the synthesis and structure of a neutral 1,8-boron-bridged naphthalene derivative which features a (di-Ao-propylamino)boron moiety bridging the two naphthalene peri-carbon atoms.33 A single-crystal analysis carried out on 26-Li(py)4 confirmed... 

Listing of compounds with two tetraorganotin moieties bridged by a carbon chain continues in Table ll6. Cyclic tetraorganotin derivatives with two tin atoms in one ring system are listed in Chapter 2.6.k. 

Table ll6. Compounds with Two Tetraorganotin Moieties Bridged by a Carbon Chain (R3Sn)aY... 

The dipyrrylacelylenedicarbaldehyde 18, which already contains, along with the acetylene moiety, two of the two-carbon bridges of the final macrotetracycle, can be prepared by a Heck-type coupling of 5-iodopyrrole-2-carbaldehydes with acetylene.8a,b The main conjugation pathway with 22n electrons is already present in the acetylene-cumulate systems 19. The expected planarity of this chromophore has been confirmed by X-ray structure analysis.8a b... 

Two-component methods represent the most widely applied principles in sulfone syntheses, including C—S bond formation between carbon and RSOz species of nucleophilic, radical or electrophilic character as well as oxidations of thioethers or sulfoxides, and cheletropic reactions of sulfur dioxide. Three-component methods use sulfur dioxide as a binding link in order to connect two carbons by a radical or polar route, or use sulfur trioxide as an electrophilic condensation agent to combine two hydrocarbon moieties by a sulfonyl bridge with elimination of water. 

The range of structural alternatives explored by valency-deficient carbon species and the subtle interplay of substituents is remarkable. Scheme 7.6 (ORTEP adapted from reference 31) illustrates an example of an X-ray structure clearly describing a localized [C-H C+] carbenium ion (A) where a symmetric bridging structure [C-H-C] + (B) could have been assumed. In this case it is proposed that a charge-transfer interaction between the resonance delocalized cation and the adjacent electron-rich carbazol moiety may be responsible for the stabilization of the localized form over the three-center, two-electron (3c-2e) bridging structure. 

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