Direct bonding between centers

The wide variety of compounds involving direct bonding between centers of coordination can be treated by one of the following practices ... 

For singlet reactions, the direction of ring closure is the one that involves the most stable 1,3-biradical, as shown in Scheme 43. This is equivalent to the assumption that in a concerted mode of reaction the bond between centers C-3 and C-5 is only weakly formed during the initial stages of the reaction. 

The only unchanging bonds so far recognized have been the shell bonds between adjacent atoms on the reaction center cycle. While longer external links between these atoms have complicated Vladutz SRSG definitions (cf. equation 7), only those which directly bond reaction center atoms across the cycle probably need to be included with the shells. These are called cross-bonds and were explicitly recognized only by some authors. ... 

Abstract This chapter summarizes the recent developments in organoaluminum compotmds containing at least one direct bond between aluminum and a s-block, p-block, d-block, or f-block metal center. General synthetic pathways to access such species are described along with their stmctural and bonding properties. 

Since the most direct evidence for specihc solvation of a carbene would be a spectroscopic signature distinct from that of the free carbene and also from that of a fully formed ylide, TRIR spectroscopy has been used to search for such car-bene-solvent interactions. Chlorophenylcarbene (32) and fluorophenylcarbene (33) were recently examined by TRIR spectroscopy in the absence and presence of tetrahydrofuran (THF) or benzene. These carbenes possess IR bands near 1225 cm that largely involve stretching of the partial double bond between the carbene carbon and the aromatic ring. It was anticipated that electron pair donation from a coordinating solvent such as THF or benzene into the empty carbene p-orbital might reduce the partial double bond character to the carbene center, shifting this vibrational frequency to a lower value. However, such shifts were not observed, perhaps because these halophenylcarbenes are so well stabilized that interactions with solvent are too weak to be observed. The bimolecular rate constant for the reaction of carbenes 32 and 33 with tetramethylethylene (TME) was also unaffected by THF or benzene, consistent with the lack of solvent coordination in these cases. °... 

It was originally hypothesized that one role of the Zn center is to help form and stabilize the pterin binding site (81). As shown in Fig. 4, the conserved Serl04, which is only three residues from one Zn ligand, directly H-bonds to the pterin. A disruption of the Zn site should, therefore, perturb interactions at the pterin site. A direct comparison between the Zn-bound and disulfide forms supports this hypothesis. In order to form the S—S bond, the entire section of polypeptide involving Serl04 must slide down and away from the pterin site, which... 

Direct coordination of a metal ion to the non-bridging oxygen might render the phosphorus center more susceptible to nucleophilic attack (electrophilic catalysis Fig. 3e) or, alternatively, hydrogen bonding between a metal-... 

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