Co-X bonds

The broken bonds (boldface = dissociated fragment) BDEs (boldface = recommended data reference in parentheses) Methods (reference in parentheses)  

The broken bonds (bold face=d issodated fragment) 

PCys salen=bis(salicylidene)-ethylene-diamine Cy = tricyclohexylphosphine 28.7 0.4 120.0 1.6  

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The CO-X bond breaking is the result of an electrophilic attack (on the carbonyl oxygen atom, hence the catalytic role of acids in these rupture reactions) or a nucleophilic one (on the carbonyl carbon atom whose positive property is due to the X electron-withdrawing property). The dangers of this type of reaction come from its speed and high exothermicity and/or instability of the products obtained in some cases. The accidents that are described below can make one believe that acid anhydrides in general and acetic anhydride in particular represent greater risks than acid chlorides since they constitute the accident factor of almost all accidents described. This is obviously related to their frequent use in synthesis rather than acid chlorides, that are rarely used. 

A quite different and complimentary approach is to assume that addition of a nucleophile to an acyl derivative (RCOX) would follow the linear free energy relationship for addition of the nucleophile to the corresponding ketone (RCOR, or aldehyde if R=H) if conjugation between X and the carbonyl could be turned off, while leaving its polar effects unchanged. This can be done if one knows or can estimate the barrier to rotation about the CO-X bond, because the transition state for this rotation is expected to be in a conformation with X rotated by 90° relative to RCO. In this conformation X is no longer conjugated, so one can treat it as a pure polar substituent. Various values determined by this approach are included in the tables in this chapter. 

In general the thiol proteases catalyze the hydrolysis of a variety of peptide, ester, and amide bonds of synthetic substrates. Employing the general formula R —NH—CHR—CO—X, cleavage of the —CO—X— bond has been demonstrated when R represents the side chain of glycine, threonine, methionine, lysine, arginine, citrulline, leucine, and tyrosine. 

The LUMO of COFj is a x(CO) antibonding orbital whilst the HOMO essentially corresponds to a non-bonding oxygen 2p orbital directed in the plane of the molecule. The SHOMO, of bj symmetry, is a CO x-bonding orbital, which is also antibonding with respect to fluorine. Between the 4b, and la orbitals, which are both very pure fluorine lone-pair... 

The variation of rates with the identity of X correlates well with the variation in thermodynamic stability of the complexes. This indicates that breaking the Co—X bond is at least important in reaching the transition state. 

The rate of substitution varies over five orders of magnitude with the nature of the leaving group X, as shown in Table 17.3. The weaker the Co-X bond, the faster the reaction. These results support a dissociative intermediate, as the loss of X occurs in the RDS of the D mechanism. 

Transition metal-catalyzed carbonylation reactions represent an enormous toolbox for CO-X bond formation (X = C, N, O, etc.). While most coupling reactions take place with heteronucleophiles nowadays, carbonylations including C-H activation are attracting more and more attention because the use of stoichiometric amounts of organometallic reagents can be avoided. 

Common intermediates are not involved in the Hg +-, Ag+-, and NO -induced aquation and base hydrolysis reactions of [Co(Metren)(NH3)X] + ions (X=CI, Br, N3, or NH3) which have structures (6)—(8). The stereochemistries of the HaO/OH-product ions were established using A-Mc C-enriched materials related to known crystal structures. An h mechanism is favoured for these reactions with H O involved prior to complete Co—X bond fission. 

CpCo(XCPh=CPhSc)] (X = S, Se) occur across the Co-X bond where X = S, this isomerises to the Co-Se bridged isomer.267... 

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