Steric and Electronic Requirements

The starting point for most of the redox chemistry considered in this review is the nickel(II) ion. The nickel(II) ion has a d8 electronic configuration and, with weak-field ligands such as H20, it forms a six-coordinate ion with approximately octahedral symmetry and a paramagnetic (two unpaired electrons) 3A2 ground state. The characteristic solution chemistry of six-coordinate nickel(II) is well documented and, in particular, the substitution behavior has been extensively studied and is the subject of recent reviews (11, 12). It is a labile ion with solvent exchange rates around 104 sec-1 at 25°C and activation parameters are consistent with dissociatively activated interchange behavior (13). 

Clearly different ligand types will favor different oxidation states. Higher oxidation states prefer hard acid donor atoms, generally first-row p-block elements, rich in electron density and capable of strong a donation. A further provision is that they should resist oxidation. Common donor chromophores which have been used are amines N, imides (including oximes and imines)I N , oxides —0 and fluorides F-. Second- and third-row p-block donors have also been used, forming bonds which are more covalent in character and creating special problems, as discussed below. 

The nickel(I) state is favored by soft donors capable of n back bonding. Although there is a rich chemistry associated with second- and third-row p-block donors to nickel(I), examples in this review will be restricted again to, primarily, nitrogen and oxygen as donor atoms. 

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Nickel (continued) polysulfide complex, 31 98 half chair conformation, 31 115 porphyrin complexes, 32 13 in proteins, 47 284-285 quadruply bridged dimers, 40 211-214 steric structures, 40 190-194 quaternary phosphonium salts of, 6 31-32 redox chemistry probes of structure, 32 243-245 steric and electronic requirements, 32 242-243... 

A number of representative nickel(II) complexes prepared with Schiff bases derived from pyridine-2-carbaldehyde, pyridine-2,6-dicarbaldehyde and related species are summarized in Table 98, together with some of their distinctive physicochemical properties and preparative routes. All of these complexes involve N and either O or S as donor atoms and exhibit various coordination numbers and geometries depending on the denticity of the ligands and on their steric and electronic requirements. 

In order to design a host that will selectively bind a particular guest, we make use of the chelate and macrocyclic effects as well as the concept of complementarity (matching of host and guest steric and electronic requirements) and, crucially, host preorganisation. 

Varying substituents in The aromatic address, and investigating the spacer s steric and electronic requirements... 

From the synthetic point of view, satisfactory cis dihydroxylations with these reagents are best achieved with electron-poor alkenes such as oc,/ -unsaturated esters and lactones. Permanganate ion mediated dihydroxylations of chiral alkenes usually afford the same sense of diastereoselection as the osmylation reaction, a result suggesting comparable steric and electronic requirements in the corresponding transition states. 

In the solid state spectra the chemical shift separation between the two resonances is 9.3 ppm, 2.7 ppm and very small, for CIO, BF4" and PF counterions, respectively. This is the result of minor differences in the geometry of the phosphorus atoms in the three solids with respect to the applied magnetic held as a consequence of the steric and electronic requirements of the counterions. Furthermore it was observed that y( Rh, P) coupling constants are highly sensitive probes for the changes in the counterions, which affect to a different extent the distorsion of the square-planar stereochemistry around Rh (Table 4). 

Rhodium carbenoids, especially the donor/acceptor carbenoids, act as very sterically demanding electrophiles. Hence, based on size alone, the favored order of reactivity of C-H bonds would be 1°>2°>3°, yet carbenoids are also very electrophilic, so they would prefer to react with more electron rich C-H bonds, thus 1°<2°<30. So, in practice, secondary C-H bonds tend to be the most active overall, because they possess the proper balance between these steric and electronic requirements [5], Furthermore, when the C-H bond is adjacent to an electron donating group such as a heteroatom or an aromatic ring, it becomes even further activated towards functionalization. Based upon these few general trends, a surprising level of control of reactivity can be achieved in carbenoid reactions with complex molecules, especially when their reactivity is attenuated with proper substituents on the carbenoid. 

The use of silyl ethers also provided a good glimpse into the steric effects of the C-H insertion [98], Relative rates were obtained for insertion a to the oxygen atom in silyl protected n-butanol. It was found that the reaction rate increased dramatically as the size of the silyl protecting group decreased, with a 100-fold rate difference between TBDPS and TMS. Complementary steric and electronic effects were observed with the tetralkoxy silane substrates [97,98], hi competition experiments, it was found that the carbenoid derived from diazo ester 99 reacted solely with tetraethoxy silane 123 to form product 126, and not the corresponding tetramethoxy or tetraisopropoxy derivatives 124 or 125 (Scheme 28). Thus, the secondary C-H bonds appear to possess the right balance between steric and electronic requirements for the insertion. 

Randall et al. s functional group survey (83) included a number of transition metal carbonyl complexes (Table XL). The authors showed that SCO depended on the local stereochemistry of the metal atom, for example, compare octahedral-(CO)sCrC(OMe)(Me) CO-ciy 217. 6 ppm CO-trans 223.6 ppm with pyramidal-(CO)3(aC5Hs)WMe CO-cis 239.2 ppm CO-trans 217.8 ppm. An additional factor influencing the carbonyl resonance was found to be the steric and electronic requirements of the other ligands in the complex. 

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