Pentacoordinate transition state

There is little mention in the literature of the use of amide salts in substitution reactions on chlorophosphazene precursors. The anilide anion was shown to be a powerful nucleophile in substitution reactions on various trimer derivatives, but investigations of such reactions with the high polymer have not been reported.22 Where strong nucleophiles (such as amide salts) with low steric requirements are employed, the usual pentacoordinate transition state (Scheme 1), may be a viable reaction intermediate which can undergo alternative modes of decomposition, perhaps involving chain cleavage and/or cross-linking. 

A model with a pentacoordinate transition state II may account for the observed fluctionality and for the reluctance of 4 and 5 to rearrange to the respective phosphinomethane. 

Several mechanisms had been proposed for the [1,2]-Wittig rearrangement, including (i) a concerted process taking place through a pentacoordinate transition state, (ii) a... 

Benzylic N-protection (entry 1) is often unsatisfactory since competitive reaction at the exocyclic methylene can occur [84JCS(PI)481 85S302]. As with bis(l-pyrazolyl)methane (Section I1,C,1)(83T4133), the nature of the isolated product is determined by the type of electrophile used, with benzyl halides and, to a certain extent, iodomethane reacting at the A-benzyl carbon atom, whereas most other electrophiles give rise to the normal C-2 alkylated products (90JHC673). This result was interpreted in terms of steric rather than thermodynamic factors, since the two species that reacted at the benzylic carbon required a bulkier pentacoordinate transition state, whereas all of the others were able to react via a lower coordinate species (90JHC673). 

The protolytic activation of the alkane is, however, only the apparent part of the reaction as long as the alkane or the acid is not isotopically labeled. When HF is replaced by DF and the isobutane-CO mixture is bubbled through the DF-SbF5 acid (6 1 molar ratio) at — 10°C, the apparent conversion based on ester or H2 formation is only 4% but the 1H/2H NMR analysis of the apparently unreacted isobutane (96%) shows extensive H-D exchange (18 atom% in the tertiary position and 9 atom% at each primary position).30 The most plausible rationalization of hydrogen exchange is via the formation of carbonium ions (here pentacoordinate transition states or intermediates) as described in Eq. (5.15). 

With this background data, modeling of a reaction coordinate for a phosphorus compound proceeding by an associative mechanism through a pentacoordinated transition state becomes attractive. 

DNA polymerases catalyze DNA synthesis in a template-directed manner (Box 16). For most known DNA polymerases a short DNA strand hybridized to the template strand is required to serve as a primer for initiation of DNA synthesis. Nascent DNA synthesis is promoted by DNA polymerases by catalysis of nucleophilic attack of the 3 -hydroxyl group of the 3 -terminal nucleotide of the primer strand on the a-phosphate of an incoming nucleoside triphosphate (dNTP), leading to substitution of pyrophosphate. This phosphoryl transfer step is promoted by two magnesium ions that stabilize a pentacoordinated transition state by complex-ation of the phosphate groups and essential carboxylate moieties in the active site (Figure 4.1.1) [2],... 

Reactions 48 (M, M and M" are Si, Ge and Sn) were investigated for both degenerate and nondegenerate situations. Back-side (132) and front-side (133) approaches of MH3 were considered. Both approaches involve pentacoordinate transition states (Scheme 5). The optimized structures of 132 and 133, M = M = M" = Si are shown in Figure 39a. 

In mechanistic terms, the PMR states that the pathways for forward and reverse reactions at equilibrium are described by the same energy surface it does not state that the profile of such a surface must be symmetrical with respect to the reaction path 2 Application of the PMR to displacement reactions at phosphorus is aided by Fig. 9, which depicts all of the possible reaction pathways for degenerate ligand exchange at tetracoordinate phosphorus that proceed either via pentacoordinate transition states or via phosphorane intermediates capable of pseudorotation. The letters a and e in Fig. 9... 

When methane is reacted with ozone in superacidic media, formaldehyde is directly formed. The pathway is considered an attack by +O3H into a C-H bond to form a very reactive methyloxenium ion (69) through the pentacoordinate transition state [Eq. (6.50)]. 

The reactions in equation 1 constitute ester exchange, hydrolysis, and their reversals. In the base-catalyzed case these reactions proceed through a nucleophilic attack on the silicon atom, resulting in a pentacoordinate transition state (12-15). 

Fig. 23.12b shows the typical situation for the carbon analog where the ionic stmcture is slightly higher in energy, relative to the HL-state at the crossing point. The consequent VB mixing leads to a single step reaction with a pentacoordinated transition state. 

Fig. 9. Model building of the pentacoordinate transition state of the reaction into the crystallographic structure of tyrosyl-tRNA synthetase. [Reprinted with permission from Ref. (/56).)...

---