Pentacoordinate intermediates

ATP is activated by coordination to magnesium ion, and nucleophilic addition of a fatty acid caiboxylate to phosphorus then yields a pentacoordinate intermediate. . ... 

The formation of the pentacoordinated intermediate was confirmed later by performing the reaction at low temperature and by using a poly-... 

Tetrahedral Intermediates Derived from Carbonyl Compounds, Pentacoordinate Intermediates Derived from Phosphoryl and Sulfonyl Compounds, and Concerted Paths Which Avoid Them... 

Phosphate esters have a variety of mechanistic paths for hydrolysis. Both C-O and P-0 cleavage are possible depending on the situation. A phosphate monoanion is a reasonable leaving group for nucleophilic substitution at carbon and so 8 2 or SnI reactions of neutral phosphate esters are well known. PO cleavage can occur by associative (by way of a pentacoordinate intermediate), dissociative (by way of a metaphosphate species), or concerted (avoiding both of these intermediates) mechanisms. 

The pentacoordinate intermediate is the analog of the tetrahedral intermediate, and stable phosphoranes are the analogs of ortho esters and related species in carbon chemistry. Ph3P(OPh)2 and P(OPh)5 were reported in 1959, and in 1958 a general synthesis of pentaalkoxy phosphoranes containing an unsaturated five-membered ring was reported. In 1964 a synthesis of pentaethoxyphosphorane was devised which led to the preparation of a number of saturated and unsaturated pentaalkoxy... 

For a number of reactions of cyclic di- and triesters of phosphoric acid, there are exchange data which can be rationalized on the assumption of trigonal bipyrami-dal intermediates which readily interconvert by pseudorotation. This constitutes a strong argument that at least these cyclic esters react by an associative mechanism and is suggestive evidence that simple trialkyl phosphates also react by this mechanism. The pH dependence of exocyclic versus endocyclic cleavage of methyl ethylene phosphate is readily interpreted in terms of the effect of ionization of the intermediate on the pseudorotation of these pentacoordinate intermediates. ... 

By linear free energy relation arguments, Williams et al. concluded that in the case of a five-membered ring sultone the reaction with a phenoxide was either stepwise or, if concerted, had a transition state close to the pentacoordinated intermediate. ... 

Here the thermodynamics still favor stepwise reaction by way of pentacoordinate intermediate, 25, but the preference is weaker than in the triester case above. A concerted path might be barely possible here but would be expected to be close in structure and transition state energy to the pentacoordinate intermediate. 

Much less is known about the thermodynamics of the pentacoordinate intermediates in phosphoryl and sulfonyl chemistry, although such species clearly exist and are intermediates in at least some of the reactions of these classes of compounds. 

J. P. Guthrie, Hydration and Dehydration of Phosphoric Acid Derivatives Free Energies of Formation of the Pentacoordinate Intermediates for Phosphate Ester Hydrolysis and of Monomeric Metaphosphate, J. Am. Chem. Soc. 1977, 99, 3391. 

A similar reducing system is created by combining dilithium catecholate and trichlorosilane at —78° in tetrahydrofuran. It is speculated that the relatively unstable pentacoordinate bis(l,2-benzenediolato)hydridosilicate (61) is formed in situ and that it is this species that can reduce aldehydes and ketones, but not esters, to alcohols when they are added to the reaction mixture at 0° (Eq. 168).93 In a like manner, the dilithium salt of 2,2/-dihydroxybiphenyl, which forms a pentacoordinate intermediate that is stable enough to react at room temperature, can also be used to promote the reduction reaction. The alkoxides of aliphatic diols... 

Fig. 9 A hypothetical More-O Ferrall Jencks diagram for the attack of methoxide on O-aryl phosphate triesters (20) and 5-aryl phosphorothioates (21). Note that the diagram for attack of a metal-coordinated methoxide would be similar, but Mx +-coordination would push the TS toward the S-corner, possibly stabilizing the pentacoordinated intermediate to the point that the reaction occurs stepwise with the likely rate-limiting step being breakdown.

A thorough investigation has shown that the Tanigawa amination of alcohols, in which the corresponding alkoxide (282) is treated in DMF with A-methyl-A-phenylamino-triphenylphosphonium iodide (283) and a secondary amine at 80 °C, does not proceed at this temperature. In situ generation of the A.A-dimethylamino analogue (284) by reaction of (283) with dimethylamine, however, does lead to a smooth reaction at 90 °C. The proposed mechanism involves a pentacoordinated intermediate (285). " ... 

A series of diaquatetraaza cobalt(III) complexes accelerated the hydrolysis of adenylyl(3 -50adenosine (ApA) (304), an enhancement of 10 -fold being observed with the triethylenetetramine complex (303) at pH 7. The pentacoordinated intermediate (305), which is formed with the complex initially acting as an electrophilic catalyst, then suffers general acid catalysis by the coordination water on the Co(III) ion to yield the complexed 1,2-cyclic phosphate (306), the hydrolysis of which occurs via intracomplex nucleophilic attack by the metal-bound hydroxide ion on the phosphorus atom. Neomycin B (307) has also been shown to accelerate the phosphodiester hydrolysis of ApA (304) more effectively than a simple unstructured diamine. 

Henry Taube I think that Dr. W ilmarth and his co-workers have presented excellent evidence for the existence of a genuine pentacoordinated intermediate in the cyano system. The result reported by them that the intermediate generated by the spontaneous reaction is different from that which may be generated by the reaction of HONO with Co(CN)5N3+3 casts doubt on some of the conclusions which Dr. Haim and I reached on the reaction of nitrous acid with the azidopentaammino-cobaltic ion. In addition, you will remember that some of the results which Ralph Pearson mentioned also cast doubt on our conclusions. 

Arthur Adamson I think Dr. Wilmarth and co-workers have probably supplied the better available evidence for the pentacoordinated intermediate in a cobalt substitution reaction. 

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