Hexacoordinate intermediates

Scheme 21 Higher order Horner-Wadsworth-Emmons olefination via hexacoordinated intermediates 67...

Quite recently, it was established that the formation of the pentacoordinated germanium and tin compounds EF4(CH2CH=CH2) from EF3(CH2CH=CH2) (E = Ge, Sn) by the addition of F is exothermic148. The nucleophilicity of the allylic y-carbon is much enhanced when the pentacoordinated EF4(CH2CH=CH2) is formed. These results are qualitatively similar to those found for the reaction of the corresponding silicon compound. The pentacoordinated Ge and Sn complexes have a significant Lewis acidity which allows them to form stable hexacoordinated intermediates in the course of the reaction. 

One of the interesting points connected with this type of coordination springs from the fact that nucleophilic substitution on phosphoranes probably occurs via the formation of an octahedral complex242-245. Further, it should be recalled that the mechanism of irregular stereomutations of pentacoordinated structures in basic media may also involve a hexacoordinated intermediate. Figure 26 shows just a few of the numerous and diverse structures known (196-203) but, even then, some general observations can be made. 

Ramirez et al.273 also used the possible formation of a hexacoordinated intermediate to account for the OMe-OPh ligand exchange between P(OPh)5 and 2,2,2-trimethoxy-2,2-dihydro-l,3,2-trioxaphospholene. 

When the bis(isopropylamino)iodocyclopropenylium iodide is reacted with platinum black in acetonitrile, the reaction takes a different course, affording mainly the trans-bis[bis(diisopropylamino)cyclopropenylidene] diiodoplatinum complex (equation 277)351. A plausible pathway for this reaction involves two consecutive oxidative additions to platinum leading to the hexacoordinated intermediate Ptlv-complex [ -Pr2N)2C3]Ptf4, followed by reductive elimination of I2 to form the product (cf Section VI. A. 1. a). 

The slow step in the process is then the attack by a second nucleophile at an R—X edge to give a hexacoordinate intermediate or transition state. The second nucleophile is the alcohol in the case of alcoholysis or the nucleophilic catalyst in the case of racemization (equations 12 and 13). 

Substitution with inversion and retention of the pentacoordinate species 33 has been interpreted in terms of two competing dissociative mechanisms Si—O cleavage and Si—NMI cleavage65. A hexacoordinate intermediate is not implicated. 

However, Corriu and coworkers66 postulate a hexacoordinate intermediate (or transition state) in the hydrolysis of organic silicates, with the rate-determining step in the reaction being the coordination of water to a pentacoordinate intermediate formed by initial nucleophilic attack. 

The possibility of ligand substitution reactions in pentacoordinate silicates Sil I31 2 and Sil FfF via hexacoordinate intermediates was studied by Fujimoto, Arita and Tamao73. Attack on each of these silicates by F or hydride produced qualitatively similar reaction pathways, leading to stable hexacoordinate intermediates, without significant breaking of the bond between silicon and the leaving group. It was concluded that a nonconcerted displacement mechanism via a hexacoordinate intermediate is likely. 

For these reactions hexacoordinated intermediates 128 were proposed which can also be approached by a totally different route via the carbanionic precursor 129. 

Route 1, attack of the nucleophilic reagent leading to a hexacoordinate intermediate or transition state, has been given preference (11,12,247,306-308). A similar pathway has been also postulated in organophosphorus chemistry (309-311). This mechanism may account for many experimental observations. It does not, however, seem to be generally accepted as the most favorable route (see, for example, Refs. 243 and 290). Activation toward nucleophilic attack intuitively should be less effective in the extracoordination structure [Eq. (72), B] than in the silylonium ion (C), because of the higher positive charge and lower steric requirements of the Si center in the latter. 

After treatment of the rhodium compound shown in Eq. (37) with CO, the ir spectrum indicated an unstable hexacoordinate intermediate ... 

Reaction of functionalized ketones such as a-hydroxy ketones or 1,3-ketones with allyltrifluorosilane in the presence of Et3N has also been investigated [95]. Although catecholate does not play a key role in the pentacoordination of silicon, chelating hexacoordinate intermediates are formed to give allylation products with high yields and selectivity (Sch. 54). 

Dynamic processes causing ligand permutation in pentacoordinate phosphorus compounds constitute a well-studied area (148). As illustrated in Sect. V-A, various authenticated penta- or hexacoordinate organosilanes have been reported. However studies of the isomerization processes of these compounds are limited. Such processes are of importance, since penta- or hexacoordinate intermediates are implicated in substitution reactions or in racemization of tetra-coordinate silicon compounds. The stereochemical outcome of such reactions is therefore related to the configurational stability of penta- or hexacoordinate intermediates. 

The intervention of hexacoordinated intermediates in intramolecular ligand exchanges can be an alternative mechanism to pseudorotation. Hexacoordinated intermediates are involved in intermolecular ligand exchanges of pentacoordi-nated silicon compounds (286) and in the hexamethylphosphoric triamide (HMPA) catalyzed F-Cl exchange of tetracoordinated silanes (355). 

In associative ligand exchange, the intervention of hexacoordinated intermediates has been demonstrated in various intermolecular ligand exchange processes at five-coordinated silicon compounds (286,355) (eq. [114]). 

No firm evidence is available to exclude either type of the intermediates (C) and (D). Stable examples of both are now well-documented in phosphorus159 as well as in silicon chemistry (Section II). The participation of pentacoordinate siliconium ions in chlorosila-cyclobutane isomerization160 and of analogous cationic species in the racemization of triorganotin halides161 has been supported. The possible intervention of hexacoordinate intermediates in substitution reactions at phosphorus has often been considered162. 

The lack of direct evidence for TBP intermediates has allowed the proposal of alternative pentacoordinate intermediates and of hexacoordinate intermediates in nucleophilic substitution reactions at tetrahedral phosphorus. Pentacoordinate phosphorus species (spirocyclic phosphoranes) have been observed in which the most stable form adopts the square pyramidal (SP) geometry (Howard et ai, 1973). Such SP species have been postulated as intermediates in substitution reactions at tetrahedral phosphorus (Boudreau et ai, 1975). 

Stable octahedral hexacoordinate oxyphosphorane ions had been isolated and characterized by X-ray structural analysis as early as 1966 (Hellwinkel, 1966). Subsequently, acyclic species, hexaphenyl, hexamethyl and hexaethyl phosphorane, were prepared and observed in solution (Lehrman and West-heimer, 1976 Denney et al., 1976). Nucleophilic substitution on pentaoxy-phosphoranes has been proposed as proceeding via addition of nucleophile to form a hexacoordinate intermediate (see e.g. Garrigues et al., 1980). Ramirez has postulated that tetrahedral phosphates undergoing substitution via TBP intermediates might also follow a hexacoordinate mechanism (Ramirez et al., 1972). One example of such a postulated hexacoordinate mechanism occurs in the hydrolysis of pentaaryloxyphosphorane in neutral and alkaline conditions. Archie and Westheimer (1973) used kinetic analysis to support a hydrolysis mechanism proceeding via an octahedral hexacoordinate intermediate or transition state. 

More recently, several reactions have been proposed in which TBP phosphoranes break down via hexacoordinate intermediates (Queen et al., 1981 Bel skii et al., 1979). Skowronska et al. (1981) claim to have observed a number of hexacoordinate intermediate species (for example, [69], [70]) in... 

The results of this analysis are compatible with the intermediacy of TBP pentacoordinate intermediates, but are unambiguously incompatible with the intermediacy of hexacoordinate intermediates (Kluger and Thatcher, 1986). However, exhaustive analysis (Thatcher, 1985) of the possible hexacoordinate species involved in this reaction demonstrates that the results may be interpreted in terms of a hexacoordinate mechanism if the following conditions are satisfied ... 

Nucleophilic attack on the pentacoordinate species may occur via a hexacoordinate intermediate or transition state. However, the nature of this... 

Alternative associative reaction pathways must also be considered. Both reaction via equatorial entry and apical departure (or vice versa) and hexacoordinate intermediates (or transition states) have been suggested. Although chemically reasonable, it is certain that unambiguous experimental support for such mechanisms will prove elusive. 

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