Electrophilic cationic centers

B. Heterolytic Cleavage of Silanes on Electrophilic Cationic Centers... 

Fewer examples of heterolysis of silane Si-H bonds are known as compared to splitting of H-H bonds. However, the Si is highly activated toward nucleophilic attack by trace water, hydroxylic or halocarbon solvents, and even fluoride from anions when silanes are coordinated to electrophilic cationic centers (103). 

Proton loss from alkyl groups a or 7 to a cationic center in an azolium ring is often easy. The resulting neutral anhydro bases or methides (cf. 381) can sometimes be isolated they react readily with electrophilic reagents to give products which can often lose another proton to give new resonance-stabilized anhydro bases. Thus the trithione methides are anhydro bases derived from 3-alkyl-l,2-dithiolylium salts (382 383) (66AHC(7)39). These... 

The familiar pattern of 2-amination with sodamide ( — 33°, 90% yield) occurs also with 1,5-naphthyridine. Greater reactivity at the 2-position is attributed, as before, to a cyclic transition state with electrophilic attack at a ring-nitrogen concomitant with nucleophilic attack adjacent to the cationic center thus formed. 

Triphenylphosphonium ylide reacts with the silylene complex 93 which has a highly electrophilic silicon center, to give the corresponding cationic adduct 94 [115]. The lengthening of the PC bond indicates a loss of the double bond character of the ylide and corresponds to the formation of a tetrahedral silicon center with four covalent bonds (Scheme 28). 

This chapter describes our studies of electrophilic systems having adjacent, stable cationic centers. We have shown in a wide variety of systems that stable cationic centers (i.e., ammonium, pyridinium, and phosphonium groups) can enhance the reactivities of some electrophiles. This enhanced reactivity is evident by reactions with weak nucleophiles, but may also involve unusual rearrangements or regiochemistry in the conversions. Using this chemistry, reactive dicationic systems can be generated and studied. 

Much of our research has involved the use of dicationic electrophiles in reactions with very weak nucleophiles, such as non-activated arenes and alkanes. By comparison to similar monocationic electrophiles, we have been able to show the extent of electrophilic activation by adjacent cationic centers. For example, carbocations show an increased reactivity with a nearby cationic charge (eqs 3-4).9 When 1,1-diphenyletheneis reacted with superacidic CF3SO3H... 

The phosphonium cationic center is well known for its ability to stabilize adjacent anionic centers (i.e. Wittig reagents), however we have shown it to be capable of activating (or destabilizing) cationic electrophilic sites.15 As noted... 

This latter compound, 36, and the isomeric 9-oxabicyclo[4.2.1]nonane, 37, were obtained as the sole products, in ca 13 87 ratio, by reaction of 3 with A-chlorosuccinimide (NCS) in protic solvents (methanol, dioxane-water mixtures)72. It is noteworthy that similar ratios of the two disubstituted bicyclononane derivatives were obtained, independently of the solvent, also by using A-bromosuccinimide (NBS) as electrophile, whereas a strongly solvent-dependent ratio was observed when A-iodosuccinimide (NIS) was used. Since these reactions should proceed through hydroxy- or alkoxyhalogenation of one of the double bonds, followed by transannular attack of the oxygen function on the cationic center which is formed on the other side of the ring by the reaction of another electrophile with the second double bond, the isomer ratio has been rationalized in terms of a different nature of the intermediates. 

Carbon-Nitrogen Bond Formation A carbon-nitrogen bond can result either from the reaction between an anodically produced cationic center and an amino group or from the reaction between a nucleophilic center and an electrogenerated electrophilic nitrogen... 

Again, the exclusive formation of six-membered rings indicates that the cyclization takes place by the electrophilic attack of a cationic center, generated from the enol ester moiety to the olefinic double bond. The eventually conceivable oxidation of the terminal double bond seems to be negligible under the reaction conditions since the halve-wave oxidation potentials E1/2 of enol acetates are + 1.44 to - - 2.09 V vs. SCE in acetonitrile while those of 1-alkenes are + 2.70 to -1- 2.90 V vs. Ag/0.01 N AgC104 in acetonitrile and the cyclization reactions are carried out at anodic potentials of mainly 1.8 to 2.0 V vs. SCE. 

The high electrophilicity of the positively charged element can be modified by intramolecular donation from remote donor substituents. This interaction leads to solvent-free cations with coordination numbers for the positively charged element > 3 and to a considerable electron transfer from the donor group to the element. Frequently used donor substituents utilize heteroatoms with lone pairs (e.g. amino, hydrazino, methoxy, carboxy, phosphino, etc.), in many cases in combination with pincer-type topology of the ligand, for the stabilization of the cationic center. These strongly stabilized cations are beyond the scope of this review and instead we will concentrate on few examples where we have weak donors such as CC multiple bonds, which stabilize the electron-deficient element atom. 

For successful reaction of [Fe2S2(NO)4]2 at transition metal centers a strongly electrophilic metal is required thus while no reaction was observed (29) with [(fy5-C5H5)Fe(CO)2Br], the more electrophilic cationic complex [( 5-C5H5)Fe(CO)2(THF)]BF4 reacted to provide a 90% yield of the air-stable [Fe2 SFe(CO)2( /5-C5H5) 2(NO)4]. 

The nature of electronic effects in cationic reactions has been probed by application of the Gassman-Fentiman tool of increasing electron demand.67 Aryl-substituted cationic centers can be made more electron demanding (i.e., electrophilic) by introduction of electron-withdrawing substituents into the aryl ring. 

When a cationic center becomes sufficiently electrophilic, it may draw on electrons from neighboring Jt- and o-bonds and thus delocalize positive charge density. The onset of participation of n- and o-bonds can be detected as a departure from linearity in a Hammett-type plot as the electron-withdrawing ability of the aryl substituent increases. In stable ion studies, 13C NMR chemical shifts are generally used as a structural probe reflecting the charge density at the cation center (in closely related homologous cations, other factors that may affect chemical shift may be assumed constant). 

However, when H2 is bound to a highly electrophilic cationic metal center, the acidity of H2 gas can be increased spectacularly, up to 40 orders of magnitude. The pK of H2 can become as low as —6 and thus the acidity of rf -H2 becomes as strong as that in sulfuric or triflic acid. As discussed in reviews by Morris (4,5) and Jia (25) and further work by Morris (26,27), such pK values are usually determined by NMR measurement of the concentrations of M H2 complexes in equilibrium with an external base such as a phosphine or amine. Electron deficient cationic and dicationic H2 complexes with strong short H-H bonds (<0.9 A) and weakly bound H2 such as [Cp Re(H2)(CO)(NO)]+ and [Re(H2)(CO)4(PR3)]+ are among the most acidic complexes (Table I). These acidic complexes typically have relatively high values of Jhd for their r 2-HD isotopomers, although pK values do not correlate... 

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