Alkoxycarbenium cations

Alkylcarboxonium (Alkoxycarbenium) Cations. The ionic structure of pyrylium salts was clearly stated by Hantzsch592 as early as 1922. In pyrylium salts 297, there is a contribution from carbocation structures, a fact apparent in the behavior toward strong nucleophiles leading to phenols. 

The presence of homopolymers, instead of the block copolymer, is probably due to side reactions, e.g. hydride transfer. The alkoxycarbenium cations, present in polymerization in equilibrium with oxonium ions, are strong hydride acceptors ... 

We focused on two types of organic cations, W-acyliminium ions and alkoxycarbenium ions,2 because they are very popular in organic synthesis. A number of reactions involving such onium ions have been developed and widely utilized for the construction of organic molecules. 

Alkoxycarbenium ions are important reactive intermediates in modem organic synthesis.28 It should be noted that other names such as oxonium ions, oxocarbenium ions, and carboxonium ions have also been used for carbocations stabilized by an adjacent oxygen atom and that we often draw structures having a carbon-oxygen double bond for this type of cations.2 Alkoxycarbenium ions are often generated from the corresponding acetals by treatment with Lewis acids in the presence of carbon nucleophiles. This type of reaction serves as efficient methods for carbon-carbon bond formation. 

Lewis acid-acetal complexes in NMR studies, but never detected alkoxycarbenium ions.29 The absence of alkoxycarbenium ions in the spectra, however, does not necessarily rule out their intermediacy in the reactions with nucleophiles. Therefore, it was imperative to accomplish the reactions of spectroscopically characterized, nonstabilized alkoxycarbenium ions with carbon nucleophiles. The cation pool method made it possible and opened a new chapter in the chemistry of alkoxycarbenium ions. 

The concept of electroauxiiiaiy is quite powerful to solve these problems. The pre-introduction of a silyl group as an electroauxiliary decreases the oxidation potential of dialkyl ethers by virtue of the orbital interaction. As a matter of fact, we demonstrated that the anodic oxidation of a-silyl ether took place smoothly in methanol.30 Selective dissociation of the C-Si bond occured and the methoxy group was introduced on the carbon to which the silyl group was attached. Therefore, a-silyl ethers seemed to serve as suitable precursors for alkoxycarbenium ions in the cation pool method. 

The thermal stability of the alkoxycarbenium ion is noteworthy. When the electrolysis was complete, the cation pool of 26 was allowed to warm up to a second temperature. After being kept there for 30 min, the cation was allowed to react with allyltrimethylsilane. The yield of allylated product 27 was plotted against the temperature. It can be seen from Fig. 7 that alkoxycarbenium ion 26 is stable at temperatures approximately below -50 °C. Above this temperature, the yield of 27 decreased significantly. Intramolecular coordination of ether functionality seems to be effective for the stabilization of alkoxycarbenium ions.33... 

The alkoxycarbenium ions generated by the cation pool method react with various carbon nucleophiles such as substituted allylsilanes and enol silyl ethers to give the corresponding coupling products in good yields. It should be noted that the reactions of alkoxycarbenium ion pools with such nucleophiles are much faster than the Lewis acid promoted reactions of acetals with similar nucleophiles. A higher concentration of the cationic species in the cation pool method seems to be responsible. 

The a-phenylthioether 28 was oxidized in the absence of a nucleophile by low temperature electrolysis (Scheme 15). The corresponding alkoxycarbenium ion pool 26 was formed, which exhibited a single set of signals in H and l3C NMR spectroscopy. The chemical shifts were quite similar to those obtained by the oxidative C-Si bond dissociation described in the previous section. Subsequently, the cation pool was allowed to react with allyltrimethylsilane to obtain the allylated product 27. 

We chose to study the generation of alkoxycarbenium ion 26 from thioacetal 28. The electrochemically generated ArS(ArSSAr)+, 37 which was well characterized by CSI-MS, was found to be quite effective for the generation of alkoxycarbenium ions, presumably because of its high thiophilicity (Scheme 17). The conversion of 28 to 26 requires 5 min at -78 °C. The alkoxycarbenium ion pool 26 thus obtained exhibited similar stability and reactivity to that obtained with the direct electrochemical method. The indirect cation pool method serves a powerful tool not only for mechanistic studies on highly reactive cations but also for rapid parallel synthesis. 

The X-ray structure of a number of alkoxycarbenium ions has been determined.66 An interesting example is 2-methoxy-l,7,7,-trimethylbicyclo[2.2.1]hept-2-ylium tetrafluroborate 326.630 It is a substituted 2-norbomyl cation and, indeed, the C(2)-C(l)-C(6) bond angle (98.8°) and the C(l)-C(6) bond distance (1.603 A) indicate G-bond charge delocalization, that is, the contribution of the 326b resonance form. 

Starting with the knowledge of carbenium ion stability and the understanding that it is necessary to decrease this stability enabling the reaction of these ions with nitriles102 (Section III.B.l), it should be assumed that acetals 209, oc-haloethers 211, enols 217 and vinyl ethers 218 are ineffective precursors since they are the sources of the highly stable hydroxy- (223) and alkoxycarbenium ions 224. In contrast, the carbonyl compound derivatives, which can produce destabilized ions (in comparison to ions 223 and 224) are the most interesting for reactions with nitriles. This includes acyloxycarbocations 225 (see Section III.B.3), halocarbocations 226, A-acyliminium ions 228 and vinyl cations 229. 

The alkoxycarbenium ion 315, obtained by electrochemical oxidation of ct-silyl ethers under standard cation pool conditions, underwent ring opening by reaction with the nucleophile cyclohexenyltrimethylsilane to give 316 (Scheme 73) <20050L4717>. 

In contrast, the onium ion intermediates formed by esters tend to release acyl cations, and those formed by acetals release alkoxycarbenium ions. 

Ion-trapping is based on the fast reactions between cations and trialkyl-phosphines leading to stable phosphonium salts, namely tertiary phosphonium salts when a proton is trapped (e.g. from/F.i) and quaternary salts when the reaction involves tertiary oxonium ions (e. g. IV.4 and/or IV.5) or alkoxycarbenium ions. 

Recently, it was shown by H-NMR on the basis of direct studies of the CH3OCH2 cation (being a model of the supposed growing alkoxycarbenium sp ies) that its exchange with the corresponding oxonium counterpart was very fast ). 

S. Suga, K. Matsumoto, K. Ueoka, and J.-I. Yoshida, Indirect cation pool method. Rapid generation of alkoxycarbenium ion pools from thioacetals, J. Am. Chem. Soc., 128 (2006) 7710-7711. 

The Stability of alkoxycarbenium ions can be increased by the introduction of an ether functionality in an appropriate position by virtue of effective intramolecular coordination of the ether oxygen to the cationic carbon.t i]... 

---