Thiocarbenium ions

The primary addition products A now undergo SN1 reactions (Figure 9.12). A reversible protonation of the OH group of A leads to oxonium ions D. Water is ejected from D without the nucleophile getting involved. Thereby the intermediate oxocarbenium ions C (Nu = OR3), thiocarbenium ions C (Nu = SR3), or iminium ions C (Nu = NR3R4) are formed. These combine with the second equivalent of the nucleophile to form an 0,0-acetal B (Nu = OR3), an S,S-acetal B (Nu = SR3), or an N,N-acetal (Nu = NR3R4). 

Cyclization reactions 422.4 Reactions Involving Thiocarbenium Ions... 

In analogy to the preparation of iminium salts, thiocarbenium ions (115) can be created by treatment of carbonyl compounds (1) with thiols," by alkylating (or Lewis acid catalyzed) cleavage of dithioace-tals U14) or by protonation of vinyl sulfides (thio enol ethers) (116 Scheme 55)." ... 

Most efficiently, however, thiocarbenium ions (115) can be prepared from sulfides (117), by 5-oxida-tion, transformation of the introduced oxygen into a good leaving group e.g. into the trifluoroacetate) and subsequent -elimination— the Pummerer-type route (Scheme 56 see also Chapter 4.7). 

Only very rarely have endo-type cyclization reactions triggered by thiocarbenium ions been achieved. One interesting example, involving an aromatic C-nucleophile, is the sulfur analog of the Pictet-Spen-gler reaction, leading to (125), presumably via (124 Scheme 60). Besides the vinyl sulfide (123), the corresponding dithioacetal can also be utilized. 

Not covered in this review are radical oxidations of alcohols," ethers, thio-ethers and amines to carbonyls, oxocarbenium ions, thiocarbenium ions, and iminium ions where in situ nucleophilic attack through an ionic mechanism either... 

The development of successful models for stereoselectivity in such cycli-zations is indirect evidence for the intermediacy of an oxocarbenium ion (and iminium ions in the related DDQ-mediated cyclizations of e.g., N-vinyl amides as well as thiocarbenium ions in the analogous sulfur-containing substrates ). Can we go further and resolve whether the mechanism is, in the earlier stages, SET then HAT, or some other combination Essentially we cannot from the published results. It is challenging to interrogate such electron transfer without computational modeling, partly because the electron-transfer events are likely to be much faster than the relatively (glacially) slow movement of molecules and atoms. 

As for the corresponding iminium and oxocarbenium ions, thiocarbenium salts can undergo endo- or exo-cyclic ring closure reactions. Again, carbon-carbon bond formation by exocyclic-type reactions lead to heterocyclic systems only by chance , namely when an additional heteroatom is located in the chain that will form the ring, e.g. the thiol-assisted annulation of (119) to give (120 Scheme 58). ... 

An important feature of these reactions is that acyloxy migration occurs intramolecularly. Moderate (30-70% ee) induction of asymmetry at the a-carbon is observed. In fact, Pummerer reactions in which chiral transfer occurs are almost exclusively restricted to this category. The very tight nature of the a-thiocarbenium-acetate ion pair and very rapid recombination (ion pair return) are considered to be two... 

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