Carbocations mercury-stabilized

Mercuric acetate and water react with alkenes via a mercury-stabilized carbocation to give a hydroxy alkyl-mercury compound. Reduction of the C-Hg bond with NaBH4 leads to the Markovnikov alcohol. 

Methyl-l-hexene is a terminal alkene, and formation of the more stable secondary cation leads to a single major product. However, oxymercu-ration of an unsymmetrical internal alkene leads to a mixture of products. Oxymercuration of 3,3-dimethylcyclopentene, for example, gives a mixture of 74 and 75. Although rearrangement did not occur, two regioisomers are formed because both of the possible carbocation intermediates are secondary. The two mercury-stabilized carbocations are essentially equal in stability, so both are formed and subsequent reaction with water leads to the mixture of alcohols shown after reduction with sodium borohydride. 

Organomercury reagents do not react with ketones or aldehydes but Lewis acids cause reaction with acyl chlorides.187 With alkenyl mercury compounds, the reaction probably proceeds by electrophilic attack on the double bond with the regiochemistry being directed by the stabilization of the (3-carbocation by the mercury.188... 

SftT 1 substitutions also proceed easily in compounds forming highly stabilized secondary carbocations such as benzyhydrol , ferrocene and analogous systems. A variety of leaving groups can be utilized in these reactions including halides, alkoxy and alkylthio in the presence of mercury salts and ethers. 

Barrett has reported a systematic study on the cleavage of multiple cyclopropanes (see e g. equation 20). Interestingly, it was found that while the mercury-mediated ring opening of monocyclopropanes usually proceeds via a concerted mechanism, in the case of multiple cyclopropanes, it occurs through a stabilized free carbocation. Furthermore, the presence of an adjacent cyclopropane increases the rate of the ring opening of the first cyclopropane. 

On the basis of theoretical and experimental results a symmetrical mercurinium ion, with most of the positive charge on mercury, has therefore been proposed in reactions of symmetrically substituted alkenes , while asymmetrical mercurinium ions or weakly bridged mercury-substituted carbocations have been proposed when there is a substituent, such as an aryl group, on the double bond -. Finally, with substituents highly capable of stabilizing carbocations, fully open intermediates have been proposed . ... 

Based on the previous analysis, 3-methyl-l-hexene reacts with mercuric acetate to give secondary carbocation intermediate 72. Is this carbocation unusual relative to previously discussed carbocations, such as 30 in Section 10.3 Note that mercury is a transition metal with d-orbitals that can donate electron density to the carbocation, stabilizing that positive center via what is known as back-donation (similar to what is observed with the halogens in the formation of halonium ions). Despite the manner in which is it drawn, this carbocation is not a three-membered ring, but rather a secondary carbocation that is stabilized by back-donation from the mercury atom. The dashed line (--) indicates significant coordination between the carbon and mercury. 

Amine 17 was prepared from4 by reaction with aniline at 80 °C. Treatment of 17 with mercury(II) acetate followed by borohydride reduction gave the azasilacycle (18) in modest yield (eq 9). This result is in accord with the 8-carbocation stabilizing effect of a silicon atom. Synthon 4 is thus a useful building block for the preparation of silicon-containing heterocycles. 

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