Chlorosulfites conversion

The reaction of primary or secondary alcohols with thionyl chloride is a general method for preparing the corresponding chloro compounds. In the first step a chlorosulfne ROSOC) is formed from which S02 is eliminated in a relatively slow step. This decomposition is facilitated by a tertiary amine, e.g. pyridine. The ammonium salt RO-SON+.Cl— formed from the chlorosulftte is subsequently attacked on carbon (in R) by CF. Since nucleophilic substitutions on propargylic carbon proceed more easily than on carbon in saturated compounds, it may be expected that the conversion of propargylic chlorosulfites into the chlorides will take place under relatively mild conditions. 

The first step is an allylic chloride displacement on a chlorosulfite ester generated from the tertiary vinyl carbinol and thionyl chloride. Attack of chloride on an allylic carbocation intermediate can equally well be envisaged. The subsequent conversion is simply an S 2 displacement by acetate ion (Scheme 10.6). 

The reaction with thionyl chloride affords a chlorosulfite, the decomposition of which may generate an alkyl chloride by the S i (substitution, nucleophilic, internal) mechanism (Scheme 2.16). This reaction, w hich may proceed by an ion pair, can lead to the retention of configuration of an asymmetric secondary alcohol in the conversion to the alkyl chloride. This is in contrast to the inversion of configuration found with the reaction with phosphorus pentachloride and with the nucleophilic displacement of a leaving group. 

Mechanism 4.3 shows the conversion of a primary alcohol to the alkyl chloride using thionyl chloride. The reaction involves the formation of an alkyl chlorosulfite that is attacked by the nucleophilic chloride ion. 

The Sni mechanism (substitution-nucleophilic-internal), as originally proposed, postulated a front-side displacement occurring via a four-center transition state. It is now clearly established that such a process has never been observed, and that all the reactions proposed to proceed by SnI mechanisms involve ion pairs. The Sni mechanism was initially suggested to explain the observed stereochemistry of chlorosulfite decomposition. Chlorosulfite esters are formed by reaction of alcohols with thionyl chloride, and are the key reactive intermediates in the conversion of alcohols to chlorides with thionyl chloride. Nucleophilic attack by the chloride of the chlorosulfite ester was considered to be concerted with cleavage of the C-O bond ... 

The conversion of alcohols to alkyl chlorides using thionyl chloride was described previously in this chapter, in Section 5.8. It was pointed out that the stereochemistry of the reaction is sensitive to the precise experimental conditions employed, and can vary from inversion of configuration to retention of configuration, depending on the circumstances attending decomposition of the intermediate chlorosulfite ester. 

The mechanism for the reaction of thionyl chloride with a carboxylic acid to form an acid chloride is similar to that presented in Section 10.5C for the conversion of an alcohol to a chloroalkane and involves initial chlorosulfite formation, followed by nucleophilic attack of chloride ion on the carbonyl carbon to give a tetrahedral carbonyl addition intermediate, which decomposes to give the acid chloride, SO2, and chloride ion. 

Because tertiary alcohols are so readily converted to chlorides with hydrogen chloride, thionyl chloride is used mainly to prepare primary and secondary alkyl chlorides. An early step in the mechanism of this reaction is the conversion of the alcohol to a chlorosulfite, which then reacts with chloride ion to yield the alkyl chloride. 

We ve already seen one general reaction of alcohols—their conversion to alkyl halides (Section 12.3). Tertiary alcohols react with HGl and HBr by an SnI mechanism through a carhocation intermediate. Primary and secondary alcohols react with SOCI2 and PBra by an Sn2 mechanism through backside attack on a chlorosulfite or dibromophosphite intermediate. 

It is tempting to employ the same steps for this problem as we used in the conversion of an alcohol into the corresponding alkyl chlorosulfite, ROSOCl, upon reaction with SOCI2 (Section 9-4), namely, displacement of chloride from sulfur by the hydroxy group of the alcohol ... 

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