Oxallyl group

The second stage of the Swem oxidation, illustrated below, involves a nucleophilic displacement of the oxallyl group from the sulfur. In this step, the nucleophile is a chloride anion, and the reaction is facilitated by the decomposition of the leaving group into carbon dioxide gas, carbon monoxide gas, and a chloride anion. 

The photo-Nazarov cyclisation of l-cyclohexenyl phenyl ketone (285) yields the hexahydrofluorenone (286). Schaffner s group has examined the mechanism of this reaction and have found evidence for some novel intermediates. They propose that the excited state of (285) decays by cis-trans isomerisation of the cyclohexenyl double bond the tx-ans-cyclohexenyl derivative (287) cyclises to the oxallyl species (288) which collapses to the relatively stable enol (289) or reacts with accumulated (289) to give the isolable enol dimer (290). When the ketone derived from (290) is photolysed it gives back (289) along with (286). The enol (290) and its corresponding ketone were both isolated and characterised, while (287) and (288) were trappable with cyclopentadiene. The enol (289) was observable by H-NMR before it ketonised and gave the isolated product (286). 

In summary, the relatively electropositive, highly Lewis acidic early transition metals form O-bound enolates, as expected according to their oxophilic character. When switching to the so-called late transition metals, located to the right of group 5 in the periodic table, the clear preference for O-bound enolate structures vanishes, and the C-bound tautomers become involved but also the i/ -bound oxallyl type. A general rule on which tautomer is favored by which metal cannot be deduced, because the type of enolate structure is mainly determined by the oxidation state, the coordination number, and in particular the individual ligands at the transition metal. 

A model proposed for rationalizing the stereochemical outcome is also shown in Scheme 5.112. It seems plausible that the skewed structure 441, typical for transition metal BINAP complexes, has just one open space that is filled by a coordination of rhodium to the carbon - carbon bond of cyclohexenone. As a consequence, the insertion of the phenyl group then occurs from the Sf-face to the enone to give the / -rhodium complex 442 that subsequently tautomerizes to the more stable oxallyl-type enolate [205a]. It seems that a transmetallation at the stage of the enolate - from rhodium to boron enolate, as indicated in cycle B (Scheme 5.111) - doesnot occur in all these rhodium-mediated domino reactions without exception. Thus, Hayashi and coworkers produced evidence in support of a rhodium enolate as an active nucleophile in the aldol step when phenyl-9-BBN 438 was reacted with acyclic enones in the presence of [Rh(OH)-(S)-BINAP]2. In this case, the catalytic cycle is maintained by a transmetallation of the rhodium to the boron aldolate [221]. 

---