3-Metalla-l,2-dioxolanes

The olefin oxygenations carried out with dioxygen seem to be metal-centered processes, which thus require the coordination of both substrates to the metal. Consequently, complexes containing the framework M (peroxo)(olefin) represent key intermediates able to promote the desired C-0 bond formation, which is supposed to give 3-metalla -l,2-dioxolane compounds (Scheme 6) from a 1,3-dipolar cycloinsertion. This situation is quite different from that observed in similar reactions involving middle transition metals for which the direct interaction of the olefin and the oxygen coordinated to the metal, which is the concerted oxygen transfer mechanism proposed by Sharpless, seems to be a more reasonable pathway [64] without the need for prior olefin coordination. In principle, there are two ways to produce the M (peroxo)(olefin) species, shown in Scheme 6, both based on the easy switch between the M and M oxidation states for... 

Compounds of the 3-metalla -l,2-dioxolane type were unknown in rhodium and iridium chemistry until 2001 [65] when Gal reported the preparation of some examples through sohd-gas reactions, quite an imcommon type of chemistry. They were initially obtained from the cationic complex [Rh(/c -tpa)(C2H4)] (tpa = Ar,Nd, -tris(2-pyridylmethyl)amine), and then extended to the iridium counterparts and the rhodium [66] compoimds [Rh(/c -dpda-Me2)(C2H4)] (Fig. 1) (the aromatic circles for the pyridine rings have been omitted for clarity). 

These solid-gas reactions represent, at the moment, the single path to 3-metalla -l,2-dioxolane complexes of rhodium and iridium. Complexes of this type have been widely proposed in catalytic cycles. However, it is unlikely that they take part in oxygenations with rhodium because of their high reactivity (see below) and the special conditions for their preparation. 

The few 3-metalla -l,2-dioxolane complexes of rhodium and iridium isolated so far have been highly reactive species. Simply by exposure to daylight they rearrange to the very unusual formylmethyl hydroxy complexes [M(/c -tpa)M(OH)(iii-CH2CHO)](X) and [Rh(/c4 dpda-Me2)(OH)(Tii-CH2CHO)] (PFe) in the solid state (Scheme 13) [84]. An alternative route to these formylmethyl hydroxy complexes is the oxidation of a 2-rhoda oxetane with hydrogen peroxide [67] (Scheme 13). 

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