2-Rhoda oxetanes

The conversion of the 6,7-oxarhoda tetracyclodecanes into the 1-hydroxy-2-metalla (5,6,7)-allyl products is more complicated since it involves the rupture of Rh - C and C - O bonds in the rhodiiun complexes. In some instances, an equilibriiun between both types of complexes, 6,7-oxarhoda -tetracyclodecanes and 2-rhoda oxetanes, has been proposed to accoimt for these results [72]. 

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). 

However, the elimination of acetaldehyde from 2-rhoda oxetanes (Scheme 14) is observed if a coordination vacancy on the metal, necessary for a /1-hydrogen elimination, is created. This is achieved by dissociation of an acetonitrile ligand [67] or by protonation [80] of the oxetane moiety. 

This 6-hydrogen elimination in 2-rhoda oxetanes is apparently favored over reductive elimination to an epoxide. Moreover, the reverse step, i.e., the oxidative-addition of epoxides to Rh and Ir results in 2-rhoda oxetanes [85] and/or hydrido formylmethyl complexes [86]. Therefore, assuming that 2-metalla oxetanes are intermediates in the oxygenation of alkenes by group VIII transition metals, the reported reactivity would account for selectivity to ketones in the catalytic reactions based on these metals. 

The oxidation of Rh (olefin) complexes with the ligand l,4,7-trimethyl-l,4,7-triazacyclononane or -butylbis(pyr-idine-2-ylmethyl)amine afforded the preparation of 2-rhoda(lIl)oxetanes 289 and 290. ... 

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