Oxetanes metalla

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. 

These l-hydroxy-2-metalla (5,6,7)-allyl complexes result directly from the reactions of the less sterically crowded complexes [lr(Cn)(cod)](OTf) (Cn = 1,4,7-triazacyclononane) [82] and [Rh(/c -Py3S3)(cod)](BPh4) [81]. Slowing down the reactions by increasing the steric crowding around the metal, the kinetic isomers 2-irida oxetane and 6,7-oxarhoda tetracyclodecane can be isolated from the reactions of [lr(Cn )(cod)](OTf) [82] and [Rh(/c -L )(cod)](Pp6) (L = Cn, dpa-R ) [72,79] with H2O2, respectively. The... 

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). 

Metalla-2-Oxetanes in [2+2] Addition Reactions of Metal Oxides to Olefins. . 125... 

The discussion about the possible formation of metalla-2-oxetanes in transition metal-mediated oxidation reactions began with the ground breaking work of Sharpless in the field of enantioselective dihydroxylation of olefins with osmium tetraoxide using cinchona alkaloids as ligands [6]. The transfer of the stereochemical information of the chiral ligand to the substrate was explained by Sharpless with a two-step mechanism for the addition reaction, which should occur rather than a concerted [3+2] addition as originally proposed [110] (Fig. 15). 

Direct evidence for the formation of a metalla-2-oxetane in a metal mediated oxidation reaction was recently reported by Sharp and coworkers [121]. The oxidation of norbornylene by the tetranuclear platinum(II) p-oxo complex 1 yielded nearly quantitative formation of the platina-2-oxetane 2 shown in Fig. 18. The compound 2 could be isolated and the structure was identified with X-ray structure analysis. However, it is unclear if the reaction of this late transition metal complex takes place via [2-1-2] addition of a metal-oxo moiety across the C=C double bond. The authors write that the formation of the C-0 bond allows considerable speculation on this process. DFT calculations are underway to help differentiate the various possibihties [121]. 

The competing formation of metalla-2-oxetane via [2+2] addition vs metalla-dioxylate formation through [3+2] addition has been theoretically studied for... 

All theoretical studies which analyzed the reaction course of the addition of metal oxides to olefins which were claimed by experimentahsts [111, 117] to yield metalla-2-oxetanes via [2-1-2] addition showed that the [3-1-2] pathway has a lower activation barrier than the [2-1-2] addition [127-136]. This made some theoreticians search for oxidation reactions where the [2-1-2] addition of an oxide to an olefin has a lower barrier than the [3-1-2] addition. The first success albeit nor for a metal oxide was reported by Houk et al. [137]. They reported about ab initio and DFT calculations which show that the [3-1-2] addition of SO3 to ethylene yielding ethylensulfite has a higher barrier than the [2-1-2] addition giving the four-membered cycHc sulfone. It follows that the formally symmetry-forbidden [2-1-2] addition can become more favorable than the [3-1-2] addition. An explanation for the reversal of the activation barriers was given in terms of the strongly polarized frontier orbitals of SO3, which has the LUMO essentially localized at sulfur and the HOMO locahzed at the oxygen atoms. 

The calculations of the transition states led to the following predictions concerning the kinetically most favorable reactions pathways [140]. The reaction of ketene with OSO4 should proceed via [3+2] addition across the C=C bond. The [2+2] addition reactions of all LReOj species to ketene have lower activation energies than the [3+2] addition. These were the first examples where calculations showed that the [2+2] addition of a metal oxide to a C=C bond has a lower barrier than the [3+2] addition. For Re07 and (HjPNlReOj it was found that the [2+2] addition to the C=0 bond is kinetically even more favorable than the addition to the C=C bond. However, the calculation predicted that the [2+2] addition of MeReOj, CpReOj and Cp ReOj should proceed via [2+2] addition across the C=C bond yielding the metalla-2-oxetane. 

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