Oxetane complexes

Oxetane undergoes ring-opening polymerization under the action of MAD in conjunction with onium salts, including quaternary ammonium and phosphonium halides, giving a narrow MWD polyether (Scheme 6.173) [221]. Use of MesAl in place of MAD resulted in no polymerization. The aluminum ate complex seemed to be an initiator, which underwent a trigger reaction involving halide transfer to the aluminum-oxetane complex. 

Fig. 12 Concerted three-center C(sp )-0 reductive elimination of olefin oxides from Pt" oxetane complexes [33]...

In order to study the reaction, they defined all the different pathways for approaching the olefin to the catalyst. They are depicted in Fig. 5a. There are three ways of approaching the olefin to the osmium tetroxide, each one directed to one of the equatorial oxygens. Thus, the different isomers of the oxetane complex can be created from the osmium tetraoxide-cinchona ligand complex by adding the olefin in a [2+2] fashion, therefore distorting an equatorial oxo... 

Fig. 8 The n-pair models of 2,5-dihydrofuran, oxetane and oxirane (first column) and the experimental geometries of their complexes with HC1 (second column) and C1F (third column), each drawn to scale. The angle 0 is almost identical in B- HC1 and B- ClF for a given B but increases from 2,5-dihydrofuran, through oxetane, to oxirane, as expected from the model (see text). The non-linearity of the hydrogen bond increases monotoni-cally from 2,5-dihydrofuran to oxirane. See Fig. 1 for key to the colour coding of atoms...

Table 1 The angles and 9 (in degrees see Fig. 8 for definitions) in complexes B- HC1 and B- ClF, where B is one of the cyclic ethers 2,5-dihydrofuran, oxetane or oxirane...

The scope of the Patemo-Buchi cycloaddition has been widely expanded for the oxetane synthesis from enone and quinone acceptors with a variety of olefins, stilbenes, acetylenes, etc. For example, an intense dark-red solution is obtained from an equimolar solution of tetrachlorobenzoquinone (CA) and stilbene owing to the spontaneous formation of 1 1 electron donor/acceptor complexes.55 A selective photoirradiation of either the charge-transfer absorption band of the [D, A] complex or the specific irradiation of the carbonyl acceptor (i.e., CA) leads to the formation of the same oxetane regioisomers in identical molar ratios56 (equation 27). 

The Patterno-Buchi coupling of various stilbenes (S) with chloroanil (Q) to yield fran -oxetanes is achieved by the specific charge-transfer photo-activation of the electron donor-acceptor complexes (SQ). Time-resolved spectroscopy revealed the (singlet) ion-radical pair[S+% Q" ] to be the primary reaction intermediate and established the electron-transfer pathway for this Patterno-Buchi transformation. Carbonyl quinone activation leads to the same oxetane products with identical isomer ratios. Thus, an analogous mechanism is applied which includes an initial transfer quenching of the photo-activated (triplet) quinone acceptor by the stilbene donors resulting in triplet ion-radical pairs. ... 

For instance, Kochi and co-workers [89,90] reported the photochemical coupling of various stilbenes and chloranil by specific charge-transfer activation of the precursor donor-acceptor complex (EDA) to form rrans-oxetanes selectively. The primary reaction intermediate is the singlet radical ion pair as revealed by time-resolved spectroscopy and thus establishing the electron-transfer pathway for this typical Paterno-Biichi reaction. This radical ion pair either collapses to a 1,4-biradical species or yields the original EDA complex after back-electron transfer. Because the alternative cycloaddition via specific activation of the carbonyl compound yields the same oxetane regioisomers in identical molar ratios, it can be concluded that a common electron-transfer mechanism is applicable (Scheme 53) [89,90]. 

Abstract This chapter focuses on well-defined metal complexes that serve as homogeneous catalysts for the production of polycarbonates from epoxides or oxetanes and carbon dioxide. Emphasis is placed on the use of salen metal complexes, mainly derived from the transition metals chromium and cobalt, in the presence of onium salts as catalysts for the coupling of carbon dioxide with these cyclic ethers. Special considerations are given to the mechanistic pathways involved in these processes for the production of these important polymeric materials. 

In 1985, Kwiatkowski et al. reported a tetramethyl ethylenediamine (TMEDA)-catalyzed dimerization of ketene giving the interesting compound 4-methylene-oxetane-2-one (diketene). This substance can be hydrogenated by either Pd/C to racemic p-BL as well as by asymmetric catalysis according to Takaya et al. using Ru complexes of (5)-BINAP as catalyst, with an ee of 92% [111] (Fig. 39). 

Although anionic polymerization of cyclic ethers is generally limited to oxiranes, there are reports of successful oxetane and tetrahydrofuran polymerizations in the presence of a Lewis acid. Aluminum porphyrin alone does not polymerize oxetane, but polymerization proceeds in the presence of a Lewis acid [Sugimoto and Inoue, 1999]. Similarly, THF is polymerized by sodium triphenylmethyl in the presence of a Lewis acid such as aluminum alkoxide [Kubisa and Penczek, 1999]. The Lewis acid complexes at the ether oxygen, which weakens (polarizes) the carbon-oxygen bond and enhances nucleophilic attack. 

Most of the work reported with these complexes has been concerned with kinetic measurements and suggestions of possible mechanisms. The [Ru(HjO)(EDTA)] / aq. HjOj/ascorbate/dioxane system was used for the oxidation of cyclohexanol to cw-l,3-cyclohexanediol and regarded as a model for peroxidase systems kinetic data and rate laws were derived [773], Kinetic data were recorded for the following systems [Ru(Hj0)(EDTA)]702/aq. ascorbate/dioxane/30°C (an analogue of the Udenfriend system cyclohexanol oxidation) [731] [Ru(H20)(EDTA)]70j/water (alkanes and epoxidation of cyclic alkenes - [Ru (0)(EDTA)] may be involved) [774] [Ru(HjO)(EDTA)]702/water-dioxane (epoxidation of styrenes - a metallo-oxetane intermediate was postulated) [775] [Ru(HjO)(EDTA)]7aq. H O /dioxane (ascorbic acid to dehydroascorbic acid and of cyclohexanol to cyclohexanone)... 

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). 

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. 

Further work by Turro and coworkers70 led to the discovery of a singlet complex in this reaction. A plot of the reciprocal of the quantum yield for oxetane formation versus the reciprocal of the concentration of 12 according to Eq. (34) p. 275 gave a slope of 2.6 but an intercept of 13.2. This suggests that oxetane formation proceeds via an intermediate (complex) which is frequently deactivated. Since the value of Ks obtained from this plot agreed... 

The mechanism of decarboxylation of /3-lactones has attracted much attention. The gas-phase decomposition of 2-oxetanone is a unimolecular first-order process. It has a considerably lower energy of activation than the pyrolysis of oxetane and a much higher entropy of activation, indicating a loose activated complex (69JA7743). The ease of the reaction is greatly affected by the electronic effect of substituents at position-4, but not at position-3. The Hammett treatment of a series of rrans-4-aryl-3-methyl-2-oxetanones gave a good correlation with [Pg.374]

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