Indium complexes alkoxides

ROP [65, 74]. The indium alkoxide (11) was a good initiator, with 100 equivalents of SS-lactide being converted to PLA in 200 min, at 50°C. When compared with the analogous aluminium initiators, the indium complexes showed comparable rates but lower-molecular-weight PLA. All the PLA formed from rac-lactide was atactic [65]. 

Okuda and Arnold have reported a series of 5- and 6-coordinate chiral indium complexes (e.g. 12), analogous to those previously discussed with yttrium (Table 4), which show varying activities for the polymerization of rac-lactide [71]. The homoleptic species was fluxional, interconverting between a 5-coordinate and fac- and mer-isomers of a 6-coordinate complex. It resulted in controlled polymerization and some stereoselectivity (max. P, = 0.63 and max. Ps = 0.64). MALDI-ToF mass spectrometry confirmed that the chiral alkoxide ligand was an initiating group. 

Complexes ( )- and (R,R)-2 are highly active and controlled for the polymerization of rac-LA yielding a linear relationship between the observed PLA molecular weights and the added monomer, assuming both alkoxides initiate the polymerization, with low molecular weight distributions (Figure 36)." The rate of polymerization with these complexes is first order in lactide concentration with values comparable with the tri-dentate diaminophenolate indium complexes desaibed in Section 2. " -The rates of polymerization are much faster than any of the known chiral aluminum salen systems, which require elevated temperatures and days to reach full conversion." -... 

The reduced yttrium complex was active for the polymerization of 55-lactide, 74% conversion in 3 h (0.2 M, THF, 25°C). When the complex was oxidized, using FcBArF, the polymerization was completely stopped, and it was restarted when the complex was reduced back. The authors provide good evidence that the polymerization switch operates by changing the electron density of the yttrium-alkoxide bond during oxidation/reduction cycles. Polymerizations with the indium analogue... 

Mixed-metal alkoxide complexes of thallium are also known. For example, Sn(/u-t-BuO)3Tl has both Sn(II) and T1(I) ions." The thallium site is unreactive as a donor for metal carbonyls. However, as indicated earlier, the indium(I) site of the indium analogue shows Lewis-base character. The Sn(IV)/Tl(I) mixed alkoxide [Sn(EtO)eTl2] exists as a one-dimensional polymer. This adduct reacts quantitatively with SnCl2 to form the homoleptic, mixed-valent [Sn2(OEt)6]n. Thallium-titanium double alkoxides have been synthesized using thallium alkoxide as one of the starting materials. ... 

The cause of this remarkable selectivity switch is thought to be due to the relative stability of the two intermediates. Normal 1,4-addition leads to an enolate anion, the charge of which may be delocalised away from the oxygen centre. The presence of indium(III), however, stabilises the 1,2-addition product (an alkoxide), presumably forming an ate complex. As the initial reaction of the TMM palladium complex with the a,p-unsaturated substrate is reversible, stabilisation of the 1,2-addition intermediate biases the equilibrium in favour of the eventual 1,2-cyclisation product. 

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