Complexities in the ring-opening polymerization of lactide by chiral salen aluminum initiators
Complexities in the ring-opening polymerization of lactide by chiral salen aluminum initiators
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DOI：
10.1021/ic7019704
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发表时间：
2008-04-07
影响因子：
4.6
通讯作者：
Zhou, Zhiping
中科院分区：
文献类型：
--
作者：
Chisholm, Malcolm H.;Gallucci, Judith C.;Zhou, Zhiping
报道了（R，R）-和（S，S）-salen Al（OR）配合物的制备，其中R = Et, (CH2Pr)- pr -i, (CH2Bu)- bu -t和CH2CH(S)MeCl。它们与丙交酯（salen = N，N'-双（3,5-二叔丁基水杨基）-1,2环己二胺）的反应。LA与salen金属配合物发生了快速可逆的配位反应，醇/醇交换的相对速率与NMR时间尺度相当，而（R，R）-salenAl（O-R-R）和（S，S）-salenAl（O-S-R）的链转移速率远快于LA单体的初始开环速率。对于伯[Al-OR]基团，rac-LA的开环速度远快于LA的开环聚合/链结，并且在初始开环事件中，非对映选择性取决于溶剂、salen配体的手性和OR基团。无论引发剂基团或溶剂如何，该体系都会达到L-和D-LA的假静态平衡浓度，这取决于salen配体手性的性质。进一步的研究表明，反式酯化反应的相对速率比LA链结的速率慢，外聚化反应的速率是体系中最慢的反应。不定水导致催化活性丧失，形成惰性氧桥化合物[(salen)Al](2)(mu-O)，其结构已被表征。
The preparation of (R,R)- and (S,S)-salen Al(OR) complexes, where R = Et, (CH2Pr)-Pr-i, (CH2Bu)-Bu-t, and CH2CH(S)MeCl, are reported, along with. their reactions with rac-lactide (salen = N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2cyclohexanediamino). Rapid, reversible coordination of LA to the salen metal complex is observed, and it is shown that the relative rates of alcohol/alkoxide exchange are comparable to the NMR time scale while the rate of chain transfer involving (R,R)-salenAl(O-R-R) and (S,S)-salenAl(O-S-R) is much faster than the initial rate of ring opening of the LA monomer. For a primary [Al-OR] moiety, the ring opening of rac-LA is much faster than the ring-opening polymerization/enchainment of LA, and in the initial ring-opening event, the diastereoselectivity is dependent on the solvent, the chirality of the salen ligand, and the OR group. Irrespective of the initiator group OR or the solvent, the system moves to a pseudostatic equilibrium concentration of L- and D-LA which is dependent on the nature of the chirality of the salen ligand. Further studies show that the relative rate of trans-esterification is slower than the rate of LA enchainment and that the rate of epimerization is the slowest reaction in the system. Adventitious water leads to loss of catalytic activity and formation of the inert oxo-bridged compound [(salen)Al](2)(mu-O) which has been structurally characterized.