Chisholm Malcolm H, Gallucci Judith, Phomphrai Khamphee
Department of Chemistry, Newman & Wolfrom Laboratories, The Ohio State University, 100 W. 18th Avenue, Columbus, Ohio 43210-1185.
Inorg Chem. 2002 May 20;41(10):2785-94. doi: 10.1021/ic020148e.
The preparation and characterization of a series of closely related magnesium and zinc compounds are reported: LMg(N(i)Pr(2))(THF), 1; LZn(N(i)Pr(2)), 2; LMg(O(t)Bu)(THF), 3; LZn(O(t)Bu), 4; and LZn(OSiPh(3))(THF), 6; where L = CH(CMeNC(6)H(3)-2,6-(i)Pr(2))(2). Their dynamic solution behavior has been examined by variable-temperature NMR studies and reveals that THF reversibly dissociates in toluene-d(8) or CD(2)Cl(2) and that exchange with free THF occurs by a dissociative process. Compounds 1-4 and 6 all initiate and subsequently sustain ring-opening polymerization (ROP) of lactides. For a related series of compounds LMX(THF)(n)(), where n = 1 or 0, the rate of initial ring-opening follows the order M = Mg > Zn and X = O(t)Bu > N(i)Pr(2) > NSi(2)Me(6) > OSiPh(3). In THF at 25 degrees C, compounds 3 and 4 polymerize 100 equiv of rac-lactide to >95% conversion in 5 and 80 min for M = Mg and Zn, respectively, and yield ca. 90% heterotactic PLA, (isi + sis tetrads). The reactions proceed faster in methylene chloride, but for M = Mg, a Bernoulian distribution of tetrads is formed from rac-lactide (3iii:2isi:sii:sis:iis) prior to trans-esterification. Polymerization of L-LA in toluene-d(8) and THF-d(8) by 3 and 4 have been studied by VT (1)H NMR spectroscopy: the resting state for zinc is proposed to be a monomeric species akin to LZn(eta(2)-OCHMeC(O)OMe), whereas the magnesium complex appears to be dimeric LMg(mu-OP)(2)MgL. None of the compounds is capable of initiating homopolymerization of propylene oxide (PO) or cyclohexene oxide (CHO), although the magnesium amide 1 effects ring-opening by allylic proton abstraction and the dimeric compound LMg(mu-OC(6)H(9)), 7, is formed. Reactions with carbon dioxide are also described, along with the characterization of LZnO(2)CN(i)Pr(2), 8, which is shown to be inert with respect to CHO and PO at room temperature. All the compounds are hydrolytically sensitive, and LZn(mu-OH)(2)ZnL, 5, has been isolated from hydrolysis of compound 4. The crystal and molecular structures are reported for compounds 1-5, 7, and 8. These results are compared with those recently reported by Coates et al.
LMg(N(i)Pr(2))(THF)，1；LZn(N(i)Pr(2))，2；LMg(O(t)Bu)(THF)，3；LZn(O(t)Bu)，4；以及LZn(OSiPh(3))(THF)，6；其中L = CH(CMeNC(6)H(3)-2,6-(i)Pr(2))(2)。通过变温核磁共振研究考察了它们的动态溶液行为，结果表明THF在甲苯-d(8)或CD(2)Cl(2)中可逆解离，且与游离THF的交换通过解离过程发生。化合物1 - 4和6均能引发并随后维持丙交酯的开环聚合(ROP)。对于一系列相关化合物LMX(THF)(n)（n = 1或0），初始开环速率顺序为M = Mg > Zn且X = O(t)Bu > N(i)Pr(2) > NSi(2)Me(6) > OSiPh(3)。在25℃的THF中，化合物3和4分别在5分钟和80分钟内将100当量的外消旋丙交酯聚合至转化率>95%，产物为约90%的间同立构聚乳酸（isi + sis四联）。反应在二氯甲烷中进行得更快，但对于M = Mg，在酯交换之前，外消旋丙交酯会形成Bernoulian四联分布(3iii:2isi:sii:sis:iis)。通过变温(1)H NMR光谱研究了3和4在甲苯-d(8)和THF-d(8)中对L - LA的聚合反应：推测锌的静止态为类似于LZn(η(2)-OCHMeC(O)OMe)的单体物种，而镁配合物似乎为二聚体LMg(μ - OP)(2)MgL。尽管镁酰胺1通过烯丙基质子夺取引发开环并形成二聚化合物LMg(μ - OC(6)H(9))，7，但这些化合物均不能引发环氧丙烷(PO)或氧化环己烯(CHO)的均聚反应。还描述了与二氧化碳的反应，以及LZnO(2)CN(i)Pr(2)，8的表征，结果表明其在室温下对CHO和PO呈惰性。所有化合物对水解均敏感，且已从化合物4的水解产物中分离出LZn(μ - OH)(2)ZnL，5。报道了化合物1 - 5、7和8的晶体和分子结构。将这些结果与Coates等人最近报道的结果进行了比较。
