Kinetics and thermodynamics of H. transfer from (eta5-C5R5)Cr(CO)3H (R = Ph, Me, H) to methyl methacrylate and styrene.
Kinetics and thermodynamics of H. transfer from (eta5-C5R5)Cr(CO)3H (R = Ph, Me, H) to methyl methacrylate and styrene.
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H.从 (eta5-C5R5)Cr(CO)3H (R = Ph, Me, H) 转移到甲基丙烯酸甲酯和苯乙烯的动力学和热力学。
DOI：
10.1021/ja034927l
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发表时间：
2003
期刊：
影响因子：
--
通讯作者：
Rappe,Anthony
中科院分区：
文献类型：
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作者：
Tang,Lihao;Papish,ElizabethT;Abramo,GrahamP;Norton,JackR;Baik,Mu-Hyun;Friesner,RichardA;Rappe,Anthony
测定了活化烯烃甲基丙烯酸甲酯-d_5和苯乙烯-d_8与Cr（η_5-C_5Ph_5）Cr（CO）_3H（2a）、（η_5-C_5Me_5）Cr（CO）_3H（2b）和（η_5-C_5H_5）Cr（CO）_3H（2c）之间的H/D交换速率。当氘代烯烃大量过量时，第一次交换在随后的交换开始之前完成，在烯烃和氢化物中的速率为一级。（除了苯乙烯和CpCr（CO）3 H外，氢化作用不明显;在大多数情况下，第一次H·转移产生的自由基受到太大阻碍，无法提取另一个H·。）统计校正给出了氢从氢化物转移到烯烃的速率常数skreinit。与MMA，kreinit大幅减少，氢化物的空间体积的增加;与苯乙烯，氢化物的空间体积几乎没有影响。在较长的时间内，MMA或苯乙烯与2的反应给出了相应的金属自由基1a作为终止剂，消耗了甲基异丁酰基自由基3或α-甲基苄基自由基4的浓度;计算机模拟[1a]作为f（t）给出了ktr的估计值，即H·从3或4转移回Cr的速率常数。这些速率常数意味着从2原子甲基丙烯酸甲酯转移氢的ΔG（50 °C）为+11千卡/摩尔，从2原子苯乙烯转移氢的ΔG（50 °C）为+10千卡/摩尔。CH 3CN pKaof 2a为11.7，意味着其Cr-H键的BDE为59.6 kcal/mol，DFT计算得出2c中Cr-H键的BDE为58.2 kcal/mol。结合动力学Δ G值、2a的实验BDE值和H·转移的计算Δ S值，表明甲基异丁酰基自由基3的C−H BDE为45.6 kcal/mol（接近DFT计算的49.5 kcal/mol），α-甲基苄基自由基4的C−H BDE为47.9 kcal/mol（接近DFT计算的49.9 kcal/mol）。一个溶剂笼模型表明，MMA聚合中的链自由基的C-H BDE为46.1 kcal/mol。
The rates of H/D exchange have been measured between (a) the activated olefins methyl methacrylate-d5and styrene-d8, and (b) the Cr hydrides (η5-C5Ph5)Cr(CO)3H (2a), (η5-C5Me5)Cr(CO)3H (2b), and (η5-C5H5)Cr(CO)3H (2c). With a large excess of the deuterated olefin the first exchange goes to completion before subsequent exchanges begin, at a rate first order in olefin and in hydride. (Hydrogenation is insignificant except with styrene and CpCr(CO)3H; in most cases, the radicals arising from the first H• transfer are too hindered to abstract another H•.) Statistical corrections give the rate constantskreinitfor H• transfer to the olefin from the hydride. With MMA,kreinitdecreases substantially as the steric bulk of the hydride increases; with styrene, the steric bulk of the hydride has little effect. At longer times, the reaction of MMA or styrene with2agives the corresponding metalloradical1aas termination depletes the concentration of the methyl isobutyryl radical3or the α-methylbenzyl radical4; computer simulation of [1a] as f(t) gives an estimate ofktr, the rate constant for H• transfer from3or4back to Cr. These rate constants imply a ΔG(50 °C) of +11 kcal/mol for H• transfer from2ato MMA, and a ΔG(50 °C) of +10 kcal/mol for H• transfer from2ato styrene. The CH3CN pKaof2a, 11.7, implies a BDE for its Cr−H bond of 59.6 kcal/mol, and DFT calculations give 58.2 kcal/mol for the Cr−H bond in2c. In combination the kinetic ΔGvalues, the experimental BDE for2a, and the calculated ΔSvalues for H• transfer imply a C−H BDE of 45.6 kcal/mol for the methyl isobutyryl radical3(close to the DFT-calculated 49.5 kcal/mol), and a C−H BDE of 47.9 kcal/mol for the α-methylbenzyl radical4(close to the DFT-calculated 49.9 kcal/mol). A solvent cage model suggests 46.1 kcal/mol as the C−H BDE for the chain-carrying radical in MMA polymerization.