第八周-金属有机氢化反应及氢甲酰化反应

1、过渡金属催化的反应过渡金属催化的反应 TurnoverTurnover Frequency (TOF)Turnover Number (TON)Chemical selectivityRegio selectivityStereo selectivityIt has been clearly shown that PPh3 is readily lost due to steric crowding and that the inner catalyst cycle with a weakly coordinated solvent molecule (not shown) is about

2、1000 times faster than the outer cycle that has 3 PPh3 ligands coordinated to the metal.This hydrogenation catalyst is compatible with a variety of functional groups (ketones, esters, carboxylic acids, nitriles, nitro groups, and ethers) and indicates that the metal hydride intermediate is primarily

3、 covalent in character.Coordinatively unsaturated cationic catalysts that were considerably more active for hydrogenation were later discovered. The reason for this is that the cationic metal center is more electrophillic and this favors alkene coordination, which is often the rate determining react

4、ion step.The ability to coordinate to the catalyst directly influences the rate of hydrogenation. Thus, unsaturated substrates containing polar functionality which can assist in binding to the catalyst have faster hydrogenation rates. Terminal alkynes are hydrogenated as well and at a faster rate th

5、an terminal alkenes (better binding and insertion). The following is the general trend in hydrogenation rates:选择性：选择性： 氢化反应首先发生在最活泼的多重键。空间跟电子效应有重要的影响。氢化反应首先发生在最活泼的多重键。空间跟电子效应有重要的影响。Directing Effects 定位效应Amide OH OR ester ketoneAsymmetric Hydrogenation95% of all hydrogenations use heterogeneous catal

6、ysts like Pd on carbon (Pd/C) or Raney Nickel. One area where homogeneous catalysis rules is asymmetric hydrogenation. This involves the use of a chiral catalyst and an alkene substrate that generates a chiral carbon center on hydrogenation.这个氢化反应的机理与其他单齿配体配合物的不同。首先发生的是单体的配位而不是氢气的氧化加成。氧化加成是限速步骤。更为重要

7、的是烯烃配位的异构体配合物（主要产物）通过X-射线表征，给出的是一种错误的信息。Halpern 证明了，那种次要异构体的反应速度比主要异构体的反应速度快580倍。 Ru 是一种很强的异裂活化氢气的催化剂。这种催化剂主要发生的是氢气的异裂而不是氧化加成。氢气与金属的配位削弱了H-H键的强度。整个过程中Ru价态保持不变。Tobin Marks reported the extraordinary activity of (Cp*2LuH)2 for the hydrogenation of alkenes and alkynes. The monometallic complex catalyze

8、s the hydrogenation of 1-hexene with a TOF = 120,000 hr1 at 1 atm H2, 25C! This is one of the most active hydrogenation catalysts known.Hydrosilylation (addition of H-SiR3)These H-X additions are very similar to hydrogenation (addition of H-X where X=H). Platinum and Palladium catalysts are most wid

9、ely used for the hydrsilylation of alkenes.氢甲酰化反应1970年之前大部分的氢甲酰化反应是通过Co催化剂来完成的；后来Rh催化剂逐渐占主导。2004年大约75%的氢甲酰化反应是用Rh催化剂来完成。增加CO分压能降低氢甲酰化反应速率，但是能降低支链产物的比例。给电子性的烷基膦取代CO，增强了Co-CO键的强度(降低了CO的分压， CO作用：稳定催化剂，避免生成Co金属); 膦的给电子性增加了HCo(CO)4中H的碱性（hydridic)，使之氢化能力更强。但是这也意味着生成的醛有可能但是这也意味着生成的醛有可能会被进一步氢化为醇会被进一步氢化为醇。给电

10、子性越差的膦配体，其催化醛到醇的比例越低。同时催化剂强的氢化能力也容易使烯烃直接氢化为烷烃。HRh(CO)(Ph3P)2 具有较高的选择性，没有醇生成，没有烷烃生成；直链醛比例高Pruett 发现当烷基膦加入过量的时候能提高直链醛的比例。他发现HRh(CO)(Ph3P)2在催化过程中容易发生解离而生成高活性但低选择性的HRh(CO)4； 过量三苯膦的加入能抑制这个分解过程。HRh(CO)(PPh3)2 选择性还跟三苯膦的浓度， H2/CO 比例有关。其它一些膦配体这是第二大工业均相羰基化处理过程，每年大概有70亿磅乙酸是通过催化甲醇羰基化制备的。1970年孟山托公司（Monsanto)产业化了Rh催化剂用于这个流程。反应速率与CO压力无关，相对与Rh、MeI也是一级反应。反应的决速步骤是碘甲烷对金属中心的氧化加成。这个流程中存在一个问题： 体系中会生成无活性的RhI4(CO)2-。