Non-oxidative vanadium-catalyzed C-O bond cleavage: application to degradation of lignin model compounds.
Non-oxidative vanadium-catalyzed C-O bond cleavage: application to degradation of lignin model compounds.
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DOI：
10.1002/anie.201001293
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发表时间：
2010-05-17
影响因子：
16.6
通讯作者：
Toste, F Dean
中科院分区：
文献类型：
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作者：
Son, Sunghee;Toste, F Dean
木质纤维素生物质作为一种可再生的燃料和化学品来源最近受到了极大的关注。[1]在不可食用的木质纤维素的三种主要组分（纤维素、半纤维素和木质素）中，已经进行了广泛的努力来将纤维素转化为乙醇和其他生物燃料。相反，对木质素转化的研究仅限于将其从生物质中去除，以提高化学品和酶对生物质其他组分的可及性，或防止纸和纸浆的光致泛黄。尽管事实上木质素相当于木质纤维素生物质的重量的30%和能量含量的40%，但很少有旨在生产高价值化合物的新方法被报道。近年来，受纸浆漂白过程的启发，有关酶促和化学氧化反应的机理和产物分布的研究报道不断涌现。[2]使用含有β-O-4键的二聚木质素模型化合物（例如1），代表木质素中最常见的子结构，[3]芳香醛作为主要产物以低收率获得。虽然这些方法显示出选择性转化木质素的前景，但需要开发全新的催化工艺以充分实现木质素作为化学原料的潜力。另外，为了成功地实现高选择性，需要彻底理解这些过程的机理。为了开发将木质素选择性转化为高度官能化的芳香族化合物的新方法，我们探索了用于转化1的各种均相钒络合物（表1）。[4]测试的大多数钒催化剂除了产生少量的C-O键裂解产物2和3（条目2-7）之外，还产生苄醇氧化产物4作为主要产物[5]。尽管产率较低，但2的形成将该反应与以前的报道区分开来：2不仅是一种新的产物，而且它也是一种氧化还原中性转化。受到这种新反应性的激发，我们探索了其他钒催化剂，发现三齿席夫碱配体比苄基氧化更有利于C-O键断裂（条目8-11）。当使用具有较大咬合角的配体时，观察到对C-O键裂解的较高选择性（条目8对9和10对11）。[6]与9相比，催化剂11增加的反应性（条目11对9）可归因于其tBu取代基，这使得来自11的中间体保持为催化活性单体物质，而不是形成不溶性聚集体。[7]因此，通过配体结构的细微变化，钒（V）-氧代催化剂的反应性从简单的醇氧化转向β-O-4碳-氧键的断裂。
Lignocellulosic biomass has recently received great interest as a renewable source of fuel and chemicals.[1] Among the three major components of non-edible lignocellulose (cellulose, hemicellulose, and lignin), extensive efforts have been made to convert cellulose to ethanol and other biofuels. In contrast, research on the conversion of lignin has been limited to its removal from biomass either to enhance the accessibility of chemicals and enzymes to other components of biomass or to prevent photo-yellowing of paper and pulp. Despite the fact that lignin corresponds up to 30% of the weight and 40% of the energy content of lignocellulosic biomass, few novel processes aimed at producing high value compounds have been reported. Recently, several reports inspired by the pulp bleaching process have been published regarding the mechanism and product distribution of enzymatic and chemical oxidation reactions.[2] Using dimeric lignin model compounds (eg 1) containing a β-O-4 linkage that represents the most common substructure in lignin,[3] aromatic aldehydes were obtained as the main products in low yield. Although these methods show promises for selective conversion of lignin, fundamentally new catalytic processes need to be developed to fully realize lignin's potential as a chemical feedstock. In addition, thorough understanding of the mechanism of these processes is necessary to successfully achieve high selectivity.Aiming to develop a novel method to selectively convert lignin to highly functionalized aromatic compounds, we explored various homogeneous vanadium complexes for the conversion of 1 (Table 1).[4] Most of the vanadium catalysts tested yielded benzylic alcohol oxidation product 4 as the major product [5] in addition to small amounts of C–O bond cleavage products 2 and 3 (entries 2–7). In spite of the low yield, the formation of 2 distinguishes this reaction from previous reports: not only is 2 a novel product, but it is also a redox-neutral transformation. Excited by this new reactivity, we explored other vanadium catalysts and found that tridendate Schiff base ligands favor C–O bond cleavage over benzylic oxidation (entries 8–11). Higher selectivity for C–O bond cleavage was observed when ligands with larger bite angles were employed (entries 8 vs. 9 and 10 vs. 11).[6] The increased reactivity of catalyst 11 compared to 9 (entry 11 vs. 9) may be attributed to its tBu substituents, which enable intermediates from 11 to remain as catalytically active monomeric species instead of forming insoluble aggregates.[7] Thus, through subtle changes in the ligand structure, the reactivity of the vanadium (V)-oxo catalyst was tuned away from simple alcohol oxidation toward the cleavage of the β-O-4 carbon-oxygen bond.