Vanadium salen catalyst

The 1,2-addition of a cyanide ion to a carbonyl compound to form a cyanohydrin is a fundamental carbon-carbon bond-forming reaction in organic chemistry, and has frequently been at the forefront of advances in chemical transformations. In 2000, Belokon and North developed a catalytic system based on a vanadium-salen complex (Scheme 9.1). The synthesis of vanadium(iv) complex 1 was accomplished by refluxing a mixture of the corresponding Schiff base with vanadium(iv) sulfate and pyridine in ethanol under an argon atmosphere. A very low catalyst loading of 0.1 mol% was employed to convert aromatic and aliphatic aldehydes to cyanohydrin silyl ethers 3 with enantioselectivities of 68-95% after 24 h. Further investigations... 

In 2006, vanadium-salen catalyst 1 was utilised as part of the total synthesis of Manassantin A, reported by Hanessian (Scheme 9.3). The reaction of aldehyde 8 with trimethylsilyl cyanide gave cyanohydrin 9 in 89% yield with 90% enantiomeric excess. Optically pure 9 was readily obtained through a single recrystallisation of the product and transformed to not only manassantin A, but also manassantin B and Bl, in 23 steps. ... 

These materials also opened the possibility to coat glassware or even stir bars with the immobilized vanadium salen catalyst. This is another approach toward a recyclable system, because such stir bars can be used in several reactions. 

The vanadium(IV) complex of salen in zeolite was found to be an effective catalyst for the room temperature epoxidation of cyclohexene using t-butyl hydroperoxide as oxidant.88 Well-characterized vanadyl bis-bipyridine complexes encapsulated in Y zeolite were used as oxidation catalysts.101 Ligation of manganese ions in zeolites with 1,4,7-triazacyclononanes gives rise to a binu-clear complex stabilized by the zeolites but allows oxidation with excellent selectivity (Scheme 7.4). 

Vanadyl salen is readily converted at 100°C with H2S in the absence of a catalyst to a vanadium sulfide and a free organic ligand (or decomposition products). Vanadyl phthalocyanine is more stable with respect to ring attack and demetallation. Rates relative to catalytic reactions have not been measured. If VO-salen is an appropriate model of vanadium binding in asphaltenes, asphaltenic metals are more readily converted to sulfides under hydrotreating conditions than the porphyrinic metals. This suggests... 

Allylic alcohols, for example geraniol, 2-methylallyl alcohol, 3,3-dimethylallyl alcohol, 3-buten-2-ol, l-octen-3-ol, and l-hexen-3-ol, are epoxidized with tert-butyl hydroperoxide in the presence of a vanadyl salen oxo-transfer catalyst in supercritical CO2. The metal catalyst was prepared in a simple two-step, Schiff base-type reaction to form the salen ligand, followed by complexation to the vanadyl group. The use of non-toxic supercritical CO2 in the presence of the new epoxidation vanadium catalyst led to yields and diastereoselectivities that were comparable to those resulting from the use of environmentally hazardous solvents such as CH2CI2 [59]. 

For metal chelate-mediated oxidation of sulfides, (salen)Mn systems such as 57," 58," ind 59" have been successfully exploited. A vanadium complex of 60 is also of some -tility. Certain multidentate ligands can prolong the lifetimes of catalysts" at the expense t enantioselectivity. 

Avecia, a former part of the Zeneca Group, developed a range of cheap and highly active titanium- or vanadium-based salen catalysts called CACHy catalysts (Scheme 23). They are based on Jacobsen s salen technology, but they are much more reactive and can, for example, be used in concentrations as low as 0.1 mol% in the cyanation of aryl aldehydes. The catalysts show similar reactivity with alkyl aldehydes and ketones and are applicable to the synthesis of the commercially important mandelic acid derivatives. 

In 2006, Khan reported the dimeric vanadium(v)-salen complex S,S,S,S)-5 (Figure 9.2), which could catalyse the asymmetric addition of a cyanide ion to aldehydes using potassium cyanide with acetic anhydride. The reaction of aromatic aldehydes as substrates with 1-5 mol% of complex 5 afforded adducts 4 in excellent yields (90-99%) and high enantioselectivities (88-95%). In addition, the catalyst was able to be precipitated and recycled by adding -hexane to the postcatalytic reaction mixture. In this way, catalyst 5 was readily recovered and reused four times. 

In 2011, Khan also reported the novel, recyclable, dimeric vanadium(v)-salen complex [R,R,R,R)-6. Dinuclear chiral complex 6 had a flexible polyether linkage and was efficient for the stereoselective O-acetylcyanation of a variety of aliphatic and aromatic aldehydes with potassium cyanide as a cyanide source, providing the chiral O-acetyl cyanohydrins 4 in excellent yields (93%-quant.) with high enantioselectivities (72-98%). As shown in Scheme 9.2, these results were achieved using only a 1 mol% catalyst loading. In addition, Khan investigated the use of ethyl cyanoformate for... 

In 2010, Kahn found that dimeric vanadium(v) salen complex [S,S,S,S)-5, which had previously been reported as a catalyst for the asymmetric synthesis of 0-acetyl cyanohydrins 4, was an effective catalyst for the asymmetric addition of cyanide to N-benzyl-protected aldimines to give (J )-amino nitriles 16 in 65-92% yields with up to 94% enantiomeric excess. Addition of a small amount of water and low temperature were essential to achieve both high conversion and high asymmetric induction for asymmetric catalysis. In this system, catalyst 5 could be reused approximately four times while maintaining its high activity. 

The asymmetric 1,4-addition (Michael addition) of a cyanide ion to nitroalkenes is a potentially useful route for preparing p-amino acids via transformation of 3-nitropropanonitriles. ° In 2013, North reported the first vanadium-catalysed, enantioselective Michael addition of trimethylsilyl cyanide to aliphatic p-nitroalkenes, which provided (S)-19, as shown in Scheme 9.9. The reaction was promoted by a 3 mol% catalyst loading of chiral vanadium(v) salen complex 18 ° providing (S)-19 in 74-88% conversions with up to 89% enantiomeric excess. North proposed that the nitro group acts as a bridge for the two vanadium metals in the possible transition... 

The base-catalysed hydrophosphonylation of aldehydes or imines (Pudovik reaction) [58] as a convenient method was widely used for the synthesis of 1-hydrox-yalkylphosphonates. Since the pioneering work of Shibuya [50] and Spilling [51] was reported, much attention has been devoted to developing enantioselective catalysts for the synthesis of chiral 1 -hydroxy alkylphosphonates. Chiral aluminium complexes were shown to be more effective chiral catalysts [59-62]. Based on the success of using A1 (salen) and A1 (salcyen) as asymmetric catalysts, Al-Schiff base complexes [63, 64] have been developed to catalyze the asymmetric addition reaction of dial-kylphosphonates and aldehydes. Tridentate Schiff base metal complexes, such as vanadium, chromium, and iron [65], have been successfully applied in many asymmetric synthetic reactions. We noticed that Al(III) complexes could catalyse the asymmetric Pudovik reaction and these ligands could be easily synthesized [66-70]. 

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