Enantioselective Synthesis ofO-Methoxycarbonyl Cyanohydrins: Chiral Building Blocks Generated by Bifunctional Catalysis with BINOLAM-AlCl

Asymmetric Carboxycyanation of Aldehydes by Cooperative AlF/Onium Salt Catalysts: from Cyanoformate to KCN as Cyanide Source

Asymmetric 1,2-additions of cyanide yield enantioenriched cyanohydrins as versatile chiral building blocks. Next to HCN, volatile organic cyanide sources are usually used. Among them, cyanoformates are more attractive on technical scale than TMSCN for cost reasons, but catalytic productivity is usually lower. Here, the development of a new strategy for cyanations is described, in which this activity disadvantage is overcome. A Lewis acidic Al center cooperates with an aprotic onium moiety within a remarkably robust bifunctional Al-F-salen complex. This allowed for unprecedented turnover numbers of up to 10⁴ . DFT studies suggest an unexpected unique trimolecular pathway in which the ammonium bound cyanide attacks the aldehyde, which itself is activated by the carbonyl group of the cyanoformate binding to the Al center. In addition, a novel practical carboxycyanation method was developed that makes use of KCN as the sole cyanide source. The use of a pyrocarbonate as carboxylating reagent provided the best results.

TBD- or PS-TBD-Catalyzed One-Pot Synthesis of Cyanohydrin Carbonates and Cyanohydrin Acetates from Carbonyl Compounds

Cyanation reactions of carbonyl compounds with methyl cyanoformate or acetyl cyanide catalyzed by 5 mol % of 1,5,7-triazabicyclo[4,4,0]dec-5-ene (TBD) were examined. Using methyl cyanoformate, the corresponding cyanohydrin carbonates were readily obtained in high yield for aromatic and aliphatic aldehydes and ketones. Similar results were obtained when acetyl cyanide was used as the cyanide source. The polymer-supported catalyst, PS-TBD, also acted as a good catalyst for this reaction. PS-TBD was easily recovered and reused with minimal activity loss.

Nonlinear effects in asymmetric catalysis

There is a need for the preparation of enantiomerically pure compounds for various applications. An efficient approach to achieve this goal is asymmetric catalysis. The chiral catalyst is usually prepared from a chiral auxiliary, which itself is derived from a natural product or by resolution of a racemic precursor. The use of non-enantiopure chiral auxiliaries in asymmetric catalysis seems unattractive to preparative chemists, since the anticipated enantiomeric excess (ee) of the reaction product should be proportional to the ee value of the chiral auxiliary (linearity). In fact, some deviation from linearity may arise. Such nonlinear effects can be rich in mechanistic information and can be synthetically useful (asymmetric amplification). This Review documents the advances made during the last decade in the use of nonlinear effects in the area of organometallic and organic catalysis.

Synthesis of α-keto esters by the rhodium-catalysed reaction of cyanoformate with arylboronic acids

An arylrhodium(I) species selectively reacts with the cyano group of ethyl cyanoformate to afford the corresponding alpha-keto ester in good yield.

Enantioenriched cyanohydrin O-phosphates: Synthesis and applications as chiral building blocks

Aluminum complexes of the chiral (R)- or (S)-3,3'-bis(diethylaminomethyl)-1,1'-bi-2,2'-naphthol (BINOLAM) ligand behave as efficient catalysts for the enantioselective cyanation-O-functionalization of aldehydes, thereby leading to enantiomerically enriched O-silyl, O-methoxycarbonyl, or O-phosphate derivatives of cyanohydrins. The enantioenriched cyanohydrin-O-phosphates are useful for the synthesis of several enantioenriched compounds such as α-hydroxy esters, β-amino alcohols, and γ-substituted α,β-unsaturated nitriles. Natural products such as (-)-aegeline and (-)-tembamide have been prepared in this manner.