Lanthanide Dithiocarbamate Complexes : Efficient Catalysts for the Cyanosilylation of Aldehydes

The addition of trimethylsilyl cyanide (TMSCN) to carbonyl compounds is one of the most effective preparation methods of cyanohydrins. Upon reaction, cyanohydrins or cyano trimethylsilyl ethers can be transformed into the corresponding α-hydroxyacids, α-hydroxyketones, αamino acids, and β-amino alcohols, which are not only important building blocks in the synthesis of natural products but also in the field of biology and pharmaceuticals. Many different catalysts have been developed for this reaction. A range of chemical catalyst classes are available for such reaction, including chiral and achiral Lewis acids and bases and transition metal complexes. In recent years, several lanthanide compounds such as triflates and organometallic derivatives have been used as catalysts for the addition of TMSCN to carbonyl compounds. Additionally, optically active catalysts based on lanthanides for the asymmetric synthesis of cyanohydrins have been reported in the literature. We describe herein a high yield and at room temperature methodology for cyanation of aldehydes by the use of lanthanide dithiocarbamate complexes 1, an inexpensive catalyst (Figure 1). The use of complexes (1) has two advantages if compared to other systems based lanthanide compounds: (i) chemical stability and easy manipulation in anhydrous form; (ii) high solubility in organic solvents. Results and Discussion


Introduction
The addition of trimethylsilyl cyanide (TMSCN) to carbonyl compounds is one of the most effective preparation methods of cyanohydrins. 1 Upon reaction, cyanohydrins or cyano trimethylsilyl ethers can be transformed into the corresponding α-hydroxyacids, 2 α-hydroxyketones, 3 αamino acids, 4 and β-amino alcohols, 5 which are not only important building blocks in the synthesis of natural products but also in the field of biology and pharmaceuticals.
Many different catalysts have been developed for this reaction. 6A range of chemical catalyst classes are available for such reaction, including chiral and achiral Lewis acids 7 and bases 8 and transition metal complexes. 9In recent years, several lanthanide compounds such as triflates 10 and organometallic derivatives 11 have been used as catalysts for the addition of TMSCN to carbonyl compounds.Additionally, optically active catalysts based on lanthanides for the asymmetric synthesis of cyanohydrins have been reported in the literature. 12e describe herein a high yield and at room temperature methodology for cyanation of aldehydes by the use of lanthanide dithiocarbamate complexes 1, an inexpensive catalyst (Figure 1).
The use of complexes (1) 13 has two advantages if compared to other systems based lanthanide compounds: (i) chemical stability and easy manipulation in anhydrous form; (ii) high solubility in organic solvents.

Results and Discussion
Since our initial studies focused on the development of an optimum set of reaction conditions, the addition of TMSCN to benzaldehyde was examined in order to optimize the reaction conditions.In this way, benzaldehyde (1.25 mmol) and TMSCN (2.5 mmol) were treated at room temperature with different lanthanide catalysts (Table 1).
As it can be seen on Table 1, the use of 1 mol% of lanthanide catalyst (1a-g) for the addition of TMSCN to benzaldehyde in THF, gave the corresponding trimethyl silyl ethers cyanohydrin (2a) at room temperature with excellent yields.Ytterbium catalyst (1c) proved to be the most effective (Table 1, entry 7).In the absence of catalyst, the reaction proceeded in longer reaction times (6 days at room temperature) to give product in 45% yield (Table 1, entry 13).Lengthy reaction time was observed when 1c was replaced by Yb(OTf) 3 (Table 1, entry 12).It was also observed that the replacements of ethyl group in lanthanide dithiocarbamate complexes (1a and 1c, Table 1, entries 3 and 7) by the phenyl group (1b and 1g, Table 1, entries 2 and 11) have small influence in the reaction yield.
The influence and the nature of the solvent are noteworthy.When the reaction was performed using ethanol as solvent no product was observed (Table 1, entry 6).The use of acetonitrile or toluene gave the desired product in lower yields (Table 1, entries 4 and 5).On the other hand, the use of dichloromethane or tetrahydrofuran greatly enhanced the product yield.
Thus, the optimal conditions for the cyanation reaction were found to be the use of Ytterbium catalyst 1c (1 mol%) and THF as solvent at room temperature for 6 hours.Moreover, the methodology was extended to other aldehydes.The results are summarized on Table 2.
The electronic effects of the aldehyde were briefly studied.All substituted aldehydes proved to have similar reactivities giving the desired products in high yields.Similar reactivity was observed for both aldehydes containing activating or deactivating groups (Table 2, Compounds 2b-2g).Acid sensitive aldehydes such as furfuraldehyde and cinnamaldehyde gave products without decomposition (Table 2, compounds 2j and 2k).When npropanal and n-heptanal were used as aldehyde source, the desired cyanohydrins were obtained in good yield and short reaction time (Table 2, compounds 2h and 2i).

General methods
5][16] The progress of reaction was monitored by thin layer chromatography on silica gel.All yields refer to isolated products. 1H NMR and 13 C NMR spectra were recorded on a Varian Unity Plus (300 MHz) spectrometer in CDCl 3 with TMS as an internal standard.Mass spectra were obtained on a Shimadzu GC-MS QP5050A instrument operating at 70 EV.

General procedure for lanthanide complexes (1) promoted addition TMSCN to aldehydes
To a solution of ytterbium diethyldithiocarbamate complex, 1c (10 mg, 0.0125 mmol) in THF (3 mL) under argon at room temperature was added aldehyde (1.25 mmol) followed by TMSCN (2.5 mmol).The resulting solution was stirred at room temperature for 6 hours before quenching with H 2 O (3 mL).The mixture was diluted with dichloromethane (5 mL) and the organic phase was separated, dried over anhydrous magnesium sulfate and concentrated in vacuum.The crude product was purified by flash chromatography eluting with n-hexane / ethyl acetate (8:2) to give the cianohydrin silyl ether.

Conclusions
In conclusion, we have demonstrated the effectiveness of the use of new lanthanide complexes as Lewis acids in the addition of TMSCN to various aldehydes.The present methodology provides the first example that utilizes the lanthanide dithiocarbamate complexes acting as Lewis acid catalysts in the cyanosilylation of aldehydes to give the corresponding cyanohydrins in good yields and short reaction times if compared with other methodologies already described for lanthanide compounds.

Table 1 .
Addition of TMSCN to benzaldehyde catalyzed by lanthanide complexes 1 in different reaction conditions a The consumption of PhCHO was observed by TLC.b Yields refer to the isolated products.c 1 mol% was used.