Si–CN Bond Cleavage of Silyl Cyanides by an Iron Catalyst. A New Route of Silyl Cyanide Formation

Abstract

The reaction of silyl cyanide (R 3 SiCN) with hydrosilane (R¤ 3 SiH) in the presence of a catalytic amount of [ Most chemical reactions consist of bond cleavage and formation. Finding suitable reaction conditions that allow breaking of a particular bond in a reagent and forming a new bond selectively is a challenging task. Bond cleavage is an important topic in organic chemistry as much as bond formation. 1Selective cleavage of weak bonds is relatively easy, whereas that of strong bonds is difficult and usually requires the use of a transition-metal catalyst to reduce the activation energy of bond breaking. In this context, we have reported a CCN bond cleavage reaction of organonitriles catalyzed by an iron complex.2 The reaction is noteworthy as the CCN bond is one of the strongest CC single bonds: the bond energy of CC bond in acetonitrile is 133 kcal mol ¹1 , whereas that of CC bond in alkanes is ca. 83 kcal mol ¹1 . The key steps of the catalytic cycle are shown in Scheme 1. The cyano group coordinates in η 1 -fashion to a 16e iron complex bearing a silyl group. Then, the organonitrile changes its coordination mode from η 1 to η 2 . The silyl group migrates from iron to nitrile nitrogen to form an N-silylimino complex. Finally, CR bond cleavage affords the iron complex bearing silyl isocyanide and R ligands. NCN bond cleavage of cyanamides (R 2 NCN) 3 and OCN bond cleavage of cyanates (ROCN) 3b,4 have been reported using a molybdenum catalyst, and a catalytic cycle similar to that for CCN bond cleavage has been proposed. In all cases above, the silyl group plays a key role because its migration triggers the cleavage of strong CCN, NCN, and OCN bonds. We reasoned that this type of reaction, called Silyl-Migration-Induced reaction (SiMI reaction), has general validity and can be applied to the cleavage of ECN bonds where E is other than alkyl, aryl, amino, and alkoxy groups. Based on this hypothesis, we investigated the cleavage of strong SiCN bonds in silyl cyanides (BDE SiCN = 85 [Fe] SiR' 3

Mechanism for Activation of the C–CN Bond of Nitriles by a Cationic CpRhIII–Silyl Complex: A Systematic DFT Study

“…For example, Jones and Hiyama independently reported a series of studies on the Ni(0)-promoted activation of nitriles in the presence of various phosphine or bis-phosphine ancillary ligands. , Chatani and Jones and their co-workers have also described some Rh(I)-promoted C–CN bond activation/subsequent functionalization reactions . Additionally, C–CN bond cleavage by complexes of some other transition metals, − such as Fe, Mo, Co, and Pd, has also been reported recently, some involving a photochemical activation strategy.…”

“…Additionally, the introduction of an activating group adjacent to the C–C bond to be cleaved is a practical way of realizing mild C–C activation reactions. Typical activating groups include carbonyls, imines, − ,− and cyanide. − Furthermore, transition-metal-catalyzed decarboxylative cross-coupling reactions have been an active field of C–C activation reactions in recent years. , These reactions involve a critical β-C elimination step to extrude CO 2 . The above strategies for C–C activation, together with other alternatives, are often synergistically combined to achieve a particular C–C activation reaction.…”

Silylpalladium Cations Enable the Cleavage of Nitrile C–CN Bonds

“…The ipso aryl carbon atoms of 2 – 4 are triplets from J PC coupling to the two chemically equivalent PCy 3 phosphorus atoms. Triplets in the downfield region of the spectra (162–166 ppm) are attributed to the isocyano carbon atoms of the EtMe 2 SiNC ligand. ,, Generation of silyl isocyanide is not unexpected, as the literature reports this is a common byproduct of metal–silyl facilitated nitrile decyanations. − Finally, the methyne carbon atoms of the PCy 3 ligands are 1:2:1 triplets due to virtual coupling between the two mutually trans phosphine ligands. − When they are taken together, the spectroscopic data for 2 – 4 indicate that these complexes adopt the structure trans -(EtMe 2 SiNC)­(PCy 3 ) 2 Pd–Ar + , where Ar is determined by the nitrile. Purification of 2 – 4 by silica gel column chromatography yielded the cyano complexes 2′ – 4′ , via hydrolysis of the silyl isocyanide.…”

“…Literature examples of metal-mediated C–CN oxidative additions typically invoke one of three key mechanistic schemes: (a) η 2 coordination of nitrile to the electron-rich metal center preceding oxidative addition, ,,− (b) Lewis acid coordination to free or η 2 -coordinated nitrile prior to oxidative addition, ,,− or (c) migratory insertion of an η 1 -coordinated nitrile into a metal–boron or −silicon bond (Scheme ). − …”

Leaving Group Effects on the Selectivity of the Silylation of Alcohols: The Reactivity–Selectivity Principle Revisited

“…The influence of the leaving group on the reaction rate was first explored for reagents containing the TBS protecting group such as tert -butyldimethylsilyl chloride (TBSCl, 1a ), tert -butyldimethylsilyl triflate (TBSOTf, 1b ), tert -butydimethylsilyl cyanide (TBSCN, 1c ), tert -butyldimethylsilyl imidazole (TBSImi, 1d ), and tert -butyldimethylsilyl- N -methyltrifluoroacetamide (MTBSTFA, 1e ) . These measurements were performed using the previously developed procedure for secondary alcohol 4b using 30 mol % of DMAP and Et 3 N (1.2 equiv) in CDCl 3 (Table ) …”