Silver complexes with isocyanides

Reaction of the Schiff base ligand A,Ar -bis(o-diphenylphosphinobenzylidene)(ethylene-diamine (49 en=P2) with AgBF4 produced a pale yellow salt. The IR spectrum of this complex showed strong bands due to the imino and BF4 group (v(C=N) 1653 cm-1, v(BFj) 1080 cm-1). The crystal structure of the Cu1 analogue was reported and the copper ion was found to adopt a severely distorted tetrahedral geometry. This strain was manifested in its reactivity since both the copper and silver complex reacted with f-butyl isocyanide. In the case of silver(I) a five-coordinate adduct was obtained, [Ag(en=P2)(Bu NC)]BF4.396... 

The ferrocenylphosphine-silver complex catalyzes the aldol-type reaction of tosylmethyl isocyanide 71 with aldehydes with higher stereoselectivity than the gold complex (Scheme 2-59) [84]. The reaction with several aldehydes produces trans-4-tosyloxazolines 72 in up to 86% ee, which can be converted into optically active l-alkyl-2-aminoethanols by reduction with LiAlH4. 

To obtain isocyanides Gautier heated silver cyanide with 0.5-1.0 equivalent of alkyl iodide for several hours on a steam-bath for low-boiling alkyl iodides a pressure vessel is used. The mixture should not become brown in this reaction. The isocyanide is precipitated as a complex with silver cyanide, which, after removal of the excess of alkyl iodide by distillation, is decomposed by addition of a concentrated solution of potassium cyanide. The isocyanide can then be isolated by distillation. 

Imidazolines are also formed in silver cyanide-catalyzed cyclization of alkyl isocyanides with aliphatic diamines (Scheme 103).169 This simple synthesis can be applied in a general way with difunctional nucleophiles and has been used to prepare benzimidazoles, oxazoles, thiazoles, and oxazines.169 It is suggested that transient carbene complexes are formed in these reactions (cf. 87 in Scheme 103) but further work is required to ascertain the mechanism and scope of these processes. 

A more conventional pathway uses the halide complexes (RNC)AuX which are reacted with 1 equiv. of the isocyanide, preferentially in the presence of a silver salt (AgS03CF3, AgBF4) for removal of the halide X as AgX (Equation (44)).222... 

The cyanide ion is an ambident nucleophile and isocyanides may be side products. If the preparation of isocyanides is desired, they can be made the main products by the use of silver or copper(I) cyanide1577 (p. 368). Vinylic bromides can be converted to vinylic cyanides with CuCN,1578 with KCN, a crown ether, and a Pd(0) complex,1579 with KCN and a Ni(0) catalyst,15 1 or with K4Ni2(CN)6.1581 Tertiary halides can be converted to the corresponding nitriles by treatment with trimethylsilyl cyanide in the presence of catalytic amounts of SnCl4 RjCCl + Me3SiCN — R3CCN.1582... 

The asymmetric aldol-type addition of tosylmethyl isocyanide (99) to aldehydes can also be catalyzed by the chiral silver(I) complex, giving almost exclusively trans-S-alkyl-4-tosyl-2-oxazolines 98 [E = S02(p-Tol)] with up to 86 % ee as shown in Sch. 26 [52]. The slow addition method described above is not necessary for this reaction system. 

The alkylation reaction involving silver cyanide is utilized for the preparation of alkyl isocyanides, because the initially formed isocyanide complex decomposes with RNC evolution while heated with KCN ... 

In analogy t 0 the Cu(II) complex systems, the silver(I) -catalyzed aldol reaction is also proposed to proceed smoothly through a Lewis acidic activation of carbonyl compounds. Since Ito and co-workers reported the first example of the asymmetric aldol reaction of tosylmethyl isocyanide and aldehydes in the presence of a chiral silver(I)-phosphine complex (99,100), the catalyst systems of sil-ver(I) and chiral phosphines have been applied successfully in the aldol reaction of tin enolates and aldehydes (101), Mukaiyama aldol reaction (102), and aldol reaction of alkenyl trichloroacetates and aldehydes (103). In the Ag(I)-disphosphine complex catalyzed aldol reaction, Momiyama and Yamamoto have also examined an aldol-type reaction of tin enolates and nitrosobenzene with different silver-phosphine complexes (Scheme 15). The catalytic activity and enantioselectivity of AgOTfi(f )-BINAP (2 1) complex that a metal center coordinated to one phosphine and triflate were relay on solvent effect dramatically (Scheme) (104). One catalyst system solves two problems for the synthesis of different O- and AT-nitroso aldol adducts under controlled conditions. 

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