Isonitriles, reaction with palladium complexes

Another variation of the oxidation reaction with dioxygen can be efficiently accomplished by a catalytic condensation of amines and isonitriles using a palladium complex catalyst and iodine at 100 °C to give dialkylcarbodirmides in yields of 35-86% [1283]. Dicyclohexylcarbodirmide was obtained in 67% yield from cyclo-hexylisonitrile 1748 and cyclohexylamine 1749 (see Section 4.5.3.5, Table 4.48). 

The indazoline products can also be made directly from the palladium complexes 78 by heating them with the isonitrile in toluene at 120CC.162 They are also formed in dicobalt octacarbonyl-catalyzed reactions of azo-arenes with isocyanides but in this case an alternative reaction pathway leading to indazolo[2,l- ]indazoles (79) is observed (Scheme 96).163 Products of the latter type are formed from sterically hindered isocyanides hence it is likely that in these cases a double metallation is favored over isocyanide insertion into a monometallated species (Scheme 97). 

Isonitriles react with oligo- and disilanes or silylstannanes to yield 1,1-addition products in which both Group 14 elements add to the same carbon atom of the substrate rather than across the C-N triple bond. Ito and coworkers report that palladium complexes, such as Pd(PPh3)4 and Pd(OAc)2, effectively mediate these insertion reactions, an example of which is shown in Eq. (27).78... 

Isonitrile ligands on palladium induce the activation of Si-Si bonds very efficiently. Thus, not only a four-membered disilane 10b, five- (14) and six-mem-bered disilanes (15,16) furnish the corresponding cyclic bis(organosilyl)Pd(II) complexes 17-20 quantitatively in the reactions with bis(tert-alkyl isonitrile) palladium(O) (Scheme 1) [20]. 

Reaction of the isonitrile-palladium complex with a spiro trisilane 21 affords binuclear palladium complex 22 in high yield (Eq. 6) [21]. 

Reaction of bis(disilanyl)dithiane 32 with the corresponding palladium(O)-isonitrile complex affords a four-membered cyclic bis(silyl)palladium(II) complex 34 quantitatively together with the formation of a disilane (Eq. 15) [30]. The formal intramolecular metathesis of the two Si-Si bonds of 32 may proceed through initial formation of tetrakis(silyl)Pd(IV) complex, corresponding to the platinum complex 33. The double oxidative addition of the two Si-Si bonds may be followed by reductive elimination of the disilane with accompanying formation of four-membered bis(silyl)palladium complex 34, due to difficulty in reductive elimination leading to formation of a three-membered cyclic disilane. 

The oxidative addition of disilanes occurs to palladium complexes of isonitrile ligands and platinum complexes of trialkylphosphine ligands as part of tiie catalytic silylation of alkynes and aryl halides. The addition of stannylboranes to Pd(0) complexes has also been reported,and the addition of diboron compounds to many metal systems, such as Pt(0) complexes (Equation 6.67), is now common. These reactions all occur with metal complexes that do not undergo intermolecular reactions with alkane C-H bonds, let alone C-C bonds. Thus, the Lewis acidic character of these reagents must accelerate the coordination of substrate and cleavage of the E-E bonds. 

Multiple, successive insertion of isonitriles into the palladium-carbon o--bond has also been found with organopalladium(II)bis(phosphine) complexes. However, unlike the Ni- catalyzed polymerization, only single, double, and triple insertion reactions of isonitriles have been reported for Pd-mediated reactions. For instance, reaction of methylpalladium(II) complex 2 with cyclohexyl isonitrile in ratios of 1 1, 1 2, and 1 3 afforded selectively single (3), double (4), and triple (5) insertion products, respectively (Scheme 3). However, no further insertion of cyclohexyl isonitrile to 5 took place. No reactivity of the triple insertion complex 5 was explained by the intramolecular coordination of the nitrogen atom of the third imino group, which formed a stable five-mem-bered chelating palladium complex. 

Besides the above-mentioned hving polymerizations mediated by the organopalladium(II) halides, successive insertion of isonitriles into sulfur-sulfur bonds catalyzed by palladium complexes was recently reported (Scheme Thus, in the presence of Pd(PPh3)4, reaction of di-p-tolyldisnlflde with 4.7 equiv of p-tolyl isonitrile afforded a mixture of ArS(C=N—Ar) SAr (Ar = p-Tol, n = 1-9) in good combined yield. Mechanistic investigations revealed that the ohgomerization may involve a reversible oxidative addition of S—S or S—C bonds onto the pahadium, foUowed by insertion of isonitriles into the resultant S— Pd or C—Pd bond, and subsequent reductive ehmination(Scheme 13). 

The reaction of palladium indenyl complexes with isonitriles leads to the synthesis of palladium fulvenes (Scheme 86). The reaction seems to occur by decoordination of the indenyl fragment, and the resulting indenide attacks a coordinated isonitrile. An analogous reaction takes place when cyclopentadienyl is used. ... 

Palladium-mediated cydization based on the reactivity of o-alkynyl or alkenyl-phenyl isonitriles have been developed [105]. On the basis of their earlier studies on the three-component synthesis of allyl aryl cyanamides [106], Yamamoto and co-workers reported a palladium-catalyzed three-component coupling reaction of 2-alkynylisocyanobenzenes 122 with allyl methyl carbonate and trimethylsilylazide leading to N-cyanoindoles 125 [107]. One of the key steps of the proposed mechanism is the formation of 7i-allylpalladium carbodiimide 123 and its isomerization to rc-allylpalladium cyanamide complex 124 (Scheme 8.50). 

Palladium isocyanide complexes react with amines and alcohols to give palladium Carbene Complexes (equation 15) in a reaction that is a nucleophihc attack on the isonitrile carbon. These complexes easily lose a halide if it is trans to the carbene hgand, since the carbene carbon has a high trans influence. The structures of these compounds have the carbene carbon and its two heteroatom groups in one plane, which is perpendicular to the coordination plane of the Pd. 

Similar generation of palladium-silylene complexes may be involved in reaction of oligosilanes with aryl isonitriles giving four-membered rearranged products 95, although the mechanism has not yet been clarified (Eq.49) [81,86]. 

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