Isocyanides insertion

In contrast to the case of CO insertion that usually allows insertion of only one CO unit into a metal-carbon bond, isocyanides undergo multiple insertions sometimes leading to polyisocyanides [53,54]. Since the inserted isocyanide units may be regarded as imines derived from carbonyl groups, the insertion products can be regarded as polycarbonyl compounds where CO units are multiply inserted into the metal carbon bonds. The multiple insertion products of isocyanides have found applications both in organic synthesis and polymer synthesis [55]. 

Isocyanide is isoelectronic with CO and a reactive compound in the presence of Pd catalysts. The heterobicyclic compound 127 is obtained by the successive insertion of 2.6-xylyl isocyanide (126) into the Pd-hydride bond formed from the hydrosilane[121. Aryl isocyanide inserts into the Si—Si bond in oligo-silanes. For example, 3 mol of 2,6-xylyl isocyanide insert into the tetrasilane 128 to give 129[122],... 

BH4 [69532-06-5]. The methyl compound has exhibited insertion reactivity, including aldehydes, ketones, nitriles, and isocyanides (29). Stable metaHacycle... 

Such titanium sulfinates are reported to increase crop yields (407—409). Isocyanides insert to yield imines as follows (410,411) ... 

Isocyanide insertion and related reactions. Y. Yamamoto and H. Yamazaki, Coord. Chem. Rev., 1972,8,225-239 (47). 

A). The alkyl complexes react rapidly with CO and isocyanides, but pure products could not be isolated. Fast reactions of CO2 and acetone with Sc(OEP)Me gave the well-characterized acetate and /-butoxide products. Sc(OEP)OAc and Sc(0EP)0-r-Bu, respectively, formed by insertion into the Sc—C bond. ... 

Rh(OEP)H reacts with CNR (R = Me, n-Bu,) to give the adduct Rh(OEP)-(H)CNR (which has no parallel in CO chemistry) which then slowly transforms to the formimidoyl insertion product, Rh(OEP)C(H)=NR. The dimer Rh(OEP))2 reacts with CNAr (Ar = 2.6-Cf,H3Mc2) in aqueous benzene to give the carbamoyl product. Rh(OEP)C(0)NHAr (characterized by an X-ray crystal structure) together with the hydride, which it.self reacts further with the isocyanide. This is suggc.sted to form via a cationic carbene intermediate, formed by attack of HiO on coordinated CNAr in concert with disproportionation to Rh(III) and Rh(l). 

The subjects of structure and bonding in metal isocyanide complexes have been discussed before 90, 156) and will not be treated extensively here. A brief discussion of this subject is presented in Section II of course, special emphasis is given to the more recent information which has appeared. Several areas of current study in the field of transition metal-isocyanide complexes have become particularly important and are discussed in this review in Section III. These include the additions of protonic compounds to coordinated isocyanides, probably the subject most actively being studied at this time insertion reactions into metal-carbon bonded species nucleophilic reactions with metal isocyanide complexes and the metal-catalyzed a-addition reactions. Concurrent with these new developments, there has been a general expansion of descriptive chemistry of isocyanide-metal complexes, and further study of the physical properties of selected species. These developments are summarized in Section IV. 

The work on insertion reactions of isocyanides was initiated by Yamamoto et ah, and described in a communication in 1968 followed by a full paper a year later (164). This study described the reaction of C5H5Ni(PPh3)CH3 with several isocyanides, from which the products C5HjNi(CNR)- C(=NR)CH3 were isolated [Eq. (2)]. 

In the reaction of Ni(CNBu )4 and methyl iodide oligomerization of the isocyanide was observed the only isolable nickel complex was (I), shown below. This product is believed to arise through sequential insertions of three isocyanides into a nickel-carbon bond. Upon further treatment with additional isocyanide at a temperature greater than 60° C one obtains a polymer (RNC) presumably through multiple isocyanide insertion reactions. The addition of benzoyl chloride to Ni(CNBu )4 gave two isolable compounds Ni(CNBu )3(COPh)Cl (74%) and (II) (8.2%). This latter reaction, and the isolation of (II) in particular, suggests that the proposed mechanism for polymerization of isocyanides is reasonable. 

Reactions of Pt(PPhj)2(R)X (R = CH3, C Hj X = Cl, Br, I) and methyl isocyanide 144) and analogous reactions of Pd(phos)2(CH3)I (phos = PPh3, PPhMc2) complexes with cyclohexyl isocyanide 169, 170) were reported about the same time. As might perhaps be anticipated, the platinum reactions were slower and one can isolate the intermediate species and observe their rearrangement to the inserted products [Eq. (8)]. The isolation of the... 

Mention was made earlier about insertion reactions into nickel alkyl bonds 108, 164), and about polymerizations of oleiins by isocyanide nickel complexes 31,174). 

A large amount of the work on palladium isocyanide complexes has been mentioned earlier, in discussions on insertion reactions 30,74,108,169,170) and on addition reactions of coordinated isocyanides 25, 33, 34, 49) the reactions of [Pd(CNBu )2] with oxygen 107) and with various olefins 29, 110) were noted. 

Insertion reactions of platinum(II) alkyl and aryl complexes (144, 153, 171), nucleophilic displacement of isocyanide from [Pt(PRj)2(CNCH3)2] (147) and additions of alcohols and related substances to isocyanides bonded to platinum (8, 9, 25, 33, 34, 100, 117) were discussed earlier. 

Much of the recent interest in insertion reactions undeniably stems from the emphasis placed on development of homogeneous catalysis as a rational discipline. One or more insertion is involved in such catalytic processes as the hydroformylation (31) or the polymerization of olefins 26, 75) and isocyanides 244). In addition, many insertion reactions have been successfully employed in organic and organometallic synthesis. The research in this general area has helped systematize a large body of previously unrelated facts and opened new areas of chemistry for investigation. Heck 114) and Lappert and Prokai 161) provide a comprehensive compilation and a systematic discussion of a wide variety of insertion reactions in two relatively recent (1965 and 1967) reviews. 

The oxidative addition of (ArS) 2 to Pd(0) and coordination of 73 to the resultant Pd(II) both lower the total energy [state (C) from (A) via (B)]. Both the insertion of isocyanide into Pd-S of 78 giving state (D) and the reductive elimination of 74 from 79 affording state (E) are reversible. The equilibrium of the insertion and de-insertion of the isocyanide favors the formation of the product of the de-insertion reaction. [State (C) is lower than state (D).] Although state (C) is more stable than state (E), the short-lived Pd(0) can be trapped by (ArS)2 to give 77 [state (E) from state (E)]. 

In the absence of an excessive driving force derived, as in the case of CO, from the oxophilicity of the metal, the reaction with Bu NC allowed one to better single out almost all of the steps of the migratory insertion of the isocyanide into the Zr-C bonds, as shown in Scheme 19.47... 

A3-Pyrrolinones have also been obtained from metal-mediated cyclooligomerization processes in which concomitant hydrolytic or carbonyl insertion occurs. For example, tert-butyl isocyanide is converted in aqueous methanol by zerovalent nickel compounds e.g., Ni(t-BuNC)4, Ni(CO)4, into a di(alkylamino)-A3-pyrrolinone in moderate yield (Scheme 34). The reaction takes a different course in anhydrous methanol in which a di-tert-butylamino)ethylene derivative is formed, albeit in poor yield (Scheme 34).62... 

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). 

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