Density Functional Theory Study of N–CN and O–CN Bond Cleavage by an Iron Silyl Complex

Abstract

Using hybrid density functional theory calculations with the B3LYP functional, the reaction mechanisms for cleavage of R 2 N−CN (R  H, Me) and MeO−CN bonds in the presence of an unsaturated iron(II) silyl complex, CpFe(CO)SiMe 3 , were studied. The following sequence of reactions was shown to be favorable: (i) coordination of a nitrile through the lone pair of electrons on the nitrile N atom (N CN ) to form an end-on complex, (ii) isomerization of the endon complex to a side-on complex, (iii) migration of the silyl group to N CN facilitated by the hypervalent character of the Si atom and its electrostatic attraction with N CN to form a stable Fe−C−N CN three-membered-ring intermediate with an Fe−N CN dative bond, (iv) dissociation of the N CN atom from Fe and coordination of an amino N atom (N NR 2 ) or methoxy O atom to Fe leading to an Fe−C−N NR 2 or Fe−C−O three-membered-ring intermediate, and (v) cleavage of the R 2 N−C or MeO−C bond to form a silyl isocyanide ligand.Step iv possesses the largest activation energy in the sequence of reactions. The activation energies for the reactions of H 2 NCN, Me 2 NCN, and MeOCN were calculated to be 29.9, 28.0, and 19.1 kcal/mol, respectively, on the basis of potential energies with zero-point energy correction. This accounts for the experimental observation that the intermediates formed by silyl group migration can be isolated. The effects of the amino and methoxy groups are discussed by comparing their reaction profiles with that for the reaction of acetonitrile. Localized orbital analysis showed that in the three-membered-ring intermediates formed in step iv, the R 2 N−C and MeO−C bonds are activated by ring strain, whereas the Me−CN bond is activated by interaction of the Me−C bond with the vacant coordination site that is produced in the dissociation of N CN .

Transformations of X (C, O, N)–CN bonds: cases of selective X (C, O, N)–C activation

“…[5] Most recently, Ni(0) and Pd(0) mediated C-CN bonds cleavage were reported. Related O-CN bond activations [6] have been realized in only a few cases while N-CN bond activation [7] is even rarer. Considering the paramount role of the elements such as nitrogen, oxygen and carbon in organic chemistry, moreover, the broad existence of cyano groups in natural ( e.g ., Cyanocycline A, Dnacin A1, Halimedin, Lahadinines A, et al) and/ or medical molecules ( e.g .,PPARgamma-active triterpenoid, 2-cyano-3,12-dioxooleana-1,9-dien-28-oic acid), along with the versatile application of cyano group towards spectacular precursor to vast functionalized groups, viz ., (i) amine; (ii) amide; (iii) acid; (iv) aldehyde; (v) ester; (vi) ketone; (vii) thioamide; (viii) hydroxyimidamide et al, reactions involved with such transformation can be of high importance.…”

“…Although other element-cyano bond cleavages, such as silyl-cyano, [12] germyl-cyano, [13] stannyl-cyano, [14] boryl-cyano, [15] sulfur-cyano, [16] and bromo-cyano [ 17 ] have been achieved, a useful synthetic transformation of oxy-cyano has not been reported yet. [6] In 2012, Nakao and co-workers disclosed palladium and triphenylborane catalyzed the intramolecular. oxycyanation of alkenes via the cleavage of O-CN bonds and the subsequent insertion of double bonds.…”

Transition Metals Catalyzed Element-Cyano Bonds Activations

“…Other element-cyano bond cleavages, such as silyl-cyano (14–17), germyl-cyano (47,48), stannyl-cyano (49–52), boryl-cyano (39–46), sulfur-cyano (53–56), and halo-cyano (57,58) have also been achieved, a useful synthetic transformation of oxy-cyano has not been reported until recently (59). …”

“…In 2012, Nakao et al reported a reaction of the palladium and triphenylborane catalyzed the intramolecular oxy-cyanation of alkenes via the cleavage of O-CN bonds and the subsequent insertion of double bonds (Scheme 47) (59a). The reaction illustrated the cyano-fuctionalization of a double bond through cleavage of an oxygen-cyano bond, which is rare.…”

“… Notes: a C sp2 in arene group; b estimated values between C-O and C=O bonds (59a, 61b); c essential information (63) …”