Theoretical Studies of Rhodium-Catalyzed Borylation of Nitriles through Cleavage of Carbon–Cyano Bonds

Abstract

Rhodium(I)-catalyzed borylation of nitriles is investigated theoretically, using the density functional theory method, to clarify the reaction mechanism, including the formation process of the catalytically active species, carboncarbon bond cleavage, and the effect of an amine additive. The initial step in this reaction is the formation of a borylrhodium(I) species, in which the rhodium center carries a significant negative charge. The most energetically favorable pathway for carbon carbon bond cleavage involves the insertion of a cyano group into borylrhodium(I) to form an iminoacyl intermediate, followed by extrusion of boryl isocyanide (iminoacyl mechanism). The calculation suggests that DABCO can react with coproduced reactive boron species, such as boryl chloride and boryl isocyanide, to form stable adducts, lowering the energy of the entire reaction system. In addition, the chemoselectivities among CCN, CBr, and CCl bonds observed in experimental studies are in good agreement with the calculated activation energies required for these bond activation processes.The selective cleavage of carboncarbon σ-bonds and their transformations are among the most fundamental challenges in organic chemistry. Several transition-metal complexes have been reported to mediate such processes.1 However, it is generally difficult, even with transition-metal complexes, to activate simple carboncarbon σ-bonds because their nonpolar character and steric bulk around the sp 3 -hybridized carbon center make interactions with metal complexes difficult. Suitable substrates are therefore largely limited to those with ring strain or directing groups. The activation of carboncyano (CCN) bonds by transition metals has long been known to occur in the absence of ring strain, despite their high bond dissociation energies (ca. 130 kcal mol ¹1 for PhCN). 2 The relative easiness of these processes is attributed to the smaller steric congestion of an sp-hybridized cyano carbon and to the metal-coordinating ability of the neighboring carbonnitrogen triple bond. Metal-mediated CCN bond activation has been successfully used in catalytic transformations of nitriles.

Copper-Catalyzed Regioselective Monodefluoroborylation of Polyfluoroalkenes en Route to Diverse Fluoroalkenes

“…A variety of substituents on the aromatic ring were tolerated; these include further transformable groups such as (pseudo)­halogeno ( 1d , 1e , and 1j ), cyano ( 1i ), and methylthio ( 1y ) groups. Notably, substrates having an unprotected phenolic hydroxy group, such as 1k , were also applicable to afford the desired products in high yields.…”

Effect of Donors on the Activation Mechanism in Ziegler–Natta Catalysis: A Computational Study

“…There are several other groups who have reported only the thermodynamicsf or reactions that show this type of conversion. [43,49,50] Thisr eaction is endergonic by 8.5 and 9.5 kcal mol À1 (the differencei ne nergy between the 3b and 3a structures in Figure 3) for the wd(TiCl 4 )a nd the eb donor cases, respectively. In the next step, transalkylation,t he interaction between as econd AlEt 3 moiety and the intermediates, 1b and 3b,i sf avorable by 4.3 and 4.9 kcal mol À1 ,r espectively,a nd leads to the formationo ft he subsequenti ntermediates, 1c and 3c,f or the wd(TiCl 4 )a nd the eb donor cases respectively.A fter this, 1c and 3c can further rearrange to form new intermediates, 1d and 3d,a fter the crossing of the four-membered transition states, TS 1c,1d and TS 3c,3d .This step is the rate limiting step for the activationp rocess, having barrierso f2 2.4 and 20.1 kcal mol À1 respectively for the two cases.…”

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 ). − …”