Catalytic Cycle for N–CN Bond Cleavage by Molybdenum Silyl Catalyst: A DFT Study

Abstract

Using hybrid density functional theory calculations with the B3LYP functional, the reaction mechanisms for cleavage of R2N–CN (R = H, Me) bonds in the presence of unsaturated molybdenum(II) silyl catalyst, Cp(CO)2MoSiMe3 (Cp = η5-C5H5), were studied. The catalytic cycle takes place in two stages; the first involves cleavage of the R2N–CN bond. The favorable sequence of reactions for this stage is as follows: (i) coordination of a nitrile through the lone pair of electrons on the nitrile nitrogen atom (NCN) to give an end-on complex; (ii) isomerization of the end-on complex to a side-on complex; (iii) migration of the silyl group to NCN to form a stable Mo–C–NCN three-membered-ring intermediate with an Mo–NCN dative bond; (iv) dissociation of NCN from Mo and coordination of an amino N atom (NNR2) to Mo, leading to an Mo–C–NNR2 three-membered-ring intermediate; and (v) cleavage of the R2N–C bond to form a silylisocyanide complex. The second stage involves the regeneration of the active catalyst through two σ-metathesis steps. In the first, Cp(CO)2MoNR2 reacts with HSiMe3 to give Cp(CO)2MoH and R2NSiMe3, and in the second, σ-metathesis of Cp(CO)2MoH with HSiMe3 regenerates Cp(CO)2MoSiMe3. Step (iv) in the first stage possesses the largest activation energy and is the rate-determining step. The activation energies for this step for the reactions of H2NCN and Me2NCN were calculated to be 36.4 and 38.3 kcal/mol, respectively, based on potential energies with zero-point energy correction. After dissociation of the silylisocyanide ligand from the silylisocyanide complex, it will be isomerized to silylcyanide, as in previous studies. The catalytic cycle for the cleavage of R2N–CN bond is compared with that of MeO–CN bond. The effects of the metal atoms are also discussed.

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.…”

Imine hydrosilylation using an iron complex catalyst: A computational study

“…η 1 ‐Complex (end‐on) 2a rearranges to η 2 ‐complex (side‐on) 2b through TS 2a‐2b , as shown in Figure . The NC bond distance in 2b (1.347 Å) is longer by 0.064 Å than that in 2a (1.283 Å) as a result of back‐donation from the filled Fe d‐orbital to the N=C π * vacant orbital [33a] . This step is endothermic by 3.7 kcal/mol because the strong σ‐coordination (end‐on) rearranges to a weak π‐coordination (side‐on) and requires an activation energy of 12.9 kcal/mol.…”

“…In the unsaturated 16 electron active Fe(II) complex CpFe(CO)SiMe 3 , 1 , there is agostic interaction between the Fe atom and one of the CH bonds in the SiMe 3 group [33a,b] . Coordination of the trans ‐imine to the Fe atom in 1 gives 2a , in which the imine ligand adopts the η 1 ‐coordination mode (end‐on) via the lone pair electrons on the N atom (FeN bond length of 1.986 Å (Fig.…”

“…This short distance favors migration of the SiMe 3 group from the Fe atom to the N atom. The high exothermicity of 14.1 kcal/mol is also beneficial, so that an activation energy for this step overall is small (2.9 kcal/mol) [32a,c,33] . The SiN bond formed in 3 is much stronger than the FeSi bond broken in 2b ( D 0 (FeSi) = 49.2 kcal/mol in 2b and D 0 (SiN) = 92.2 kcal/mol in PhCH 2 N(Me)(SiMe 3 )), thus making the reaction exothermic.…”

“…Nakazawa et al found that during the desulfurization of thioamide in the presence of CpFe(CO) 2 Me and triethylsilane, assisted by the silyl group migration induced reaction, imine RNCHPh was formed and a portion of this imine was catalytically transformed to the silylated amine R(Et 3 Si)NCH 2 Ph (R = Me, Ph) [32f,g] according to the following equation:…”