Diastereoselective intermolecular rhodium-catalyzed [4 + 2 + 2] carbocyclization reactions: computational and experimental evidence for the intermediacy of an alternative metallacycle intermediate.

Intermolecular rhodium-catalyzed [m + n + o] reactions of 1,6-enynes and various pi-components (carbon monoxide, alkynes, 1,3-butadienes, etc.) provide an expeditious approach for the construction of polycyclic fragments that represent important synthons for target-directed synthesis. We present computational and experimental evidence for the existence of a previously undescribed reaction pathway for the rhodium-catalyzed [4 + 2 + 2] reaction involving a 1,6-enyne. This model clearly demonstrates the origin of the excellent diastereoselectivity in this type of reaction and the remarkable tolerance of both (E)- and (Z)-isomers within the 1,6-enyne, which is generally prone to competitive ene-cycloisomerization.

Proton as the simplest of all catalysts for [2 + 2] cycloadditions: DFT study of acid-catalyzed imine metathesis.

The mechanism of imine metathesis was studied as a prototype reaction for the impact that heteroatom substitution has on thermally forbidden [2 + 2] addition reactions using high-level density functional theory in combination with a continuum solvation model. The intuitively expected high activation barriers were confirmed for N-alkyl- and N-aryl-substituted imine reactants with transition state free energies of 78.8 and 68.5 kcal/mol, respectively, in benzene. The computed reaction energy profiles were analyzed to discover possible strategies for lowering the transition state energy. Protonation of the imine nitrogen was proposed as a possible catalytic route and was explicitly modeled. The computed reaction energy profile shows that protonation of one of the imine reactants has an enormous effect on the overall rate of metathesis and lowers the activation barrier by as much as 37.3 and 30.6 kcal/mol for the N-alkyl and N-aryl reactants, respectively. These results suggest that acid-catalyzed imine metathesis should be amenable at elevated temperatures. Furthermore, the protonation of both reactants of the metathesis reaction is predicted to be not productive owing to electrostatic repulsion of the reactants, thus suggesting that there should be an optimum pH for the catalytic turnover. A detailed analysis of the catalytic mechanism is presented, and the primary driving force for the catalysis is identified. Upon protonation of the imine nitrogen, the key [2 + 2]-addition step becomes asynchronous and one of the two intermolecular N-C bonds is formed before traversing the transition state, resulting in a substantial net decrease of the overall energy requirement. The general applicability of this intuitively understandable mechanism for designing structural features for lowering the energy of transition state structures is explored.

Iminophosphorane mediated imine metathesis.

The iminophosphorane Cl(3)P[double bond]NAr (1a, Ar = 2-fluorophenyl) reacts metathetically with imines at 80 degrees C to produce [double bond]NR exchange products. Compound 1a effectively catalyzes imine/imine and imine/carbodiimide cross-metathesis. The observation of [double bond]NR exchange products as well as spectroscopic evidence for the existence of diazaphosphetidine type intermediates suggests that a [2 + 2] addition/elimination mechanism is the primary pathway for substrates with N-alkyl substituents and a secondary pathway for N-aryl imines. In contrast to previously studied carbodiimide systems, the resting state of the catalyst is the iminophosphorane and not the diazaphosphetidine. For N-aryl imines, Lewis-acid catalysis appears to be the dominant mechanism, not addition/elimination. For N-alkyl imines, a decomposition pathway, involving HCl elimination from a phosphorus intermediate, is competitive in some cases.

Dichloro(isopropylamido)bis(isopropylamine)(isopropylimido)tantalum(V), a monomeric TaV compound with imido, amido and amino moieties.

The title compound, [Ta(C(3)H(7)N)(C(3)H(8)N)Cl(2)(C(3)H(9)N)(2)], is the first monomeric example of a metal complex that features imido, amido and amino moieties in the same molecule. The Ta atom has distorted octahedral coordination, with the imido moiety trans to chlorine and the pseudo-axial ligands bent away from the imido moiety. Principal dimensions include Ta=N = 1.763 (8) A, Ta-N(H) = 1.964 (7) A, and Ta-N(H(2)) = 2.247 (7) and 2.262 (7) A.