Conformational preferences for some 2-substituted N-methoxy-N-methylacetamides through spectroscopic and theoretical studies

The analysis of the IR carbonyl band of the 2-substituted N-methoxy-N-methylacetamides Y-CH(2)C(O)-N(OMe)Me (Y = F1, OMe 2, OPh 3, Cl 4), supported by B3LYP/6-311++G(3df, 3pd) calculations along with the NBO analysis for 1-4, indicated the existence of cis-gauche conformers i.e. (c) and (g) for 1 and 3, (c(1), c(2)) and (g(1), g(2)) for 2, and (c) and (g(1), g(2)) for 4. In the gas phase, the g conformer population prevails over the c one, for 1 and 3, the (c(1) + c(2)) population prevails over the (g(1) + g(2)) one for 2, and the (g(1) + g(2)) conformer population is more abundant than (c) one for 4. In n-hexane solution, the cis conformer is more abundant for 1-3. The occurrence of Fermi resonance in the nu(CO) region, in n-hexane, precludes the estimative of relative populations of the (c, g(1), g(2)) conformers for 4. The SCI-PCM calculations agree with the solvent effect on the nu(CO) band component relative intensities for 1-3. NBO analysis showed that the n(N) -> pi.(CO), orbital interaction is the main factor which stabilizes the gauche (g, g(1), g(2)) conformers for 1-4 into a larger extent relative to the cis (c, c(1), c(2)) ones. The n(y) -> pi(.)(Co,) sigma(C-Y) -> pi.(CO,) pi(CO) -> sigma(C-Y) and 7co orbital interactions still contribute, but into a minor extent for the stabilization of the gauche conformers relative to the cis ones. The existence of some pyramidalization at the nitrogen atom of the Weinreb amides 1-4 is responsible for the occurrence of Y(delta)-(4)center dot center dot center dot O(delta)-(9) and Y(delta)-(4)center dot center dot center dot N(delta)-(7) short contacts in the gauche (g, g(1), g(2)) conformers, which originates strong repulsive Coulombic interactions, acting in opposition to the large orbital stabilization of the gauche conformer with respect to the cis one. Therefore, a delicate balance of the Coulombic and orbital interactions seems to be responsible for the observed stabilization of the gauche (g, g(1), g(2)) and cis (c, c(1), c(2)) conformers, both in the gas phase and in the solution for 1-4. However, the cis conformer predominance, in non polar solvents, for the 2-substituted N-methoxy-N-methyl acetamides 1-3, bearing in a first raw (fluorine and oxygen) atoms, is in the opposite direction to the gauche conformer preference for the corresponding 2-substituted N,N-dialkyl-acetamides. (C) 2010 Elsevier B.V. All rights reserved.

Electronic synergism in [RuCl(2)(PPh(3))(2)(amine)] complexes differing the reactivity for ROMP of norbornene and norbornadiene

The reactivity of the new complex [RuCl(2)(PPh(3))(2)(3,5-Me(2)piperidine)], complex 1, was investigated for ring opening metathesis polymerization (ROMP) of norbornene (NBE) and norbornadiene (NBD) in the presence of ethyl diazoacetate (EDA) in CHCl(3). The aim is to observe the combination of PPh(3) and an amine as ancillary ligands concerning the steric hindrance and the electronic perturbation in the properties of the N-bound site when replacing the amines. Thus, the results with 1 were compared to the results obtained when the amine is piperidine (complex 2). Reaction with 1 provides 70% yield of isolated polyNBE (M(n) =8.3 x 10(4) g/mol; PDI = 2.03), whereas 2 provides quantitative reaction (M(n) = 1.2 x 10(5) g/mol; PDI = 1.90) with [NBE]/[Ru] = 5000, [EDA]/[Ru] = 48 and 1.1 mu mol of Ru for 5 min at 25 degrees C. The resulting polymers showed c.a. 62% of trans-polyNBE, determined by (1)H NMR, and T(g) = 32 degrees C, determined by DSC and DMTA. For ROMP of NBD, 1 showed quantitative yield with PDI =2.62 when [NBD]/[Ru] = 5000 for 20 min at 25 degrees C, whereas the reaction with 2 reached 55% with PDI = 2.16 in the same conditions. It is concluded that the presence of the two methyl groups in the piperidine ring provides an increase in the induction period to produce the Ru-carbene species justifying better polyNBE results with 2, and a greater amine(sigma)-> Ru(pi)-> monomer synergism which contributed to the best activation of less tensioned olefin as NBD. (C) 2010 Elsevier B.V. All rights reserved.