CHAIN structure, Reaction coordinate

Regarding bis-NHC chelating ligands, several structures that differ in the motifs used for the enlargement of the tether have been proposed as catalysts for the Mizoroki-Heck reaction. They range from non-functionalised aliphatic chains [23-25] to phenyl [26], biphenyl [27], binaphthyls [28] and to chains containing additional coordination positions like ethers [29], amines [30], and pyridines in an evolution towards pincer complexes [31-35], In most cases, the activity of aryl bromides in Mizoroki-Heck transformations was demonstrated to be from moderate to high, while the activation of chlorides was non-existent or poor (Scheme 6.7). 

ABF was probed through the reversible unfolding of a short peptide, deca-L-alanine, in vacuo [52] (see Fig. 4.10). The reaction coordinate, , is the distance separating the first and the last Ca carbon atom of the peptide chain. was varied between 12 and 32 A, thereby allowing the peptide to sample the full range of conformations between the native a-helical structure and the extended structures. The force acting along was accrued in bins 0.1 A wide. 

Recent advances in the development of well-defined homogeneous metallocene-type catalysts have facilitated mechanistic studies of the processes involved in initiation, propagation, and chain transfer reactions occurring in olefins coordi-native polyaddition. As a result, end-functional polyolefin chains have been made available [103].For instance, Waymouth et al.have reported about the formation of hydroxy-terminated poly(methylene-l,3-cyclopentane) (PMCP-OH) via selective chain transfer to the aluminum atoms of methylaluminoxane (MAO) in the cyclopolymerization of 1,5-hexadiene catalyzed by di(pentameth-ylcyclopentadienyl) zirconium dichloride (Scheme 37). Subsequent equimolar reaction of the hydroxyl extremity with AlEt3 afforded an aluminum alkoxide macroinitiator for the coordinative ROP of sCL and consecutively a novel po-ly(MCP-b-CL) block copolymer [104]. The diblock structure of the copolymer... 

The mechanism probably involves a transition structure TS. Rotation of the allylic chain in TS and N-2 extrusion leads to a biradicalar complex BC1 with the terminal carbon of the allylic chain (C-l) very distant from N-l. Subsequent reaction coordinate calculations consider that C-l approaches N-l to give a biradicalar complex, BC2, thermodynamically more stable by 19.1 kcalmol-1. The formation of pyrimidinone 290 from BC2 ensues through 1,2-migration of hydrogen and exothermic formation of a new G(l)-N(l) single bond (Scheme 35) <2005TL6757>. 

The phase (or rather reaction ) boundaries of the dimer and chain polymer phases have not yet been determined, and only the reaction coordinates for the two experiments reported are shown in Fig. 18. Also, for C70 the drawing of a reaction map is complicated by the topochemical requirements for polymerization described above. Dimers can be formed in both fee and hep crystals, but ordered chain structures can only form in hep crystals, and different initial structures thus probably also lead to different final structures. Although it has been reported that initially hep C70 reverts to fee after high-pressure treatment (see above), it is not known which of these two structural phases is more stable under pressure and whether a change in the stacking sequence can be induced directly by pressure and/or temperature. 

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