Concerted coupling

A new process developed by Institut Francais du Petrole produces butene-1 (1-butene) by dimerizing ethylene.A homogeneous catalyst system based on a titanium complex is used. The reaction is a concerted coupling of two molecules on a titanium atom, affording a titanium (IV) cyclic compound, which then decomposes to butene-1 by an intramolecular (3-hydrogen transfer reaction. ... 

A soluble titanium-based modified Ziegler-Natta catalyst [Ti(OR)4-Et3Al, R = n-Bu, isoPr] is employed in the reaction.42 Since similar catalysts may be used for the oligomerization and polymerization of ethylene, the nature and oxidation state of the metal and reaction conditions determine selectivity. Ti4+ was found to be responsible for high dimerization selectivity, whereas polymerization was shown to be catalyzed by Ti3+. According to a proposed mechanism,42,43 this catalyst effects the concerted coupling of two molecules of ethylene to form a metal-lacyclopentane intermediate that decomposes via an intramolecular p-hydrogen transfer ... 

Strong evidence in support of a concerted, coupled mechanism for this reaction, even though the transition state is strongly dissociative, is provided by the interaction coefficient p (equation 4). An increase in the dependence... 

Three types of mechanisms are reported for the dimerization of olefins degenerated polymerization, concerted coupling, and reductive dimerization [1]. 

Figure 13.12 A concerted mechanism for GTP hydrolysis by Ga in which transfer of a proton to the y phosphate is coupled to deprotonation of the attacking water by Gin 200. (Adapted from J. Sondek et al., Nature 372 276-279, 1994.)...

A concerted [2 + 2] cycloaddition pathway in which an oxametallocycle intermediate is generated upon reaction of the substrate olefin with the Mn(V)oxo salen complex 8 has also been proposed (Scheme 1.4.5). Indeed, early computational calculations coupled with initial results from radical clock experiments supported the notion.More recently, however, experimental and computational evidence dismissing the oxametallocycle as a viable intermediate have emerged. In addition, epoxidation of highly substituted olefins in the presence of an axial ligand would require a seven-coordinate Mn(salen) intermediate, which, in turn, would incur severe steric interactions. " The presence of an oxametallocycle intermediate would also require an extra bond breaking and bond making step to rationalize the observation of trans-epoxides from dy-olefms (Scheme 1.4.5). 

A sequence stretch 300 base pairs upstream of the transcriptional start site suffices for most of the transcriptional regulation of the IL-6 gene (Fig. 1). Within this sequence stretch several transcription factors find their specific recognition sites. In 5 to 3 direction, AP-1, CREB, C/EBP 3/NF-IL6, SP-1 and NF-kB can bind to the promoter followed by TATA and its TATA binding protein TBP. Most enhancer factors become active in response to one or several different stimuli and the active factors can trigger transcription individually or in concert. For example, AP-1 is active upon cellular stress, or upon stimuli that tell cells to proliferate CREB becomes also active if cells experience growth signals, but also upon elevation of intracellular levels of cyclic adenosine monophosphate (cAMP), which occurs upon stimulation if so called hormone-activated G protein-coupled receptors. 

Most GPCRs interact with and activate more than one G-protein subfamily, e.g., with Gs plus Gq/n (histamine H2, parathyroid hormone and calcitonin recqrtors), Gs plus G (luteinising hormone receptor, 32-adrenoceptor) or Gq/11 plus G12/13 (thromboxane A2, angiotensin ATb endothelin ETA receptors). Some receptors show even broader G-protein coupling, e.g., to Gi, Gq/n plus Gi n ( protease-activated receptors, lysophosphatidate and sphingosine-1-phosphate receptors) or even to all four G-protein subfamilies (thyrotropin receptor). This multiple coupling results in multiple signaling via different pathways and in a concerted reaction of the cell to the stimulus. 

Kishimoto et al. (1974, 1981) found a general acid catalysis by protonated pyridines in coupling reactions of the 1-naphthoxide ion if weakly electrophilic diazonium ions were used. In this case it is likely that the general acid protonates the carbonyl oxygen of the o-complex, with a concerted or stepwise deprotonation at the 4-position (transition stage 12.150). 

From these data, some key information can be drawn in both cases, the couple methane/pentane as well as the couple ethane/butane have similar selectivities. This implies that each couple of products (ethane/butane and methane/pentane) is probably formed via a common intermediate, which is probably related to the hexyl surface intermediate D, which is formed as follows cyclohexane reacts first with the surface via C - H activation to produce a cyclohexyl intermediate A, which then undergoes a second C - H bond activation at the /-position to give the key 1,3-dimetallacyclopentane intermediate B. Concerted electron transfer (a 2+2 retrocychzation) leads to a non-cychc -alkenylidene metal surface complex, C, which under H2 can evolve towards a surface hexyl intermediate D. Then, the surface hexyl species D can lead to all the observed products via the following elementary steps (1) hydrogenolysis into hexane (2) /1-hydride elimination to form 1-hexene, followed by re-insertion to form various hexyl complexes (E and F) or (3) a second carbon-carbon bond cleavage, through a y-C - H bond activation to the metallacyclic intermediate G or H (Scheme 40). Under H2, intermediate G can lead either to pentane/methane or ethane/butane mixtures, while intermediate H would form ethane/butane or propane. 

For acyl transfer, phosphoryl transfer, and sulfonyl transfer, the primary kind of evidence in favor of concerted mechanisms for some reactions is a linear Bronsted plot of log k versus for a range of nucleophiles, spanning p/ir/ = - p r s = 0, coupled... 

Reduction is probably stepwise when 0 = 90°. (The plot in fact resembles very closely in form the familiar Karplus plot of the magnitude of vicinal proton-proton coupling constants as a function of 0 48>. The half-wave potentials of 43 and 44 are representative the half-wave potential of a related monohalide (45) is given for reference 46>. These investigators did not examine the products of electrochemical reduction of the vicinal dihalides which they studied. If reduction is concerted, the products should... 

A new oxidation-dehydrogenation-Heck coupling catalyzed by Pd on porous glass or Pd(OAc)2 also proceeded under microwave heating in the MBR (Scheme 2.18) [36]. With an excess of Phi, saturated alcohols including 1-propanol and 3-phenylpro-panol afforded 3,3-diphenylpropenal as the major product, and trans-2,3-diphenylpro-penal by a concerted process taking 10 min at 220 °C. The yields of this remarkable transformation were low, however, and further work into several aspects is required to make them more respectable. 

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