Dimerization, catalytic

Mechanistic studies41"43 by Dixon and Jones excluded the possibility of dimeric catalytic species because a linear dependence was observed between the catalyst s enantiopurity and the reaction s enantioselectivity.43 The test reaction was the desymmetrization of meso-imide 22 using chiral oxazaborolidine catalysts. The sense of the enantioselectivity of the reduction was established by conversion of hydroxy lactam 23 to the known ethoxy lactam 24 (Scheme 17.6). 

The isomer ratios are often close to the equilibrium composition at the particular reaction temperature, indicating isomerization as well as dimerization catalytic activity. The two most extensively studied zeolite catalysts contain nickel and rhodium, incorporated via ion exchange, and will be discussed separately. 

A similar dimerization catalytic system has been investigated [42] in a continuous flow loop reactor in order to study the stability of the ionic liquid solution. The catalyst used is the organometallic nickel(II) complex (Hcod)Ni(hfacac) (Hcod=cyclooct-4-ene-l-yl and hfacac=l,l,l,5,5,5-hexafluoro-2,4-pentanedionato-0,0 ) and the ionic liquid is an acidic chloroaluminate based on the acidic mixture of l-butyl-4-methylpyridinium chloride and aluminum chloride. No alkylaluminum is added, but an organic Lewis base is added to buffer the acidity of the medium. 

Figure 11.16 Regulation of glycogen phosphorylase by phosphorylation/dephosphorylation The major enzymatic system in the regulation of glycogen phosphorylase (i.e. phosphorylase) by the multicyclic phosphorylation/dephosphorylation cascade is shown. Abbreviations used are Pr, protein PrK, Protein kinase phosphorylaseK, phosphorylase kinase (C2R2 where C2 and R2 are dimeric catalytic and regulatory subunits respectively) PPrP, phosphoprotein phosphatase (G), G-subunit of phosphoprotein phosphatase and p-(G), phopsho-G-subunit.

Several early structural models of AHAS were proffered based on homology to other known ThDP-dependent enzymes, such as POX from Lactobacillus planta-rum, and carefully planned site-directed mutagenesis studies. Many of the features of the early models were borne out by the first X-ray crystal structure at 2.6-A resolution of the dimeric catalytic subunit of AHAS from S. cerevisiae [15,... 

On activation by BF3 OEti, Ni(PPh)4 anchored on brominated polystyrene exhibits high dimerization catalytic activity for ethylene [230]. The catalyst is reused without loss of activity. 

Oxidative cleavage of the complex 549 with CuCri affords 2,3-bis(chloro-methyl)-1,3-butadiene (550) and regenerates PdCri. Thus the preparation of this interesting dimerization product 550 can be carried out with a catalytic amount of PdCl2 and two equivalents of CuCb in MeCN[495], Similarly, treatment of allene with PdBr2 affords the dimeric complex 551. Treatment of this complex with 2 equiv, of bromine yields the dibromide 552. The tetra-bromide 553 is obtained by the reaction of an excess of bromine[496]. Similarly,... 

An active catalytic species in the dimerization reaction is Pd(0) complex, which forms the bis-7r-allylpalladium complex 3, The formation of 1,3,7-octa-triene (7) is understood by the elimination of/5-hydrogen from the intermediate complex 1 to give 4 and its reductive elimination. In telomer formation, a nucleophile reacts with butadiene to form the dimeric telomers in which the nucleophile is introduced mainly at the terminal position to form the 1-substituted 2,7-octadiene 5. As a minor product, the isomeric 3-substituted 1,7-octadiene 6 is formed[13,14]. The dimerization carried out in MeOD produces l-methoxy-6-deuterio-2,7-octadiene (10) as a main product 15]. This result suggests that the telomers are formed by the 1,6- and 3,6-additions of MeO and D to the intermediate complexes I and 2. 

Carbonyiation of butadiene gives two different products depending on the catalytic species. When PdCl is used in ethanol, ethyl 3-pentenoate (91) is obtained[87,88]. Further carbonyiation of 3-pentenoate catalyzed by cobalt carbonyl affords adipate 92[89], 3-Pentenoate is also obtained in the presence of acid. On the other hand, with catalysis by Pd(OAc)2 and Ph3P, methyl 3,8-nonadienoate (93) is obtained by dimerization-carbonylation[90,91]. The presence of chloride ion firmly attached to Pd makes the difference. The reaction is slow, and higher catalytic activity was observed by using Pd(OAc) , (/-Pr) ,P, and maleic anhydride[92]. Carbonyiation of isoprcne with either PdCi or Pd(OAc)2 and Ph,P gives only the 4-methyl-3-pentenoate 94[93]. 

The strong catalytic activity of anhydrous hydrogen fluoride results from the abiUty to donate a proton, as in the dimerization of isobutylene (see Butylenes) ... 

Diborane [19287-45-7] the first hydroborating agent studied, reacts sluggishly with olefins in the gas phase (14,15). In the presence of weak Lewis bases, eg, ethers and sulfides, it undergoes rapid reaction at room temperature or even below 0°C (16—18). The catalytic effect of these compounds on the hydroboration reaction is attributed to the formation of monomeric borane complexes from the borane dimer, eg, borane-tetrahydrofuran [14044-65-6] (1) or borane—dimethyl sulfide [13292-87-0] (2) (19—21). Stronger complexes formed by amines react with olefins at elevated temperatures (22—24). 

Cycloaliphatic Diene CPD—DCPD. Cycloatiphatic diene-based hydrocarbon resias are typically produced from the thermal or catalytic polymerization of cyclopeatadieae (CPD) and dicyclopentadiene (DCPD). Upon controlled heating, CPD may be dimerized to DCPD or cracked back to the monomer. The heat of cracking for DCPD is 24.6 kJ / mol (5.88 kcal/mol). In steam cracking processes, CPD is removed from C-5 and... 

Ethyleneknine dimer has been synthesized using catalytic amounts of an alkaU metal amide of ethyleneknine under alkaline conditions (89,90). 

IFP Process for 1-Butene from Ethylene. 1-Butene is widely used as a comonomer in the production of polyethylene, accounting for over 107,000 t in 1992 and 40% of the total comonomer used. About 60% of the 1-butene produced comes from steam cracking and fluid catalytic cracker effluents (10). This 1-butene is typically produced from by-product raffinate from methyl tert-huty ether production. The recovery of 1-butene from these streams is typically expensive and requires the use of large plants to be economical. Institut Francais du Petrole (IFP) has developed and patented the Alphabutol process which produces 1-butene by selectively dimerizing ethylene. 

The most common oxidatiou states and corresponding electronic configurations of rhodium are +1 which is usually square planar although some five coordinate complexes are known, and +3 (t7 ) which is usually octahedral. Dimeric rhodium carboxylates are +2 (t/) complexes. Compounds iu oxidatiou states —1 to +6 (t5 ) exist. Significant iudustrial appHcatious iuclude rhodium-catalyzed carbouylatiou of methanol to acetic acid and acetic anhydride, and hydroformylation of propene to -butyraldehyde. Enantioselective catalytic reduction has also been demonstrated. 

Rhodacarborane catalysts have been immobilized by attachment to polystyrene beads with appreciable retention of catalytic activity (227). A 13-vertex /oj iJ-hydridorhodacarborane has also been synthesized and demonstrated to possess catalytic activity similar to that of the icosahedral species (228). Ak-oxidation of closo- >(2- P((Z [) 2 - i- > l[l-Bih(Z, results in a brilliant purple dimer. This compound contains two formal Rh " centers linked by a sigma bond and a pak of Rh—H—B bridge bonds. A number of similar dimer complexes have been characterized and the mechanism of dimer formation in these rhodacarborane clusters have been studied in detail (229). 

A number of reductive procedures have found general applicability. a-Azidoketones may be reduced catalytically to the dihydropyrazines (80OPP265) and a direct conversion of a-azidoketones to pyrazines by treatment with triphenylphosphine in benzene (Scheme 55) has been reported to proceed in moderate to good yields (69LA(727)23l). Similarly, a-nitroketones may be reduced to the a-aminoketones which dimerize spontaneously (69USP3453279). The products from this reaction are pyrazines and piperazines and an intermolecular redox reaction between the initially formed dihydropyrazines may explain their formation. Normally, if the reaction is carried out in aqueous acetic acid the pyrazine predominates, but in less polar solvents over-reduction results in extensive piperazine formation. 

A dimer-dimer (DD) surface reaction scheme of the type (1/2)A2 + B2 B2A has been proposed in order to mimic the catalytic oxidation of hydrogen A2 is O2, B2 is H2, AB is OH and B2A is H2O. The model reaction proceeds according to the Langmuir-Hinshelwood... 

K. M. Khan, K. Yaldram, A. Ahmad. Kinetics of a dimer-dimer irreversible catalytic surface reaction. J Chem Phys (in press). 

Enamines containing one -hydrogen atom react with the lactone dimer of dimethylketene to form aminocyclohexanediones 116). Polycondensation of acetone diethyl ketal takes place by treating it with morpholine and a catalytic amount of p-toluenesulfonic acid while distilling off the ethanol formed 117-119). The resulting spiran, bicyclo, and cyclooctadienone products differ from the known polycondensation products of acetone, and hence their formation probably involves enamine intermediates 119). 

Alternatively, thermolysis yields the terminal alkene RCH=CH2. Note that, if propene or higher alkenes are u.sed instead of ethene, then only single insertion into Al-C occurs. This has been commercially exploited in the catalytic dimerization of propene to 2-methylpentene-1, which can then be cracked to isoprene for the production of synthetic rubber (cu-1,4-polyisoprene) ... 

During studies on ditryptophan derivatives, an interesting acid-induced cy-cHzation has been discovered. The 10-membered ring 37 was thus subjected to acidic conditions to produce the indolocarbazole derivative 38 (Scheme 6). Interestingly, calculations performed on the precursor 37 indicated that the lowest energy conformation resembled that of the diastereomer of 38, which was never observed. An additional experiment furnished the parent system 1 on treatment of 38 with a catalytic amount of acid. A TFA-induced formation of an indolo[2,3-<3]carbazole was also observed from a related acyclic 2,2 -connected tryptophan dimer (99JOC8537). 

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