Branching ratios alkenes

Entry TOFb Linear aldehyde0 (%) Branched aldehyde0 <%) Alkene isomers0 <%) Linear alcohol0 (%) l b ratio 1-Octene conversion <%)... 

The chiral sites which are able to rationalize the isospecific polymerization of 1-alkenes are also able, in the framework of the mechanism of the chiral orientation of the growing polymer chain, to account for the stereoselective behavior observed for chiral alkenes in the presence of isospecific heterogeneous catalysts.104 In particular, the model proved able to explain the experimental results relative to the first insertion of a chiral alkene into an initial Ti-methyl bond,105 that is, the absence of discrimination between si and re monomer enantiofaces and the presence of diastereoselectivity [preference for S(R) enantiomer upon si (re) insertion]. Upon si (re) coordination of the two enantiomers of 3-methyl-l-pentene to the octahedral model site, it was calculated that low-energy minima only occur when the conformation relative to the single C-C bond adjacent to the double bond, referred to the hydrogen atom bonded to the tertiary carbon atom, is nearly anticlinal minus, A- (anticlinal plus, A+). Thus one can postulate the reactivity only of the A- conformations upon si coordination and of the A+ conformations upon re coordination (Figure 1.16). In other words, upon si coordination, only the synperiplanar methyl conformation would be accessible to the S enantiomer and only the (less populated) synperiplanar ethyl conformation to the R enantiomer this would favor the si attack of the S enantiomer with respect to the same attack of the R enantiomer, independent of the chirality of the catalytic site. This result is in agreement with a previous hypothesis of Zambelli and co-workers based only on the experimental reactivity ratios of the different faces of C-3-branched 1-alkenes.105... 

Alkene Epoxide Poly(tartrate) % branching Molar ratio alkene Ti tartrate Temperature (°C) Time Isolateda Yield (%) Ee(%)... 

There was a thermodynamic preference for the reaction to take place at the terminal alkene carbon, which favors the yield of linear aldehyde, but the TS to linear aldehyde path was higher than the TS for the branched aldehyde path. Regioselectivity was evaluated from the products relative stability, i.e. considering that the reaction is under thermodynamic rather than under kinetic control. The linear to branched ratio (l b) of 94 6 was in excellent agreement with the ratio 95 5 reported for PPh3 [25], However, this nice coincidence must be viewed cautiously because the model is simple, reaction paths were partially considered, so a subtle cancellation of errors may have been made. 

For the hydroformylation, (PPli j) Rli( H)(CO) with host 11 was used as the catalyst. An excess of PPhj (stemming from the catalyst precursor) was needed to avoid isomerization, as was found when phosphine-free precursors were used (at the concentrations used even bidentates should be added in excess to prevent substantial exchange with carbon monoxide). Linear to branched ratios of 2 1 were obtained and no isomerized alkene could be detected. These results are similar to those obtained by Kalck and coworkers [41]. As expected, catalysis for 11 is slower than that for (PPh3)3Rh(H)(CO) as the host is a bidentate phosphine catalysis with (PPh3)3Rh (H)(CO) strongly depends on the concentrations of rhodium and PPh3 and comparison of the rates of the two systems does not make sense. 

The introduction of rhodium has allowed the development of processes which operate under much milder conditions and lower pressures, are highly selective, and avoid loss of alkene by hydrogenation. Although the catalyst is active at moderate temperature, plants are usually operated at 120°C to give a high n/iso (linear/ branched) ratio. The key to selectivity is the use of triphenylphosphine in large excess which leads to >95% straight chain anti-Markovnikov product. The process is used for the hydroformylation of propene to n-butyraldehyde, allyl alcohol to butanediol, and maleic anhydride to 1,4-butanediol, tetrahydrofuran, and y-butyrolactone. 

Under these conditions, the linear to branched aldehyde ratio for the hydroformylation of 1-octene was 1.9 1. Starting with 4-octene one still gets a 1.2 1 linear to branched ratio. Thus, one can start with a considerably less-expensive mixture of terminal and internal alkenes and get a product distribution favoring the linear aldehyde. The product distribution in Scheme 3 can be nicely explained by invoking facile alkene isomerization, with the fastest hydroformylation occurring for double bonds in the 1-position. Labeling studies have shown that alkene isomerization generally occurs without dissociation of the alkene from the cobalt catalyst. ... 

Thermal deazetization of pyrazolines results in the formation of cyclopropanes and alkenes, illustrated in Figure 44 for the parent compound (18). This reaction is of interest in that one could imagine that it would involve the same trimethylene biradical (19) proposed to be an intermediate in cyclopropane stereomutation (Section III.A). Supporting this notion is the observation that the parent pyrazoline gives 89% cyclopropane and 11 % propylene at 250° C. If one took this product ratio as a reflection of the branching ratio from a common trimethylene intermediate, it should then be possible to compare these figures with the relative rates of stereomutation and propylene formation from cyclopropane-d2 . Interestingly, they are identical. 

Casey observed a linear/branched ratio of 66 1 using a BISBI-Rh complex to hydroformylate 1-hexene. This is a huge increase over the linear/branched ratio of only 2.6, which Casey observed when a Rh-dppe40 complex is used for hydro-formylation of 1-alkenes.41 The (3n of BISBI is 113-120°, whereas that of dppe is much lower, at 85°. 

One explanation for the regioselectivity is related to the stereochemistry of the diphosphine-Rh catalyst complexed with the alkene. We have seen earlier that high linear-to-branch ratios in Rh-catalyzed hydroformylation, as with Co-catalyzed hydroformylation, stem from the alkene complexation-insertion step of the catalytic cycle, so differences in steric and electronic factors as a function of stereochemistry could play a significant role in the regiochemical result of the reaction. There are two trigonal bipyramidal possibilities for the diphosphine-Rh-alkene complex, which are denoted a-e (apical-equatorial isomer 18) and e-e (diequatorial isomer 19). 

The complexes [RuCl( 2.pjj pcH2PPh2)Cp] and [RuCl /2-HC(PPh2)3 -Cp] are extremely poor hydroformylation catalysts. They showed less than 0.5% conversion of alkene to aldehyde after 30 hours, linear to branched ratios of one or less, and undesirable amounts of alkene isomerisation and hydrogenation products. 

A rhodium complex with phosphine 61 in hydroformylation of 1-decene and 1-hexadecene gave linear-to-branched 1-alkene ratios from 4.8 1 to 5.9 1 at a turnover frequency of 136 h . For 1-hexene, this quantity under similar conditions was 1456 h .and the catalyst remained active up to the substrate-to-rhodium ratio of 200,000 1 [174]. 

The oxidation of alkanes, alkenes and simple aromatics at 293 K under NOx rich tropospheric conditions has been studied using laser pulse initiation combined with cw laser long path absorption/LIF for the detection of OH and NO2. In the case of aliphatic hydrocarbons the absolute yield and the kinetics of the formation of these products have been found to be sensitive indicators for the reaction behaviour of the oxy radicals RO. In combination with mechanistic simulations rate constants for individual reactions as well as branching ratios have been derived, which permit the evaluation of the compound specific NO/NO2 conversion factors (NOCON - factors) for the first oxidation steps. In the case of benzene and toluene oxidation the results indicate that reaction of the primary formed X cyclohexa-dienyl radical (X = Cl, OH) with O2 is the dominant pathway, although the rate coefficients were found to be lower than 2 x 10" cmVs. 

Hydroformylation is a metal-catalyzed reaction in which an olefin, CO, and H react to produce an aldehyde. The reaction was discovered at BASF by Otto Roelen, and was called hydroformylation by Adkins. This transformation is also sometimes referred to as the "oxo" process. In a formal sense, the elements of formaldehyde are added across a C=C bond. Common side reactions include aUcene hydrogenation, aldehyde hydrogenation, and alkene isomerization. Hydroformylation is one of the largest volume reactions conducted with homogeneous catalysts in the chemical industry. It is used to produce over 14 billion pounds of aldehydes per year (two pounds per year for every person on Earth ). The aldehydes are converted to alcohols, acids, and other materials as useful end products. One large-volume use of hydroformylation is the conversion of propene to a mixture of -butyraldehyde and /-butyraldehyde (Equation 17.2). Since the desired product is n-butyraldehyde, a great deal of effort has been expended to maximize the n i (noimal to iso, or often also called l/b for linear to branched) ratio of aldehydes and to understand the factors that control it. 

Conditions CO/H2 = 1, P(CO/H2) = 20 bar, ligand/Rh = 5, substrate/Rh = 637, [Rh] = 1.00 mM, number of experiments = 3. In none of the experiments was hydrogenation observed. Linear over branched ratio, percentage linear aldehyde, percent isomerization to 2-octene, and turnover frequency were determined at 20% alkene conversion. Turnover frequency = (mol of aldehyde) (mol of Rh)-1 h-1. Calculated from Jp-h NMR data. 

Ionic liquids have been used for the hydroformylation of higher alkenes. Here the lipophilic character of the catalyst phase can be adjusted by proper choice of the ionic liquid. In the recent past good results in hydroformylation of 1-octene have been reported by P. Wasserscheidt and coworkers. Linear branched ratios up to 16 with special ionic ligands have been achieved [10]. Bahrmann discovered that certain amines form ionic liquids with TPPTS. In this instance the ligand itself is the ionic liquid [11]. 

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