Sodium product branching ratio

Selectivity refers to the fraction of raw material alkene that is converted to product aldehyde, but since hydroformylation typically gives both a linear and branched isomer, selectivity also refers to the relative amounts of each. The linear branched (l b) ratio is highly catalyst dependant. One must simultaneously consider whether the proposed catalyst will give the desired l b selectivity and also whether the proposed catalyst is feasible for use with the catalyst/product separation technologies. For example, water extraction of a polar product, such as in the hydroformylation of allyl alcohol to give 4-hydroxybutanal, would not work well with a sodium salt of a sulfonated phosphine since both are water soluble. 

The first enantioselective, iridium-catalyzed allylic substitution was reported by Helmchen and coworkers soon after the initial report by Takeuchi. Helmchen studied catalysts generated from phosphinooxazoline (PHOX) ligands and [Ir(COD)Cl]2 for the reactions of sodium dimethylmalonate with cinnamyl acetates (Scheme 2) [50]. The alkylation products were isolated in nearly quantitative yield and were formed with ratios of branched-to-Unear products up to 99 1 and with enantioselectivities up to 95% ee. In this and subsequent studies with PHOX ligands [51,52], Helmchen et al. demonstrated that the highest yields and selectivities were obtained with a PHOX ligand containing electron-withdrawing substituents and... 

In the reactions discussed and exemplified above, reactants, transient species and products are related by linear sequences of elementary reactions. The transient species can be regarded as a kinetic product and, if isolable, subject to the usual tests for stability to the reaction conditions. Multiple products, however, may also occur by a mechanism involving branching. Indeed, the case shown earlier in Fig. 9.5b, where the transient is a cul de sac species, is the one in which the branching to the thermodynamic product P and kinetic product T occurs directly from the reactant. In the absence of reversibility, the scheme becomes as that shown in Scheme 9.8a, where the stable products P and Q are formed as, for example, in the stereoselective reduction of a ketone to give diastereoisomeric alcohols. The reduction of 2-norbornanone to a mixture of exo- and cndo-2-norbornanols by sodium borohydride is a classic case. The product ratio is constant over the course of the reaction and reflects directly the ratio of rate constants for the competing reactions. The pseudo-first-order rate constant for disappearance of R is the sum of the component rate constants. 

Prandi [50] reported the preparation of methyl a-D-caryophylloside, a natural 4-C-branched sugar, in which the key step was diiodosamarium-promoted coupling reaction. As illustrated in O Scheme 30, C-C bond formation between the cmde acid chloride 106 and ketone 107 was mediated smoothly by Sml2 in tetrahydropyran (THP). Expected products were isolated in 63% yield and in a 8 Idiastereoisomeric ratio. Reduction of the major diastereomer 108 with sodium borohydride in methanol at 0 °C was very slow, but the expected 109 was eventually obtained in 73% yield after 24 h at room temperature. 

The hydroformylation of the water-soluble substrates, 4-penten-l-ol and 3-buten-l-ol in aqueous solution using HRh(CO)(TPPTS)3 as the catalyst was investigated. Activation parameters and reaction selectivity for the hydroformylation of 4-penten-l-ol were found to depend on the ionic strength of the solution. As sodium sulfate was added, the activation energy increased. The linear-branched selectivity was strongly influenced by the ionic strength and temperature. The reaction could be directed to yield a product distribution of modest linearity (75%) or an exceptionally high ratio of the branched product (98%) as a cyclic 2-hydroxy-3-methyltetrahydropyran [33]. 

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