Anti-Markovnikov products hydroformylation

It is obvious that such equilibria would exist for all the other catalytic intermediates. The result of all this is coupled catalytic cycles and many simultaneous catalytic reactions. This is shown schematically in Fig. 5.5. The complicated rate expressions of hydroformylation reactions are due to the occurrence of many reactions at the same time. As indicated in Fig. 5.5, selectivity towards anti-Markovnikov product increases with more phosphinated intermediates, whereas more carbonylation shifts the selectivity towards Mar-kovnikov product. This is to be expected in view of the fact that a sterically crowded environment around the metal center favors anti-Markovnikov addition (see Section 5.2.2). 

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. 

Oxo reaction or hydroformylation reaction involves addition of a hydrogen atom and a formyl group (-CHO) to C=C double bond of an olefin making both anti—Markovnikov and Markovnikov products ... 

Of the isomeric aldehydes indicated in Eq. (7.1), the linear aldehyde corresponding to anti-Markovnikov addition is always the main product. The isomeric branched aldehyde may arise from an alternative alkene insertion step to produce the [RCH(Me)Co(CO)3] or [RCH(Me)Rh(CO)(PPh3)2] complexes, which are isomeric to 2 and 8, respectively. Alternatively, hydroformylation of isomerized internal alkenes also give branched aldehydes. The ratio of the linear and branched aldehydes, called linearity, may be affected by reaction conditions, and it strongly depends on the catalyst used. Unmodified cobalt and rhodium carbonyls yield about 3-5 1 mixtures of the normal and iso products. 

A hydroformylation reaction in diene polymers introduces a formyl group which is an extremely reactive functional group. Sibtain and Rempel [65] carried out hydroformylation of SBR using HRh(CO)(PPh3)3 and reported anti-Markovnikov addition product. Hydroformylation takes place preferentially in the 1,2 unit. As the degree of hydroformylation increases new absorption bands appear at 1724 cm"1 due to v(C=0) and at 2700 cm 1 due to v(C-H) in CHO. Bhattacharjee and co-workers [66] carried out hydroformylation of NBR and observed new peaks at 1724 and 2700 cm"1 which are characteristics of CHO groups. 

Since the desired product from propylene hydroformylation is -butyralde-hyde, considerable attention has been devoted to increasing the selectivity this focussed attention on the mechanism, especially the step where the propylene inserts into the Co-H bond, since this can be either Markovnikov or anti-Markovnikov. 

The hydroformylation reaction (Section 9-2), shown in equation 9.5, demonstrates regioselectivity, whereby one regioisomer forms preferentially over another. The Co catalyst may be modified to increase the proportion of linear (anti-Markovnikov) over branched (Markovnikov) product. 

It is this exothermic step that probably is the source of the preference for linear hydroformylation products over branched ones. The structure of the comparable 18-electron branched intermediate 7 is about 2 kcal/mol less stable than 7, according to Jiao s calculations. This difference leads ultimately to the anti-Markovnikov, linear aldehyde over the branched-chain isomer. Although -elimination is possible now, the high partial pressure of CO present in the reaction vessel tends to stabilize 7 and prevent loss of CO that would generate the vacant site necessary for elimination to occur. 

The hydroformylation of propylene provides two types of products, n- and isobutyraldehydes depending on the insertion modes of propylene into the M-H bond. As shown in Scheme 1.18a and b, where R = H, the anti-Markovnikov type addition of M-H to the double bond in (a) gives the linear propyl, whereas the Markovnikov type addition gives the isopropyl group bound with the metal. Further insertion of CO yields the linear and branched acyl groups. 

In contrast to propyne, allene hydroformylation presents a kinetic and thermodynamic preference for the anfi-Markovnikov insertion, as indicated by the lower activation free energy (20.1 vs. 33.5 kJ/mol) and the higher stability of the insertion product (—122.2 vs. -54.0 kJ/mol) for anfi-Markovnikov path with respect to Markovnikov path. In terms of these energy differences, a ratio of 98 2 for anti-Markovnikov to Markovnikov products is predicted kinetically. Furthermore, from... 

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