Hydroformylations Markovnikov addition

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. 

Figure 5.1 The basic catalytic cycle for the hydroformylation of propylene with Rh/ PPh3-based catalyst. In step 5.3 to 5.4 anti-Markovnikov addition is assumed.

The two different ways of inserting an alkene into a metal-hydrogen bond, as shown by 5.4 and 5.5, are called anti-Markovnikov and Markovnikov addition, respectively. Insofar as hydroformylation with high selectivity to n-butyralde-hyde is concerned, it is considered to be primarily an effect of steric crowding around the metal center. The normal alkyl requires less space and therefore formed more easily than the branched one in the presence of bulky ligands. 

However, the balance between sterically demanding ligands and their ability to remain coordinated so that the product selectivity could be influenced is a fine one. This aspect is discussed in more detail in Section 5.2.4. Although not directly related to hydroformylation, it is appropriate to note here that Markovnikov additions accompanied by /3-hydride elimination is a general pathway for alkene isomerization. This is shown in Fig. 5.2 for the isomerization of both terminal and internal alkenes. 

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). 

As an efficient route for converting alkenes and synthesis gas (CO -1- H2) into the corresponding aldehydes, hydroformylation has developed into an extremely important industrial process [1, 3, 15-20]. In the latest decades, over 6 million tons per year of 0x0 products are obtained accordingly. Because alkene insertion into the M-H bond can occur in two ways, i.e., the ant/ -Markovnikov and Markovnikov addition (Fig. 3) [23], the product of terminal alkene hydroformylation is usually a mixture of linear (normal) and branched (iso) isomers (Fig. 2) [24-26]. Therefore, controlling the addition direction is vital in many commercial hydroformylation reactions. 

The so-called LIM ligands were claimed by Sasol for cobalt-catalyzed hydroformylation (Scheme 2.6) [21]. They represent P-alkyl derivatives of 4,8-dimethyl-2-phospha-bicyclo[3.3.1]nonane and can be produced as a mixture of diastereomers by radical-mediated /iti-Markovnikov addition of PHj to (S)- or (/ )-limonene and final reaction of the yielding LIM-H with a long-chain terminal olefin [22]. The last step can be initiated by a radical chain initiator (AINB) or proceeds with the assistance of a strong base [23]. 

X-Terpineol can be considered as a hydrate of limonene. Indeed, it is available by Markovnikov addition of trifluoroacetic acid to the latter, followed by hydrolysis [141]. The alcohol has a pleasant odor similar to lilac and is a constituent of cajuput, pine, and petitgrain oil. For technical applications, it is produced from a-pinene by acid-catalyzed isomerization/hydration [142]. Hydroformylation of a-terpineol has been conducted with an unmodified Rh catalyst at 69bar (Scheme 6.45) [132]. Under the conditions applied, besides the expected cyclic carbaldehydes also a linear aldehyde with a tertiary alcoholic group were obtained. The reaction product was distilled, and the main fraction collected showed a woody and nutty aroma with minty and floral topnotes. 

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 ... 

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. 

The catalytic system Co2(CO)gH-H2 forms the complex CoH(CO)4, which has strongly acidic character in polar solvents. Therefore, the complex CoH(CO)4 is more likely to be added to 1-alkenes according to the Markovnikov rule than RhH(CO)(PPh3)3. Based on the studies of hydroformylation of deuterated olefins, it was proposed that the addition-elimination process proceeds according to the concerted mechanism (13.60)... 

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