Project Assignment on Hydroformylation of Alkenes and Mechanistic Investigation

CONTENT

• Introduction
• Importance of hydroformylation
• Various catalysts employed in hydroformylation reaction
•  Cobalt Catalysts for Hydroformylation-

                                i.        Catalytic cycle using HCo(CO)₄

                               ii.        Disadvantages of cobalt carbonyl complex catalyst

                              iii.        Phosphine Modified Cobalt Catalysts

                              iv.        Steric effect of PR₃

•         Rhodium – Phosphine Catalysts

                           i.        Catalytic cycle using HRh(CO)(PPh₃)₃ catalyst

                          ii.        Drawback of HRh(CO)(PPh₃)₃ catalyst

•   Factors Affecting the n/iso Ratio of Hydroformylation Products
•  Water-soluble rhodium catalysts
•  Bidendate Phosphine Rh Catalyst
•  Other Aspects of Hydroformylation
•  Enantioselective Hydroformylation
•   Conclusion
•  Bibliography 

Experimental setup with reactor system

INTRODUCTION

In 1938, Otto Roelen discovered the hydroformylation – often called the oxo process, one of the first commercially important homogeneous catalytic reactions. He found that a Cn alkene can be converted to a Cn+1 aldehyde by the addition of H2 and CO to an olefinic double bond catalyzed by cobalt or rhodium carbonyl complexes.

In simple terms, the equation can be written as

Aldehydes produced by hydroformylation are usually reduced to alcohols that are used as solvents plasticizers and in the synthesis of detergents.

IMPORTANCE OF HYDROFORMYLATION

The reaction was first discovered by Otto Roeln at the Ruhrchemie industry in Germany while studying recycling of olefins. Since then, the reaction has been developed by industry and account for more than 7 × 10⁶  tons/year of aldehydes. It is one of the largest homogeneous catalytic process worldwide.

 Actually, aldehydes are not the end product, they get further reduced to alcohols; sometimes in the same plant or by other heterogeneous catalysts. The short chain alcohols thus obtained are extensively used as solvents in the lacquer industry for making plasticisers and the long chain alcohols are used in the manufacture of synthetic detergents.

As the alkene also has equal propensity to react with H₂, it is interesting to study the thermodynamics involving both hydrogenation and hydroformylation reactions. The changes are in entropy and the free energy of these two reactions for propene under standard conditions are given below.

CH₃CH=CH₂ + CO + H₂ →  CH₃CH₂CH₂CHO

∆H = -150 kJ/mol

CH₃CH=CH₂ + H₂  →  CH₃CH₂CH₃

∆H = -126 kJ/mol; ∆G = -88kJ/mol

It becomes evident that the alkane is the thermodynamically favoured productand not the aldehyde. Since the aldehyde is the major product, the entropy loss is more important and ∆G becomes less negative. Also, the reaction is exothermic and if conducted under adiabatic conditions, the temperature rises and ∆G becomes close to zero.

VARIOUS CATALYSTS EMPLOYED IN HYDROFORMYLATION REACTION:

1) Cobalt Catalyst: HCo(CO)₄

2) Cobalt Phosphine-Modified Catalyst: HCo(CO)(PR₃)₃

3) Rhodium Phosphine Catalyst: HRh(CO)(PPh₃)₃

4) Aqueous phase Rhodium Catalyst: TPPTS (Triphenylphosphinetrisulfonate)

5) New generation of Rhodium Catalyst: bidentate phosphine ligands

Over the years, four catalytic processes have come into prominence for hydroformylation reaction. These are:

·          The Co₂(CO)₈ catalysed process,

·         The Co₂(CO)₈/PR₃ catalysed process,

·         The HRh(CO)(PPh₃)₃ catalysed process and

·         The biphasic HRh(CO)(PR₃)₃ process (R = m- C₆H₄SO₃Na)

The major difference between these processes are-

1. The operating temperatures and pressures,
2. The ratio of the product formed, n-aldehyde (n/iso ratio),
3. The rate of reaction and the control of side reactions such as hydrogenation and 
4. Ease of recovery of catalyst.

Currently C₃ to C₁₅ aldehydes are produced by the oxo process and the are subsequently converted into amines, carboxylic acids and most importantly to primary alcohols.

 Some of the major industrially important end products are butanol, 1,4-butanediol ( for THF synthesis), vitamin A and 2- ethylhexanol. 2-Ethylhexanol is used in making diethylhexyl phthalate (DEHP),also lnown as dioctyl phthalate(DOP),which is the most widely used plasticiser in the world.

COBALT CATALYSTS FOR HYDROFORMYLATION:

Cobalt catalyste dominated the hydroformylation industry till the early 1970s after which the triarylphosphine rhodium based catalysts took over.The latter are especially good with C₈ or lower alkenes when a higher selectivity of the linear aldehyde is required. Under H₂/CO pressure, cobalt salts produce HCo(CO)₄ as the active catalytic species.

The most widely accepted mechanism for the catalytic cycle for cobalt based catalyst Co₂₍CO)₈ was proposed by Heck and Breslow in 1961.

Kinetic studies support a general rate expression as given below:

d[aldehyde]/dt = k[alkene][Co][pH₂][pCO]^-1

•         Inversely proportional to CO concentration because CO dissociation from the coordinatively saturated 18e- species is required
•         Using a 1:1 ratio of H2/CO, the reaction rate is independent of pressure
•         HCo(CO)₄ is only stable under certain minimum CO partial pressures at a given temperature
•         CO pressure ↑ → reaction rate ↓ & high ratio of linear to branched product
•         CO pressure ↓ → reaction rate ↑ & branched alkyl ↑ (reverse ß-elminination)

Catalytic cycle using HCo(CO)₄:

In the catalytic cycle, the general catalyst is 16-electron, four coordinate Co¹ comples, HCo(CO)₃. This species is not readily available, instead catalytic precursor Co₂(CO)₈ is introduced that form HCo(CO)₃ under reaction conditions.

For this catalytic cycle, Co₂(CO)₈ and H₂ introduced that form HCo(CO)₄( an 18 electron species) which then lossesa CO to give HCo(CO)₃ (a 16 electron species) and creates a vacant coordinatin site required for alkene.

The alkene coordinates to this vacant site to form an 18-electron complex whic undergoes migratory insertion of the olefin into the C-H bond and therefore, another 16-electron complex having vacant coordination site is formed.

A CO ligand is then coordinated to the vacant site to form RCH₂CH₂Co(CO)₄ complex. Now insertion of a CO ligand of

RCH₂CH₂Co(CO)₄ occurs into the alkyl-cobalt bond to give the acyl-cobalt complexRCH₂CH₂CO-Co(CO)₃. The reactant H₂ is added oxidatively to the coordinatively unsaturated cobalt-acyl comlex to give a Co(III) complex which finally undergoes reductive elimination of product, RCH₂CH₂CHO and regenerates the active catalyst HCo(CO)₃.

The general relative reactivity of alkenes  for hydroformylation is as follows:

Disadvantages of cobalt carbonyl complex catalyst:

     ·         It operates at high temperature (140 – 175^oC) and high pressure (200 – 250 atm.).

• Straight chain(n -) as well as branched chain (iso -) aldehydes are formed. The n:iso ratio is found to be 3:1 which is not a good ratio.The n:iso ratio should be high because straight chain aldehydes are more biodegradable than the branch ones.

If the catalyst HCo(CO)₄ is modified by replacing one CO by PBu₃ (Bu = n – butyl) to give HCo(CO)₃(PBu)₃, the selectivity of the catalyst increases to give hihg n : iso ratio ( 9 : 1), but this catalyst reduces  the rate of reaction. Therefore, this catalyst operates at higher temperature (175^oC) and higher pressure (50 – 100 atm.).

Phosphine Modified Cobalt Catalysts:

The addition of PPh₃ ligands to the cobalt carbonyl catalyst brought about a dramatic change in the rate of the reaction and its regioselectivitydue to electronic and steric effect of substitution of PR₃. When a CO is substituted by the electron donating PR₃ group, the back donating from the metal to the rest of the CO group increases, thereby increasing the thermal stability of the catalyst against decomposition.If R=Bu, the n/iso ratio is ( 9:1 ).

Steric effect of PR₃:

Bulky PR₃ group influences the insertion direction of alkene to Co complex and geometry of intermediate (favors Anti-Markovnikov; Hydrogen transferred to carbon with bulkier R group).

The CO partial pressure required to stabilise the catalyst comes down considerably from 200–300 to 50–100 bars. Also, the hydridic nature of hydrogen increases as there is more electron density on the metal. The catalyst can even convert the aldehyde formed to alcohol by hydrogenation, but the presence of less electron donating phosphines like PPh₃ on the catalyst checks this process and produces  less of the alcohol.

A highly active catalyst has as additional drawback since it also hydrogenates the alkene and some alkene is wasted in the formation of unwanted products. Higher stabliity of the catalyst also means lesser activity.

RHODIUM – PHOSPHINE CATALYSTS:

In 1965, Osborn, Wilkinson and others reported that Rh(I) catalysts with PPh₃ affect not only hydrogenation but also hydroformylation with high regioselectivity near ambient conditions. As halides were found to be inhibitors for hydroformylation, the original Wilkinson’s catalyst was modified to contain no halides. HRh(CO)(PPh₃)₃ and Rh(acac)(CO)₂ are two commonly used catalyst precursors for hydroformylation.

The catalytic cycle shown that the step are analogous to Heck’s mechanism for hydroformylation using HCo(CO)₄. Kinetic studies on the rhodium catalyst showed that unlike the cobalt catalyst, there is no inverse dependence of the rate on CO concentration.

Rate α [propylene][Rh][pH₂]

Drawback of HRh(CO)(PPh₃)₃ catalyst:

The main drawback of Rhodium – PPh₃ catalyst is a problem related to the industrial process. Since a high temperature is required for separation of the long chain aldehyde products, the catalyst decomposes at that temperature. So the application has been limited to C₃ and C₄ alkenes.

This problem was solved by using a water soluble phosphine along with the catalyst and also by resorting to biphasic catalysis.

Catalytic cycle using HRh(CO)(PPh₃)₃ Catalyst :

Tables 1 and 2 summaries the reaction parameters of four catalysts and their advantages and disadvantages

Table 1: Reaction parameters and n/iso ratio obtained with different catalysts

Catalyst (active form)	Reaction parameters	(n/iso) ratio maximum
Co2(CO)8 [HCo(CO)4] Co2(CO)8/PR3 R = n-Bu and other similar groups HRh(CO)(PPh3)3	Pressure 200-300 bar Temp. 110-160oC Cat. Concentration* 0.1-1.0 Pressure 50-100 bar Temp. 160-200oC Cat. Concentration* 0.6 Pressure 15-25 bar	3:1 7:1 16:1

HRh(CO)(PR3)3 R = mC6H4SO3Na

Temp. 80-120oC Cat. Concentration* 0.01-0.05 Pressure 15-25 bar Temp. 80-120oC Cat. Concentration* 0.01-0.05

19:1

^*percentage of metal/olefin

Table 2: Advantages and disadvantages of various hydroformylation catalysts

Catalyst (active form)	Advantages	Disadvantages
HCo(CO)4 Co2(CO)8/PR3 R = n-Bu and other similar groups HRh(CO)(PPh3)3 HRh(CO)(PR3)3 R = mC6H4SO3Na (water soluble), biphasic catalysis	Relatively less alkene hydrogenation(< 2%). Catalyst decomposition redused due to increased thermal stability of catalyst. Better n/iso selectivity due to increased hydritic character of H. Low pressure (15–25 bar) aand low temperature. High n/iso ratio selectivity (94%) Easy catalyst recovery and less loss, low catalyst concentration required. Less olefin	Thermal instability and volatility of (HCo(CO)4 leads to the deposition of Co or its oxide on the reactor. High pressure of CO (200-300 bar) required to prevent this brings in operational difficulties. Rate of reaction α 1/[CO]. So, increase in CO pressure reduces rate. Low n/iso ratio Pressures and temperatures still on the higher side. Lower reaction rate (at 180oC, the rate is only 20% of the rate of HCo(CO)4 operating at 145oC). Increased hydrogenation of alkenes (up to 15% loss of alkenes). Good for production of 2-ethylehexanol from propylene (up to 85% yield in a single reactor). Applicable only to C3 and C4 olefins as catalyst is thermally unstable at the high temperatures required for the removal of products by distillation. High cost of Rh in comparison to Co. Low rate of reaction due to reduced miscibility to higher alkenes with the aqueous phase of catalyst. Pressure required is on the higher side

hydrogenation(< 2 %). Applicable to long chain olefins as well

in comparison to HRh(CO)(PPh3)3.

FACTOR AFFECTING THE n/iso RATIO OF HYDROFORMYLATION PRODUCTS:

One of the directions of recent research in hydroformylationreactions has been to improve the  n/iso ratio of aldehyde products.The first major development was the discovery of a chelating biphosphine BISBI developed by Eastman Kodak..

For example, Rh with BISBI as ligand gave an n/isp ratioof 96:4 under mild condition. Studies on related biphosphines led to an even better n/iso ratio; for example, when PPh₃ groups in BISBI were replaced with dibenzophosphole units, it resulted is an n/isp ratio of 99.4:0.6. However, there was a problem with the catalyst stability.

Piet van Leeuwen and coworkers carried out systematic studies on a series of biphosphines by varying their bite angle. They observed that larger natural bite angles in the vicinity of 120^o favoured a higher n/isp ratio of hydroformylation products.

The use of phosphites as ligands instead of phosphines also led to a higher n/iso ratio. The percentage of linearity obtained for PPh₃ and P(OPh)₃ is very similar at low ligand concentrations. However, at higher ligand concentrations, phosphites give a better n/iso ratio of products. The electronic and steric effects of the substituents on the phosphites play a significant role in deciding the rate and selectivity of the reactions. One major finding from a comparative study is that higher the χ value of the phosphine/phosphite, higher the selectivity towards linear products.

Table 3 gives some examples indicating this observation. Based on these two findings, a series of biphosphite ligands having large bite angles were prepared and found to be useful for realising a high n/iso ratio of hydroformylation products.

Table 3: Product selectivity in hydroformylation with rhodium phosphite and phosphine catalysts for 1-heptene at 90^oC temperature and 7 bar pressure of CO/H_2.

R3P:R =	χ value	Linearity of product(%)
Ph PhO n-Bu n-BuO 4-Cl-C6H4O CF3CH2O	13 29 4 20 33 39	82 86 71 81 93 96

WATER-SOLUBLE RHODIUM CATALYSTS:

Ø  Water soluble catalyst are made using sulfonated PR₃ ligands (3,3′,3″-phosphanetriyltris(benzenesulfonic acid) trisodium salt; TPPTS)

Ø  Runs at mild conditions (at 18 bar and 85- 90°C)

Ø  Easily separated because water-soluble catalystsremain in aqueous phase and aldehyde is separated into organic phase with higher regioselective ratio between linear and branch.

BIDENDATE PHOSPHINE Rh CATALYSTS

The present invention relates to a bidentate phosphine ligand, a phosphine phosphorus atom connected by a bridge group, the bridge group bidentate phosphine ligand containing one ortho position by two aryl groups fused ring system consisting of, an aryl group which are connected by two bridges, the first bridge with a -O- or -S- atom composed of a second bridge containing one oxygen, sulfur, nitrogen, silicon or carbon atom or a combination of these atoms one group, the two phosphorus atoms linked to two aryl groups in the ortho -O- or -S- atom of a bridge. This also relates to bidentate phosphine ligand containing catalyst systems for further one kind of a transition metal compound, this system can be used for thehydroformylation.

In Rh-catalyzedhydroformylation, the n:iso ratio increases with the bite angle = (preferred P–M–P angle) of a chelate phosphine, probably because these ligands facilitate the RE step in the mechanism. The Rh complex (9.27) of the wide bite angle ligand, BISBI, has proved particularly useful.

Ø  Over the past 20 years, research was focused on bidentate ligands because of remarkably increased regioselectivity between n/iso ratio of  aldehydes.

Ø   High regioselectivity is the related to the stereochemistry of complex combined with the electronic and steric factors of bidendate PR_3.

OTHER ASPECTS OF HYDROFORMYLATION: The overall effectiveness of other metals are compared with Co and Rh.

                             Rh >    Co >Ir>  Ru >Os>Mn>  Fe >  Cr, Mo, W, Ni, Re

Rel. Reactivity: 10⁴-10³    1      10^-1  10^-2    10^-3    10^-4    10^-6< 10^-6

 

ENANTIOSELECTIVE HYDROFORMYLATION:

Enantioselective hydroformylation is a relatively recent development in hydroformylation reactions. It is interesting to note that chiral aldehyde will be formed only when the addition of H₂/CO to the alkene occurs in the Markownikoff manner. In contrast to normal hydroformylation, a better n/iso ratio is preferred in enantioselective hydroformylation as the n isomer will be nonchiral. Initially, platinum based catalysts were tried; however, these gave poor n/iso ratio and were plagued by hydrogenation. The isomerisation of the alkene also occurred.It is interesting to note that hydroformylation in the Markownikoff sense will form only the chiral aldehydes.

Rhodium based chiral catalysts such as HRh(CO)₂(R,S)-BINAPHOS have been developed which give high n/iso ratio as well as good enantiomeric excess.

CONCLUSION:

·    Through the catalyzedhydroformylation reaction, olefins are converted into aldehydes; mechanism and corresponding energy calculation were demonstrated.

·    The different type of phosphine ligands and cobalt- and rhodium-based catalysts were introduced; bidendate phosphine Rh catalyst showed the highest ratios of linear to branched aldehyde even at ambient conditions.

·   Enantio- and regio-selectivity can be increased if specifically designed ligands on Rh§ catalysts are used

BIBLIOGRAPHY

•    B D Gupta and A J Elias, Basic Organometallic Chemistry : Concepts, Synthesis and   Application, Second Edition,Universities Press( India) Private Limited,2010, 2013, pp   245-252.
•    Ajai Kumar, Organometallic and Bioinorganic Chemistry, First Edition,Aaryush     Education, 2014, pp 7-9 to 7-11.
•   Robert H. Crabtree, The Organometallic Chemistry of the Transition Metals, Sixth     Edition, John Wiley & Sons, 2014, pp 242-245.
•   "Organometallic Chemistry", Spessard and Miessler
•   Chem. Rev. 2012, 112, 5675 6 – 5732

•    L. H. Slaugh and R. D. Mullineaux11 , J. Organometal. Chem., 1968, 13, 469

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