Hydroformylation of internal alkenes

1 Hydroformylation ofless reactive internal and functionalized alkenes. 

Bulky monophosphite hgands proved to be very useful for the functionalization of very unreactive substrates. Already in their first study van Leeuwen and Roobeek obtained relatively high rates for the hydroformylation of substrates such as cyclohexene and limonene. [8]. Van Rooy performed a systematic study to the rhodium catalyzed hydroformylation of substituted alkenes and compared the reaction rates with the triphenylphosphine system [42]. The bulky monophosphite derived catalyst was up to two orders of magnitude faster and gave acceptable rates using substrates for which the Wilkinson hydrofomylation catalyst gave hardly any activity. 

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Recently, a new class of phosphabarrelene/rhodium catalysts has been developed, which for the first time allows for hydroformylation of internal alkenes with very high activity and which proceeds essentially free of alkene isomerization [36-38]. Two examples, results of hydroformylation of an acyclic and a cyclic internal alkene substrate, are depicted in Scheme 2. 

Scheme 2 Position-selective hydroformylation of internal alkenes with a rhodium(I)/-phosphabarrelene catalyst...

Figure 7.3 gives an overview of the reactions involved in the hydroformylation of internal alkenes to linear products. It has been suggested that cobalt, once attached to an alkene, runs along the chain until an irreversible insertion of CO occurs. Thus, the alkene does not dissociate from the cobalt hydride during the isomerisation process. There is no experimental support for a clear-cut proof for this mechanism. In alkene polymerisation reactions this type of chain running has been actually observed. 

They constitute the first rhodium phosphine modified catalysts for such a selective linear hydroformylation of internal alkenes. The extraordinary high activity of 32 even places it among the most active diphosphines known. Since large steric differences in the catalyst complexes of these two ligands are not anticipated, the higher activity of 32 compared to 31 might be ascribed to very subtle bite angle effects or electronic characteristics of the phosphorus heterocycles. 

Since hydroformylation of internal alkenes is also relatively fast with this catalyst the overall linearity obtained may become rather low (20-30%). This explains the low linearity of a non-optimised reaction using a similar bulky-phosphite R = 2,6-Me2C6H30 quoted in Table 8.5. 

Kuil, MSoltner, T., van Leeuwen, P.W.N.M. and Reek, J.N.H. (2006) High-precision catalysts Regioselective hydroformylation of internal alkenes hy encapsulated rhodium complexes. J. Am. Chem. Soc., 128, 11344-11345. 

A particularly interesting problem is to develop catalysts that exhibit satisfactory activity in the hydroformylation of internal alkenes and produce linear aldehydes preferentially. The transformation of a technical mixture of octenes containing all isomers into octanal is of practical importance ... 

The carbonyl [Ru3(CO),2] is a good cocatalyst for the low pressure hydroformylation of internal alkenes using the classic rhodium phosphine [HRh(CO)(PPh3),] system in the presence of an excess of triphenylphosphine (P/Rh = 200) (22). Starting from a mixture of hex-2- and hex-3-ene, the addition of [Ru3(CO),2l (Rh/Ru = 1/1) increased both the reaction rate and the n/iso ratio of heptanals. More recently, Poilblanc and coworkers (23) have prepared a mixed ruthenium-rhodium complex formulated as [CIRh(/i-CO)(//-dppm)2Ru(CO)2] (dppm is Ph2PCH2PPh2). This species shows catalytic activity in the hydroformylation of pent-l-ene at 40 bar (H2/C0= 1/1) and 75°C. Conversion to hexanals was 90% in 24 hours and the linearity reached 70%. No further report has appeared to determine the role of the two metals in this catalysis. 

The 2-alkyl species shown in Figure 23 can also be formed from 2-alkenes and this species should have the same energy. We know that in most catalytic systems the hydroformylation of internal alkenes is one or two orders of magnitude slower. From the viewpoint of microscopic reversibility this leads to an interesting point. It would seem therefore that in most systems a relatively fast hydrogen exchange must take place for internal alkenes while no or little hydroformylation occurs. 

Isomerization of alkenes by these cobalt catalysts takes place faster than hydroformylation itself. Mixtures of similar proportions of 1-hexanal and 2-hexanal are obtained irrespective of whether the starting material is 1-pentene or 2-pentene. 3-Heptene affords 1-octanal as the major product. Hydroformylation of internal alkenes such as 3-heptene is difficult, because of steric hindrance. Although 3-heptene is the most stable isomer and the formation of 1-heptene from it by isomerization is thermodynamically disfavoured, any 1-heptene formed can be hydroformylated very much faster than 2- or 3-heptene. 

A new class of phosphabarrelene (13)-rhodium catalysts was described that allow the hydroformylation of internal alkenes with very high activity and proceed free of alkene isomerization [36]. 

A new class of phosphabarrelene/rhodium catalysts was found to display very high activity toward hydroformylation of internal alkenes with unusually low tendency toward alkene isomerization (Equation 7.6) [54]. 

Highly selective halide anion-promoted palladium-catalyzed hydroformylation of internal alkenes to linear alcohols was studied. A (bcope)Pd(OTf)2 complex (bcope) = bis(cydooctyl)phosphinoethane with substoichiometrically added halide anion was found to be a highly efficient homogeneous catalyst to selectively convert internal linear... 

Beller et al. used NAPHOS 27, a BISBI analog backbone, substituted with 3,5-bistrifluoromethylphenyl groups as the ligand in rhodium-catalyzed hydroformylation of internal alkenes and obtained high selectivities to the linear product. 

Breit B, Fuchs E (2004) Phosphabarrelene-nrhodium complexes as highly active catalysts for isomerization free hydroformylation of internal alkenes. Chem Commun 694-695... 

Fuchs E, Keller M, Breit B (2006) Phosphabarrelenes as ligands in rhodium-catalyzed hydroformylation of internal alkenes essentially free of alkene isomerization. Chem Eur J... 

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