Origin of the regioselectivity Influence of the nature of the substrate
As previously shown by deuterioformylation experiments, when the reaction is carried out at low temperature, the formation of alkyl-metal intermediates is not a reversible step. Under these conditions the regioselectivity observed for aldehydic isomers is directly determined in the step at which the alkyl metal intermediates are formed. Taking into account
the structure of the linear and branched alkyl-metal intermediates it is possible to explain why the branched aldehydes are strongly favored in the case of styrene or other functionalized substrates, whereas in the case of simple 1 -alkenes approximately equal amounts of aldehydic isomers are formed.
As shown in Figure 9, the metal-carbon bond in the alkyl-rhodium intermediates is polarized with a partial positive charge on the metal and a partial negative charge on the carbon atom. When this carbon atom is bonded to a strongly polarizable group (i.e. -C6H5) or to an electron withdrawing group (i.e. -F, -OR, -CH2OR, -CF3), the partial negative charge on the carbon atom is better delocalized owing to the inductive effect in the branched isomer 2bthan in the linear one 2i [12b-c].
When R is an electron donor group (n-alkyl group), no delocalization of the partial negative charge occurs for either isomer. As a consequence the branched and linear alkyl intermediates are formed in similar amounts, hence so are the corresponding aldehydes (Figure 9).
However, when the alkyl group bonded to the vinyl moiety has a secondary or tertiary structure, steric hindrance plays in crucial role on the regioselectivity, causing the linear aldehyde to predominate.
R " alkyl
Figure 9. Stabilization of alkyl-rhodium intermediates arising from the hydroformylation of different alkenes
As far as the vinylidenic substrates are concerned, deuterioformylation of phenyl substituted vinylidenic alkenes gives interesting information about the formation of a tertiary alkyl intermediate under reaction conditions. Indeed, the formation of vinylidenic alkenes deuterated at the terminal position to a larger extent than the linear aldehyde, demonstrates that the branched alkyl predominates over the linear one. As previously mentioned, this is due to the higher stabilization induced by the two phenyl groups adjacent to the carbon-rhodium bond. However the migratory insertion on to the CO coordinated to the metal in the case of tertiary alkyls is prevented by steric reasons. Thus it seems evident that the behavior of the two isomeric alkyl-rhodium intermediates is completely different: while the primary one is converted into the linear aldehyde, the tertiary one exclusively undergoes P-hydride elimination, regenerating the starting alkene [1le].
In conclusion, the 2H-NMR analysis of crude deuterioformylation products derived from vinyl or vinylidenic aromatic substrates is a direct and simple way to detect the different behavior of a primary, secondary and tertiary alkyl-metal intermediate, related to the P-hydride elimination process under typical hydroformylation conditions.
2.4.2 Influence of the reaction parameters
As far as the influence of reaction parameters, observed for vinyl and allyl substrates, is concerned, the increase of linear aldehyde with increasing temperature can be easily explained on the basis of the different behavior of the alkyl-rhodium intermediates under the reaction conditions. Thus the linear alkyl mainly undergoes the migratory insertion process and, hence, gives the linear aldehyde. In contrast, the branched one undergoes carbonylation only partially, mainly providing P-hydride elimination. It is to be noted that the n complex derived from the above elimination process regenerates both the linear and the branched alkyls. Thus the whole process brings about a partial isomerization of the branched alkyl isomer to the linear one and hence determines an increase of linear aldehyde. The different behavior of isomeric alkyl-rhodium intermediates could account also for the increase of linear aldehyde with decreasing CO and H2 pressure. At high gas pressure both the intermediate alkyls are forced to take part in the carbonylation to provide the aldehydic products. At low pressure, the P-elimination process becomes competitive with the acyl formation and with the subsequent oxidative addition of H2. Because the above elimination process is favored in the case of branched alkyl-metal species, the final result will be an increase of linear aldehyde.
On this basis it is possible to explain the results obtained by Garland in the hydroformylation of styrene under relatively mild reaction conditions. It is plausible that the b:l ratio = 66/34 obtained at 40 °C, which is lower than the one observed for the acyl-rhodium species (3b/3i = 87.5/12.5), is due to the P-elimination process which is much more favorable for the branched alkyl intermediate than for the linear one.
Phosphine ligands, when employed in excess with respect to rhodium, generally block the P-elimination process, as shown by deuterioformylation experiments carried out by Casey [27] and Takaya [7a], thus accounting for the low variation of regioselectivity with temperature obtained in the presence of phosphine-modified precursors [20b].
It is to remark that in the hydroformylation of styrene, the most investigated vinyl aromatic substrate, the predominance of the branched aldehyde at room temperature is higher with unmodified rhodium precursors than with phosphine-modified ones [4, 7c, 28]. In this context, when hydroformylation of styrene with chiral phosphines occurs without asymmetric induction and with a large prevalence of the branched aldehyde (> 96%), it is likely that unmodified rhodium-catalysts are also present in the reaction mixture [ 14, 29, 30].
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