Elsevier

Inorganica Chimica Acta

Volume 357, Issue 10, 20 July 2004, Pages 3079-3083
Inorganica Chimica Acta

Rhodium-catalyzed hydro(deuterio)formylation of vinylidenic olefins containing a phenyl and a pyridyl group: crucial role of the β-hydride elimination in determining regio- and chemoselectivity

https://doi.org/10.1016/j.ica.2004.03.015Get rights and content

Abstract

Deuterioformylation of the vinylidenic substrates 1-phenyl-1-(n-pyridyl)ethenes, in the presence of phosphine modified Rh4(CO)12 as catalyst precursor, was carried out at 100 atm of CO and D2 (1:1), 80 °C and at partial substrate conversion.

The direct 2H NMR analysis of the crude reaction mixture allowed to conclude that, under these conditions, the branched alkyl rhodium intermediate is almost exclusively formed. It can β-eliminate, undergo migratory insertion or oxidative addition of deuterium in a different degree depending on the position of the nitrogen atom with respect to the olefinic double bond, accounting for the observed different chemo- and regioselectivity.

The products distribution in the rhodium catalyzed hydro(deuterio)formylation of the isomeric vinylidenic substrates 1-phenyl-1-(n-pyridyl)ethenes depends on the different evolution of the exclusively formed tertiary alkyl rhodium intermediate. Deuterioformylation experiments demonstrated that a β-elimination process occurs for 3 and 4 isomers but not for 2 isomer accounting for the observed different chemo- and regioselectivity.

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Introduction

It is known that, under rhodium catalyzed hydroformylation, ethenes 1,1-disubstituted with a phenyl and/or an alkyl group exclusively give the linear aldehyde via the attack of the formyl group on the double bond terminal carbon atom [1], [2]. By contrast, Botteghi et al. [3] reported that vinylidenic olefins 1,1-disubstituted with both a phenyl and a pyridyl group, give, under the same experimental conditions, different chemo- and regioselectivity depending on the relative position of the double bond and the annular nitrogen atom. Thus, while 1-phenyl-1-(2-pyridyl)ethene (2Py) gives the tertiary aldehyde isomer only, 1-phenyl-1-(3-pyridyl)ethene (3Py) mainly gives the linear aldehyde as well as 1-phenyl-1-(3-pyridyl)ethene (4Py). In all cases, a consistent amount of hydrogenation product was obtained which is the almost exclusively formed species in the case of 4Py (Scheme 1).

Taking into account the electronpoor character of the pyridyl group and the presence of a positive charge on both annular positions 2 and 4, the different behavior described above is quite surprising. In order to obtain better insight on this aspect, we submitted the three ethene isomers under rhodium catalyzed deuterioformylation experiments interrupted at partial substrate conversion. As previously reported for vinyl substrates [4], [5] and vinylidenic ones [2], the examination of the 2H NMR spectra of the crude reaction mixture gave direct informations on the occurrence of the β-hydride elimination process, accounting for the observed chemo- and regioselectivity.

Section snippets

Results

Deuterioformylation reactions of the vinylidenic olefins 1-phenyl-1-(2-pyridyl)ethene (2Py), 1-phenyl-1-(3-pyridyl)ethene (3Py), and 1-phenyl-1-(4-pyridyl)ethene (4Py) were carried out in benzene, in a stainless steel autoclave (25 ml), at 80 °C and 100 atm total pressure (CO/D2=1:1).

Rh4CO12 modified with triphenylphosphine (Rh/P=1/4) was used as a catalyst precursor. The analysis of the reaction products was carried out by GC, GC-MS and 2H NMR techniques in order to establish the chemo- and

Discussion

The results reported above clearly point out that the insertion of the olefin into the Rh–D (H) bond mainly or exclusively gives the tertiary alkyl rhodium complex in all cases. In spite of their consistent steric hindrance, the presence of a polarizable and electronwithdrawing group on the double bond accounts for the preference for the tertiary alkyls with respect to the linear ones [7]. Depending on the relative position of the annular nitrogen atom and the vinylidenic group, the above

Experimental

Benzene was dried over molecular sieves and distilled under nitrogen. Rh4(CO)12 was prepared according to a well-known procedure [9]. 1-phenyl-1-(n-pyridyl)ethenes nPy were prepared as described in the literature [3](b), [10], [11]. GC analyses of the reaction mixtures were performed on a Perkin–Elmer 8500 chromatograph equipped with a 12mx0.22 mm BP1 capillary column, using nitrogen as carrier gas. 2H NMR spectra of the crude reaction mixture in benzene were recorded on a Varian VXR 300

Hydroformylation and deuterioformylation experiments: general procedure

A solution of nPy (2.75 mmol), Rh4(CO)12 (0.0106 mmol), and triphenylphosphine (corresponding to the desired P/Rh=1/4 molar ratio) in benzene (5 ml) was introduced by suction into an evacuated 25 ml stainless-steel autoclave. Carbon monoxide was introduced, the autoclave was then rocked and heated to 80 °C, and hydrogen or deuterium was rapidly introduced to 100 atm total pressure. When the gas absorption reached the value corresponding to the desired conversion, the reaction mixture was

Acknowledgements

Financial support by MIUR is gratefully acknowledged.

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