Rhodium styrene derivatives

Hydroboration of styrene derivatives has been extensively studied, and perhaps these are the best substrates to consider in a discussion of the efficiency and selectivity of the catalysts (Table 1-1). A neutral rhodium-phosphine complex... 

The two-substituted-Quinazolinap-derived rhodium complexes proved extremely efficient catalysts for the hydro-boration-oxidation of vinylarenes (Table 6). For styrene derivatives, in most cases quantitative conversions were obtained after just 2 h at the relevant temperature (entries 1-6). Higher enantioselectivities were afforded with a 4-methoxy substituent (up to 95% ee, entry 3) compared to the 4-chloro or unsubstituted styrene analogs (entries 5 and 1), a trend also observed in hydroboration with rhodium complexes of QUINAP 60. This highlights that both the electronic nature of the substrate combined with the inherent steric properties of the catalyst are important for high asymmetric induction. It is noteworthy that in most cases, optimum enantioselectivities were afforded by the... 

Enantioselective Alkene Hydroboration. Substituted styrene derivatives undergo rhodium-catalyzed hydroboration in the presence of a catalytic amount of the title reagent. However, optimal regio- as well as enantioselection is attained by using the corresponding reagent derived from pseudoephedrine (eq 2). ... 

Similar transformations using diphenylphosphine oxide and diethyl thio-phosphite have also been performed. Rhodium prolinate second generation complex Rh2(S-TISP)2 has been used as a very effective catalyst promoting conversion of dimethyl aryldiazomethyl phosphonates (192) into the stereo-chemically defined donor/acceptor substituted rhodium carbenoid intermediates. The latter are capable of cyclopropanation of various styrene derivatives affording cyclopropylphosphonates (193) in high yields (85-96%), diastereo-selectivity (>98% de), and enantioselectivity (76-92% ee) ( Scheme 52). A... 

P. Knochel and co-workers used diphosphines as ligands in the rhodium-catalyzed asymmetric hydroboration of styrene derivatives." The best results were obtained with the very electron rich diphosphane, and (S)-1-phenylethanol was obtained in 92% ee at -35 °C, with a regioselectivity greater than 99 1 (Markovnikoff product). A lower reaction temperature resulted in no reaction, while a higher temperature resulted in lower enantioselectivity and regioselectivity. The regioselectivity was excellent in all cases. Irrespective of the electronic nature of the substituents, their position and size had a profound effect on the enantioselectivity. 

Demay, S., Volant, F., Knochel, P. New C2-symmetrical 1,2-diphosphanes for the efficient rhodium-catalyzed asymmetric hydroboration of styrene derivatives. Angew. Chem., tnt. Ed. Engl. 2001,40,1235-1238. 

Optically active alcohols, amines, and alkanes can be prepared by the metal catalyzed asymmetric hydrosilylation of ketones, imines, and olefins [77,94,95]. Several catalytic systems have been successfully demonstrated, such as the asymmetric silylation of aryl ketones with rhodium and Pybox ligands however, there are no industrial processes that use asymmetric hydrosilylation. The asymmetric hydrosilyation of olefins to alkylsilanes (and the corresponding alcohol) can be accomplished with palladium catalysts that contain chiral monophosphines with high enantioselectivities (up to 96% ee) and reasonably good turnovers (S/C = 1000) [96]. Unfortunately, high enantioselectivities are only limited to the asymmetric hydrosilylation of styrene derivatives [97]. Hydrosilylation of simple terminal olefins with palladium catalysts that contain the monophosphine, MeO-MOP (67), can be obtained with enantioselectivities in the range of 94-97% ee and regioselectivities of the branched to normal of the products of 66/43 to 94/ 6 (Scheme 26) [98.99]. 

New inq)etus for asymmetric hydroformylations came primarily from Takayas phosphine-phosphinite ligand (BINAPO) 4 which constitutes an enormous breakthrough l In combination with rhodium the BINAPO ligand gave enantioselectivities up to 95% and i/n ratios > 86/14 in the hydroformylation of substituted styrene derivatives. Conversions are > 99% at substrate/catalyst ratios between 300 and 2000. Shortly afterwards, similar catalytic results were reported by Union Carbide with chiral diphosphinite ligands, e.g. 5 . After many years of stagnation these new catalysts now point the way towards fixture developments in asymmetric hydroformylation. 

Rhodium-catalysed hydroboration is a powerful tool for introducing chirality into a styrene-derivative. (Figure 1.1)[2] This was in competition to the established route based on chiral resolution using separation of diastereomers formed from reaction of the racemic amines with homo-chiral acids (natural pool). However, although the process appeared favourable from the chemical synthetic route, the process was practically impossible owing to there being no supplier of catecholborane on large scale at the time. 

Figure 1.1 Rhodium-catalysed hydroboration of a styrene derivative.

Dubbaka and Vogel reported on rhodium-catalysed Mizoroki-Heck-type reactious employing arenesulfonyl chlorides as inexpensive electrophiles. Thereby, styrene derivatives were arylated under base-free conditions. Again, rhodium-alkene complex 89 was found to be significantly more active than phosphine-derived rhodium species (Scheme 10.32) [60],... 

Kostas [46] tested the P,Af-ligand 1 in the rhodium-catalyzed HAM of styrene with morpholine (Figure 5.17). Beller discovered 2-(dicyclohexylphosphino)-l-(2-methoxyphenyl)-l//-imidazole (2) as a highly efficient co-catalyst in ruthenium-catalyzed HAM of various olefins with piperidine [44,47]. Dppf was suggested for iso-regioselective HAM of styrene derivatives with anilines [48]. 

The obtained stereoselectivities are in the same range as those of other ruthenium- or rhodium-based catalysts. As the carbene addition occurs with trans (exo) diastereoselectivity, the more thermodynamically stable trans (exo) isomer is preferentially formed. It was observed that two steric effects have influence on the stereoselectivity the steric bulk of the 4-position of the styrene derivatives and the steric bulk of the diazoacetate used. In both cases the cisitrans ratio decreases with the... 

Rhodium-supported catalysts were prepared by impregnating rhodium(I) and rhodium(III) complexes with and without heteropolyacids for the hydroformylation of styrene derivatives. A clear effect of the heteropolyacid H3PWi204o-yH20 in increasing the catalytic activity of the rhodium-supported catalyst was found [89]. 

Chelucci et al. [41] synthesized further chiral terpyridines derived from (-)-yd-pinene, (-i-)-camphor, and (-l-)-2-carene and tested their ability to chelate copper or rhodium for the asymmetric cyclopropanation of styrene. The copper catalysts were poorly efficient and selective in this reaction. The corresponding rhodium complexes led to the best result (64% ee) with the ligand derived from (-l-)-2-carene (ligand 33 in Scheme 17). 

In 2000, Claver et al. reported the synthesis of novel chiral S/P ligands with a xylofuranose backbone. These thioether-phosphite ligands derived from carbohydrates were investigated for the rhodium-catalysed hydroformylation of styrene but, in spite of good conversions (>99%) combined with excellent... 

Cyclopropanation of C=C bonds by carbenoids derived from diazoesters usually occurs stereospeciflcally with respect to the configuration of the olefin. This has been confirmed for cyclopropanation with copper 2S,S7,60 85), palladium 86), and rhodium catalysts S9,87>. However, cyclopropanation of c -D2-styrene with ethyl diazoacetate in the presence of a (l,2-dioximato)cobalt(II) complex occurs with considerable geometrical isomerization88). Furthermore, CuCl-catalyzed cyclopropanation of cis-2-butene with co-diazoacetophenone gives a mixture of the cis- and trans-1,2-dimethylcyclopropanes 89). 

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