Aryl transfer reactions

The development of ferrocene 9 was part of our studies on planar-chiral compounds, which also involved the synthesis of other scaffolds such as chromium-tricarbonyl arenes [15], sulfoximidoyl ferrocenes [16], and [2.2]paracyclophanes [17]. In aryl transfer reactions, however, ferrocene 9 proved to be the best catalyst in this series, and it is still used extensively today. 

The same synthetic strategy as in the synthesis of planar-chiral ferrocenes was applied to the preparation of rheniumtricarbonyl 14, which has also been studied as a catalyst in aryl transfer reactions [21], Subsequently, this chemistry has been extended, and various catalytic applications of cyrhetrenes 15, 16 (AAPhos), and related derivatives have recently been demonstrated [22]. 

Later, the oxazolines 25 were examined to study the effects of matched/mismatched combinations of stereogenic centers on catalyzed aryl transfer reactions to aldehydes. Of these mandelic acid-derived catalysts, 25b gave the best results in terms of enantioselectivity (up to 35% ee), while diastereomer (l ,S)-25b proved to be superior to (S,S)-25b with respect to catalyst activity [29]. With both compounds, the absolute configuration of the product was determined by the oxazo-line moiety. 

The use of hydroxy oxazolines 26 in catalytic aryl transfer reactions was investigated subsequently (specifically, addition of BPhs to p-methoxybenzaldehyde in the presence of dimethyl poly(ethylene glycol) (DiMPEG) for details of this... 

Two strategies for the synthesis of enantiomerically enriched diaryl methanols 27 are apparent first, asymmetric reductions of the corresponding diaryl ketones 36 [33], and, second, enantioselective aryl transfer reactions to the respective benzaldehyde derivatives 37 (Scheme 2.1.2.5) [34, 35]. 

Although the precise reason for the MPEG-effect [50] is still unknown, we assume that catalytically active achiral metal species are inactivated by trapping with the polyether and that the catalyzed asymmetric reaction path thereby becomes more dominant. This phenomenon had already been observed in aryl transfer reactions catalyzed by MPEG-bound ferrocene 39 [44], and it can be used to perform catalyses with lower catalyst loadings [50]. 

This protocol has attracted much attention recently, and several groups (those of Chan, Katsuki, Braga, and Zhao) have applied their catalysts to aryl transfer reactions from boronic acids (or derivatives thereof) [51]. 

With the aim of improving understanding of the mechanism of the ferrocene-catalyzed aryl transfer reactions from organozincs to aldehydes, DPT calculations were performed in close collaboration with Norrby [64]. As a result, the experimentally observed higher reactivity of the phenyl transfer to aldehydes compared... 

After screening several compounds, including ferrocene 9 or cyrhetrene 14, which had previously been applied in the aryl transfer reactions to aldehydes and led to excellent enantioselectivities, we found, in collaboration with Erase, that only [2.2]paracyclophane 48 was capable of forming a catalyst system (Scheme 2.1.2.15). Subsequently, the substrate scope was evaluated, and several diaryl-methylamines 47 were obtained with excellent enantioselectivities (up to 97% ee) [69, 70]. 

For a general review of catalyzed asymmetric aryl transfer reactions, see C. Bolm, J. P. Hildebrand, K. Muniz, N. Hermanns, Angew. Chem. 2001, 113, 3382 Angew. Chem. Int. Ed. 2001, 40, 3284. 

E. Asymmetric Aryl Transfer Reaction to Carbonyl Compounds. 566... 

The absolute configuration of 1 was set by conjugate addition to the oxazoline 3 followed by trapping of the product anion, following the precedent of Meyers. Reduction of the oxazoline to the alcohol, followed by thionocarbonate formation then set the stage for the key aryl transfer reaction. 

The construction of a borole ring that is part of a metal complex from zirconium-coordinated 1,3-dienes and tris(pentafluorophenyl)borane has been recently introduced. Tris(pentafluoro-phenyl)borane is widely used as an activator of metallocene-based polymerization catalysts because of its resistance to aryl-transfer reactions, it acts as the boron source. The reaction utilizes the zwitterionic 14-electron bis(allyl)zirconium complexes 72 and 73 which are converted to the corresponding complexes 74 and 75 via activation of one of the C-H bonds of the B-CH2 moiety... 

Oxone has been used with triarylbismuth and copper salts to effect aryl transfer reactions. Aryl groups can be transferred to alcohols to makephenyl ethers (eq 91). However, two coordination sites on the substrate are necessary. 

Scheme 7.45 Use of arylboronic acids in the Fe-catalyzed asymmetric aryl transfer reaction, as reported by Bolm and Rudolph [74].

F. Nawaz, K. Mohanan, L. Charles, M. Rajzmann, D. Boime, O. Chuzel, J. Rodriguez, Y. Coquerel, Temporary intramolecular generation of pyridine carbenes in metal-free three-component C-H bond functionalization/aryl-transfer reactions, Chem. Eur. J. 19 (2013) 17578-17583. 

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