Paracyclophane ligand

When we first ventured into the field of [2.2]paracyclophane ligand synthesis, successful applications of such ligands were relatively rare [2]. The most prominent example was clearly the PHANEPHOS ligand developed by Rossen and Pye [3], who have found several successful applications in asymmetric hydrogenation reactions. A comprehensive survey of [2.2]paracyclophane-based ligands can be found in recent reviews [4, 5]. 

The structures of 204-207 have been established in the solid state by single crystal X-ray diffraction. Clusters 205 and 206 are based on the Ru6C octahedral skeleton. The [2.2]paracyclophane ligand in 205 coordinates over a triangular metal face, and in 206 one [2.2]paracyclophane adopts a similar face-capping bonding mode and the other coordinates in a terminal mode to an apical Ru atom. The metal polyhedron in 204 is relatively open in comparison to the octahedron having only nine Ru-Ru bonds. A carbide atom occupies the central cavity and interacts with five of the six Ru atoms. 

Several new oxazolinyl ligands were synthesised and applied in asynunetric synthesis. Paracyclophane ligand 177 was applied to the enantioselective alkylation of aldehydes with diethylzinc <01TA529>. 

Axial interaction of an aryl unit with the [Ru-Ru] bond tends to increase the metal-metal distances. Petrukhina et al. isolated two complexes by codeposition of 2 and 8 with [2.2]paracyclophane to yield (17) and (18), respectively (Scheme 11) [68]. A sandwich structure with the aromatic moiety entrapped between two dimetal imits is observed. The [Ru-Ru] distance increases from 2.627(9) A in 8 to 2.656(3) A in 18 on axial coordination of the arene moiety. Similarly, a change of [Ru-Ru] distance from 2.673(1) A in 2 to 2.678(3) A in 17 was also observed. The inter-centroid distances between the two rings in [2.2]paracyclophane group are shorter (2.974(4) A in 17 and 2.982(5) A in 18) compared to the free [2.2] paracyclophane ligand (3.09 A). This supports the hypothesis that coordination of aryl group to the electrophilic ruthenium centers allows the aromatic decks to move closer which also increases the [Ru-Ru] bond distances. 

Transition metal supramolecular arrays can be constructed with the aid of n-bonding interactions by using various [2n]cyclophanes and polymetallic clusters. This area is only in its infancy and its potential is illustrated by the formation of a supramolecular hexagonal two-dimensional network by using metal centers linked through the [2.2.2]paracyclophane ligand [102],... 

Cross-coupling reactions between amines and aryl halides or pseudohalides have been employed for the preparation of a number of chiral, nonracemic ligands for asymmetric catalysis. For example, early studies by Buchwald illustrated that chiral amino binaphthol derivatives could be generated by Pd-catalyzed Af-arylation of binaphthol-derived triflates (Eq. 74) [417]. A similar strategy was employed by Erase for the synthesis of planar-chiral [2.2]paracyclophane ligands (Eq. 75) [418]. The A -arylation of [2.2]paracyclophane-derived triflates has also been used for the construction of planar-chiral benzimidazoles [419]. The IV-arylation of a substituted pyrrolidine with 4-bromopyridine played a key role in the synthesis of a chiral nucleophilic catalyst related to DMAP [420]. 

In 2004, Bolm et al. reported the use of chiral iridium complexes with chelating phosphinyl-imidazolylidene ligands in asymmetric hydrogenation of functionalized and simple alkenes with up to 89% ee [17]. These complexes were synthesized from the planar chiral [2.2]paracyclophane-based imida-zolium salts 74a-c with an imidazolylidenyl and a diphenylphosphino substituent in pseudo ortho positions of the [2.2]paracyclophane (Scheme 48). Treatment of 74a-c with t-BuOLi or t-BuOK in THF and subsequent reaction of the in situ formed carbenes with [Ir(cod)Cl]2 followed by anion exchange with NaBARF afforded complexes (Rp)-75a-c in 54-91% yield. The chela-... 

Scheme 1.29 Test reaction with [2.2]paracyclophane-type ligands with oxazoline pendant.

In recent years, many related ligands have been produced. Bolm and coworkers have produced new carbenoid catalysts 13 and 50 (Fig. 30.12) based on a paracyclophane backbone [23, 36]. To date, however, enantioselectivities have been modest with these catalysts. 

Pye and Rossen have developed a planar chiral bisphosphine ligand, [2.2]PHANE-PHOS, based on a paracyclophane backbone (Scheme 1.6) [69]. Moreover, the ortho-phenyl substituted NAPHOS ligand, Ph-o-NAPHOS, has been successfully applied for the rhodium-catalyzed hydrogenation of a-dehydroamino acid derivatives [70]. 

Earlier, [2.2]paracyclophane-based N,0-ligands had been employed in the dialkylzinc and the alkenylzinc addition to aldehydes a) S. Dahmen, S. Erase, Chem. Commun. 2002, 25 b) S. Hofener, F. Lauterwasser, S. Erase, Adv. Synth. Catal. 2004, 346, 755 c) review S. Erase,... 

Substituted [2.2]Paracyclophane Derivatives as Efficient Ligands for Asymmetric 1,2- and 1,4-Addition Reactions... 

The element of planar chirality plays a pivotal role in many modern ligand systems. The particularly huge success of ferrocenyl ligands has not been matched by any other chiral backbone to date. Metallocene and metal-arene-based ligand backbones exhibit the common feature that they become planar chiral only upon addition of (at least) two substituents on one ring fragment. [2.2]Paracyclophanes, however, need only one substituent (Fig. 2.1.3.1) to be chiral. 

The use of planar-chiral [7] and central-chiral ligands based on paracyclophane systems was still a relatively unexplored frontier, with notable exceptions in the reactions examined by the Rozenberg group [8] and the Berkessel group [9]. 

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