Phosphane catalysis

Based on nucleophilic addition, racemic allenyl sulfones were partially resolved by reaction with a deficiency of optically active primary or secondary amines [243]. The reversible nucleophilic addition of tertiary amines or phosphanes to acceptor-substituted allenes can lead to the inversion of the configuration of chiral allenes. For example, an optically active diester 177 with achiral groups R can undergo a racemization (Scheme 7.29). A 4 5 mixture of (M)- and (P)-177 with R = (-)-l-menthyl, obtained through synthesis of the allene from dimenthyl 1,3-acetonedicar-boxylate (cf. Scheme 7.18) [159], furnishes (M)-177 in high diastereomeric purity in 90% yield after repeated crystallization from pentane in the presence of catalytic amounts of triethylamine [158], Another example of a highly elegant epimerization of an optically active allene based on reversible nucleophilic addition was published by Marshall and Liao, who were successful in the transformation 179 — 180 [35], Recently, Lu et al. published a very informative review on the reactions of electron-deficient allenes under phosphane catalysis [244]. 

Keywords Group 13 metals (aluminum, gallium, indium, thalhum), Ambidentate ligands. Phosphorus-nitrogen bidentate ligands, Pyridyl phosphanes, Aminoiminophosphoranes, Lewis acid catalysis... 

One of the limitations of the use of asymmetric catalysis comes from the difficulties of separating the chiral catalyst from the reaction medium and recycling it. Such systems are generally formed with chiral phosphane and/or... 

The ring closure to form butenolides by palladium(O) catalysis can be combined with C,C bond linking, as shown by Ma and co-workers. If using tetrakis (triphenyl -phosphane)palladium(O), the products 272 are obtained from 268 (R1 = alkyl, R2 = H) and vinyl iodides or aryl bromides and iodides R3X [304]. The authors assume that... 

The intramolecular hydroamination of substrates 170 is catalyzed by a Pd(0) catalyst which is generated in situ from a Pd(II) precursor and a phosphane. One equivalent of acetic acid has to be added for efficient catalysis this is a hint of a hydropallada-tion mechanism. Meguro and Yamamoto obtained good yields of the vinyltetrahy-dropyrroles or the vinylhexahydropyridines 171 in that way (Scheme 15.53) [109]. 

Until 1998, only gold(III) was believed to be effective for catalyzing these processes because, as mentioned previously, only the gold(I) compound K[Au (CN)2] was tested and it was inert to catalysis. Fortunately, Teles et al. reported very strong activity in the addition of alcohols to alkynes when they used cationic gold( I) -phosphane complexes [14]. In this study, the aforementioned authors tested for the first time the suitability of nucleophilic carbenes that displayed even greater activity than other gold complexes, but they were unable to synthesize the subsequent cationic derivatives. 

The electrochemical allylation of carbonyl compounds by electroreductivc regeneration of a diallyltin reagent from allyl bromide and a Sn species leads to formation of homoallylic alcohols in yields of 70-90 % even in methanol or methanol/water (Table 7, No. 12) Bisaryl formation is possible also from aryl iodides or bromides in the presence of electro-generated Pd phosphane complexes (Table 7, No. 13) In the presence of styrenes, 1,3-butadienes, or phenyl acetylene the products of ArH addition are formed in this way (Table 7, No. 14) . The electroreductivc cleavage of allylic acetates is also possible by catalysis of an Pd°-complex (Table K No. 15)... 

For quite a long time chloroarenes were considered as too unreative for catalysis. However the significant improvements in various coupling reactions of chloroarenes by the use of electron rich phosphanes have rendered this statement as no longer valid. The use of these cheap intermediates as coupling partners in the synthesis of higher functionalized molecules of industrial relevance is now within reach. Since some of these ligands are already commercially available it s probably only a question of time when we will see the first industrial applications of these improved procedures. 

The status for homogeneous catalysis by gold in solution was dramatically transformed in 1998 by the results of Teles et al.11 This BASF group described the use of cationic Au1 complexes of the type [L-Au+] (where L is a phosphane, phosphite or arsine) for the addition of alcohols to alkynes in the presence of a Lewis or Brpnsted acid as a co-catalyst in methanol at 313 K. An example of such a reaction is the conversion of 3-hexyne to 3,3-dimethoxyhexane ... 

In recent years the synthesis of chiral and achiral tripodal phosphines and their application in homogeneous catalysis has been studied in more detail [2]. Enantiomerically pure tripodal ligands were synthesized from the corresponding trichloro compounds and chiral, cyclic lithio-phosphanes, e.g. 17, (Scheme 6) [21,22], Using a rhodium(I) complex of ligand 18, an enantiomeric excess of 89 % was obtained in the asymmetric hydrogenation reaction of methyl acetami-docinnamate (19). 

One of the major goals of our investigations has been the synthesis of phosphane-free water-soluble metal complexes with stable a-bonding that are suitable for two-phase catalysis. 

The description of the steric properties of phosphanes using the Tolman cone angle [113] proved to be an excellent concept capable of explaining many phenomena in the coordination chemistry of phosphanes and their applications, especially in homogenous catalysis. That there is a steric influence connected with NHC was noticed very early, in fact it was thought that the steric hindrance introduced by the N-mesityl substituents was a contributing factor in the isolation of the first stable carbene in 1991 as opposed to dimerisation to the known tetraaminoethylenes [1],... 

---