Ligands in hydrocyanations

The steroidal 1,4-diphosphanes 3x- and 3/ -(diphenylphosphino)-2a-[2-diphenylphosphi-no)ethyl]-5a-cholestane and their 5//-benzo[h]phosphindole derivatives are also used as chiral ligands in hydrocyanation. While the diphenylphosphane ligands lead to moderate optical yields in the hydrocyanation of norbornene in the presence of Pd(BDA)2 (21% yield, 98% product selectivity, up to 39 % ee), the phosphole ligands only give trace amounts of product with no asymmetric induction detectable38. 

A variety of ligands other than phosphites are of course also important in hydrocyanation. The isolation of the 16-electron complex Ni[P(0-o-tolyl)3]3 (17) provided a remarkable opportunity for the study of how various components of the catalytic system interact with nickel. The addition of nitriles to this complex led to the observation of the first nitrile complex of zero-valent nickel (25). When the phosphite ligands are very bulky, as in the case of P(0-o-tolyl)3, a nitrile complex of formula (RCN)NiL3 is formed nearly quantitatively even in the presence of excess L, and is identified easily... 

For hydrocyanation of MVN with bidentate phosphinites zero-order in substrate and in HCN was again observed. Yet first-order in catalyst was obtained because of the bidentate ligand. The hydrocyanation followed in time should, in theory, fit a simple zero-order model (d[RCN]/dt= ki[Ni]). The catalyst is, however,... 

Spectrophotometry. The theory of spectra is far advanced. In many cases, compounds can be unambiguously identified by their ultraviolet, visible, or infrared spectra (e.g., see Smith s book [43]). As an example, the double bond of a CO ligand in a complex has a strong characteristic infrared vibration frequency whose exact value depends on the electronic properties of the coordinating metal these, in turn, are affected by the other substituents. In homogeneous catalysis by transition-metal complexes in particular, foremost among them hydrogenation, hydroformylation, and hydrocyanation, spectra have contributed much to the identification of reaction intermediates and thus of pathways. 

Fig. 2. Prototypical phosphinites used as ligands in asymmetric hydrocyanation Table 1 Hydrocyanation of 2-methoxy-6-vinylnaphthalene effect of sugar backbone...

Olefin hydrocyanation using palladium catalysts has been less well studied than with nickel. Nevertheless, zerovalent complexes of palladium, particulrly triarylphosphite complexes, hydrocyanate a wide range of olefins in useful yields (see Table 1). Early work reported the merit of excess phosphorus ligand to promote the reaction, and further paralleling the observations with nickel, Lewis acids have been used to improve catalytic activity. However, addition of ZnClj fails to improve nitrile product yield . Asymmetric induction in hydrocyanation results in optical yields of 30% in the synthesis of exo-2-cyanonorbomane using the chiral ligand DIOP, and studies on the stereochemistry of HCN and DCN addition to terminal alkenes and a substituted cyclohexene with the same catalyst have been reported. ... 

Trifluoroacetic acid hydrolysis of the isopropylidene protecting group in 79 without loss of the tosyl group provides (25,3 5)-1-0-tosyl-L-threitol (116), which can be cyclized with basic resin to afford (25,3 S)-l,4-anhydro-L-threitol (117) in good overall yield. Treating 117 with triphenylphosphine in pyridine affords the chiral diphosphinite diphin (118), which has been utilized as a ligand in asymmetric hydrogenation, hydrocyanation, and hydroformylation reactions [48] (Scheme 26). 

Allyl complexes have contributed significantly to the development of the organometallic chemistry of nickel and the applications of nickel complexes in organic synthesis, for example, nucleophilic attack on coordinated allyl ligands. In addition, allylnickel complexes have been identified as key intermediates in the oligomerization and cyclization of olefins and dienes. For example, the Ni(0)-catalyzed hydrocyanation of butadiene to adiponitrile, the main component of a major commercial process for the production of nylon, involves Ni (7r-allyl) intermediates. Moreover, the 77-rearrangements of allylnickel species have helped explain the facile isomerization of olefins in the presence of nickel complexes. The Ni-catalyzed homoallylation of carbonyl compounds with 1,3-dienes also involves Ni(7r-allyl) complexes this subject has been reviewed recently. New applications include the cleavage of G-G bonds in the deallylation of malonates, the preparation of cyclopentenones by carbonylative cycloaddi-... 

Coeriz, W., Kamer, P.C.J., van Leeuwen, P. W.N.M. and Vogt, D. (1997) Application of chelating diphosphine ligands in the nickel-catalysed hydrocyanation of alk-l-enes and omega-unsaturated fatty acid esters. Journal of the Chemical Society, Chemical Communications, 1521. 

Most of the major developments in homogeneous transition metal-catalyzed hydrocyanation since the early 1980s have been reviewed 62 67 considerable effort in this area has focused on enantioselective reactions.68 In this regard, the emphasis has been on complexes of the group 10 metals (particularly Ni and Pd) incorporating new classes of phosphine, phosphite, and phos-phinite ligands. 

Isomerisation is also an important step in the DuPont process for making adiponitrile (Chapter 11) in which internal pentenenitriles must be converted to the terminal alkene. The catalyst is the same as that used for the hydrocyanation reaction, namely nickel(II) hydrides containing phosphite ligands. 

The transition metal catalysed addition of HCN to alkenes is potentially a very useful reaction in organic synthesis and it certainly would have been more widely applied in the laboratory if its attraction were not largely offset by the toxicity of HCN. Industrially the difficulties can be minimised to an acceptable level and we are not aware of major accidents. DuPont has commercialised the addition of HCN to butadiene for the production of adiponitrile [ADN, NC(CH2)4CN], a precursor to 1,6-hexanediamine, one of the components of 6,6-nylon and polyurethanes (after reaction with diisocyanates). The details of the hydrocyanation process have not been released, but a substantial amount of related basic chemistry has been published. The development of the ligand parameters % and 0 by Tolman formed part of the basic studies carried out in the Du Pont labs related to the ADN process [1],... 

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