Norbornene hydrocyanation

Hydrocyanation is also catalyzed by [Pd(PPh3)4] (103) and [Pd P(OPh), 4] (132), again in both cases in the presence of excess ligand.604 Complex (132) is an effective catalyst for the addition of hydrogen cyanide to cyclic monoenes and dienes such as norbomene and norbornadiene 605-606 ethylene also reacted readily. The product obtained from norbornene was the exo isomer (equation 165). When norbornadiene was the substrate, some of the endo product was formed.605... 

Asymmetric hydrocyanation has now been achieved using norbornene and norbornadiene as substrates. The reduction of either [PdCl2(+)-DIOP] or PdCl2 in presence of (+)-DIOP led to a palladium(O) species formulated simply as [Pd(+)-DIOP]. This gave, in reaction (164), an optical yield of 30% for the 2-exo-cyanonorbornane formed. Norbornadiene with the same catalyst gave 2-exo-cyanonorborn-5-ene with an optical purity of 17%. When the ligand CHIRAPHOS (51) was used, the catalytic activity was greatly diminished.608,609 In addition to the review of the early work already mentioned, two more recent reviews of hydrocyanation have appeared.610,611... 

Figure 2. Chiral ligands used in the asymmetric hydrocyanation of norbornene.

Similar results arc obtained with addition of hydrogen cyanide or deuterium cyanide to dcutcr-ated and nondeuterated alkenes, norbornene and anchored cyclohexenes in the presence of Pd(Diop)2 as the catalyst precursor20-24. Steric requirements in the substrate are crucial. Thus, 7,7-dimethylnorbornene is not hydrocyanated and 4-/m-butylcyclohexene only gives the equatorial product (vide infra)10-24. 

In further studies of asymmetric hydrocyanation the phosphinite [(+ )-(35,4S )-tetrahydro-furan-3,4-diyl]bis(diphenylphosphinite [( + )-DIPHIN] was prepared in several steps from (+)-(/ ./ )-tartaric acid via the corresponding diol, and used as a chiral ligand. With a palladium catalyst precursor prepared from Pd(BDA)2 and this ligand, hydrocyanation of norbornene was achieved with only 7% op and 10% yield. Under the same conditions Diop gives 90% yield and 24% ee21. 

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. 

Similar results for the asymmetric hydrocyanation of norbornene (up to 70% yield with up to 38% ee) were obtained using an air-stable chirally modified Ni(0)L2 complex as the catalyst precursor39. The chiral (7 )-2,2 -binaphthol-derived aryl diphosphite (L1), contains three 1,1 -binaphthyl units. Its Ni(0)L2 complex is prepared from Ni(cod)2 and can be activated with trip hen ylboron39. Use of a similar chiral monophosphite ligand [L2, also derived from (/ )- 2,2 -... 

In conclusion, until recently asymmetric hydrocyanation was limited to only a few substrates, mainly norbornene systems, giving only low to medium asymmetric inductions. The stereoselective syn hydrocyanation of alkynes and the new results with vinylarenes, however, offer... 

The mechanism of the related palladium-catalyzed asymmetric hydrocyanation of alkenes has also been examined in detail using [Pd(RR-diop)(C2H4)] and related chiral species as precursor complexes. These additions gave the exo nitrile product with up to 40% enantiomeric excess, indicating stereoselective complexa-tion of norbornene to the Pd(0) via the exo face. The plausible reaction intermediates [Pd(diop)(norbornene)] and the hydrido cyanide (60) were characterized by and P NMR spectroscopy. Oxidative addition of HCN to Pd(0) is believed to precede rate-determining alkene binding. 

---