Hydrocyanation catalyst systems, particularly pertaining to the hydrocyanation of ethylenically unsaturated compounds, have been described.  For example, systems useful for the hydrocyanation of 1,3-butadiene (BD) to form pentenenitrile (PN) isomers and in the subsequent hydrocyanation of pentenenitriles to form adiponitrile (ADN) are known in the commercially important nylon synthesis field. 
1.  Improvements in the zero-valent nickel catalyzed hydrocyanation of ethylenically unsaturated compounds with the use of certain multidentate phosphite ligands are also disclosed.  Such improvements are described, for example, in U.S. Pat.  Nos.  5,821,378; 5,981,772; 6,020,516; and 6,284,865. 
The hydrocyanation of activated ethylenically unsaturated compounds, such as with conjugated ethylenically unsaturated compounds (e.g., BD and styrene), and strained ethylenically unsaturated compounds (e.g., norbornene) proceed at useful rates without the use of a Lewis acid promoter.  However, hydrocyanation of unactivated, ethylenically unsaturated compounds, such as 1-octene and 3PN, requires the use of a Lewis acid promoter to obtain industrially useful rates and yields for the production of linear nitriles, such as n-octyl cyanide and ADN, respectively. 
The use of a promoter in the hydrocyanation reaction is disclosed, for example, in U.S. Pat.  No.  3,496,217.  This patent discloses an improvement in hydrocyanation using a promoter selected from a large number of metal cation compounds as nickel catalyst promoters with a wide variety of counterions.  U.S. Pat.  No.  3,496,218 discloses a nickel hydrocyanation catalyst promoted with various boron-containing compounds, including triphenylboron and alkali metal borohydrides.  U.S. Pat.  No.  4,774,353 discloses a process for the preparation of dinitriles, including ADN, from unsaturated nitriles, including pentenenitriles, in the presence of a zero-valent nickel catalyst and a triorganotin promoter.  Moreover, U.S. Pat.  No.  4,874,884 discloses a process for producing ADN by the zero-valent nickel catalyzed hydrocyanation of pentenenitriles in the presence of a synergistic combination of promoters selected in accordance with the desired reaction kinetics of the ADN synthesis.  Furthermore, the use of Lewis acids to promote the hydrocyanation of pentenenitriles to produce ADN using zero-valent nickel catalysts with multidentate phosphite ligands is also disclosed.  See, for example, U.S. Pat.  Nos.  5,512,696; 5,723,641; 5,959,135; 6,127,567; and 6,646,148. 
It is reported in the prior art that, concomitant with the hydrocyanation of 3PN and 4PN to produce ADN, some isomerization of 3PN to cis- and trans-2PN can occur.  However, in the process of hydrocyanating 3PN and 4PN using nickel catalysts derived from monodentate phosphite ligands, such as Ni[P(OC6H5)3]4, U.S. Pat.  No.  3,564,040 states that the presence of cis- or trans-2PN, even in low concentrations, is detrimental to catalyst efficiency and the production of 2PN is undesirable since they constitute a yield loss as well as a poison for the catalyst. 
In order to address this issue, U.S. Pat.  No.  3,564,040 describes a method to maintain the steady-state concentration of 2PN below 5 mole percent as based on the nitriles present in the reaction mixture.  Because trans-2PN is difficult to separate from a mixture of 3PN and 4PN by distillation due to their close relative volatilities, the disclosed method involves the catalytic isomerization of trans-2PN to cis-2PN followed by fractional distillation of the mixture of pentenenitrile isomers to remove the more volatile cis-2PN isomer.  The catalyst systems used to isomerize trans-2PN to cis-2PN are those that also serve to hydrocyanate pentenenitriles to ADN, in particular, nickel catalysts derived from monodentate phosphite ligands as described in U.S. Pat.  Nos.  3,496,217 and 3,496,218. 
Alternative catalyst systems for the isomerization of trans-2PN to cis-2PN are disclosed in U.S. Pat.  Nos.  3,852,325 and 3,852,327.  The primary advantage of the catalyst systems described therein is in avoiding appreciable carbon-carbon double bond migration in the pentenenitrile isomers, which allows for the isomerization of trans-2PN to cis-2PN without substantial further isomerization of the 3PN to 2PN.  The catalysts described in U.S. Pat.  No.  3,852,325 are compounds of the general formula R3C—X, such as triphenylmethyl bromide, wherein R is an aryl radical having up to 18 carbon atoms and —X is of the group consisting of —H, —Cl, —Br, —I, —SH, —B(C6H5)4, —PF6, —AsF6, —SbF6 and —BF4, while the catalyst systems described in U.S. Pat.  No.  3,852,327 are Lewis acid/Lewis base compositions, such as combinations of zinc chloride with triphenylphosphine. 
A different method of removing the 2PN from mixtures of pentenenitrile isomers containing 3PN and 4PN is disclosed in U.S. Pat.  No.  3,865,865.  The 2PN and/or 2-methyl-2-butenenitriles (2M2BN) can be selectively separated from a mixture of pentenenitrile isomers containing 3PN and 4PN by contacting the mixture of nitriles with an aqueous solution of a treating agent comprising sulfite and bisulfite ions and ammonium or alkali metal cations to produce an aqueous phase containing the bisulfite adduct of the 2PN and/or 2M2BN and an organic phase containing the 3PN and 4PN, substantially free of 2PN and 2M2BN.  The recovered organic phase can provide a feed material of pentenenitriles for further hydrocyanation to produce ADN with greatly reduced amounts of the undesired by-product 2PN that is detrimental to catalyst efficiency. 
U.S. Pat.  No.  6,127,567 discloses nickel catalyst precursor compositions derived from bidentate phosphite ligands and processes for the hydrocyanation of monoethylenically unsaturated compounds which are more rapid, selective, efficient, and stable than prior processes using nickel catalysts derived from monodentate phosphites.  U.S. Pat.  No.  5,688,986 discloses that at least one member of this class of catalysts is capable of hydrocyanating olefins conjugated to nitrites, for example 2PN.  The present invention provides novel processes for the hydrocyanation of pentenenitriles to produce dinitriles, in particular ADN, using certain catalyst precursor compositions described in U.S. Pat.  No.  6,127,567.  Such processes can overcome the detrimental effect of 2PN on catalyst efficiency and can greatly reduce or eliminate yield losses to 2PN in the pentenenitrile hydrocyanation reaction.