Organometallics 1989,8, 2871-2875 binuclear rhodium and iridium complexes containing both dppm and dmpm ligands." Most of the Rh and Ir compounds can only be prepared with the very electron-deficient alkynes such as hexafluoro-2-butyne and dimethyl acetylenedicarboxylate. The anticipated increase in basicity in moving from Rh(1) or Ir(1) to Ru(0) apparently allows the reaction to occur with the less electron-deficient alkyne, diphenylacetylene.
Summary We have described the high yield preparation of a new, (17) (a) Davidson, J. L.; Harrison, W.; Sharp, D. W. A.; Sim, G . A. J. Organomet. Chem. 1972, 46, C47. (b) Dickson, R. S.; Johnson, S. H.; Kirsch, H. F.; Lloyd, D. J. Acta Crystallogr.,Sect. B. 1977, B33,2057. (c) Balch, A. L.; Lee,C.-L.; Lindsay, C. H.; Olmstead, M. M. J. Organomet. Chem. 1979,177, C22. (d) Koie, Y.; Shinoda, S.; Saito, Y.; Fitzgerald, B. J.; Pierpont, C. G. Inorg. Chem. 1980,19,770. (e) Cowie, M.; Southern, T. G. J. Organomet. Chem. 1980,193, C46. (f) Cowie, M.; Dickson, R. S. Znorg. Chem. 1981,20, 2682. (9) Cowie, M.; Southern, T. G. Inorg. Chem. 1982,21, 246.
2871
reactive binuclear ruthenium dimer, R~,(dmpm),(CO)~, in one step from R U ~ ( C O )and ~ , dmpm. Its structure was determined by X-ray diffraction methods and was found to contain trans phosphines in axial positions and one bridging and four terminal carbonyls in the equatorial plane. Initial reactivity studies found that this highly basic dimer reacts with protons to give [HRu,(dmpm),(CO),]BF,, and with activated or unactivated alkynes. The structure of R~,(dmpm),(C0)~(PhCCPh) was found by X-ray diffraction studies to contain a bridging a,-alkyne. Additional studies of the reactivity of R ~ ~ ( d m p m ) , ( C O ) ~ are in progress.
Acknowledgment. This research was supported by a grant from the National Science Foundation (CHE8714326). Supplementary Material Available: Tables of the H atom ~ i t i o n thermal s~ parameters,and bond angles (20 pages); hth@ of structure factors (75 Pages). Ordering h-h"ation is given On any current masthead page.
Lewis Acid Effects on Selectivity in Nickel-Catalyzed Pentenenitrile Hydrocyanation. Triorganotin Salts as Tunable Lewis Acid Promoters Ronald J. McKinney' and William A. Nugent Central Research and Development Department and Petrochemicals Department, E. I. do Pont de Nemours and Company, Experimental Station, P.0. Box 80328, Wilmington, &la ware 19880-0328 Received May 9, 1989
Anhydrous triorganotin salta, &SnX, have been synthesized and utilized in exploring steric and electronic effects on selectivity in nickel-catalyzed pentenenitrile hydrocyanation. Steric effects are found to dominate the selectivity in the competition both between 3- and 4-pentenenitrile (3PN and 4PN) hydrocyanation and between Markovnikov and anti-Markovnikov addition of HCN to 4PN. Electronic effects, i.e., Lewis acidity, effect only the activity of the catalyst, but in the complex hydrocyanation system, this can result in yield changes to adiponitrile.
Introduction The Du Pont adiponitrile (ADN) process1 involves nickel-catalyzed hydrocyanation of 3-pentenenitrile (3PN) and produces 2-methylglutaronitrile (MGN), ethylsuccinonitrile (ESN), and 2-pentenenitrile (2PN) as byproducts (eq 1-4). The selectivity to adiponitrile is CHSCH=CHCH,CN e CHXHZCHZCH2CN (1) 3PN 4PN 3PN 4PN
+ HCN
-
+ CH3CH,CH=CHCN 2PN
(2)
ks
NCCH,CH&H2CHZCN ADN
-
+ NCCHzCHzCH(CH3)CN MGN
(3)
3PN + HCN NCCH2CH,CH(CH,)CN + NCCH2CH(C,H5)CN (4) MGN ESN strongly influenced by the choice of Lewis acid promoter? k4
7 Contribution
No. 4931.
0276-7333/89/2308-2871$01.50/0
For example, Lewis acid promotion of the hydrocyanation of 3PN a t 50 "C with NiL4 [L = P ( O - p - t ~ l y l ) catalyst ~] gives the following selectivities to A D N B(C8H5)3,96%; ZnCl,, 82%; AlC13, 50%.3 In this paper, we have undertaken a systematic study of the role that the Lewis acid plays in controlling the selectivity of addition of HCN to pentenenitrile. In order to accomplish this goal, we have utilized triorganotin salts as tunable Lewis acids and in the process have developed synthetic methods and isolated the first anhydrous triorganotin cations in which the anion is not coordinated to the tin.4 As eq 2-4 indicate, in the hydrocyanation of 3PN, byproducts arise by way of three different reactions, i.e., 2PN from eq 2, MGN from eq 3, and MGN and ESN from eq 4. The isomerization reactions, eq 1-2, have been the (1) Chem. Eng. News 1971,49,30. (2) (a) Tolman, C. A.; McKmey, R. J.; Seidel, W. C.; Druliier, J. D.; Stevens, W. R. Adu. Catal. 1985, 33, 1-46. Recent references on the mechanism of nickel-catalyzed olefin hydrocyanation include: (b) Backvall, J. E.; Andell, 0. S. Organometallics 1986, 5, 2360. (c) McKinney, R. J.; Roe,D. C. J. Am. Chem. SOC.1986,108,5167. (3) Based on eq 3 and 4 only; yield l w from 2PN not included here. (4) Nugent, W. A.; McKinney, R. J.; Harlow, R. L. Organometallics 1984, 3, 1315.
0 1989 American Chemical Society
2872 Organometallics, Vol. 8, No. 12, 1989
McKinney and Nugent
Table I. Elemental Analyses of Triorganotin Salts calcd, % tin compd (MW) C H N C (CH,)3Sn(CH3CN)2SbF6 (481.5) 17.5 3.1 5.8 17.5 (CZHF,)~S~(CH,CN)~S~F, (523.5) 22.9 4.0 5.4 22.8 (i-C3H7)3Sn(CH3CN)2SbFB (565.5) 27.6 4.8 4.9 25.9 (i-C8g)3Sn(CH3CN)SbF6 (566.5) 29.7 5.3 2.5 28.1 5.5 4.6 (t-C,Hg)SSn(CH3CN)2SbF6 (607.5) 31.6 31.6 ( ~ ~ O - C ~ H ~ ~ ) ~ S ~ ( C H(608.5) , C N ) S ~ F , 33.5 5.9 2.3 33.2 ( C - C B H , ~ ) ~ S ~ ( C H ~(686.5) CN)~S~F 38.5 5.7 4.1 38.4 3.0 (C6H6),Sno3SCF(498.7) 45.7 0.0 45.6 (C&6)3Sn0zCCF3(462.7) 51.9 0.0 3.2 51.9 subject of a preliminary report5 in which it was suggested that the Lewis acid controls the equilibria of eq 5 and 6. HNiL,(CN-A) i- HNiL3+ CN-A(5) HNiL3(CN-A) @ HNiL,(CN-A) L (6) The product of eq 5 has been implicated as the active catalyst for olefin isomerization whereas that of eq 6 catalyzes olefin hydrocyanation. The role of Lewis acids in isomerization will not be discussed in detail in this paper. However, since hydrocyanation is kinetically first order in olefin concentration,2 changes in the ratio of 4PN to 3PN through isomerization will clearly effect the relative yields of the dinitriles.
+
Experimental Details All reaction mixtures were prepared in a Vacuum Atmospheres
drybox and reactions carried out under nitrogen or argon in thermostated oil baths unless otherwise noted. Gas chromatographic analysis of reaction mixtures was carried out on a Carbowax 20M capillary column (0.2 mm (i.d.) X 25 m). Hydrogen cyanide was obtained in 200-mL cylinders from Fumico, Inc. (Amarillo,TX). Caution! Hydrogen cyanide (HCN) is very volatile and highly toxic and should be used only in a well-ventilated fume hood or drybox. Distilled HCN is prone to very exothermic oligomerization when heated and should be kept at 0 OC or lower at all times. The commercially available samples contain strong acidic inhibitors. Sensibleprecautions include not working alone and having available proper first aid equipment. Excess HCN may be disposed of by burning or by slowly adding to aqueous basic sodium hypochlorite.6 3-Pentenenitrile (Du Pont) was dried by passing through a column of acidic alumina and then sparged with, and stored under, nitrogen. Tetrakis(tri-p-tolyl phosphite)nickel(O) was prepared by literature procedures.' 4-Pentenenitrile is thermodynamically disfavored with respect to 2- and 3-pentenenitrile and cannot be separated from them by distillation. A 50-mL three-neck flask equipped with a water-cooled condensor,an overhead stirrer, and a dropping funnel (pressureequalized) was charged with dimethyl sulfoxide (DMSO) and sparged with nitrogen. (Caution! DMSO is readily adsorbed through the skin and may carry toxic chemicals such as KCN or bromobutene through the skin also.) 4-Bromobutene (50 g) was added to the mixture under nitrogen. KCN (35 g) was dissolved in H20 (50 mL) with warming, briefly sparged with nitrogen, added to the dropping funnel, and then rapidly added to the reaction mixture. (Caution! The reaction/mixing is exothermic!) The temperature increased to about 90 OC, and as it started to cool again, the mixture was placed in an oil bath and maintained at 75 "C for 45 min. The mixture was cooled and transferred (in air) to a 1-L conical flask, and H20 (350 mL) was added (the mixture warmed again, and all remaining salts dissolved). After cooling, extraction was done with diethyl ether (3 X 200 mL). The extracts were combined and extracted once with water (100mL). The ether fraction was dried over MgSO,. The sample was ( 5 ) McKinney, R. J. Organometallics 1985,4, 1142.
(6) For additional details, see: Prudent Practices for Handling Hazardous Chemicals in Laboratories;National Academy Press: Washington, DC, 1981; pp 45-47. Safety literature is also delivered with commercial samples from Fumico, Inc. (7) Ittel, S. D. Inorg. Synth. 1977, 17, 118-120.
found, % H 3.4 4.2
N
5.5 5.1 4.6 2.6 4.4 1.8 3.9 0.1
4.6 5.4 5.3 6.3 5.6 3.1 0.0 3.3 distilled under nitrogen (atmospheric pressure) and the product collected between 135 and 145 OC. ll9SnNMR spectra were obtained on a Nicolet 300 instrument operating at 134.6 MHz. Samples were dissolved in CD3CNand chemical shifts measured against Me&. Synthesis of Triorganotin Cations (Typical Procedures). Silver-Mediated Metathesis. Bis(acetonitri1e)trimethyltin Hexafluoroantimonate. A solution of trimethyltin bromide8 1.00 g, 4.1 mmol) in dry acetonitrile (30 mL) was treated with a solution of silver hexafluoroantimonate (1.35 g, 3.9 mmol) in acetonitrile (10 mL) under dry nitrogen with stirring for 3 h. The resulting silver bromide was separated by filtration and the volume of the filtrate reduced by evaporation under vacuum. After a small amount more of silver bromide was separated, toluene (20 mL) was added and the volume reduced under vacuum. White crystals separated, which were isolated by filtration. The crystals were washed with toluene and then pentane and dried under high vacuum. The yield was 1.54 g (81% yield on limiting AgSbF6). Similar results were obtained with a variety of trialkyltin bromides; however, with several of the larger alkyl derivatives,diethyl ether was a more effective solvent for crystallization. Elemental analyses are found in Table I. Bis(acetonitri1e)tri- tert -butyltin Hexafluoroantimonate. Despite suggestions in the literature that tri-tert-butyltin chloride may not be isolable: we were able to prepare it in modest yields as follows. The starting material, di-tert-butyltin dichloride was obtained either from a commercial source (Strem Chemical Co.) or by treatment of a toluene/ether solution of tin(1V) chloride with a tetrahydrofuran solution of tert-butylmagnesium chloride followed by recrystallization from hexamethyldisiloxane at -25 "C. To a solution of this dichloride (4.2 g, 13.8 mmol) in hexanes (30 mL) was added a 2.6 M solution of tert-butyllithium in After 0.5 h, the mixture was filtered pentane (5.5 mL, 14.3 "01). and the solvent distilled under reduced pressure. The product was separated from the brown residue by distillation (60-70 "C, 0.4 Torr) with use of an air-cooled condenser. The product (1.07 g, 24%), a colorless liquid, freezes just below room temperature. The 90-MHz 'H NMR spectrum (c&) is a singlet at 15 1.27 with satellites at J = 69 and 74 Hz due to "'Sn and llgSn. Silver hexafluoroantimonate (0.91 g, 2.65 mmol) was added to a solution of tri-tert-butyltin chloride (1.00 g, 3.07 mmol) in nitromethane (15 mL) under dry dinitrogen. After 0.5 h, vacuum was applied briefly for evaporative cooling and 0.36 g of precipitate was removed from the black suspension by filtration. Removal of solvent from the resultant yellow solution gave an oily residue that was redissolved in acetonitrile (2 mL). Removal of the acetonitrile afforded a solid that was washed with hexanes (2 X 20 mL) and dried in vacuo to afford the crude product (1.39 g, 86%). This material was recrystallized by dissolution of 1.0 g of material in a mixture of ether (10 mL) and acetonitrile (0.85 mL) and cooling to -25 "C to produce 0.40 g of white needles. Acid-Mediated Metathesis. Triphenyltin Trifluoromethanesulfonate. Triphenyltin cyanide (5.00 g, 13.3 mmol) suspended in acetonitrile (100 mL) was treated with trifluoromethanesulfonic acid (triflic acid, CF3S03H;2.00 g, 13.3 mmol) under dry nitrogen until all solids dissolved. The solvent and HCN were removed in vacuo and the white solid washed with pentane (8) Use of triorganotin chlorides often led to incomplete precipitation of silver chloride in solvent acetonitrile. This could sometimes be circumvented by carrying out the initial metathesis in nitromethane and a subsequent ligand exchange. (9) Prince, R. H. J. Chem. SOC.1959, 1783.
R#nX Lewis Acid Effects on PN Hydrocyanation and then dried under high vacuum. The isolated yield was 6.13 g. This method works nicely for any readily available triorganotin cyanide in combination with an anhydrous acid that is stronger than HCN. Silver Oxidation of Tin Hydride.lo Triphenyltin Trifluoromethanesulfonate. Silver trifluoromethaneadfonate (1.03 g, 4.0 mmol) was added to a solution of triphenyltin hydride (1.40
g, 4.0 mmol) in acetonitrile (30 mL). After 15 min, the mixture was filtered to remove precipitated silver metal (0.45 g versus 0.43 g theory). On concentrating to ca. 3 mL, additional solids separated, which were filtered off prior to removal of the remaining solvent affording the product as a white solid (1.15 g, 58%). Hydrocyanation of Pentenenitriles. Catalytic reactions were carried out in a "semi-batch" manner; all reagents except HCN were placed in a reaction vessel under nitrogen and then HCN was fed slowly to the mixture either by vapor transfer or by feeding a solution of HCN by syringe pump. A convenient method of controlling vapor transfer is to pass a stream of nitrogen through liquid HCN (maintained at 0 "C in an ice bath); the nitrogen flow may be controlled by needle valve and flow meter. HCN solutions were made at 15% v/v in toluene. 3-Pentenenitrile Hydrocyanation. A typical reaction was carried out as follows: A catalyst solution was prepared by dis(2.50 solving Ni[P(O-p-tolyl),], (2.94 g, 2.0 mmol), P(O-p-t~lyl)~ mL, 7.8 mmol), and C6H5CN(2.50 mL as internal standard) in dry 3PN (95 mL, 1.02 mol). A 25-mL, three-neck flask fitted with reflux condensor, nitrogen bubbler, magnetic stirring bar, and rubber septum was charged with 6.0 mL of the catalyst solution (0.08 C Hg,~ 0.18 ) ~ Smmol) ~ F ~and heated and ( C - C ~ H ~ ~ ) ~ S ~ ( N C in a thermostated oil bath at 50 "C. Nitrogen gas was passed through liquid HCN (at 0 "C) at 5.5 mL/min, through a P205trap, and introduced by way of a syringe needle through the rubber septum intothe reaction mixture. Samples were taken periodically by syringe, quenched by dilution in air-saturated acetone, and analyzed by capillary gas chromatography (Carbowax 20M column, 25 m, 0.2-mm i.d.). 4-Pentenenitrile Hydrocyanation. A solution was prepared (1.00 g, 0.68 "01) under nitrogen by dissolving Ni[P(O-p-tolyl)314 and P(O-p-tolyl), (1.00 mL, 3.3 mmol) in a mixture of acetonitrile (15 mL) and 4PN (10 mL, 100 mmol). An aliquot (6.0 mL) was (0.12 g, 0.25 mmol) and treated with (CH3)3Sn(CH3CN)2SbF6 heated under nitrogen to 50 "C in an oil bath. HCN was fed by vapor transfer (see above) with a nitrogen flow of 5 mL/min. Samples were taken periodically by syringe, quenched by dilution in air-saturated acetone, and analyzed by capillary gas chromatography. The reaction was carried out to about 50% conversion of PN's to dinitriles (DN's). Selectivity for the first 30% conversion was 87% ADN and 13% MGN. In a similar manner, selectivities from 4PN were determined for (c-C6Hll),Sn(CH3CN)2SbF6(5% MGN), (neo-CSHll),Sn(CH,CN)SbF, (6% MGN), and (C6H5)3Sn03SCF3 (6% MGN).
Results Synthesis of Triorganotin Salts. Anhydrous triorganotin salts were prepared4 by the treatment of either a triorganotin bromide* or a triorganotin hydride'O in acetonitrile with 1equiv of the appropriate silver salt, e.g., silver hexafluoroantimonate or silver trifluoromethanesulfonate (triflate), eq 7 and 8, or by treatment of the triorganotin cyanide with an anhydrous acid stronger than HCN, e.g., trifluoromethanesulfonic acid or trifluoroacetic acid, eq 9. In general, we found that the method of eq R3SnBr + AgSbF, R3SnH + AgSbF,
MeCN
+
R&~I(NCM~)~(S~F AgBr ,) (7)
MeCN
R3Sn(NCMe)2(SbF6)+ Ag
+ l/zH2 (8)
Organometallics, Vol. 8, No. 12, 1989 2873
7 worked well for R = alkyl but that for R = aryl the method of eq 8 was superior. The method of eq 9 was preferable whenever the tin cyanide was readily available because it does not leave any silver residues that may effect subsequent catalysis. In some cases, attempted recrystallization produced amorphous solids with nonintegral amounts of acetonitrile. Whereas this created problems with respect to elemental analyses, it had no effect on the catalytic selectivity analyses described below; selectivity is insensitive to changes in the Lewis acid concentration. In the case of bis(acetonitri1e)tricyclohexyltin hexafluoroantimonate, the identity has been confirmed by X-ray crystal structure determinati~n.~ The synthesis of bis(acetonitri1e)tri-tert-butyltinhexafluoroantimonate is noteworthy especially with regard to the previous suggestion that the intermediate tri-tert-butyltin chloride might not be i ~ o l a b l e .Treatment ~ of ditert-butyltin dichloride with tert-butyllithium produced a low yield of tri-tert-butyltin chloride that was purified by distillation and subsequently used in a silver metathesis reaction. Hydrocyanation of Pentenenitrile. Interaction of triorganotin salts with the nickel catalyst is believed to occur by tin coordination to the cyanide ligand of a hydridonickel cyanide intermediate as illustrated in eq 10. NiL4 + HCN
+ R3SnX
-
HNiL3(CN-SnR3-X)
(10)
Dissociation of ligand L on the nickel provides substrate accessibility to the catalyst. Convincing evidence for coordination of the Lewis acid to the nickel cyanide moiety has been provided through a combination of labeling and NMR studies with other Lewis acids.ll Variation of both R and X in a series of R3SnX promoters leads to changes in selectivity in the hydrocyanation of 3PN and 4PN in the presence of NiL4 [L = P(0-p-tolyl),] at 50 "C. As indicated above, selectivity changes arise through competition between eq 3 and 4 and by direction of HCN addition to the double bond in eq 3 and 4. Determination of selectivity separately in eq 3 and 4 is complicated by the fact that MGN is produced in both of them. Fortunately, previous workers12J3have established that, for several Lewis acid promoters, the amount of MGN arising from eq 4 is approximately equal to the amount of ESN obtained from eq 4; Le., there is little or no preference for the direction of HCN addition to the double bond of 3PN. Therefore, doubling the amount of ESN provides the amount of dinitrile arising from eq 4, or alternatively subtracting the amount of ESN from MGN gives the approximate amount of MGN arising from eq 3. We have demonstrated that this approximation holds for four of the tin Lewis acids used for this study; in Table 11,the final column, the values in parentheses were determined by hydrocyanation of 4PN directly; i.e., the hydrocyanation was carried out starting with pure 4PN. We therefore feel justified in using this approximation throughout this paper to help identify the role of Lewis acid in both eq 3 and 4. Selectivity data for a series of pentenenitrile hydrocyanations using tin Lewis acid promoters is given in Table 11, along with computed values for the amount of MGN arising from eq 3 and the amount of dinitrile (MGN + ESN) arising from eq 4. As mentioned in the Introduction, the fmt-order dependency means that changes in 3PN and
MeCN
R3SnCN + CF3S03H R3Sn(NCMe),03SCF3 + HCN (9) (10)Oxidation of triorganotin hydrides by silver salts has been known for some time: Anderson, H. H. J . Am. Chem. SOC.1957,79,4913-4915.
(11)Druliner, J. D.; Engliih, A. D.; Jesson, J. P.; Meakin, P.; Tolman, C . A. J . Am. Chem. SOC.1976,98,2156. (12)Tolman, C. A.;Seidel, W. C.; Druliner, J. D.; Domaille, P. J. Organometallics 1984,3,33. (13)Druliner, J. D. Organometallics 1984,3,205.
2814 Organometallics, Vol. 8, No. 12, 1989
McKinney and Nugent
Table 11. Dinitrile Distribution for Triorganotin Promoters. R&nX
R
X
ESN 4.1 1.1 1.2 3.4 3.6 2.2 4.6 6.5 3.8 3.7 1.7 3.4 2.3 2.3 2.1 2.2 1.7
SbF6
C6H5 C6H5 C6H6
CsH5 p-FC6H4
P-CH&sH4 P-CHsOCsH, CH3 C2H5 n-C3H7 i-C3H7 n-C4H9 i-C4H9 s-C~H~ t-CdH9
CFBCO2 CH&H,SO3 CF3S03 CFSS03 CF3SO3 CFSSOB SbF6
neo-C5Hll C-C6H11
MGN 11.2 7.8 7.4 9.9 10.9 9.0 11.8 17.1 13.1 12.4 9.1 13.9 11.4 10.0 7.8 9.9 8.5
'2 X % ESN. b ( 2 X % ESN)([4PN]/[3PN])/(% ADN ESN). dFrom hydrocyanation of 4 P N (see text).
ADN 84.7 91.1 91.4 86.7 85.5 88.8 83.6 75.8 83.1 83.9 89.2 82.7 86.3 87.7 90.1 87.9 89.8
% ESN
+ MGN"
from ea 4 8 2 2
I I 4 9 13 8
I 3
I 5 5 4 4 3
+ % MGN - % ESN).
'100
X (%
% MGNc
k 4 h b
x104 7.1 7.0 5.6 5.5 6.6 6.6 6.7 15.1 10.4 10.3 7.0 10.4 10.0 6.4 5.5 6.1 5.8
from ea 3 8
MGN - % ESN)/(% ADN
+ % MGN - %
14PN1/13PN1 0.008 0.031 0.024 0.008 0.0085 0.014 0.0066 0.010 0.014 0.013 0.019 0.014 0.021 0.013 0.014 0.014 0.016
7 6 7 (6)d 8 7 8 13 (13)d 10 10
8 11 10 8 6 8 7 (5Y
4PN concentrations must be factored out of selectivity results. This can be done by utilizing eq 11 and 12 (DN = dinitrile). The ratio k4/k3,provided in the pentultimate ( % DN from eq 4)/( % DN from eq 3) = k4[3PNl/k,[4PNl (11)
I:
k 4 / k 3 = [4PN](% DN from eq 4)/ [3PN](% DN from eq 3) (12) column of Table 11, is a measure of selectivity between eq 3 and 4 independent of the pentenenitrile concentrations. Note that, in general, 4PN hydrocyanation is favored over 3PN by 3 orders of magnitude; most likely this arises from favored binding of sterically less hindered terminal olefins (4PN) over internal olefins (3PN). The last column of Table 11, the percent MGN arising from eq 3, provides a measure of selectivity in the direction of HCN addition to the double bond of 4PN. Variation of X in R3SnX. The nucleophilicity of X may be expected to effect the electronic nature, i.e., acidity of the tin center. In the limiting case of a nonnucleophilic anion, e.g., X = SbF6-, we can expect the anion to be displaced completely by an organonitrile group as was shown for [(C6H11),Sn(NCCH3)2]SbF6.4 However, 119Sn NMR for the series of tin compounds (C&I5)3SnX, X = SbF6, CF3S03, and CF3C02,in acetonitrile has revealed that the position of the rapid equilibrium illustrated in eq 13 is strongly effected by the nature of X. The tin chemical R3SnX
R,Sn(NCR')X
F!
R,SII(NCR')~+X- (13)
shifts along the series X = CF3C02 (-196 ppm), CF3S03 (-209 ppm), SbF6 (-218 ppm) are consistent with the decreasing nucleophilicity of these anions. (As expected, the ll9Sn chemical shifts correlate with the pK, of the conjugate acids HX.) It is therefore noteworthy that when these tin compounds are used as promoters for ADN synthesis, the selectivity decreases monotonically along this same series: CFBC02(91%) > CF3S03(87%) > SbF, (84.5%). However the first four entries of Table I1 reveal that upon factoring out the changing [4PN]/[3PN] ratio, the ratio k 4 / k 3is essentially unchanged across the series and that very little if any effect is observed on the percent of MGN in eq 3 (last two columns). The changes in ADN yield are therefore the result of changing P N ratios due to isomerization. Kinetic studies14 have revealed that rate con(14) McKinney, R. J., Unpublished results.
4
R=
0 CH3
1
C2Hs
2 3 I - C ~ H ~ t-CdH9
No of Alpha Branches
.,
Figure 1. Selectivity for a series of R3SnSbF6-promotednickel-catalyzedpentenenitrilehydrocyanations wherein the number of methyl substituents on the carbon attached to tin is varied (so-called a branching series). Key: the ratio ( k 4 / k 3 )X lo4 revealing the change in selection of 4PN vs 3PN, 0,the percent of MGN arising from hydrocyanation of 4PN.
stants for eq 3 and 4 increase by more than 1 order of magnitude across this series. Variation of R = Aryl in R3SnX. A second way in which to observe electronic effects is to vary aryl substituents. This allows results to be compared against Hammett constants. Results obtained from the set of tin compounds, (4-Y-C,&)3Sn03SCF3, where Y = F, H, CH,, and CH30, are given in Table 11. Again we find that selectivity as measured by the ratio k 4 / k , or the percent of MGN in eq 3 is insensitive to electronic changes. However, qualitative observations of increased activity for the more electronegatively substituted species are once again noted. Variation of R = Alkyl in R@X. In an attempt to probe steric effects, a series of (alkyl),SnSbF6compounds were evaluated. In the absence of suitable measures of steric infl~ence,'~ correlations were sought with the number branches a t a given carbon. Figure 1 shows that there is significant effect due to branching on the carbon adjacent to the tin (a carbon), Le., R = CH,, CzH6,i-C3H7,t-C4H,. We fmd that both the ratio k 4 / k , and the percent of MGN in eq 3 correlate with (15) Attempts to establish cone angles gave unsatisfactory results.
Organometallics, Vol. 8, No. 12, 1989 2875
R3SnX Lewis Acid Effects on PN Hydrocyanation
Scheme I l l4
6
t
1
l
6
1”
mCN 3PN
H N i L l CN .A-
ll
mCN 4PN
HNi(CN*A)L,
‘
t2NiI CN
41
0
R=
C,H,
1
2
n-C3H7
i-C4H9
3
4
neo-C5HI1
No. of Beta Branches
.,
Figure 2. Selectivity for a series of RaSnSbF6-promotednickel-catalyzed pentenenitrile hydrocyanationswherein the number of methyl substituents on the carbon @ to tin is varied (so-called j3 branching series). Key: the ratio (k4/ks)104 revealing the change in selection of 4PN vs 3PN, 0,the percent of MGN arising from hydrocyanation of 4PN.
the number of branches on the a carbon. Whereas this is strongly suggestive of steric influence on the selectivity, it must be remembered that electronic as well as steric changes are occurring across the series. A series of tin compounds in which steric change is produced by branching a t the /3 carbon should minimize electronic changes.16 In Figure 2, we find that in the series R = CzH5, n-C3H7,i-C4H9,and neo-C5H11 little change occurs until the very bulky neopentyl group is used.
Discussion Scheme I illustrates the pathways believed to account for the observed pentenenitrile hydrocyanation products. Many things remain unclear about this picture, e.g., the rate-limiting step(s),the importance of nitrile coordination to the nickel, and kinetic versus thermodynamic control in determining selectivity a t branch points a-c. However, this study has clarified certain features. Having reconfirmed that it is reasonable to assume that selectivity in the direction of HCN addition to 3PN (branch point b) is insensitive to general changes in the Lewis acid promoter, we have found that competition between eq 3 and 4 (branch point a) and selectivity in the direction of HCN addition to 4PN (branch point c ) are insensitive to electronic changes, i.e., Lewis acidity, but are sensitive to the size of the Lewis acid with the paths leading to ADN being favored by greater steric bulk. It should come as no surprise that as crowding increases around the nickel catalyst, coordination of an a-olefin like 4PN should be preferred over a sterically more demanding internal olefin like 3PN a t branch point a. Similar argu(16) Nugent, W. A.; Kochi, J. K. J . Am. Chem. SOC.1976,98, 5979.
I
L,Ni
\CN
h
\CN-A C N
1
1
ESN
MGN
I ADN
mente apply to branch point c. The improvement in selectivity to linear product from propylene in the presence of bulky Lewis acids has been observed previously by Tolman et a1.;12 they have proposed that it results from a “buttressing effect”17whereby increased steric crowding around the nickel increases steric strain energy associated with larger R groups. In accordance with this argument, we conclude that the equilibrium between the branched and linear alkylnickel species resulting from branch point c, will shift in favor of the linear alkylnickel species as steric bulk is increased around the nickel.
Conclusions Selectivity at each of the branch points a-c, in Scheme I, is insensitive to electronic changes in the Lewis acid promoter (“Lewis acidity”) a t similar 4PN:3PN ratios. However, we wish to reemphasize that due to the first order-rate dependence on 4PN and 3PN, changes in the 4PN:3PN ratio result in significant changes in selectivity a t branch point a and therefore in the final product distribution. Changes in the 4PN:3PN ratio arise from changes in the rate of isomerization (eq 1and 2) relative to the rate of hydrocyanation (eq 3 and 4) and will be the subject of a subsequent paper. On the other hand, selectivity is effected by changes in steric requirements of the Lewis acid promoter in particular a t branch points a and c. In both cases, increased steric bulk improves the selectivity to the desired production of ADN by (1)increasing the preference for 4PN at branch point a and (2) increasing the proportion of “linear” addition of HCN to 4PN a t branch point c. (17) Brown, H. C. Boranes in Organic Chemistry;Comell University Press: Ithaca, NY, 1972; pp 71, 102, and 282.
