J. Phys. Chem. 1993,97, 3918-3921
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Stereochemical Consequences of Halogen Atom Substitution. 3. Recoil I8F-for-X (X = F, CI) Reactions in Gaseous Diastereomeric 2,3-Dihalobutanes Richard A. Ferrieri,',? Ram B. Sharma,* Alfred P. Wolf,? and Edward P. Racks Department of Chemistry, Brookhaven National Laboratory, Upton, New York 11973, and Department of Chemistry, University of Nebraska- Lincoln, Lincoln, Nebraska 68588-0304 Received: October 13, I992
Studies have been carried out in the gas phase that address the stereochemical consequences of hot I8F-for-X ( X = F, C1) substitution in (2S,3R)-meso- and (2S,3S)-dl-difluorobutane, (2S,3R)-dl- and (2S,3S)-dlchlorofluorobutane, and (2S,3R)-meso- and (2S,3S)-dl-dichlorobutane.Two features of reaction are revealed from this work when the results are compared to those of earlier work on W1-for-X ( X = F, C1) substitution in the same systems. First, the overall reaction efficiency is sensitive to the size and mass of the leaving group, but only when the hot atom itself is relatively small. Secondly, steric hindrance to attack a t the asymmetric carbon site is not important when reactions involve small hot atoms like IEF. However, for larger atoms such as 3 T l , this is an important feature.
Introduction on the stereochemical consequences While our previous of hot (SH"~)homolytic bimolecular halogen-for-halogen atom substitution in gaseous compounds comprised of single asymmetric carbon sites provides a strong foundation to support the hypothesis that substitution proceeds through a two-channel mechanism involving either front-side attack or back-side attack at the asymmetricsite, thereis some question as to whether substitution in diastereomeric molecules proceeds by the same process. The present work represents the third paper in a series of four that will be published on this subject. The first paper in this series6 was concerned with conformer effects on the stereochemical consequences of 38C1-for-X (X = F, Ci) substitution in gaseous diastereomeric 2,3-difluorobutanes (DFB), 2,3-chlorofluorobutanes (CFB), and 2,3-dichlorobutanes (DCB). These studies showed a systematic variation in the inversion product yields between the (S,R)and (S,S) configuration ofeach substrate. On theother hand, the retention product yields were relatively constant between configuration sets. We concluded from this work that two channels for substitution were in fact operating (front-side attack yielding retention products and back-side attack yielding inversion products), and that systematic variations in the degree of steric hindrance to back-side attack imposed by differences in the equilibrium rotational conformer populations for the (S,R) and (S,S)configurations could account for such systematic variation in the inversion product yields. The second paper in this series' provided additional evidence in support of this mechanism by demonstrating that the retention and inversion product yields arising from exothermic reactions had very different energy dependences. Similar behavior was noted by us in earlier work on I8F substitution in 2(S)-(+)chloropropionyl chloride.5 The present study returns to our original interest on the effects of conformer populations on reaction stereochemistry. While such effects were significant in altering the degree of steric hindrance to back-side attack in the 3 T 1 work, our premise is that they should be relatively unimportant if the size of the hot atom is sufficiently small. To test this, we carried out parallel studies on the stereochemical consequences of IsF-for-X substitution in gaseous diastereomeric DFBs, CFBs, and DCBs where Brookhaven National Laboratory. Present address: William Beaumont Hospital, 3601 W. Thirteen Mile Rd., Royal Oak, MI 48073-6769. University of Nebraska-Lincoln. +
1
the l*F hot atom represents a significant reduction in size and mass over that of our original work with 38Cl.
Experimental Section Materials. A detailed description of the synthesis of mesoand dl-2,3-difluorobutanes can be found in the first paper of this series.6 In thegeneral meth0d,8-~2,3-butanediol (K & KChemical Co.) was converted to 2,3-bis@-toluenesulfonoxy)butanes and then reacted with anhydrous potassium fluoride to yield the difluorobutanes. Compound authenticity was checked by N M R and IR, and purity checked by GCMS. Individual diastereomers were purified by preparative GLC using a column packed with 15% di-n-decyl phthalate (DDP) on Chromosorb P. GLC assignments were based on earlier work of Speranza et a1.I0 (2S,3R)-dl-Chlorofluorobutane and (2S,3S)-dl-chlorofluorobutane were prepared through the net addition of ClF across the double bonds of trans- and cis-2-butenes (Phillips Petroleum Co.) I 1-12 The chloride ion source was N-chlorosuccinimide, while the fluoride ion source was H F in pyridine (Aldrich Chemical Co.). Details of this synthesis can be found in our earlier paper.6 Compound authenticity was checked by N M R and IR, and purity checked by GCMS. Since the synthesis yielded a 99.9% purity for the (2S,3R)-dl isomer and 98.9% purity for the (2S,3S)-dl isomer, no further purification was needed. meso-2,3-Dichlorobutane and dl-2,3-dichlorobutane was obtained from an isomeric mixture that was commercially available (Columbia Organic Chemical Co., Inc.). The mixture was subjected to preparative GLC using both the DDP column and another column packed with 10%tricresyl phosphate (TCP) on Chromosorb P. The neon target gas used as the I8Fprecursor was purchased from Matheson Gas Co. (Research purity; 99.999%) and used without further purification. Sample heparation. The samples were prepared, withstandard vacuum-line techniques, by introducing the desired amount of liquid substrate into quartz vessels so that a partial pressure of 30 Torr would be exerted when the sample was heated to 65 OC during irradiation. An additional 30 Torr of neon gas was introduced into the vessel while the substrate was maintained in a frozen state. This amount of neon provided adequate I8Factivity for counting purposes, yet it was not present in significant enough concentrations to cause major perturbations in the recoil atom's kinetic energydistribution prior tochemical reaction. The quartz irradiation vessels used were 30 mL in volume and were equipped
0022-3654/93/2097-3918504.00/0 0 1993 American Chemical Society I
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Halogen Atom Substitution with Teflon-typestopcocks and 0.25-mm-thick quartz entrance windows to allow for particle beam penetration. Sample Irradiation. All irradiations were carried out at the Brookhaven National Laboratory 60-inch cyclotron. Typically, a 13-MeV deuteron beam of 1-2 pA intensity was employed producing I8Fby the *ONe(d,ar)l8Freaction. Irradiations were of 3-5-min duration, during which the vessel was maintained at 65 OC to ensure that the substrate remained in the gas phase. Checks on possible neon effects were carried out .on the DFB system by irradiatingtargets devoid of neon with a 33-MeV proton beam thus producing 18Fby the 19F(p,pn)18Freaction. While these checks were complicated by the additional production of IC, this had no bearing on the individual substitution products. No significantdifference was found between the 18Fsubstitution product yields arising from the two methods described. Radioassay and Absolute Yield Measurements. After irradiation, the gaseous constituentsof the target were condensed with liquid nitrogen and dissolved in dichloromethane. Aliquots of the above solution were taken with use of a liquid syringe and subjected to radiogas chromatography for separation and measurement of the labeled dihalobutaneproducts. The dihalobutane products were identified by comparison of their elution times with those of authenticsampleson three different columns: 10.4 m X 6.4 mm 0.d. copper column packed with 10% tricresyl phosphate (TCP) on Chromosorb P (80-100 mesh); 9.0 m X 6.4 mm 0.d. copper column packed with 27% DC-7 10with 3%stearic acid on Chromosorb P (80-100 mesh); and a 5.0 m X 6.4 mm 0.d. copper column packed with 15% di-n-decyl phthalate on Chromosorb P (60-80 mesh). Labelled products eluting from the chromatography column were trapped in glass tubes packed with 40/60 mesh activated charcoal. In addition, the more volatile products were subjected to on-line radioactive effluent counting using a heated external proportionalcounter. This served as a check on charcoaltrapping efficiency. Radioactivity for each of the charcoal-trapped productswas measured with a NaI(T1) detector. In all instances the activity was corrected for radioactive decay, detector efficiency, detector background, and sample fraction. At least three separate determinations were made for each system studied. The total 18Factivity in the gas phase was determined for each sample by removing a known fraction of the target contents that remained dissolved in dichloromethane after the freezing step and assaying it in a NaI(T1) detector. After radioactive decay and fraction correctionswere made, a measureof the total volatile activity (TVA) was obtained. The quartzvesselswere also washed two times with NaHS03 solution to remove the nonvolatile 18F activity from the walls. Aliquots were also counted in the same fashion as described above. This measurement depicted the level of nonvolatile activity (NVA) in the target. The total 18Factivity (TA) was calculated as the sum of the TVA and NVA. The fraction of product yields was then calculated by dividing the corrected product activities by TA.
Results and Discussion Percent retention of configuration values for I8F-for-F and I8F-for-C1substitution in DFB, CFB, and DCB are listed in Table I. The results show clearly that I8Fatom substitution proceeds predominantly with retention of configuration in all systems. However, the results also show substantial yields of inversion products, which cannot be attributed to radiation-induced processes. As a check for radiation damageand radical reactions we added 1,3-butadiene to samples as a scavenger but saw no significant effect on reaction stereochemistry. In addition, the isomeric purity of all substrates remained constant throughout the irradiations. Changes in isomeric purity as a result of the irradiation is usually a sign of heavy radiation damage to the sample.
The Journal of Physical Chemistry, Vol. 97, No. 15, 1993 3919
TABLE-I: Percent Retention of Configuration for Gaseous 1*F-for-XSubstitutions at the Asymmetric Carbon Sites of mese and dkDifluorobutanes, (2S,3R)-dk and (2S,3S)-dkChlorofluorobutanes, and meso- and dkDichlorobutanesa 5% retention of configurationb
substrate
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Abbreviations: DFB = 2,3-difluorobutane; CFB = 2,3-chlorofluorobutane; DCB = 2,3-dichlorobutane. Errors reported represent standard deviations of at least three independent experimental runs.
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It is interesting to note that the retention values listed in Table I are nearly the same regardless of the nature of the leaving group. In addition, these values are nearly the same as those observed from our earlier 38Clwork in the same systems.6 This is surprising in view of the fact that past studies, at least those dealing with substrate molecules possessing single asymmetric carbon have shown systematicbehavior in thedependence of reaction stereochemistryon the nature of the hot atom, as well as on the nature of the leaving group. Absolute yields for 18F-for-X(X = F, C1) substitution in the 2,3-dihalobutanes are presented in Figure 1 for the retention products and in Figure 2 for the inversion products. Results in both figures are presented according to the nature of the reaction. Two key features of reaction are apparent from these measurements. On the other hand, both the retention and inversion product yields are consistently higher for the 18F-for-Freacthn
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Ferrieri et al.
The Journal of Physical Chemistry, Vol. 97, No. 15, 1993 0.3
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than for the I8F-for-C1reaction. Note that yields from the DFB and DCB systems should be halved prior to comparison with CFB results. This corrects for the fact that the two asymmetric sites are available for reaction in the DFB and DCB systems which are identical in the sense that the leaving groups are the same. However, this treatment was not carried out on the data the way it was presented here. Contrary to the above behavior, product yields measured in our earlier studies on 38C1-for-X (X = F, C1) substitution in the same dihalobutanes were somewhat invariant between substratesystems.6 We attribute the behavior seen in the 18Freactions to the small size and mass of the fluorine atom. Because of this feature, momentum transfer during substitution is more easily affected by the nature of the leaving group. Therefore, bulkier atoms stand less of a chance of being displaced. On the other hand, the much larger size and mass of the 38Clatom probably overwhelmed these subtle effects in our earlier work, and therefore probably obscurred this behavior. The second key feature of our present work is the lack of systematic dependence of the inversion product yields on the configurationof the substrate (see Figure 2). Again, our earlier 38Clwork showed consistentlylower inversion product yields from the (S,R) configuration than from the (SJ)configuration of each substratebecause the 38Cl atom was more stericallyhindered from attack at the back-side by molecules in the (S,R) configuration. We illustrate these results in Figure 3 to faciliate comparison with the present data. Our observationshere support the hypothesis that steric hindranceto back-side attack, imposed by changes in the equilibrium rotational conformer populations becomes an unimportant feature of reaction for sufficientlysmall hot atoms.
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Conclusions In light of our earlier 38Cl work, results from our present studies on 8F-for-X substitution in gaseous diastereomeric 2,3-dihalobutane reveal two general featuresof this reaction. On the one hand, the reaction efficiency is sensitive to the size and mass of the leaving group, but only if the hot atom itself is small. We attribute this behavior to the efficiency with which momentum is transferred during reaction. It is interestingto see that both front-sideand back-sidechannels of reaction in the 18F work exhibit the same leaving group dependence. Although each channel differs by which side the hot atom attacks the asymmetriccarbon site, these observations suggest that their dynamical aspects are probably very similar. A second feature of these reactions is that while rotational conformer populations can strongly affect the degree of steric hindrance to back-side attack, these effects are generally unimportant for small hot atoms like I8F.
Acknowledgment. This research was supported by the US. Department of Energy, Fundamental Interactions Branch, Division of Chemical Sciences, under Contract DE-AC0276CH00016 (Brookhaven National Laboratory) and Contract DE-FG02-84ER13231.A004 (University of Nebraska). References and Notes (1) Wolf, A. P.; Schueler, P.; Pettijohn, R. R.; To, K.-C.; Rack, E. P. J. Phys. Chem. 1979,83, 1237. (2) To, K.-C.; Rack, E. P.; Wolf, A. P. J. Chem. Phys. 1981, 74, 1499. (3) To, K.-C.; Wolf, A. P.; Rack, E. P. J. Chem. Phys. 1983.87.4929. (4) Firouzbakht,M.L.;Ferrieri, R. A.; Wolf, A. P.; Rack, E. P. J.Phys. Chem. 1986, 90, 5339.
Halogen Atom Substitution ( 5 ) Firouzbakht. M. L.; Ferrieri, R. A.; Wolf, A. P.; Rack, E. P. J . Am. Chem. SOC.1987. 109. 2213.
(6) Sharma,'R. B.; Ferrieri, R. A.; Meyer, R. J.; Rack, E. P.: Wolf, A. P. J . Phys. Chem. 1990, 94, 2316. (7) Sharma. R. B.; Ferrieri. R. A,; Rack, E. P.; Wolf, A . P. Radiochim. Aria 1990. 50, 91. ( 8 ) Edgell, W. F.; Parts, L. J . Am. Chem. SOC.1955, 77, 4899.
The Journal of Physical Chemistry, Vol. 97, No. 15, 1993 3921 (9) Hoffmann, F. W. J . Org. Chem. 1949, 14, 105, (IO) Angelini. G.; Speranza, M . J . Am. Chem. SOC.1981, 103, 3792. ( I I ) Bower, A.; Ibanez. L. C.; Deriot, E.; Becerra. R. J . Am. Chem. Soc. 1960.82. 4001.
(12) Sheppard. W. A.; Shartz, C. M. Organic Fhorine Chemistry; Benjamin: New York. 1969; p 127.
