638
-2
0
-I lop K
Figure 1. Contour diagram showing breakdown o f the Swain-Schaad relation. The full lines are contours for different values of c (the percentage breakdown of the Swain-Schaad relation) as a function of @ I / & (the ratio of fractionation factors for the two steps) and K (which describes the extent to which one step is cleanly rate limiting). The dashed lines are contours for $l/$I1.2 (which is approximately equal to the obseroed deuterium kinetic isotope effect) as a function of @ 1 / $ 2 and K . The square boxes show typical free energy levels (14, f u l l lines; D,dashed lines) for the two transition states. Transition state I (on the left of the boxes) has a bound proton ($1 N I ) and there is a difference in the zero point energies; transition state 2 (on the right) has the proton i n flight ($12 < 1) and the H and D levels are much closer.
to 10. For the range 10 > 41/62 > 1, therefore, we construct Figure 1, which is a contour diagram for c calculated from eq 8. In addition, the broken lines show contours for 4 , / 4 1 ,(from ~ eq 5 ) . Since 41does not involve a proton transfer, this ratio may be estimated from the experimental d a t a 3
That is, 41/41,2is approximately equal to the observed deuterium isotope effect. Thus Figure I displays how the breakdown of the SwainSchaad relation and the obserued deuterium isotope effect depend on the properties of the transition states. The insets illustrate typical free energies associated with the transition states and the isotope effects. On the right of the diagram ( K = k-l/kz >> 1) the proton transfer transition state is cleanly rate limiting. Under these conditions c tends to zero, and the ratio 41/41,2tends toward the ratio of the single factors 41/42. On the left of the diagram ( K 2 days)'
98 (60 m i n )
Zr
AI
QlfR 0
d
97 (45 m i n )
-
e
98 ( 4 5 min)
0 a Yield
determined by VPC. b Isolated yield. C l-Octene, cis-2-octene, trans-2-octene,cis4octene, fuans4-octene (1 :0.5 : O S : 1 : 1 ) .
Communications to the Editor
640 prepared directly from acetylenes) followed by acylation, a high yield of a,&unsaturated ketones is obtained. Through this sequential use of two reactive organometallic species, ketone is formed stereospecifically by overall cis addition of acyl-H to an alkyne; regioselectivity in the ketone prepared is the same as that in its Z r precursor.2,'2 The mechanism of transmetalation from Z r to A1 has been briefly investigated. In a competitive procedure involving equimolar amounts of alkyl complex la, vinylic complex Id, and a deficiency of AIC13, we have found that transmetalation occurs faster for the vinylic substituent than for the alkyl one. In addition, results involving stereospecifically labeled dideuterio complex 3' indicate that transmetalation from Z r to
point of departure for the rational development of new types of sequential organometallic reagent systems, be they stoichiometric or catalytic ones, in which the ability to transfer organic ligands readily from one metal to another provides a pathway to accomplish a synthetic transformation which is impossible (or is a t best tedious) for any one given type of organometallic species alone.
Acknowledgments. The authors acknowledge generous support for this work given by the National Science Foundation (CHE74-08081 A02) and the National Institutes of Health ( G M I 9658-03). References and Notes
H,
D. W. Hart and J. Schwartz, J. Am. Chem. SOC.,96,6115(1974). D. W. Hart, T. F. Blackburn, and J. Schwartz, J. Am. Chem. SOC.,97,679
D'
C. A. Bertelo and J. Schwartz, J. Am. Chem. SOC.,98,262(1976). C. A. Bertelo and J. Schwartz, J. Am. Chem. SOC., 97,228(1975). T. F. Blackburn, J. A. Labinger, and J. Schwartz, Tetrahedron Lett., 3041
(1975).
(1975).
A1 proceeds predominantly with retention of configuration at (as it does with vinylic substrates). These results suggest a mechanism for transmetalation which is indeed analogous to the one reported' for electrophilic cleavage of alkylzirconium complexes; the transition state (or intermediate) for this reaction involves a transition metal-main group metal complex containing a bridging alkyl (or vinylic) group. Such bridging alkyl or vinylic groups are well known in main group chemistryI3 and have been postulated to occur in intermediatesI7.l8or in isolable species19.20in transition metal chemistry as well. Shuttling of a a-bonded substituent from one metal to another via such a bridged intermediate could be expected to proceed more rapidly for the bridging vinylic compared to the bridging alkyl case; it is well known in organoaluminum chemistry, for example, that vinylic bridges between two A1 atoms are more stable than are alkyl ones.21-22 The sequence shown in reaction 1 suggests that acylation of 1 by acid chlorides could be catalyzed by AIC13. Transmetalation succeeds, however, only if AlC13 is not complexed with a Lewis base. Unfortunately, the ketonic product of acylation is a strong enough Lewis base to coordinate an equivalent amount of AIC13; this slows further transmetalation. Using 15% AIC13 (based on la, 0 OC, I O h), a 40% yield of ketone is produced; heating this reaction mixture (70 OC, 0.5 h) results in a higher yield (83%). Transmetalation does not proceed readily in Lewis basic solvents (such as ethers) for similar reasons. It is known that t r i a l k y l a l ~ m i n u malkylmagnesium ,~~ halide,23or a l k y l l i t h i ~ mcompounds ~~ will alkylate CpzZrC12, and preliminary results show that 1 will alkylate ZnC1224 or HgC12.6 On the basis of these observations, the following reactivity sequence of alkylating abilities of organometallic alkyls toward metal halides is derived: RLi, RMgX, or R3Al > CpzZr(C1)R > RAIC12; CpzZr(C1)R > RZnCl or RHgC1. Transmetalation results presented herein suggest that, as a general rule, 1 should be able to alkylate metal salts in which the metal (with its complement of ligands) is more electronegative than is the CpzZrCl-moiety (for example, Cu(1) halides) and that alkenyl complexes should be more reactive in this regard than alkyl ones. It is important to note here that, notwithstanding the utility of organoaluminum species in their own right, our results presented herein are significant in that they demonstrate that an easily prepared transition metal complex can serve as the convenient precursor of other reactive organometallic species. These studies, therefore, represent a Journal of the American Chemical Society
/
99:2
/ January
(a) D. W. Hart, Ph.D. Thesis, Princeton University, 1975;(b) R. A. Budnik and J. K. Kochi. J. Organomet. Chem., 116,3C (1976). J. A. Labinger, D. W. Hart, W. E. Seibert Ill, and J. Schwartz, J. Am. Chem. SOC.,97,3851 (1975). All reactions were performed under an atmosphere of dry argon. All solvents were dried by distillation from an appropriate drying agent under inert atmosphere. Organozirconiumcomplexes could be purified by crystallization from hexane. a color change from orange to yellow was observed. NMR analysis For IC and not color is, of course, a more reliable method for monitoring the transmetalation reaction. For la, for example, in CHZCIP,6cP 6.25,~ z ~ - c1.08, H ~ ~ A ~ 0.25; - c ~ for~ example, for (CH~CH~AICIZ)~, in CH2CI2, 6 ~ l - c0.58. ~ ~ The Zr product formed by transmetalation depends on the amount of AIC13 present relative to 1. A stoichiometric amount of AICi3 reacts with 1 to give free Cp2ZrC12. If an excess of AIC13 is used (
