Isotope effects, secondary deuterium kinetic

Indicate mechanisms that would account for the formation of each product. Show how the isotopic substitution could cause a change in product composition. Does your mechanism predict that the isotopic substitution would give rise to a primary or secondary deuterium kinetic isotope effect Calculate the magnitude of the kinetic isotope effect from the data given. 

The use of secondary deuterium kinetic isotope effects in mechanistic studies of olefin bromination... 

The density functional theory calculations of primary 14C KIE and secondary deuterium kinetic isotope effects (SKIE)220 did not reproduce satisfactorily all the experimentally determined 14C KIE and deuterium (4,4-2H2)- and 6,6-2H2-SKIE, though the non-local DFT methods provide transition state energies on a par with correlated molecular orbital theory221. 

The primary hydrogen-deuterium kinetic isotope effect is obtained from the percent cw-2-butene obtained from the deuterated and undeuterated stannanes. This is possible because a hydride and a deuteride are transferred to the carbocation when the undeuterated and deuterated stannane, respectively, forms c -2-butene. The secondary deuterium kinetic isotope effect for the hydride transfer reaction is obtained from the relative amounts of fraws-2-butene in each reaction. This is because a hydride is transferred from a deuterated and undeuterated stannane when trans-2-butene is formed. 

Secondary Deuterium Kinetic Isotope Effects and Transition State Structure... 

A secondary deuterium kinetic isotope effect is observed when substitution of a deuterium atom(s) for a hydrogen atom(s) in the substrate changes the rate constant but the bond to the deuterium atom is neither broken nor formed in the transition state of the rate-determining step of the reaction. Several types of secondary hydrogen-deuterium (deuterium) KIEs are found. They are characterized by the position of the deuterium relative to the reaction centre. Thus, a secondary a-deuterium KIE is observed when an a-hydrogen(s) is replaced by deuterium [equations (1) and (2)], where L is either hydrogen or deuterium. 

Isotope effects on both the carbon and hydrogen of the breaking C-H bond have been measured. However, for this reaction both forward and reverse commitments are sizable so the three equations corresponding to Equation 11.48 have four unknowns the forward and reverse commitments and two intrinsic isotope effects. Measurements of the secondary deuterium kinetic isotope effect (at position 4 of nicotinamide ring of NADP+) and the carbon kinetic isotope effect with the secondary position deuterated introduce two additional equations, but only one more unknown ... 

A different experimental approach to the relative importance of one-center and two-center epimerizations in cyclopropane itself was based on the isomeric l-13C-l,2,3-d3-cyclopropanes165"169. Here each carbon has the same substituents, one hydrogen and one deuterium, and should be equally involved in stereomutation events secondary carbon-13 kinetic isotope effects or diastereotopically distinct secondary deuterium kinetic isotope effects may be safely presumed to be inconsequential. Unlike the isomeric 1,2,3-d3-cyclo-propanes (two isomers, only one phenomenological rate constant, for approach to syn, anti equilibrium), the l-13C-l,2,3-d3-cyclopropanes provide four isomers and two distinct observables since there are two chiral forms as well as two meso structures (Scheme 4). Both chiral isomers were synthesized, and the phenomenological rate constants at 407 °C were found to be k, = (4 l2 + 8, ) = (4.63 0.19)x 10 5s l and ka = (4kl2 + 4, ) = (3.10 0.07) x 10 5 s 1. The ratio of rate constants k, kl2 is thus 1.0 0.2 both one-center and two-center... 

These recent calculations for cyclopropanes and trimethylene transition structures, and for isotopically labeled analogs, have provided vibrational frequencies from which secondary deuterium isotope effects have been calculated. Getty, Davidson and Borden found that reactions dependent upon C,(ts) at 422.5 °C should be associated with secondary deuterium kinetic isotope effects favoring access to 1,2-d2-Cj(ts) over l,3-d2-C (ts) structures by a factor of 1.13269. 

An unusually large inverse secondary deuterium kinetic isotope effect (1.53-2.75, depending on the reaction conditions) has been reported for bromination of the sterically congested olefin 74. This behaviour can be rationalized by decreased steric hindrance due, in particular, to the ewrfo-placement of the deuterium atoms relative to the double bond135. 

Pham TV, Fang Y-R, Westaway KC (1997) Using secondary deuterium kinetic isotope effects to determine the symmetry of SN2 transition states. J. Am. Chem. Soc. 119 3670-3676... 

These results are compatible with stereochemical predictions derived through orbital symmetry theory, assuming a one-step n2s - -n2a addition. But secondary deuterium kinetic isotope effects on the allene plus allene thermal (2+2) cycloaddition seem to require a two-step mechanism with formation of an intermediate 44>, and as Moore and coworkers fully realized 83> stereoselective formation and reactions of 2,2 -biallylene intermediates will equally well account for the product ratios. In their rationale, two allenes approach and distort through simultaneous conrotatory twistings to give the perpendicular 2,2 -biallylene intermediate, which closes to form products in a disrotatory fashion. The experimentally observed stereochemical selectivity is equally compatible with a reversed order of rotatory motions disrotatory joining of two allenic reactants followed by conrotatory closure to create the 1,2-dimethylenecyclobutane products 83>. 

It would have been possible to rescue the concerted hypothesis at this point by claiming that the transition state was structurally similar to the diolate, that is, that the alkene extrusions involved little C O bond cleavage prior to the transition state. However, the measurement of a significant secondary deuterium kinetic isotope effect (kHJkw = 1.25 0.05 at 100°C) gave evidence of significant C—O bond cleavage, whatever the mechanism. 

Gajewski, J. J. Peterson, K. B. Kagel, J. R. Huang, Y. C. J. Transition-statestructure variation in the Diels-Alder reaction from secondary deuterium kinetic isotope effects. The reaction of nearly symmetrical dienes and dienophUes is nearly synchronous, 7. Am. Chem. Soc. 1989, 111, 9078-9081. 

The secondary -deuterium kinetic isotope effects in reactions of -perdeuterated ethyl-, isopropyl- and rm-butylmagnesium halides with four different ketones were described [24] and were found to be small (within 5%). There, too, hyperconjugative stabilization was supposed to play a role, but this effect was opposed by the steric effects. The role of hyperconjugation in reactions of Grignard reagents, therefore, seems complicated and requires further studies. 

Secondary Deuterium Kinetic Isotope Effects. Deuterium substitution has been employed to probe for bridging in the transition state of 2-norbornyl brosy-late solvolyses. The secondary a-, (3-, and 7-deuterium kinetic isotope effects for exo-and endo-norbornyl brosylate are shown in formulas 721) and (722), respectively. The overall pattern for the mfo-compounds (722), with an a-effect close to the limiting value (1.22, cf. Section 7.2.3)506), with nil effect of C(1)-DS07 a modest... 

Resume. Alkyl participation in the solvolysis of exo-2-norbornyl derivatives is not unambiguously detected by rate measurements. Neither monocyclic analogs nor the corresponding enr/o-2-norbornyl compounds appear to be good models for comparison. Secondary deuterium kinetic isotope effects, however, point to distinct ionization mechanisms of exo- and -norbornyl derivatives. 

When imidazole or methanol was introduced to the dichloromethane solution of 36, instantaneous decomposition of 36 and the epoxide formation were observed. Thus, in the presence of either methanol or imidazole, the rate-determining step in the reaction of 14 and olefin was changed to the formation of 36. Under these conditions, secondary deuterium kinetic isotope effects on epoxi-dation were examined by a- and /3-deuterio-p-chlorostyrenes. For both the a- and the /3-positions of styrene, kn/fco = 1 was observed. The isotope effect and substituent effect on the formation of 36 suggest that both the a- and /8-carbons remain planar (sp hybridized) at the transition state and that a positive charge forms on the a-carbon. Accordingly, the formation of an olefin cation radical by an electron transfer from the olefin to 14 is indicated in the formation of 36 (Scheme XX). 

The secondary deuterium kinetic isotope effect is comprised of two secondary p-deuterium kinetic isotope effects of 1.22/jg-D and a secondary -deuterium kinetic isotope effect of 1.02. This kinetic isotope effect is identical to those found in several protic and dipolar aprotic solvents such as sulphuric acid-water, trifluoroethanol and methylene chloride. As a result, it has been concluded that the phenyl diazonium salt decomposes by the same mechanism in all solvents. 

The formation of the vinyl cation-silver ion complex in the slow step of the reaction is consistent with the observation of an inverse secondary deuterium kinetic isotope effect, because the terminal C—H bond undergoes a hybridization change from sp to sp in the rate-determining step of the reaction. 

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