Heterogeneous Rate Constants for Indium Mediated Allylations

Article pubs.acs.org/JPCA

Heterogeneous Rate Constants for Indium Mediated Allylations: Cinnamyl Chloride in Ethanol/Water Mixtures Alexa N. Hill, Katherine M. Delaney, Tessa R. Sullivan, Gabriella Mylod, Katrina H. Kiesow, and Walter J. Bowyer* Department of Chemistry, Hobart and William Smith Colleges, Geneva, New York 14456, United States ABSTRACT: Indium mediated allylation (IMA) offers a powerful tool to synthetic chemists for creating carbon−carbon bonds. However, its rate limiting step, the heterogeneous reaction of allyl halides at solid indium surfaces, is still poorly understood. For example, solvent effects, especially the presence of water, on IMA are dramatic. We report for the first time rate constants for the heterogeneous rate limiting step of IMA. The rate constant for reaction of cinnamyl chloride on indium decreases from 5.5 × 10−4 cm/s in 80% ethanol/20% water to 1 × 10−4 cm/s in 99.8% ethanol/0.2% water. In addition, the percent water has a dramatic effect on induction time. This study further establishes photomicroscopy as a powerful tool for the determination of heterogeneous rate constants.

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rate limiting step7,8 is the heterogeneous reaction of the allyl halide on solid indium metal to generate an allyl indium intermediate (vide infra).9−12 The intermediate then goes on to couple with a wide range of electrophiles, often by way of a chelated transition state.13−17 The nature of the solvent exerts unusually high influence on the outcome of the reaction, including the nature of the product, the yield, and the rate.8,14−25 Studies have identified at least five steps where solvent can influence the outcome of IMAs. Solvent can affect step 1, the rate of mass transport of the allyl halide to the indium surface, and step 2, the rate of reaction on the surface. For example, Paquette and colleagues have demonstrated that allylation of many aldehydes proceeds very quickly in water but slowly or not at all in anhydrous THF.8,17,26 They demonstrated that this effect occurs in step 1 and/or 2 by synthesizing the organoindium before introduction of the aldehyde and observing that the coupling reaction goes very quickly indeed in either solvent.8,27 The structure of the organoindium intermediates in solution (step 3) is affected by solvent. Early NMR tube reactions were interpreted to suggest that allyl indium sesquihalide was formed in DMF, and allylindium(I) was formed in water.1,28 However, in a recent elegant series of studies, Baba et al. have shown that a mix of diallylindium halide (R2InX) and allylindium dihalide (RInX2) are produced in either water or organic solvents.10−12,29,30 They demonstrated that diallylindium halide (R2InX) has a relatively short lifetime in water, confirming the conclusions of Singaram and his students.6 Furthermore, studies by Koszinowski using a combination of ESI-MS, variable temperature NMR, and conductivity have demonstrated the existence of a variety of other intermediates including InR2+, InRX+, InRX3−, InR2+, and In2R4X+.31 The

INTRODUCTION Indium mediated allylation (IMA, Scheme 1 illustrates a simple example) is a powerful tool for synthetic chemists because it Scheme 1

offers a wide range of benefits. IMAs typically proceed as a onepot reaction easily with no promoters, and they enjoy very high yields. Furthermore, they often are remarkably regio-, diastereo-, and enantioselective. They tolerate a wide range of functional groups, obviating the need for protection− deprotection. Unlike many metals, the indium surface is unreactive to water and atmospheric oxygen. Perhaps most importantly, IMAs often proceed in water and other environmentally friendly solvents.1−6 The mechanism has been demonstrated to proceed by way of an organoindium intermediate. As illustrated in Scheme 2, the Scheme 2

Received: April 23, 2013 Revised: July 22, 2013 Published: July 23, 2013 © 2013 American Chemical Society

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dx.doi.org/10.1021/jp404027p | J. Phys. Chem. A 2013, 117, 8826−8835

The Journal of Physical Chemistry A

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and cinnamyl bromide with indium in ethanol/water are mass transport controlled under a variety of geometries, and their heterogeneous rate constants were shown to be greater than 1 × 10−2 cm/s.39,62 We now report rates for the slower reaction of cinnamyl chloride, which is kinetically controlled under conditions of rapid mass transport. These represent the first heterogeneous rate constants determined for indium mediated allylations. The heterogeneous rate constants depend on the percent of water in the ethanol/water mix. In addition, the induction time for the reaction depends greatly on the percent water, which can drastically affect apparent rates if not accounted for.

polarity and donor strength of the solvent was shown to determine the distribution of organoindium species. Step 4, the coupling of the organoindiuim intermediate with the electrophile, has been extensively studied because of its implications for the chemo-, regio-, and stereoselectivity of the reaction.8,9,14,17,23,32,33 Evidence is excellent that the coupling often proceeds by a chelated, cyclic transition state. In many cases, the nature of the solvent, especially aqueous vs organic, can reverse the selectivity by determining the degree of solvation of the indium ion, which determines the structure of the transition state.14,16,18,32−36 Finally, in step 5 the product of the coupling reaction sometimes can rearrange or further react depending on the solvent, especially protic vs aprotic solvents.37,38 Furthermore, disproportionation−conproportionation between In3+, In+, and In0, which is sensitive to solvent, may affect product distribution and yield.6,39−43 Of these five steps, the heterogeneous reaction of allyl halide at the indium surface, step 2, is the least studied. Given the importance of IMA, it is remarkable that the rate limiting step has been so slightly studied.7,8 Semiquantitative studies, by monitoring the reaction for completion by TLC, have demonstrated that the heterogeneous reaction is rate limiting and that water accelerates the reaction.8 In some cases, special preparation of the indium surface, for example, nanoparticles or electrodeposited indium, can increase the overall rate of reaction, further demonstrating the importance of the heterogeneous reaction.7,44 Fundamental kinetic studies are the most powerful tool for exploring mechanisms, including solvent effects, and they often assist in the rational selection of conditions for synthesis.9,45,46 For example, measuring rates has explained the dramatic effects of solvent on the Diels−Alder reaction.47,48 However, determination of the kinetic parameters for the formation of allyl indium intermediates (step 2) is challenging. In general, heterogeneous reactions are difficult to study because of complications at the surface, and several factors have been shown to drastically affect the apparent rate of heterogeneous reactions: i. Reactions at metal surfaces can be difficult to initiate, and induction times can dramatically extend the time required for reaction.49,50 ii. Heterogeneous reaction rates depend on surface area, and surfaces often are not uniformly reactive. Further complicating the rate, the reactive surface often changes as the reaction consumes the metal.55−58 iii. Under some conditions, the rate of mass transport to the surface may determine the overall rate of the heterogeneous reaction while under other conditions the rate of reaction, the breaking and forming of chemical bonds, determines the reaction rate.39,46,49−59 Electrochemists have a wide array of analytical tools for measuring rates of electron transfer at surfaces, but other types of heterogeneous reactions offer significant challenges. Many types of microscopy have been shown to be useful for a range of heterogeneous reactions.52,53,57−67 For example, we have applied simple photomicroscopy to determine rates of reactions at metal surfaces that are being consumed.46,60−62 Recently, we quantified rates for steps 1 and 2 using in situ photomicrography to measure the distance of retreat of the indium surface, x, as a function of time, t.39,62 To form the allylindium intermediate, two things must happen: the allyl halide must be transported to the indium surface and then the reaction, the breaking and forming of bonds, must occur. Either can be rate limiting. The reactions of allyl iodide, allyl bromide,

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EXPERIMENTAL SECTION Reagents. Indium spheres [99.99%, diameter = 1.0 (±0.05) mm] were obtained from Alfa Aesar. Cinnamyl chloride (CinnCl) was obtained from Sigma Aldrich and stored at 4 °C. (Caution: cinnamyl chloride is highly toxic.) Ethanol (200 proof, undenatured, USP/ACS grade) was purchased from Pharmco-AAPER (Brookfield, CT) and contains approximately 0.2% water. All other reagents and solvents were obtained from Sigma Aldrich or Fisher Scientific and used as received. Reactions were performed at 22 ± 1 °C. Solvent Mixes. One major advantage of indium mediated allylations is that they can be performed in water. However, the reactants are only very slightly soluble in water, and for kinetic measurements, concentrations must be controlled.62 Thus, we use a mix of water and ethanol (also a common solvent for indium mediated allylations): ethanol volume%/water volume % = 80%/20%; 90%/10%, 95%/5%, and 99.8%/0.2%. The solutions were acidified with either 0.1 M HCl or 0.1 M acetic acid (HOAc) to keep the metal surface uniformly reactive by preventing precipitation of indium hydroxide. Earlier work has shown that these acids do not react with indium metal on the time scale of our experiments.39,62 For this study, we reconfirmed that conclusion by reacting indium with water/ ethanol/0.1 M acid under the conditions described below; again no retreat of the surface was observed. Image Recording and Analysis. For measurement of indium surfaces, a trinocular Nikon SMZ-U Zoom 1:10 microscope at 56× magnification, a Diagnostic Instruments monochromatic video camera, Spot version 4.6 software, and an MVI Model NCL150 light source were used to obtain and make measurements on the photomicrographs. The calibration of the image system was confirmed each day by photographing a Meleemeter (Edmund Scientific) and measuring the line spacing. Diameters were measured by fitting a circle to the image of the sphere, which resulted in a repeatability of about ±0.01 mm on multiple measurements. Measuring Reaction Rates. Photomicrographs were taken at time intervals such that four to eight images were obtained over the course of the reaction. Indium metal is reactive toward allyl halides over the entire surface, and we use two geometries. Geometry 1: convection to indium spheres (d = 1.0 mm) in a Teflon cell using stir bar and stir plate. Earlier, we described a “MAG” cell, a Teflon reaction vessel with a depression to contain the stir bar and a second depression to contain the soft, easily deformed indium beads.39 For this study, we describe a modified “mini-MAG”, illustrated in Figure 1A (well diameter = 2.1 cm, depth = 1.0 cm, and stirbar = 7.2 × 2.3 mm). The smaller mini-MAG has the advantage of achieving slightly higher mass transport rates and using only 3 mL of solution. The top of the cell is sealed with a 2 × 2 inch microscope slide 8827



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heterogeneous rate constant and concentration of the allyl halide. [x = dimension perpendicular to reacting surface (cm); t = time (sec); ks = heterogeneous rate constant (cm/s if n = 1); C = concentration of allyl halide (mol/cm3); n = reaction order; and Vm = molar volume of indium (cm3/mol).] Thus, a plot of the radius of the sphere vs time would be linear under kinetic control in both geometries 1 and 2. dx /dt = ksC nVm

(1)

Mass Transport Control. In contrast to kinetic control, if mass transport controls the rate of reaction, then increasing the rate of convection will increase the rate of reaction and thus the slope of x vs t. In the mini-MAG cells (Figure 1A), the equations for mass transport control are not easily described. However, if the concentration and stir rate remain constant, a plot of radius of the sphere vs time would be linear under mass transport control, similar to kinetic control. Mass transport to a planar rotating disk is well-defined by the Levich equation, eq 2.68

Figure 1. Geometries for mass transport to indium surfaces. (A) Convection to 1 mm spheres with stir plate and stir bar. (B) One millimeter spheres on a rotating shaft.

and silicone grease, allowing in situ photomicroscopy to monitor the retreat of the indium surface. Photomicrographs were taken at intervals for up to five hours, after which further reaction significantly depleted the CinnCl concentration in the cell. To improve precision for comparability at different stir rates, we constructed pairs of matching cells and ran paired experiments with one cell stirring on “high” and the other on “low”. This allowed a test of diffusion vs kinetic control by comparing the effect of stir rate on the rate of the reaction under otherwise similar conditions. Geometry 2: indium spheres on a rotating shaft, illustrated in Figure 1B. A Pine Instruments MSR Speed Control and Analytical Rotator (designed for rotating ring-disc electrode measurements) were used to achieve very high mass transport rates with rotations of 100−2000 rpm. A layer of glue approximately 0.5 mm thick was spread over the surface of a Teflon cap (r = 8 mm), and four to six indium spheres (d = 1.0 mm) were placed in a circle 2−4 mm from the edge of the cap. Typically, the spheres protrude 0.5 mm from the glue.39 In both geometries, in order to maintain effectively constant allyl halide concentrations, the extent of reaction compared to the amount of reactants was such that less than 10% of starting materials are consumed over the course of the measurements. Similarly, the acid concentration was not changed significantly by the reaction.

flux = 0.620AD2/3 ω1/2ν−1/6C

(2)

(flux = flux of allyl halide to the surface (mol/sec), A = area (cm2), D = diffusion coefficient of allyl halide (cm2/s); ω = angular frequency of rotation (s−1), and ν = kinematic viscoscity (cm2/s) of the solvent.) Thus, under mass transport control, eq 2 predicts that the reaction rate will be linearly proportional to the square root of the angular frequency, ω1/2. In eq 2, flux is the rate of arrival of allyl halide to the surface, while the rate of retreat of the indium surface is experimentally measured. Thus, to convert flux into the rate of reaction of indium, flux is multiplied by the stoichiometric ratio, 2 In/3 allyl halide, as described in eq 3 [rateD = diffusive component of rate of removal of In atoms (mol/s)].62 rateD = (2 mol In/3 mol allyl halide)flux

(3)

As demonstrated earlier,39 the rate of removal of indium in mol/s can be converted into its change of volume (dV/dt where V = volume of reacting indium in cm3) using the molar volume of indium (Vm = 15.7 cm3/mol), eq 4. The rate of retreat of the surface, dx/dt where x is the dimension perpendicular to the indium surface, is related to dV/dt by the reactive surface area, A in cm2, eq 5.

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THEORY For allyl halide to react with indium, first the allyl halide must diffuse to the surface, and then bonds must be broken and formed as the reactants surmount the energy of activation barrier. If the energy of activation is sufficiently small, then mass transport will be rate limiting. However, with large energies of activation and/or fast mass transport, the kinetics of the chemical reaction will be rate limiting, allowing measurement of heterogeneous rate constants. In between the two regimes, both the diffusion component and kinetic component contribute significantly. We have derived the equations that describe the radius, x, of a reacting surface as a function of time under kinetic control and under mass transport control.39,46,62 Here, we briefly summarize those results and extend them for application in this study. Kinetic Control. Under kinetic control, the rate of retreat of the surface is independent of the geometry of the indium and independent of the rate of mass transport. Equation 1 describes the rate of retreat of the indium surface as a function of the

rate = (dV /dt )/Vm

(4)

A(dx /dt ) = dV /dt

(5)

Combining eqs 2−5 and converting angular frequency to rpm (ω = 2πf = 2πrpm/60) results in eq 6a. This can be simplified to eq 6b, which describes the relationship between the rate of retreat of the surface under diffusion control, (dx/ dt)D, and the rotation rate in rpm (min−1). rateD = A(dx /dt )/Vm = (2/3)0.620AD2/3 (2π rpm/60)1/2 ν−1/6C

(6a)

(dx /dt )D = 0.1336VmCD2/3ν−1/6(rpm)1/2

(6b)

However, the Levich equation does not apply exactly to geometry 2 (Figure 1B) because the indium hemispheres extend beyond the surface of the rotating plane. A variety of rotating hemisphere electrodes have been developed and the corresponding Levich equation derived, all of which are similar to eq 4, having only a slightly different coefficient (e.g., 0.474 8828



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Figure 2. Photomicrographs of indium bead reacting with 0.10 M CinnCl in 80% ethanol/20% water/0.1 M HOAc at slow stir in a mini-MAG cell. Time = 0 h (left) and 4.0 h (right).

instead of 0.620 for a hemisphere on an insulating plane).52,69−73 Unlike the rotating hemispherical electrodes treated, the indium hemispheres are not centered on the axis of rotation. Nevertheless, it seems reasonable that the form of eqs 4 and 6 should apply with a different value of the coefficient. In fact, in an earlier study,39 we have shown that, when allyl iodides and bromides are reacting with indium hemispheres under mass transport control, the reaction rate is proportional to ω1/2 and approximately two times faster than predicted by the Levich equation for a rotating disc. Thus, for this study, as an approximation, we use the slope, m, of the plot of rate of retreat of the surface under diffusion control vs the square root of rotation rate for cinnamyl bromide, which was evaluated empirically earlier.39 (dx /dt )D = m(rpm)1/2

1/(dx /dt )tot = 1/(dx /dt )D + 1/(dx /dt )K

Now substituting eqs 1 and 7 into eq 11 yields eq 12, which combines both the kinetic and diffusion components of the reaction and describes the dependence of the rate of retreat of the surface on the rotation rate. 1/(dx /dt )tot = 1/ksC nVm + 1/[m(rpm)1/2 ]

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(7)

RESULTS Mini-MAG Cells: Convection with Stir Plate and Stir Bar. Figure 2 illustrates photomicrographs of an indium bead reacting with 0.10 M CinnCl in 80% ethanol/20% water/0.1 M HOAc. The diameter has decreased from 0.995 mm to 0.920 mm over the course of four hours. (The black and white feature at the center of the bead on the left is due to reflectance of the light source off of the smooth metal surface. As the reaction proceeds, there is a slight roughening of the surface as indicated by the change in reflectivity. We attribute this to differential reactivity at grain boundaries,53,73 which is more easily seen on indium foil.62) In all of the mini-MAG cell experiments, the radius of the spheres decreased linearly with time as they reacted. In every experiment, the spheres in the high stirring cell reacted more quickly than low stirring, suggesting significant contribution of mass transport rate to the rate of reaction. For example, Figure 3 illustrates the decrease in diameter of spheres vs time while reacting with 0.125 M CinnCl in 80% ethanol/20% water/0.1 M HOAc. In both cells, the relationship is linear; the slope (reaction rate) in the cell with faster stirring (squares) is larger than the cell with slower stirring (circles). The scatter in the data is typically about 0.01 mm, attributable to the reproducibility of the measurement of diameter on the photomicrograph. To determine the order of reaction, we measured rates of reaction at concentrations of CinnCl from 0.03 to 0.2 M. As expected, a plot of the rate of reaction vs concentration is linear, and the best fit line passes very close to the origin; see Figure 4. The scatter of the data indicates the reproducibility from one

(8)

In the same study, it was shown that the Levich equation does accurately predict the dependence of reaction rate on rotation rate, and plots of log(dx/dt)D vs log(rpm) yielded straight lines with slopes of 0.49−0.55 for allyl iodide, allyl bromide, and cinnamyl bromide.39 Mixed Kinetic and Mass Transport Control. Under conditions where the kinetics and mass transport rates are similar (within a factor of 10−100), both processes contribute to the overall rate. For electrochemical reactions, this is described by eq 9, where itot = the total or observed current (amps), iD = the current corresponding to that due to diffusion component, and iK = the current corresponding to the kinetic component.68 1/i tot = 1/iD + 1/iK

(9)

The corresponding equation for the rate of a heterogeneous reaction in which no current flows, rate, (in this case, the rate of loss of indium atoms from the surface, mol/s) is eq 10. 1/rate tot = 1/rateD + 1/rateK

(12)

Equation 1 is for the general case of a retreating surface. In geometry 1, the diameter of the bead is obtained experimentally. Thus, to apply eq 1 to geometry 1, we divide dd/dt by 2 to obtain dr/dt, which corresponds to dx/dt. In geometry 2, the radius of the indium hemisphere is obtained experimentally, and r corresponds directly to x in eq 12.

Taking the log of both sides of eq 7 and rearranging yields eq 8. This allows for a quantitative testing of the hypothesis of diffusion control by determining the slope of the plot of log(dx/dt) vs log(rpm). log(dx /dt )D = 1/2log(rpm) + log(m)

(11)

(10)

Substituting eqs 4 and 5 into eq 10 yields the equation in terms of the rate of retreat, eq 11. 8829



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differences are very highly significant with confidence levels greater than 99.9%. Thus, the data clearly support a significant contribution of mass transport to control the rate of reaction in those two solvent mixtures. In 99.8% ethanol, the difference is not statistically significant (confidence level ≈ 80%), suggesting that mass transport rate does not contribute in this solvent. We return to the difference in solvents below. To further test the hypothesis of mass transport control with CinnCl, we measured rates of reaction of allyl bromide (ABr) under similar conditions. ABr has been shown to react at mass transport controlled rates with ks ≥ 0.01 cm/s.39 Thus, if CinnCl is reacting at mass transport controlled rates, ABr should react at close to the same rate under the same convection conditions. In contrast, in mini-MAG cells with fast stirring in 80% ethanol/20% water, for ABr the kapp = 9.0(±0.9) × 10−4 cm/s, more than four times faster than reactions of cinnamyl chloride under identical conditions. This difference is highly significant, again with a confidence greater than 99.9%. Because the convection rate is the same in this comparison of ABr to CinnCl in 80/20, the lower apparent rate constant for CinnCl must be attributed to partial kinetic control. Taken together, all of these data are consistent with a mix of kinetic and mass transport control of reaction rate at the convection rates achievable in geometry 1 with fast stirring. In order to determine absolute rate constants, rather than apparent rate constants, we need to have kinetic control be the major contributor. To achieve this, we use the higher convection rates achievable in geometry 2. Reaction Rates at Indium on a Rotating Shaft. To further explore the possibility of mixed kinetic and mass transport rate control, we turned to measurement of reaction rates at indium beads on a rotating shaft, geometry 2, with higher rates of mass transport than can be achieved by the miniMAG cells. As we found in geometry 1, reaction rates are proportional to CinnCl concentration, again consistent with a first order reaction. Figure 5 illustrates a bead of indium glued to a rotator cap initially and after 2.0 h of reaction with 0.20 M CinnCl in 80% ethanol/20% water at 1500 rpm. (Again, the white features in the bead on the left are due to reflectance of the light source.) Over the course of the reactions, the beads largely retained their hemispherical shape as illustrated in Figure 6. In contrast, when reacting with cinnamyl bromide at mass transport controlled rates, the beads acquired a characteristic “shark-fin” shape due to variable flow rates over the surface of the bead resulting in variable reaction rates.39 Thus, the shape of the bead after reaction is not consistent with mass transport control of rate for the reaction of CinnCl. Under mass transport control, reaction rates are predicted to increase linearly with the square root of the rotation rate (eq 7). However, with kinetic control, there should be no dependence of reaction rate on rotation rate. Figure 6 illustrates the effect of rotation rate on reaction rates of CinnCl with indium in 90% ethanol/10% water (solid diamonds). A line fit to the points has a very small positive slope, suggesting that mass transport contributes minimally to the reaction rate. Furthermore, the data do not extrapolate to the origin, as would be predicted by eq 7 under mass transport control. To further test for mass transport control, we compare the reaction rate of CinnCl to that expected under mass transport control. The dashed line in Figure 6 represents the slope of the corresponding reaction rates for cinnamyl bromide, which reacts at mass transport controlled rates.39 CinnCl would be

Figure 3. Diameter of indium spheres reacting in matched mini-MAG cells with 0.125 M CinnCl in 80% ethanol/20% water/0.1 M HOAc (circles, slow stir; squares, fast stir).

Figure 4. Rates of reaction (dd/dt) of spheres vs concentration in 80/ 20/0.1 with [CinnCl] = 0.03 to 0.2 M (circles, slow stir; squares, fast stir).

experiment to another, which is limited by the relatively poor control of stir rate with traditional stir plates. In 80% ethanol/ 20% water, the fast stir cells react about two times faster than slow stir cells. Similar linear plots of reaction rate vs concentration are found for CinnCl in 95% and 99.8% ethanol although the difference between rates in fast vs slow stir cells is not as large; see Table 1. Table 1. Values of kapp and 95% Confidence Interval in MiniMAG Cells in Three Solvent Mixes (All with 0.10 M HOAc) solvent viscosity (cp)

stir rate

average kapp/10−4 cm/s

confidence interval

80/20

2.1

95/5

1.46

99.8/0.2

1.11

fast slow fast slow fast slow

2.1 1.0 1.4 0.9 1.3 1.0

±0.18 ±0.13 ±0.24 ±0.11 ±0.36 ±0.18

%EtOH/%H2O

It is possible to convert the rate of reaction, dd/dt, into dr/dt and then to calculate an apparent rate constant, kapp with eq 1. If mass transport contributes significantly to the reaction rate, the value of kapp represents the minimum value of ks. The values of kapp are collected in Table 1 for the three solvent mixtures. Each average represents 10−15 independent experiments, and the 95% confidence interval is calculated from eq 13 where t = t-statistic for Student’s t test, s = standard deviation, and N = number of replicates. confidence interval = ±ts /N1/2

(13)

The Student’s t test was used to determine whether stir rate has a significant effect on rotation rate. In 80/20 and 95/5, the 8830



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Figure 5. Photomicrographs of indium bead glued to a rotator reacting with 0.20 M CinnCl in 80% ethanol/20% water/0.1 M HOAc. Time = 0 h (left) and 2.0 h (right). For scale, the width of the field is 2.4 mm.

Table 2. Values of kapp Determined from Indium Spheres on a Rotator for Four Different Compounds compd CinnCl CinnCl CinnCl CinnCl CinnBr ABr AI

EtOH%/H2O% 80/20 90/10 95/5 99.8/0.2 90/10 60/40 60/40

acid HOAc HCl HOAc HOAc HCl HCl HCl

kapp/10−4 cm/s c

4.7(±0.6) 2.8(±0.5) 1 1 110(±10)a 90(±10)a 70(±7)a

slopeb 0.16 0.16

0.55a 0.49a 0.52a

a

Values from ref 39. bSlope of log(dr/dt) vs log(rpm). cParentheses enclose confidence interval at 95% confidence.

reactions of cinnamyl bromide, allyl bromide, and allyl iodide, the slopes of the log−log plots are 0.55, 0.49, and 0.52 (Table 2).39 As a final confirmation of this model, Figure 6 illustrates the reaction rates predicted by combining the kinetic and mass transport components of the rate using eq 12. The predicted overall rates (open triangles) were calculated using ks = 3.0 × 10−4 cm/s (the average value of ks from the highest rotation rates) and m = 3.66 × 10−7 cm/s (the slope of the dashed line in Figure 6, determined for cinnamyl bromide under similar conditions, which is completely dominated by mass transport control). The match is quite acceptable within the precision of the data.75 In 80% ethanol, the results are very similar to 90% ethanol, leading to the same conclusion: kinetics determine the reaction rates at the highest rotation rates. Again, using eq 12, the simulation in 80% ethanol resulted in ks = 5.5 × 10−4 cm/s. Calculating the average ks (for all rotation rates and at concentrations from 0.05 to 0.2 M) yields ks = 2.8 × 10−4 cm/s in 90% ethanol/10% water and ks = 4.7 × 10−4 cm/s in 80% ethanol/20% water. Although the reproducibility of the data in Figure 6 is not excellent (the percent relative standard deviation is about 30%), reasonably narrow confidence intervals are achieved with a relatively large number of replicates; typically N = 10 to 20. A Student’s t test was used to determine whether the apparent difference of ks in 80% vs 90% ethanol is

Figure 6. Reaction rate (dr/dt) vs rotation rate1/2 for 0.10 M CinnCl reacting in 90% ethanol/10% water (diamond points). The dashed line represents the slope for CinnBr under similar conditions.39 The open triangles are the predicted slopes using eq 12.

expected to diffuse at close to the same rate as CinnBr, yet the chloro analogue reacts approximately 20 times slower, an observation inconsistent with mass transport control for CinnCl. Thus, these data strongly suggest that the reaction rate is dominated by kinetic control. Under these conditions, higher rotation rates should lead to complete kinetic control,68,74 consistent with closer inspection of Figure 6 (solid diamonds). At the lowest values of rotation rate, the reaction rate of CinnCl appears to depend very slightly on rotation rate. At the highest rotation rates, there is no apparent dependence. These data are consistent with a time regime where the reaction rate is dominated by kinetics, but where mass transport perhaps contributes slightly at lower rotation rates. To further confirm this conclusion, the data in Figure 6 were plotted as the log of reaction rate vs the log of the rotation rate, which results in a slope of 0.16 (Table 2), significantly smaller than the value of 0.5 predicted for mass transport control (eq 8) but slightly greater than the value of 0 expected for complete kinetic control. In contrast, for the mass transport controlled 8831



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significant. In fact, the difference is highly significant with confidence levels greater than 99.9%. Thus, the data clearly support the conclusion that decreasing water in the solvent mix results in a smaller ks. We have similarly determined rate constants for the reaction of CinnCl with indium in 95% and 99.8% ethanol, and these values are reported in Table 2. However, the induction time (see following section) in these solvents complicates the measurements on a rotator, and we report those values with only one significant figure, consistent with their lower precision. We note with satisfaction that these rotator results (geometry 2) are consistent with those from the mini-MAG cells (geometry 1), Table 1. If geometry 2 is dominated by kinetic control with some mass transport contribution at the slowest rotation rates, measurements at the longer time scale of the mini-MAG cell are predicted to have lower values of kapp; this is precisely what is observed. Furthermore, in geometry 1, the solvent mix that shows the least evidence of mass transport control (99.8% ethanol in which the difference between slow and fast stir rates is not significant) has the smallest rate constant. This observation is consistent with the slower rate constant in 99.8% ethanol, which apparently has minimal mass transport contribution even in geometry 1. The observed kapp in mini-MAG cells in 95% and 99.8% ethanol (1.4 and 1.3 × 10−4 cm/s, Table 1) corresponds reasonably well with the values of ks obtained by the rotator (1 × 10−4 cm/s, Table 2), increasing our confidence in those values in spite of the imprecision of the ks values determined by the rotator. As can be seen in Figures 2 and 5, there is a slight roughening of the surface. This is more easily observed on indium foil and is attributable to differential reactivity at grain boundaries39,73 and at the various faces of the indium microcrystals.49 Thus, the surface area is slightly greater than that calculated simply from the radius, meaning that our calculated heterogeneous rate constants are slightly high. However, we have never seen more than 10% variability in the rate of reaction at different locations on a single surface, suggesting that this is a relatively small effect. The effects of convection rate, halide (Br vs Cl), and viscocity of solvent are all consistent with a mix of kinetic control and diffusion control, with kinetic control dominating. At the highest convection rates, kinetics completely dominate, allowing determination of absolute rate constants for reaction of cinnamyl chloride at indium. Importantly, the rate constants for the reaction depend on % water in ethanol, decreasing by a factor of 5 as the % water decreases from 20% to 0.2%. Induction Times. While making measurements with miniMAGs (geometry 1), we observed that at low percent water, there is a significant delay between the mixing of the reactants and the start of reaction. At 5% or 0.2% water, we often observed that the diameter of the sphere did not change (indicating no reaction) for a period of time. Once the reaction starts, the radius decreased linearly with time. By extrapolating the initial, level portion of the plot to the linear portion during reaction, we determined induction time with a precision of about ±5 min. However, in fact, the induction time is less reproducible than that. Figure 7 illustrates induction times for the three solvent mixtures over a range of concentrations of CinnCl. (The ranges of concentrations do not entirely overlap. In 80% ethanol/20% water, CinnCl does not dissolve above 0.2 M. In 99.8% ethanol/0.2% water, CinnCl is much more soluble, and measurements at low concentrations were not done because the long induction time makes measurements

Figure 7. Induction times for three different solvent mixtures as a function of CinnCl concentration. Open triangles = 80% ethanol/20% water. Solid squares = 95% ethanol/5% water. Open diamonds = 99.8% ethanol/0.2% water.

impractical.) Although the straight lines in Figure 7 fit the data reasonably well within its scatter, they are not intended to imply a linear relationship. However, they do illustrate that induction times decrease with higher % water and at higher molarity of the CinnCl. The induction times illustrated in Figure 7 show a great deal of variation, which is not uncommon for this phenomenon. Metal surfaces are complex with crystal strain, adsorbates, passification, and other phenomena affecting reactivity.57 For example, highly variable induction times have been observed in reactions ranging from deposition of zinc52 to corrosion of steel.53,54 More closely related to IMA, the induction time of the Grignard reaction is highly variable. Trace amounts of water in the organic solvent react with the magnesium surface, affecting its reactivity. Thus, the induction time for the Grignard reaction varies greatly, depending on the dryness of the solvent.46 However, even under rigorously anhydrous conditions, such reactions can have significant and highly variable induction times.50,76 Of course indium surfaces do not react with water as magnesium does, and water is having the opposite effect on IMA, decreasing induction time. We hypothesize that the water increases the surface reactivity by solubilizing trace impurities that otherwise absorb on the surfaces. However, we continue to explore what those impurities are and why the induction times are so variable.

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DISCUSSION Taken together, the evidence demonstrates that under sufficiently high mass transport rates the reaction of cinnamyl chloride at indium surfaces is first order and kinetically controlled. The heterogeneous rate constant, ks, for this reaction in ethanol/water depends on percent water and ranges from 1 × 10−4 cm/s at 0.2% water to 5.5 × 10−4 cm/s at 20% water. The reaction of cinnamyl chloride is much slower than for the corresponding cinnamyl bromide, whose heterogeneous rate constant, ks, is greater than 1 × 10−2 cm/s.39 This difference in rates between the chloro and bromo analogues is not surprising and suggests that the carbon-halide bond is significantly broken in the rate determining step. 8832



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Ketones and Mechanistic Investigation of the Organoindium Reagents. J. Org. Chem. 2010, 75, 642−649. (7) Li, J.; Zha, Z.; Sun, L.; Zhang, Y.; Wang, Z. The Employment of Indium Nanoparticles in Barbier-Type Reaction of Allylic Chloride in Water. Chem. Lett. 2006, 35, 498−499. (8) Paquette, L. A.; Mitzel, T. M. Addition of Allylindium Reagents to Aldehydes Substituted at CαCβ with Heteroatomic Functional Groups. Analysis of the Modulation in Diastereoselectivity Attainable in Aqueous, Organic, and Mixed Solvent Systems. J. Am. Chem. Soc. 1996, 118, 1931−1937. (9) Dam, J. H.; Fristrup, P.; Madsen, R. Combined Experimental and Theoretical Mechanistic Investigation of the Barbier Allylation in Aqueous Media. J. Org. Chem. 2008, 73, 3228−3235. (10) Yasuda, M.; Haga, M.; Baba, A. Isolation and Characterization of a Nucleophilic Allylic Indium Reagent. Organometallics 2009, 28, 1998−2000. (11) Yasuda, M.; Haga, M.; Baba, A. Isolation and Crystallographic Characterization of Allylindium Species Generated from Allyl Halide and Indium(0). Eur. J. Org. Chem. 2009, 5513−5517. (12) Yasuda, M.; Haga, M.; Nagaoka, Y.; Baba, A. Characterization of the Nucleophilic Allylindium Species Generated from Allyl Bromide and Indium(0) in Aqueous Media. Eur. J. Org. Chem. 2010, 5359− 5363. (13) Isaac, M. B.; Chan, T.-H. Indium Mediated Coupling of Aldehydes with Allyl Bromides in Aqueous Media. The Issue of Regioand Diastereo-Selectivity. Tetrahedron Lett. 1995, 36, 8957−8960. (14) Lu, W.; Chan, T. H. Indium-Mediated Organometallic Reactions in Aqueous Media. Stereoselectivity in the Crotylation of Sulfonimines Bearing a Proximal Chelating Group. J. Org. Chem. 2001, 66, 3467−3473. (15) Lu, W.; Chan, T. H. Organometallic Reactions in Aqueous Media. Indium- and Zinc-Mediated Allylation of Sulfonimines. J. Org. Chem. 2000, 65, 8589−8594. (16) Araki, S.; Shiraki, F.; Tanaka, T.; Nakano, H.; Subburaj, K.; Hirashita, T.; Yamamura, H.; Kawai, M. Allylindation of Cyclopropenes in Organic and Aqueous Media: Switching the Regio- and Stereoselectivity Based on the Chelation with a Hydroxyl Group and the Crystal Structure of the Cyclopropylindium Product. Chem.Eur. J. 2001, 7, 2784−2790. (17) Paquette, L. A.; Lobben, P. C. Evaluation of Chelation Effects Operative During Diastereoselective Addition of the Allylindium Reagent to 2- and 3-Hydroxycyclohexanones in Aqueous, Organic, and Mixed Solvent Systems. J. Org. Chem. 1998, 63, 5604−5616. (18) Araki, S.; Kameda, K.; Tanaka, J.; Hirashita, T.; Yamamura, H.; Kawai, M. Umpolung of Vinyloxiranes: Regio-and Stereoselectivity of the In/Pd-Mediated Allylation of Carbonyl Compounds. J. Org. Chem. 2001, 66, 7919−7921. (19) Tan, K.-T.; Chng, S.-S.; Cheng, H.-S.; Loh, T.-P. Development of a Highly α-Regioselective Metal-Mediated Allylation Reaction in Aqueous Media: New Mechanistic Proposal for the Origin of αHomoallylic Alcohols. J. Am. Chem. Soc. 2003, 125, 2958−2963. (20) Zhao, J.-F.; Tsui, H.-Y.; Wu, P. J.; Lu, J.; Loh, T.-P. Highly Enantioselective Carbonyl-ene Reactions Catalyzed by In(III)_PyBox Complex. J. Am. Chem. Soc. 2008, 130, 16492−16493. (21) Chung, W. J.; Higashiya, S.; Oba, Y.; Welch, J. T. Indium- and Zinc-Mediated Allylation of Difluoroacetyltrialkylsilanes in Aqueous Media. Tetrahedron 2003, 59, 10031−10036. (22) Goeta, A.; Salter, M. M.; Shah, H. New Indium-Mediated Cyclization Reactions of Tethered Haloenynes in Aqueous Solvent Systems. Tetrahedron 2006, 62, 3582−3599. (23) Kargbo, R.; Takahashi, Y.; Bhor, S.; Cook, G. R.; Lloyd-Jones, G. C.; Shepperson, I. R. Readily Accessible, Modular, and Tuneable BINOL 3,3′-Perfluoroalkylsulfones: Highly Efficient Catalysts for Enantioselective In-Mediated Imine Allylation. J. Am. Chem. Soc. 2007, 129, 3846−3847. (24) Law, M. C.; Cheung, T. W.; Wong, K. Y.; Chan, T. H. Synthetic and Mechanistic Studies of Indium-Mediated Allylation of Imines in Ionic Liquids. J. Org. Chem. 2007, 72, 923−929.

The dependence on solvent of the values of ks is perhaps most simply interpreted by hypothesizing that the transition state of the reaction is highly polar or charged. If so, water would decrease the energy of activation by stabilizing the transition state and simultaneously destabilizing the nonpolar reactant, allyl halide. However, further studies, for example, in a wider variety of solvents, will be needed to test this hypothesis. To the synthetic chemist, the more important effect of solvent on reaction time may be the induction time, which can exceed six hours at low concentrations of water and cinnamyl chloride on indium beads. Surprisingly, to our knowledge there is no measure of induction time for IMA reactions in the literature although it has been reported that some IMAs do not occur at all in the absence of water, an observation that may be due to long induction times; e.g., ref 9. There are several cases in the literature where the rate of the reaction is estimated from the time it takes to completely consume starting material; e.g., refs 17 and 26. Our observation of a significant induction time means that simply measuring the completion time conflates at least two effects: rate and induction time. Thus, this study demonstrates that to understand the effect of solvent on rates of indium mediated allylation, it is necessary to discriminate between solvent effects on mass transport rate, on reaction rate, and on induction time. Photomicroscopy is demonstrated to be a powerful tool for these studies.

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CONCLUSIONS Reaction of cinnamyl chloride with indium in ethanol/water proceeds under kinetic control at high rates of convection. The percent water in ethanol has two important effects on the rate. The heterogeneous rate constants decrease from 5.5 × 10−4 cm/s at 20% water to 1 × 10−4 cm/s at 0.2% water. In addition, percent water has a dramatic effect on induction time. These results are summarized in Table 2.

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AUTHOR INFORMATION

Corresponding Author

*(W.J.B.) E-mail: [email protected]. Phone: 315-781-3608. Fax: 315-781-3860. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work supported by the National Science Foundation under Grant No. CHE-1007510. We thank Peter Spacher and Kathy Slentz of Hobart and William Smith Colleges for generous technical assistance.

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REFERENCES

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Heterogeneous Rate Constants for Indium Mediated Allylations

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