The hydroboration of substituted cyclopropanes has been investigated using the B3LYP density functional method employing 6-31G** basis set. Borane moiety approaching the cyclopropane ring has been reported. It is shown that the reaction proceeds via a three-centered, “loose” and “tight,” transition states when boron added to the cyclopropane across a bond to a substituents. Single point calculations at higher levels of theory were also performed at the geometries optimized at the B3LYP level, but only slight changes in the barriers were observed. Structural parameters for the transition state are also reported.
Hydroboration of substituted alkenes has been investigated theoretically and experimentally. Brown and Zweifel [
When electron withdrawing groups are attached to the alkene preferential formation of the Markownikoff addition products has been reported. Phillips and Stone [
We have theoretically investigated the hydroboration of cyclopropane [
Optimization of all the geometries of stationary structures involved in the reaction was carried out using 6-31G** basis set at DFT/B3LYP [
Six substituted cyclopropanes (Figure
Optimized geometries of borane (BH3) and six kinds of substituted cyclopropane at B3LYP/6-31G** level.
There are two possibilities to be considered in connection with each substituted cyclopropane: first with the carbon atom bearing substituents denoted by C1, addition takes place across the C1–C3 bond and second addition takes place across the C2–C3 bond.
Optimization led to two types of transition states. In one case the BH3 group is closer to the ring (“tight” TS) and is on the side opposite the fluorine, whereas in the other the BH3 group is somewhat farther (“loose” TS) and on the side of the fluorine. The BH3 group has lost its planarity in both, but the distortion from planarity is more pronounced in the first case. These transition states are shown in Figure
Transition states optimized at B3LYP/6-31G** in the case of 1-fluorocyclopropane along the plane of cyclopropane ring.
IRC plot for the loose transition state for the addition of borane to 1-fluorocyclopropane along the plane of cyclopropane ring.
IRC plot for the tight transition state for the addition of borane to 1-fluorocyclopropane along the plane of cyclopropane ring.
On the other hand in the case of 1-chlorocyclopropane, we obtain both “tight” and “loose” transition states but in this case the path through the “tight” transition state leads to the Markownikoff product and the path through the “loose” transition state leads to the anti-Markownikoff product. Of the other substituted cyclopropanes studied it is found that cyano and isocyano cyclopropanes behave like chlorocyclopropane, while methyl and 1,1-dimethyl cyclopropanes behave like fluorocyclopropane in this respect (see the Supplementary Material).
For each of the substituted cyclopropanes, the structures of the transition structures at the B3LYP/6-31G** level were optimized. The “loose” transition state is preceded by an intermediate complex, while the “tight” transition state is apparently not. The structures of the complexes, “loose” transition structures, and products are shown in Figure
(a) B3LYP/6-31G** optimized structural parameters (units in Å for bond length) for the –F substituted cyclopropanes, intermediate complexes (LM-CX), “loose” transition structures (TS), and products (LM) along the plane of cyclopropane ring. (b) B3LYP/6-31G** optimized structural parameters (units in Å for bond length and in degree for angle) for the “tight” transition structures (TS) and products (LM) along the plane of cyclopropane ring.
C1–C3 | C1–B1 | C3–B1 | B1–H8 | |
---|---|---|---|---|
C3H5F | 1.493 | — | — | — |
LM-CX1 | 1.506 | 3.094 | 2.922 | 1.190 |
TS1 | 1.912 | 1.991 | 1.829 | 1.209 |
LM1 | 2.529 | 3.263 | 1.559 | 2.955 |
C3H5Cl | 1.498 | — | — | — |
LM-CX2 | 1.505 | 3.350 | 3.141 | 1.190 |
TS2 | 1.893 | 1.883 | 1.929 | 1.206 |
LM2 | 2.498 | 1.565 | 3.417 | 3.166 |
C3H5CN | 1.522 | — | — | — |
LM-CX3 | 1.528 | 3.325 | 3.209 | 1.190 |
TS3 | 1.958 | 1.863 | 1.989 | 1.206 |
LM3 | 2.546 | 1.590 | 3.231 | 2.914 |
C3H5NC | 1.512 | — | — | — |
LM-CX4 | 1.519 | 3.309 | 3.127 | 1.190 |
TS4 | 1.947 | 1.926 | 1.910 | 1.207 |
LM4 | 2.532 | 1.592 | 3.273 | 2.970 |
C3H5CH3 | 1.509 | — | — | — |
LM-CX5 | 1.522 | 3.166 | 2.894 | 1.191 |
TS5 | 2.012 | 1.767 | 2.042 | 1.215 |
LM5 | 2.580 | 3.303 | 1.558 | 2.960 |
C3H4(CH3)2 | 1.511 | — | — | — |
LM-CX6 | 1.519 | 3.493 | 2.942 | 1.192 |
TS6 | 2.040 | 2.216 | 1.764 | 1.216 |
LM6 | 2.591 | 3.304 | 1.558 | 2.915 |
C1–C3 | C1–B1 | C3–B1 | B1–H8 | |
---|---|---|---|---|
C3H5F | ||||
TS7 | 2.251 | 1.620 | 1.759 | 1.264 |
LM7 | 2.515 | 1.572 | 3.430 | 3.186 |
C3H5Cl | ||||
TS8 | 2.293 | 1.809 | 1.609 | 1.266 |
LM8 | 2.522 | 3.247 | 1.560 | 2.954 |
C3H5CN | ||||
TS9 | 2.310 | 1.841 | 1.605 | 1.276 |
LM9 | 2.546 | 3.261 | 1.561 | 2.930 |
C3H5NC | ||||
TS10 | 2.309 | 1.837 | 1.606 | 1.267 |
LM10 | 2.535 | 3.257 | 1.561 | 2.929 |
C3H5CH3 | ||||
TS11 | 2.278 | 1.761 | 1.622 | 1.266 |
LM11 | 2.579 | 1.558 | 3.272 | 2.971 |
C3H4(CH3)2 | ||||
TS12 | 2.040 | 2.216 | 1.764 | 1.216 |
LM12 | 2.591 | 1.566 | 3.211 | 2.894 |
It is seen that the C1–B1 and C3–B1 distances found in the complexes are significantly longer in the case of all substituted cyclopropanes compared to the unsubstituted case; for example, in case of fluoro substitution the C1–B1 and C3–B1 distances are 3.094 and 2.922 Å against 2.905 Å in unsubstituted case pointing to weaker complexation. In the complex with –F, –Cl, –CN, and –NC substituted cyclopropanes, the boron is nearly symmetrically disposed with respect to C1 and C3 (
The C1–C3 distance in the “loose” transition structure for the reaction between cyclopropane and BH3 is 1.994 Å. In case of substitutions by –F, –Cl, –CN, and –NC this distance is less than this value but in the methyl and dimethyl case it is greater, pointing to a weaker C1–C3 bond in these cases.
In the case of unsubstituted cyclopropane, in the transition state the C1–B1 distance is greater than C3–B1 (see Table
The selected geometrical parameters for the “tight” transition structures are shown in Table
The molecular orbital plots in Figure
HOMOs of the complexes and “loose” transition structures for the hydroboration of –F substituted cyclopropanes at B3LYP/6-31G** level along the plane of cyclopropane ring.
HOMOs of the complexes and “tight” transition structures for the hydroboration of –F substituted cyclopropanes at B3LYP/6-31G** level along the plane of cyclopropane ring.
The energies of the optimized intermediate complex (LM-CX), the “loose” transition structures (TS), and the addition product along with the product type (Markownikoff or anti-Markownikoff) are shown in Table
B3LYP/6-31G** optimized total energies (in kcal/mol) for the intermediate complex, “loose” transition structure, and product for substituted cyclopropanes for addition across C1–C3 bond along the plane of cyclopropane ring.
LM-CX | TS | Product | Product type | Gibbs energy ( |
Entropy change ( |
|
---|---|---|---|---|---|---|
C3H5F + BH3 |
|
23.62 |
|
AM |
|
|
C3H5Cl + BH3 |
|
28.62 |
|
M |
|
|
C3H5CN + BH3 |
|
30.74 |
|
M |
|
|
C3H5NC + BH3 |
|
29.65 |
|
M |
|
|
C3H5CH3 + BH3 |
|
24.63 |
|
AM |
|
|
C3H4(CH3)2 + BH3 |
|
23.52 |
|
AM |
|
|
Relative energies for the parent cyclopropane are LM-CX = −1.97 kcal/mol, TS = 25.17 kcal/mol, LM = −40.15 kcal/mol,
The reactants proceed without barrier to an intermediate complex and cross over a barrier between 23.52 and 30.74 kcal/mol to form the product, which is more stable than the reactants by 35.59 to 43.39 kcal/mol, depending on the substituent. The intermediate occurs at shallow minima, stabilized by 0.50 to 2.00 kcal/mol relative to the reactants.
The loose transition states in these cases all correspond to barrier comparable to the case of unsubstituted cyclopropane; the barriers are slightly lower (than for cyclopropane) in the case of fluoro, methyl, and dimethyl substitutions whereas they are slightly higher for the others and the nature of the products also differs in the two cases. It is thought that the high electronegativity of fluorine causes the carbon to which it is bonded to be more positive overall, hence facilitating the abstraction of a hydride (or hydrogen with net negative Mulliken charge), thus leading to the anti-Markownikoff product. In the case of methyl substitution steric influence of the methyl group(s) may be what causes the larger BH2 moiety to move to the less substituted carbon.
In the case of “tight” transition structure no intermediate complex has been observed. The relative energies are listed in Table
B3LYP/6-31G** optimized total energies (in kcal/mol) for the “tight” transition structure and product for substituted cyclopropanes for addition across C1–C3 bond along the plane of cyclopropane ring.
TS | Product | Product type | Gibbs energy ( |
Entropy change ( |
|
---|---|---|---|---|---|
C3H5F + BH3 | 6.60 |
|
M |
|
|
C3H5Cl + BH3 | 9.02 |
|
AM |
|
|
C3H5CN + BH3 | 9.26 |
|
AM |
|
|
C3H5NC + BH3 | 9.75 |
|
AM |
|
|
C3H5CH3 + BH3 | 8.97 |
|
M |
|
|
C3H4(CH3)2 + BH3 | 10.64 |
|
M |
|
|
Single point calculations at the DFT optimized geometries have also been carried out on all the key species studied at CCSD, CCSD(T), QCISD(T), MP2, and MP4D levels. The single point energies obtained are shown in
These “tight” transition structures are an interesting anomaly in that their energies are uncharacteristically low; that is, they correspond to very low barriers compared to the other cases, being comparable to reported values for hydroboration of ethylene. The tightly bound structure being stabler is to be expected and we find that the BH3 moiety is distorted farther from planarity than in the “loose” structure. The hydrogen on the carbon atoms also assumes a nearly planar disposition.
In summary, we have investigated the stationary structures involved in the hydroboration of substituted cyclopropanes with borane. Our study posits three-centered transition states for these reactions. It is also hoped that studies on reactions involving cyclopropane and its derivatives with other reagents will clarify the situation. Of the reactions studied three-centered transition states are encountered in the case of BH3 adding to cyclopropane with an in-plane approach. We are led to suspect that the electronic structure and high reactivity of borane are the major causative factors involved.
The authors declare that there is no conflict of interests regarding the publication of this paper.
One of the authors (Satya Prakash Singh) is grateful to the Ministry of Human Resources and Development (MHRD), Government of India, for the award of a fellowship.