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DFT and ab initio studies of the addition step of alkyne bromoboration ... anion on the opposite side of acetylene, transition structures have been obtained that.
DFT and ab initio studies of the addition step of alkyne bromoboration 1

1

Jakub Stošek and Markéta L. Munzarová 1

Department of Chemistry, Faculty of Science, Masaryk University. Kotlářská 2,CZ-611 37 Brno Email: [email protected] Results 1: acetylene + BX3 (X ∊ {Br, Cl, I})

Introduction • • • •

Br

Alkyne bromoboration reaction used by synthetic organic chemists since 1963. Just 1 theoretical analysis of its mechanism: a study of Wang and Uchyama from 2012. Reaction with acetylene provides an anti-adduct. Reactions with all other terminal alkynes tested provide syn-adducts.

Br

B

Br

Br

+ H

Br B

H

H

Br

H

Table 1: Results of single coordinate driving for an addition of BBr3 to acetylene and MP2/6-31+G* method Distance [Å] Energy [kcal mol-1] Distance [Å] Energy [kcal mol-1] Distance [Å] Energy [kcal mol-1] Distance [Å] Energy [kcal mol-1]

Figure 1: General scope of alkyne haloboration (FG = function group) (see Reference 1) • Wang and Uchyama explain this observation via a subsequent isomerization of the syn-adduct promoted by another BBr3 molecule.

3.5274

0.000

2.9774

2.711

2.4274

9.473

1.8774

12.924

3.4774

0.115

2.9274

3.171

2.3774

10.106

1.8274

12.928

3.4274

0.249

2.8774

3.672

2.3274

10.691

1.7774

12.973

3.3774

0.404

2.8274

4.215

2.2774

11.220

1.7274

13.071

3.3274

0.581

2.7774

4.798

2.2274

11.680

1.6774

-17.907

3.2774

0.781

2.7274

5.417

2.1774

12.069

1.6274

-21.284

3.2274

1.009

2.6774

6.066

2.1274

12.379

1.5774

-22.902

3.1774

1.273

2.6274

6.739

2.0774

12.614

1.5274

-23.321

3.1274

1.573

2.5774

7.427

2.0274

12.776

3.0774

1.913

2.5274

8.121

1.9774

12.871

3.0274

2.293

2.4774

8.807

1.9274

12.914

15 10

Energy [kcal mol-1]

5 0 4

3,5

3

2,5

2

1,5

-5

1.7274 Å 13.071 kcal mol-1

-10 -15

3.5274 Å 0 kcal mol-1

-20 -25

Figure 4: Optimized structures of transition states (H – white, B – pink, C – black, Cl – green, Br – red, I – purple) Figure 3: Results of single coordinate driving for an addition of BBr3 to acetylene and MP2/6-31+G* method B-C distance [Å]

Results 2: acetylene + BX3 + X- (X ∊ {Br, Cl, I}) Br

Br

B

Br Br

+

Br B

H

H -

+ Br

H H

+

Br

-

Figure 2: Mechanism of alkyne bromoboration by Wang and Uchyama taken from Reference 2

Br

• Theoretical hypothesis of Wang and Uchyama in contradiction with experimental mechanistic studies of Jan Polášek and Ctibor Mazal from 2014. • Latter experimental study suggested a possibility of an alternative mechanism, involving a direct addition of BBr3 under the participation of Br– anion into the anti-arrangement.

10 0 3,5

Methods

3

2,5

2

1,5

Energy [kcal mol-1]

-10

• Starting structures of BX3 (X = Cl, Br and I), acetylene, propyne and van der Waals complexes built in the program Avogadro. • Geometry optimization of these structures. • Guesses of all transition states estimated using the single coordinate driving method. • Guesses of the transition states then optimized and followed by the frequency analysis to verify the optimized transition states. • All calculations carried out at the B3LYP and MP2 level of theory as implemented in the Gaussian09 quantum chemical software. • Just 1 basis set used with MP2: 6-31+G*; 7 different basis sets used with B3LYP: 6-31+G*, 6-31++G*, 6-31+G**, 6-311++G*, 6-311+G**, 6-311++G** and cc-pVDZ. • SVP basis set has been used for Br an I. • Calculations provided for molecules in CH2Cl2 using SCRF model of implicit solvent. • Calculations done on the cluster perian (NCBResearch, MU), a part of the Czech supercomputing center MetaCentrum.

-20 -30 -40 -50

1.9497 Å -1.641 kcal mol-1

3.4997 Å 0 kcal mol-1

-60

B-C distance [Å]

Figure 5: Results of single coordinate driving for an addition of BBr3 to acetylene and B3LYP/6-31+G* method Figure 6: Optimized structures of transition states (H – white, B – pink, C – black, Cl – green, Br – red, I – purple)

Conclusion • Bimolecular reaction involving only acetylene and one molecule of BX3 seems to lead to synadduct. • Additional molecule involving halogen is needed on the opposite side of acetylene for formation of anti-adduct. • Using X– anion on the opposite side of acetylene, transition structures have been obtained that could lead to a reaction mechanism with two intermediates and three transition structures or to an alternative reaction mechanism with one intermediate and two transition structures in a case of acetylene chloroboration. • Intermediates supporting these reaction mechanisms have been obtained as well. • Unfortunately, neither all transition structures nor all structures of intermediates have been obtained using both methods (MP2 and B3LYP) and all basis sets used.

References [1] Wang, Ch.; Uchyiama, M. Mechanistic Understanding of Alkyne Haloboration: An Ab initio study. Eur. J. Org. Chem. 2012, 33, 6548-6554. [2] Polášek, J. Reinvestigation of acetylene bromoboration reaction. Bachelor thesis, Brno, Czech Republic, 2014. [3] Semrád, H.; Stošek, J.; and Munzarová, L. M. Ab initio studies of the acetylene bromoboration mechanism. Conference abstract, 52nd Symposium on Theoretical Chemistry – Chemistry in Solution, Bochum, Germany, 2016.

Br Br

B

Proposed mechanisms of haloboration

Br

Br

Br

+

Br

-

Br B

H

Br

-

Br B

Br B

H

+

H

-

+ Br

CH H

+

Br

Br

H

+

H

Br

-

-

Br

Br

Figure 7: A proposed reaction mechanism of acetylene haloboration Cl Cl

B

Cl Cl

+

-

Cl B

H

Cl

Cl Cl

B

H

+ Cl

H H

+

Cl

H

-

Cl

-

Cl Figure 8: An alternative reaction mechanism of acetylene chloroboration

Figure 9: Intermediates in proposed reaction mechanisms