Indian Journal of Chemistry Vol. 19A, July 1980, pp. 611-614 Nature of the Transition States in Hydroboration Reactions of Ethylene, Acetylene & Hydrocyanic Acid M. K. DAlTA, S. DASGUPTA & RANJAN DAlTA* Department of Chemistry, Jadavpur University, Calcutta 700032 Received 28 June 1979; revised and accepted 27 September 1979 A semi-empirical molecular orbital study has been carried out to establish the nature of the transition states in hydroboration reactions of ethylene, acetylene and HCN. The results suggest that the reactions proceed through a three-centre transition state. The nature of bonding and the MO interactions involved in the formation of the transition states have been discussed. HE mechanism of hydroboration had been the R R' a subject of quite a number of investigations BH " / T since the sixties and the first picture that emer- R-C=C-R' ~ /C = C" ged out of the works of Brown and co-workers H BH2 was that of the reaction proceeding through a four- centre transition state (1)1-3. A number of kinetic R-CH2-C (BHzhR' (major product) ... (1) and isotope effect studies+" appeared to support Similar reactions are also given by compounds having the above mechanism. Streitwieser et a/.9 disputed heteropolar double and triple bonds19,20 where the the four-centre transition state and suggested on the boron atom is known to add normally to the more basis of stereochemical evidences a three-membered electron rich atoms. For instance organocyanides transition state closely resembling a triangular form stable adducts with borane at low temperatures rc-complex as the reaction pathway. There were (Eq. 2) other attempts to rationalise the stereochemical results on the basis of slight variations of Brown's H H model-v+', but the four-centre nature of the transi- R-CEN -{.-H - B( __ / R-CH=N-B" tion state in general appeared to be satisfactoryw=. However, MO symmetry considerations-s show H H that the four-centre transition state usually proposed ... (2) has significant symmetry barriers and the results of a gas phase kinetic study" of the reaction between In view of the tendency of the trivalent boron borane and ethylene indicated the transition state atom to accept a pair of electrons from a suitable to be a loose three-membered activated complex (II). donor molecule in order to complete its outer shell In our attempt to find the nature of the transition of electrons, the adduct may initially be formed by state for the hydroboration reactions we made a semi- donor-acceptor bond formation between nitrogen empirical MO study of the reaction between ethylene and boron, followed by hydrogenation by intramo- and borane-" and our findings were in favour of a lecular hydride migration (Eq. 3) three-centre intermediate rather than a four-centre R-CE N+BHa -_ [R-C=N:BHaJ one. Such, three-centre two-electron bonds are not uncommon in borane chemistry and may be regarded --+ R-CH=N-BH2 ... (3) as being formed by the interaction of the borane Yet another possibility is that of the borane moiety moiety with the ethylene rc-bond, such that the two attacking the rc-bond between the carbon and nitro- P.-orbitals of the ethylene carbon atoms overlap gen resulting in a rc-complex having a three-centre with the vacant 2p-orbital of the boron atom. two-electron bond of the type envisaged in case of Acetylenes which have two such re-bonds perpen- reactions with ethylene and acetylene. This may be dicular to each other undergo both mono- and di- followed by a concerted conversion of the intermediate hydroboration as expected-t-" (Eq. 1). to product, when the three-centre bonding orbital of the complex becomes a boron-nitrogen a-bond. Recently, Dewar et al.2l have reported the MNDO (not MINDO) optimised geometry of re-complexea formed by reactions of'borane with ethylene, acetylene and some of their derivatives. In this paper, we have studied the nature of bonding in adducts formed by III the reactions of borane with ethylene, acetylene and hydrogen cyanide. The transfer of charge and the 611 INDIAN J. CHEM., VOL. 19A, JULY 1980 as a consequence of the approximations inherent in the CNDO calculations, they reflect the relative stabilities of the adducts very well. The three-centre transition state adduct of BHa,C2.H4 (A) is found to be more stable than the four-centre C2H4.BHa (B) by about 50 kcal/mol while the three-centre HCN. BHa (E) is stabler than the other one having linear C-N-B bond (F) by over 70 kcal/mol, These may therefore be regarded as the preferred pathways for the gas phase hydroboration reactions. In BH3. C2H2 also, the three-centre geometry leads to greater stability and therefore we have subsequently consider- ed only the three-centre geometries of the adducts. The charge density on different atoms (Fig. 1) after adduct formation indicate transfer of charge taking place during donor-acceptor bond formations. The boron atom of the free borane molecule acquires ~] a negative charge after adduct formation, while the 7t-bonded carbon and nitrogen atoms in ethylene, acetylene and HCN become positively charged. The AO population analysis reveals the expected involve- ment of the vacant B2". orbital of borane. How- ever, the charge gained by the boron 2". AO is found to be more than what has actually been transferred by the ligands, probably due to back donation from other MOs of the BHa moiety as a result of their interaction with the orbitals of the ligands (vide infra). Fig. 1 - Optimised geometries of the adducts of BH3 with Some significant interaction also takes place between ethylene, acetylene and hydrocyanic acid [Bond lengths are a carbon atom of the ligand and the borane hydrogen given in atomic units (a. u.)] oriented closest to it. MO interactions involved in the formation of the The various bonds present in the adducts can be three-centre complexes have been discussed. clearly recognized from the sets of localized MOs generated. For the adducts formed with ethylene Methods and acetylene, the notable feature is the existence Throughout these calculations we have used the well of a three-centre two-electron bond between the two known CNDOj2 method 22 with usual parameters. carbon atoms and a boron atom, the contributions The eigenvectors of each of the adducts were from the carbon 2". and boron 21>' orbitals being transformed into a set of localised molecular orbitals most pronounced in their formation. Even in the (LMO) by the self-energy localisation procedure as case of the ethylene-borane adduct with a four-centre suitable for CNDO type of caiculations23,24. The geometry, the LMO eigenvectors show a strong analysis of charge transfer between the ligands tendency to form a three-centre C-B-C bond instead of an anticipated four-centre bond involving C,C,B C2H4, C2.H2.' HCN and BHa have been made by carrying out the single configuration analysis tech- 25 nique of Fukui et al. , in which the MOs of the TABLE 1 - ENERGIES AND HEATS OF FORMATION OF THE ADDUCT:> adducts have been re-expressed in terms of the MOs of BHa and the ligands as they exist in the adducts. Systems Energy in a.u. 6£ The calculations were carried out in the Burrough's (kcal/rnolr] 6700 system at the Regional Computer Centre, BHa - 5.98319 Jadavpur, Calcutta. (Doh symmetry) C.H. -17.07313 C.H. -15.34464 Results and Discussions HCN -19.15705 The optimised geometries of the adducts are shown BHa.C.H. (3-centre) -23.56314 318.0 in Fig. 1. In all the adducts, previously planar [A]· BHa.C.H. (4-centre) -23.48231 267.3 BHa molecule (D:J1I symmetry) are distorted consi- [B]" derably to C3V symmetry. The C-C and C-N bond BHa.C.H2 (3-centre) -21.80675 300.5 lengths increase considerably from their correspond- [C]* ing values in the free Iigandsw, The C-H bond BHa.C2H,.BH. (3-centre) -23.27990 670.8 [D]* lengths are least affected. It may be mentioned HCN.BHa (3-centre) -25.56955 269.5 that a rotation of the BHa moiety around its C3V [E]* -axis has been performed in each case and the lowest HCN.BH. (linear) -25.45576 198.0 energy configurations are shown in Fig. 1. [F]· The calculated energies and heats of formation ·These refer to the corresponding diagrams in Fig. 1. of the adducts (6.E) are given in Table 1. Although tl atomic unit (a.u.) = 627.49 kcal/rnol. 'the calculated 6.E values appear to be unduly large 612 DAlTA et 01. : TRANSITION STATE IN HYDROBORATION REACI10NS and H. With BHa.C2H2, apart from the familiar surprising but may be explained as arising due to C-B-C bond, the other 7; -bond between the two some back donation from the le-orbital of BH3. carbon atoms is clearly recognizable, which is re- Also, the higher lying Mas of both BHa and C~H4 placed by another equivalent C-B-C three-centre that were previously unoccupied are found to be bond in the dihydroborated species (BHa.C2H2.BHs). slightly populated, probably due to self-excitation of Similarly, in HCN. BH3, the LMOs clearly show a the lower lying MOs. three-centre C-B-N bond. The remaining n-bond MO interactions occurring in CZH2.BH3 are similar between carbon and nitrogen and the nitrogen lone in pattern. One of the two n -orbitals in acetylene pair are also easily discernible. participate in the formation of the C-B-C bond, The transfer of charge and the MO interactions while the other re-bond prependicular to it cannot involved in the formation of the three-centre comple- be so involved due to the symmetry of the adduct.