Hydroboration: MNDO study of hydroboration of phosphaethenes by borane and dimethylborane

25

Journal of Molecular Structure (Theo&em), 276 (1992) 2&33 Elsevier Science Publishers B.V.. Amsterdam

Hydroboration: MNDO study of hydroboration phosphaethenes by borane and dimethylborane

of

L.V. Ermolaeva and A.S. Ionkin Arbuzov Institute of Organic and Physical (Russian Federation)

Chemistry,

Arbuzov

St. 8, 420083 Kazan

(Received 26 August 1991; in final form 10 January 1992)

Abstract Transition structures for the hydroboration of phosphaethenes R’P = CHR” (R’ = H, CH, or C,H,; R” = NH,) by borane and dimethylborane have been located with MNDO calculations. The first stage of the reaction of borane with phosphaethene and with (E)-C-aminophosphaethene is the formation of a boranephosphaalkene complex. These complexes transform predominantly to those adducts containing a EC bond. Dimethylborane reacts directly with the ICsystem without forming an intermediate complex. Substituent effects on the activation energies are in agreement with the experimental data.

INTRODUCTION

The hydroboration of alkenes has become a well-known reaction in synthetic organic chemistry. Recently, it has been shown that members of the rather new class of compounds with a di-coordinated phosphorus atom and containing a P=C double bond react with boron hydrides [1,2]. This reaction involves the P=C bond and leads to the formation of a EC or a Pa bond [l]. H R=H

BCC#ll~z

*I I C6HSF--CH2

R + HBCC6HllD2

,,M< 'BH5

H CC6H1132;S -_ R = NR;,

;

C6H6P--CHR OR',

SR'

Correspondence to: L.V. Ermolaeva, Arbuzov Institute of Organic and Physical Chemistry, Arbuzov St. 6, 420063 Kazan, Russian Federation.

0166-1280/92/$05.00 0 1992 Elsevier Science Publishers

B.V. All rights reserved.

26

L. V. Ermolaeva and AS. Ionkin/J. Mol. Struct. (Theochem) 276 (1992) 25-33

The P=C bond is similar to the C=C bond with respect to its electronic structure and chemical behaviour [3]. However, the presence of a P(II1) atom with its lone electron pair should affect the regiochemistry of the final products, e.g. in addition reactions. The regioselectivity in such reactions presents an interesting problem. The stereocontrolled formation of the P-B bond has great importance in synthetic organoelement chemistry. In order to understand the observed regiochemistry in the reaction of boron hydrides with phosphaalkenes we have performed a quantum chemical study of the parent reaction J R'P=CHR"

+

HE3RZ

-I R = H,CH3

_!2..

R'= H.CH3,C6Hs

BR2 H II R'P -CHR" R"= H, CE3-NH2

For our analysis we preferred the MNDO method [4]. According to the investigations by Egger and Keese [5], the MNDO simulation of the addition of borane to ethylene gives a reaction profile which closely resembles the ab initio results at the RHF/6-31G’ level. The best ab initio calculations - with polarization functions and correlation energy corrections (MP2/6-31G”) predict that the hydroboration of ethylene by borane proceeds without activation energy [6]. Nevertheless Houk and coworkers come to the conclusion that RHF transition states of reaction may be compared in order to see the influence of the substituents on the geometries and enthalpies of the transition states [S]. METHODOFCALCULATION

Calculations were performed with the AMPAC program [7]. All geometries were optimized by the DFP method [8]. Transition states were approximately located using a suitable reaction coordinate, then refined by minimizing the scalar gradient according to McIver and Komornicky [9]. All stationary points were characterized by calculating and diagonalizing the force constant matrix. A transition state on the potential surface has only one negative eigenvalue of the final force constant matrix. RESULTSANDDISCUSSION

Hydroboration of phosphaethene and C-aminophosphaethene

by borane

We first studied the reaction of borane with phosphaethene (Ph=CH,) (1)

L.V. Ermolaeva

and A.S. Ionkin/J. Mol. Struct. (Theo&em)

276 (1992) 25-33

27

2 Fig. 1. Structures

of the hydroboration

complexes

of PH=CH,

(1) and PH=CHNH,

(2) with

BH,.

and cis-C-aminophosphaethene ((E)-PH=CHNH,) (2), the latter being an example of a phosphaalkene with an electron-donating group. We investigated the approach of borane to phosphaalkenes 1 and 2 using the P3 and C-33distances as reaction coordinates. Starting values of 9.OA were used. This calculation showed that the reactions of 1 and 2 with borane proceed first by the formation of the boran+phosphaalkene complexes la and 2a, respectively. The boron atom is coordinated with a phosphorus lone pair (Fig. 1). The heats of complexation were found to be - 8.5 kcalmol-’ for complex la and - 13.3 kcal mol-l for 2a. There is almost free rotation about the boron-phosphorus dative bond. The geometries and energies of the complexes and the transition structures are summarized in Table 1. Some of the results for hydroboration of 1 and 2 are shown in Fig. 2. These complexes may convert to one of two final products: a product with a P-B bond or a product with a C33 bond. The adduct containing a Pa bond is the result of a 1,3-hydrogen shift in la or 2a; we could not find the transition state corresponding to this conversion for the complex la. The

L.V. Ermolaeva and A.S. Ionkin/J. Mol. Struct. (Theochem) 276 (1992) 2&33

28 TABLE 3

MNDO results for bond lengths (A), bond angles (degrees), complexation energies EC and activation energies E. (kcalmol-‘) for the reaction of borane with phosphaethene (1) and cis-C-aminophosphaethene (2) Complex

P=C

P-B

L BPC

la 2a

1.569 1.648

2.997 1.961

139 116

Transition structure

P=C

BC

BH

WI

TSl TS2 TS3 TS4

1.596 1.697 1.665 1.724

2.429 1.917” 1.829 1.879”

1.165 1.239 1.186 1.259

2.397 l.65Sb 2.224 1.668b

186 118

L BCP 61 67 59 69

LBPCH

LHBCP - 25 1’ 43 0”

EC - 8.5 - 13.3 E. 10.8 34.8 14.4 15.3

“BP distance. bP-H distance. ’ HBPC dihedral angle.

HP=CHa l 083

+--a 4a

EC= -8.5

(a)

TSl BE'= to.8

W-HP4Wt44, -OH,

AH’=-23.r

AHH?-39.0

TS2 AE*=34.1

. J-4 AH’=-‘/03

(b)

AE*=14.4

E, = -13.3

Fig. 2. MNDO complexation energy, adduct formation energy and activation energy (kcalmol-‘): (a) the reaction of borane with phosphaethene (1);(b)the reaction of borane with C-aminophosphaethene (2). (+) Indicate the form of the normal mode corresponding to the negative eigenvalue of the force constant matrix.

L..V. Erm~~eva

and A.S. ~~nk~~lJ. MOE.Struct. ~~~~~rn)

276 (1.992) 25-33

29

latter transfers only to the adduct with a B-C bond via the transition structure I‘S1 (Fig. 2(a)) BH, 9 PH=CH, + PH&H,BH, la-+TSl-+l The origin of an activation barrier is understandable from the following considerations. Complex la contains a P-B dative bond. In order to form a B-C bond from this complex it is necessary to break the dative bond and place the molecules in a position favourable for the interaction of the vacant boron orbital with the z system. The final adduct may undergo a rearrangement via TS2 to the structure containing the Pa bond PH&H,

BH, --) PHBH,CH, l-+TS2+2

The activation energy of this process is high (34.8kcalmol-‘). We have studied the substituent effects in hydroboration of electrondonating groups for both the complex and the transition structure. Another picture emerges for the reaction of borane with 2 (Fig. 4). The borane-Caminophosphaethene complex (2a) differs from la in its geometry. In the former the boron atom lies in the plane bisecting the x and D planes of the P=C fragment. The boron is coordinated not only with a phosphors lone pair, but also with the x system, the boron atom being out of the (r plane of the P=C fragment by MA. Complex 2a may form an adduct with the BC bond via a transition state TS3. In addition, a hydrogen atom may migrate to the carbon atom through a four-centre transition state TS4 (Fig. 2(b)). The TS3 transition structure is favoured by 0.9 kcal mall’. Thus, there are two possible reaction paths for the interaction of borane with 2: H,PXHBH,

t BH,

l

PH=CHNH,

+-HPBH,-CH,-NH,

I NJ%

3

t

TS3

t2at

TS4:

4

In the case of unsubstituted phosphaethene, however, the path leading to the adduct with the EK bond is the only one found. The reason for this lies in the different geometries of the complexes with borane. Nevertheless, the path leading to B-C bond formation is preferable. The reaction of dimethylborane with substituted phosphaalkenes It has been shown experimentally that hydroboration of phosphaalkenes with boron hydrides is regioselective [l]. We have studied the substituent effects on the energy and geometry of the complexes and the transition states of the reaction R’P=CHR”

+ HB(CH~)~

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L. V. Ermolaeva and A.S. IonkinlJ. Mol. Strut.

(Theochem) 276 (1992) 25-33

TABLE 2 MNDO results for several bond lengths (A), bond angles (deg.), complexation energies E, and activation energies E. (kcal mol-‘) for the reaction of dimethylborane with phosphaethene (l), P-methylphosphaethene (3) P-phenylphosphaethene (4) and P-phenyl-C-aminophosphaethene (5) Complex

P=C

P-B

L BPC

1 3 4 5

1.569

1.574 1.575 1.616

2.143 2.141 2.195 2.148

134 122 123 126

Transition structure

P=C

BH

d,

d,

TCl TPl TC3 TP3 TC4 TP4 TC5 TP5

1.594 1.609 1.599 1.613 1.598 1.610 1.647 1.730

1.187 1.189 1.186 1.195 1.181 1.188 1.194 1.281

2.216 2.174 2.174 2.207 2.189 2.274 2.035 1.936

2.148 2.076 2.194 2.029 2.240 2.071 2.145 1.607

u 71 68 73 66 76 63 71 74

LBPCH 180 180 180 180

EC 1.5 1.1 2.4 1.7

LHBCP

E.

- 10 lb - 16 Ob - 13 5b - 17 Sb

15.8 15.0 18.1 18.4 18.6 19.2 25.9 20.5

“For a definition of the parameters, see Fig. 4. bHBPC dihedral angle.

for R’ = H and R” = H (1);R' = CH, and R” = H (3); R’ = C,H, and R” = H (4); R’ = C&H, and R” = (E)--NH, (5). The above phosphaalkenes also coordinate with substituted borane. The boron atom lies in the o-plane of the P=C bond in all complexes. In contrast to the borane coordination, the complexation energies are positive (Table 2). The PB distance is in the range 2.14-2.19 A. Methyl substitution on the boron atom tends to lengthen the complexing bond length and to destabilize the a-complex. NB coordination in (E)-C-aminophosphaethene is less favoured than P-B coordination. This is due to the activation barrier of the former being 7 kcal mall’ higher than the barrier for PB complexation. The latter barrier is equal to 2.6 kcal mall’. Both cases are shown in Fig. 3. This is in agreement with the experimental fact that the hydroboration of C-substituted phosphaalkenes involves only the P=C fragment [l]. For dimethyl or other sterically hindered boranes, no complexes are expected to be formed, and boron hydrides react directly with the K system of phosphaalkenes; this is analogous to the reaction with ethenes [lO,ll]. The geometries and energies of the transition states are presented in Fig. 4 and Table 2. In all transition states the BH bond is nearly parallel to the P=C bond. According to calculations, substituents have effects on the transition

L. V. Ermolaeva and A.S. IonkinlJ. Mol. Struct. (Theochem) 276 (1992) 25-33

Fig. 3. Optimized geometries complex: (a) P-B coordination;

31

of the dimethylborane-P-phenyl.C-aminophosphaethene (b) NB coordination.

state geometries and energies. The more hindered P-substituted phosphaalkenes (3, 4) have higher activation barriers, corresponding to formation of a P-33bond, compared to the barrier leading to adducts with a IX bond. The predicted route for the addition of a borane atom to P-phenylphosphaethene (4) and P-phenyl-C-aminophosphaethene (5) is consistent with the experimental data [l]. Substitution on the phosphorus atom tends to increase the activation barrier of hydroboration and to change the mode of addition of borane hydride. Therefore, steric interactions’ are important in the phosphaalkene transition structure. Electron-donating substituents on the carbon atom of the P=C fragment (as can be seen from NH, group calculations) reduce the activation barrier for the addition that leads to P-3 formation. The energy of the second transition state is raised a little. CONCLUSION

For the reaction of borane with phosphaethene and C-aminophosphaethene, the borane-phosphaethene o complex lies on the reaction path and

32

L. V. Ermolaeva and A.S. lonkin/J.

Mol. Struct. (Theochem) 276 (1992) 25-33

Fig. 4. The two possible transition states for hydroboration of phosphaalkenes by dimethylborane.

controls the regiochemistry of addition. Dimethylborane reacts directly with the n system of the P=C fragment without preliminary formation of a 0 complex. The path leading to complexation is kinetically, but not thermodynamically favoured. Sterically hindered substitution on the phosphorus atom favours the path leading to an adduct containing a Bc bond, whereas electron-donating groups on the C atom of the alkene increase the probability of P-3 bond formation. ACKNOWLEDGEMENTS

We are grateful to B.A. Arbuzov and Professor P.v.R. Schleyer for fruitful discussion and encouragement. REFERENCES AS. Ionkin, S.N. Ignat’eva, V.M. Nechoroshkov, Yu.Yu. Efremov and B.A. Arbuzov, Phosphorus Sulfur Silicon, 53 (1990) 1. R.W. Miller, K.J. Donaghy and J.T. Spencer, Phosphorus Sulfur Silicon, 57 (1991) 287. L.N. Marcovski, V.D. Romanenko and A.V. Ruban in N. Dumka (Ed.), Chemistry of acyclic compounds of two-coordinated phosphorus atoms, A.V. Kirsanov, Kiev, 1988, p. 54. M.J.S. Dewar and W. Thiel, J. Am. Chem. Sot., 99 (1977) 4967. M. Egger and R. Keese, Helv. Chim. Acta, 70 (1987) 1843. X. Wang, Y. Li, Y.-D. Wu, M.N. Paddon-Row, N.G. Rondan and K.N. Houk, J. Org. Chem., 550 (1990) 2601. M. J.S. Dewar, E.G. Zoebisch, E.F. Healy and J.J.P. Stewart, J. Am. Chem. Sot., 107 (1985) 3902. M.J.S. Dewar, E.G. Zoebiech, E.F. Healy and J.J.P. Stewart, AMPAC (IBM), Version 3.0,

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Program 527, QCPE, University of Indiana, Bloomington, IN, 1987. (a) W.C. Davidson, Comput. J., 10 (1968) 406. (b) R. Fletcher and M.J.D. Powell, Comput. J., 6 (1964) 163. 9 J.M. McIver and A. Komornicky, Chem. Phys. Lett., 10 (1971) 303. 10 T. Clark, D. Wilhelm and P.v.R. Schleyer, J. Chem. Sot., Chem. Commun., (1983) 606. 11 K.N. Houk, N.G. Rondan, Y.-D. Wu, J.T. Metz and M.N. Paddon-Row, Tetrahedron, 40 (19&1) 2257. 8