Borepin and its valence isomers

Tetrahedron Letters,Vol.24,No.l8,pp Printed in Creat Britain

1863-1866,1983

004n-4039/83/181863-04$03.00/0 01983 PerTamon Press Ltd.

BOREPIN AND ITS VALENCE ISOMERS

Raymond L. Disch, Michael L. Sabio, and Jerome M. Schulman" Department of Chemistry, City University of New York, Queens College, Flushing, New York 11367 Summary: A_b initio calculations show borepin to be planar and more stable than its valence isomers boranorbornadiene and boranorcaradiene by a considerable amount; the latter have C, symmetry. Borepin (I) is perhaps the most conspicuously absent member of the family of monocyclic 4n+2 pi-electron systems.

In contrast to the isoelectronic tropylium

cation, which is stable in aqueous solution, even alkylborepins are quite 4 unstable. l-3 Stable heptaarylborepins are known , however, as are benzo-fused borepins.5-7 In order to understand the energetics of the borepin system we have undertaken ah inif&

theoretical studies of borepin and

its valence isomers 7-borabicyclo[4.1.0lhepta-2,4-diene clo[2.2.11hepta-2,5-diene

I

(II) and 7-borabicy-

(III).

II

III

The geometries have been optimized at the STO-3G SCF level of calculation. Results for borepin have been reported previously8, while those for II and

1863

1864

III

are

given

CC bond

I.

69.1’

of

a tub-shaped and 4.7’,

i

1 are

in

1.521

i

1.526

CaCg

into

and C3C4

reasonable ii,

= 1.344

of

and C3Cq

p-pi

to

alternation

1.

The bond

with

their

elevation

II

lengths

X.

For

is

greater

in

at

is

II

a

we

of

we found than

BCI = 1.529

geometry

There

angles

structure in

alternating

orbital.

counterparts

The local

with

1.451

q

atomic

the

bond

respectively.

structure

bow and stern

similar

agreement

Q,,

1,

boron

with

1.481

q

a (planar)

the

quite

The extent

1.548

and

0.28e

respectively,

1.321

q

1;

has

$ structure

norcaradiene’. C2C3

Borepin

C2C3 = 1.345

delocalization

obtained

I:

Table

lengths:

modest

for

in

in

1 and ClC6

q

borirane”,

B in

II

is

nearly

planar. For

III

BPQ, where

angle 87.8'.

The

canting

of

P and Q are

the

midpoints

H7BP is

boron

to

X

short,

larger

1.88

remains

the

to

firmly

carbon-boron

levels

(a)

the

to

in

the

relative

several

(a),

which

clear

that

borepin

against

is

is

the

1-methylborepin3. tropilidene, we have

more

proposed Borepin which

found

is that

to

of

be

than

conversion is only

ca.

tropylium

length

both

of

1.353

il is

relatively

however,

the

almost

exactly

the

for II

and

least

stable

the

ca.

than

more is

more

(d)

in

Table

boron

37

II

6-31G*(SCF) energetics

compounds,

suggested

7 kcal/mol

cation

given

strained-ring

by at

more

and III

Discounting

(I-11)

markedly

of

X;

4-31C(RMP2)

drawn.

erroneous

stable

II,

I,

(c)

be

The

of

BC5 and BC6 are

1,

is

and an

a subject

2.48

The

8.

energies

conclusions

borirene

C5C6 bond

1.588

q

Cg=C6.

C5C6.

back-donation

BC2 and BC3 value,

4-31C(SCF);

known to

the

BC1

toward

to

with

toward

CICq and C5C6,

cation,

The distances

dimethylborane

(b)

of

counterpart

p orbital,

Cl and C4 since

length

of

level

However,

with

canted

pointed

7-norbornadienyl

boron ~2C3.

boron

hydrogen

Concommitant

of

STO-3G(SCF);

allows

argues

that

bonded bond

the

pure

compared

Examination the

nearly

the

a homoaromatic

studies.

than

i,

respective

177.4’with

furnishes

and theoreticall

0.041

with

with

counterpart

electrons

for

a Cs structure

angle

isoelectronic nmrl’

we obtained

kcal/mol.

at it

This

for II

stable stable

when compared than

norcaradiene.

than the

is

1866

hypothetical bicyclo[4.1.0lhepta-2,4-dien-7-yl

cation (the analog of II) by

101.5 kcal/mol and 112.7 kcal/mol at levels (b) and (cl, respectively.

This

suggests that borepin has 40-43% of the delocalization energy of tropylium, a conclusion consistent with that found in our earlier study.8 While the gap between I and II is unlikely to change substantially at higher levels of approximation, that between I and III undoubtedly would decrease further;

this is because inclusion of d-orbitals and electron

correlation work in tandem to diminish the relative energy of III.

Finally, the

asymmetric structure of III due to canting of the boron bridge probably adds somewhat to the complexity of the nmr of p-tolyl derivatives of III observed by Eisch and Galle13.

Table II. Relative Energies of Borepin and its Valence Isomers (kcal/mol) ----------.------~---__molecule STO-3G(SCF) 4-31G(SCF) 4-31GcRMP2) 6-3lG*(SCF) _______________________________~~~~~~_______~~~~~~_~_~~~~~~~~~~~~~~~~~~~~ I 0.0 0.0 0.0 0.0 II III

25.7

43.7

44.8

37.7

1.6

37.4

26.9 --__------

23.6

---

---------

References and Notes 1. 2. 3. 4. 5. 76:

8. 9.

IO.

11.

12. 13.

D. F. Halpern, Ph.D. Thesis, Queens College, CUNY, 1971. G. Axelrad and D. Halpern, Chem. Commun., 291 (1971). A. J. Leusink, W. Drenth, J. G. Noltes, and G. J. M. van der Kerk, Tetrahedron w., 1263 (1967). S. M. van der Kerk, J. Boersma, and G. J. M. van der Kerk, J. Organometal. Cu., a, 303 (1981). J. J. Eisch and J. E. Galle, J. A_m. Cm. &$., p3, 4436 (1975). E. E. van Tamelen, G. Brieger, and K. G. Untch, Tetrahedron w., 8, 14, (1960). R. van Veen and F. Bickelhaupt, J. Organomet. Cb., a,C51 (1971). P. Jutzi, Angew. Chemie, 83, 912 (1971). Ss., J. M. Schulman, R. L. Disch and M. L. Sabio, J. Am. Cm. 104, 3785 (1982). J. M. Schulman, M. L. Sabio and R. L Disch (to be published). K. Krogh-Jespersen, D. Cremer, J. D. Dill, J. A. Pople and &. , lu, 2589 (1981). P. v. R. Schleyer, J. &. Ca. (a) P. R. Story and M. Saunders, -J. Amer. Cb. Ss., 82, 6199 (1960); &,, 4876(1962); (b) P. R. Story, L. C. Snyder, D. C. Douglass, E. W. Anderson, and R. L. Kornegay, ibid, &, 3630 (1963); (c) R. K. Lustgarten, M. Brookhart, and S. Winstein, ibid, w, 2347 (1972). S. Yoneda, Z. Yoshida, and S. Winstein, Tetrahedron, 23, 2395(1972). m, C9 (1977). J. J. Eisch and J. E. Galle, J. Qraanometal. Ca.

(Received in USA 28 January 1983)