A synthesis of aromatic five- and six-membered B-N heterocycles via ring closing metathesis

Article abstract of DOI:10.1021/ol0001113

equation presented The ring closing metathesis on appropriate vinyl or allyl aminoboranes (1 or 2) gives azaboracycloalkenes (3 or 4) which can be converted to azaborolides (5) or azaborines (6).

Full text of DOI:10.1021/ol0001113

ORGANIC
LETTERS
2000
Vol. 2, No. 14
2089-2091
A Synthesis of Aromatic Five- and
Six-Membered B−N Heterocycles via
Ring Closing Metathesis
Arthur J. Ashe, III,* and Xiangdong Fang
Department of Chemistry, UniVersity of Michigan, Ann Arbor, Michigan 48109-1055
ajashe@umich.edu
Received April 28, 2000
ABSTRACT
The ring closing metathesis on appropriate vinyl or allyl aminoboranes (1 or 2) gives azaboracycloalkenes (3 or 4) which can be converted
to azaborolides (5) or azaborines (6).
Aminoboranes 7 have substantial B-N π-bond character
which makes them isoelectronic with olefins.1-6 The B-N
Scheme 1
heterocycles, 1,2-azaborolide (5) and 1,2-azaborine (6), are
isoelectronic with cyclopentadienide and benzene, respec-
tively (Scheme 1). In 1980 Schmid reported the synthesis
of N-tert-butyl- and N-trimethylsilyl-1,2-azaborolide 5a and
5b, which have been used as replacement ligands for Cp in
a number of transition metal complexes.7,8 The recent report
in the patent literature that Zr complex 8a is a good
(1) Wiberg, E. Naturwissenschaften 1948, 35, 182.
(2) Greenwood, N. N.; Thomas, B. S. In ComprehensiVe Inorganic
Chemistry; Bailar, J. C., Emele´us, H. J.; Nyholm, R.; Trotman-Dickenson,
A. F., Eds.; Pergamon: Oxford, 1973; Vol. 1, pp 916-949.
precatalyst for the polymerization of olefins has increased
(3) Pelter, A.; Smith, K. In ComprehensiVe Organic Chemistry, Vol. 3;
the interest in 5.9 1,2-Azaborines were reported in the early
Barton, D., Ollis, W. D., Eds.; Pergamon: Oxford, 1979; pp 925-32.
(4) Hoffmann, R. J. Chem. Phys. 1964, 40, 2474.
(5) Dill, J. D.; Schleyer, P.v.R.; Pople, J. A. J. Am. Chem. Soc. 1975,
1960s by Dewar10 and by White.11 However, the claims of
97, 3402.
(6) Budzelaar, P. H. M.; Kos, A. J.; Clark, T.; Schleyer, P.v. R.
Organometallics 1985, 4, 429.
(9) Nagy, S.; Krishnamurti, R.; Etherton, B. P. PCT Int. Appl. WO9634,-
021; Chem. Abstr. 1997, 126, 19432j.
(7) (a) Schulze, J.; Schmid, G. Angew. Chem., Int. Ed. Engl. 1980, 19,
54. (b) Amirkhalili, S.; Boese, R.; Ho¨hner, U.; Kampmann, D.; Schmid,
G.; Rademacher, P. Chem. Ber. 1982, 115, 732. (c) Schmid, G.; Boltsch,
O.; Bla¨ser, D.; Boese, R. Z. Naturforch 1984, 39b, 1082. (d) Schmid, G.;
Schu¨tz, M. J. Organomet. Chem. 1995, 492, 185 and earlier papers in this
series.
(10) (a) Dewar, M. J. S.; Marr, P. A. J. Am. Chem. Soc. 1962, 84, 3782.
(b) Davies, K. M.; Dewar, M. J. S.; Rona, P. J. Am. Chem. Soc. 1967, 89,
6294.
(11) White, D. G. J. Am. Chem. Soc. 1963, 85, 3634.
(12) Fritsch, A. J. Chem. Heterocycl. Compd. 1977, 30, 381.
(13) Massey, S. T.; Zoellner, R. W. Int. J. Quantum Chem. 1991, 39,
787.
(8) Schmid, G. In ComprehensiVe Heterocyclic Chemistry II; Shinkai,
I., Ed.; Pergamon: Oxford, 1996; Vol. 3, p 739.
(14) Kranz, M.; Clark, T. J. Org. Chem. 1992, 57, 5492.
10.1021/ol0001113 CCC: $19.00 © 2000 American Chemical Society
Published on Web 06/17/2000

aromaticity of 6 were supported only by slender experimental
data. Although a number of ring-fused derivatives of 6 have
been reported, investigation of monocyclic 1,2-azaborines
has been minimal.12 However, several recent computational
studies on 6 suggest it has considerable stability.13,14
substitution patterns, we chose initially to investigate the
synthesis of the B-phenyl N-methyl derivative 5c (Scheme
3). Treatment of tributylvinyl tin18 (10) with phenylboron
A good general synthesis of 5 and 6 would greatly
facilitate the study of these interesting heterocycles. The
Schmid synthesis of 5a,b involved LTMP deprotonation of
3a or 3b, while the White synthesis of 6d involved
Pd-catalyzed dehydrogenation of 9d. On this basis it seemed
likely that azaboracycloalkenes 3 and 4 should be good
precursors to 5 and 6, respectively. In principle, 3 and 4
should be available using the ring closing metathesis (RCM)
on appropriate vinyl or allyl aminoboranes (Scheme 2).15,16
Scheme 3a
Scheme 2
a Key: (a) PhBCl2, pentane; (b) (C3H5)MeNH, Et3N; (c)
(Cy3P)2(PhCH)RuCl2, CH2Cl2; (d) LDA; (e) [Cp*RuCl4].
dichloride gave phenyl vinyl boron chloride (11) in 78%
yield. Reaction of 11 with allylmethylamine in pentane
containing triethylamine afforded a 95% yield of amino-
borane 1c. The substantial π-bond character of the B-N
bond makes rotation about the bond slow on the NMR time
1
scale.19 The H and 13C NMR spectra of 1c are consistent
with it existing as a 1:1 mixture of B-N rotomers. However,
the two rotomers are readily interconverted since the mixture
undergoes RCM in high yield. Thus, upon treatment of 1c
with 5 mol % of Grubbs catalyst in CH2Cl2 at 25 °C for 10
h, cyclization occurred smoothly to give 3c in 82% yield.
Reaction of 3c with LDA in ether gave 81% conversion to
1
azaborolide 5c. The H, 11B, and 13C NMR chemical shift
values for the ring atoms of 5c closely follow those reported
for the corresponding positions of 5a and 5b.7,8 Reaction 5c
with [Cp*RuCl]4 afforded expected sandwich complex 12c.7c
Azaborine 6e was prepared by an analogous route il-
lustrated in Scheme 4. Allyltributyltin20a reacted with BCl3
in pentane at -78 °C to afford allylboron dichloride (13)
which was not isolated.20b In situ addition of ethylallylamine
RCM using the Grubbs catalyst ((Cy3P)2(PhCH) RuCl2) is
tolerant of a variety of oxygen, sulfur, and nitrogen functional
groups and has been used to prepare alkenylboronates,17 but
to the best of our knowledge, it has not previously been
applied to organoboron heterocycles. We wish to report that
5c and 6e can be obtained in high yield by easy reaction
sequences involving RCM.
The Schmid azaborolide syntheses were limited to N-tert-
butyl and N-trimethylsilyl derivatives.7 To examine other
Scheme 4a
(15) (a) Fu, G. C.; Nguyen, S. T.; Grubbs, R. H. J. Am. Chem. Soc.
1993, 115, 9856. (b) Grubbs, R. H.; Miller, S. J.; Fu, G. C. Acc. Chem.
Res. 1995, 28, 437.
(16) (a) Schmalz, H.-G. Angew. Chem., Int. Ed. Engl. 1995, 34, 1833.
(b) Schuster, M.; Blechert, S. Angew. Chem., Int. Ed. Engl. 1997, 36, 2036.
(c) Grubbs, R. H.; Chang, S. Tetrahedron 1998, 54, 4413. (d) Snapper, M.
L.; Randall, M. L. Strem Chemiker 1998, 17 (No 1), (e) Phillips, A. J.;
Abell, A. D. Aldrichimica Acta 1999, 32, 75.
(17) (a) Renaud, J.; Ouellet, S. G. J. Am. Chem. Soc. 1998, 120, 7995.
(b) Blackwell, H. E.; O’Leary, D. J.; Chatterjee, A. K.; Washenfelder, R.
A.; Bussmann, D. A.; Grubbs, R. H. J. Am. Chem. Soc. 2000, 122, 58.
(18) Seyferth, D.; Stone, F. G. A. J. Am. Chem. Soc. 1957, 79, 515.
(19) Imbery, D.; Jaeschke, A.; Friebolin, H. Org. Magn. Reson. 1970,
2, 271. Friebolin, H.; Rensch, R.; Wendel, H. Org. Magn. Reson. 1976, 8,
287.
(20) (a) Halligan, N. G.; Blaszczak, L. C. Organic Syntheses; Wiley:
New York, 1993; Collect. Vol. VIII, p 23. (b) Singleton, D. A.; Waller, S.
C.; Zhang, Z.; Frantz, D. E.; Leung, S.-W. J. Am. Chem. Soc. 1996, 118,
9986.
a Key: (a) BCl3, pentane; (b) (C3H5)EtNH, Et3N; (c) PhLi; (d)
(Cy3P)2(PhCH)RuCl2; (e) DDQ, pentane.
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Org. Lett., Vol. 2, No. 14, 2000

at -78 °C followed by triethylamine gave adduct 14 which
was isolated by distillation in 68% yield. The reaction of 14
with phenyllithium afforded an 81% yield of aminoborane
2e. Both 14 and 2e display complex NMR spectra which
indicate the presence of B-N rotomers. The treatment of
2e with 5 mol % of Grubbs catalyst in CH2Cl2 at room
temperature for 10 h gave an 86% yield of the ring-closed
product 4e. To our delight, the reaction of 4e with DDQ in
pentane at 35 °C for 24 h afforded azaborine 6e as a pale
yellow oil which was contaminated with a small amount of
biphenyl. On column chromotography, pure 6e was obtained
Figure 1. Experimental 11B and 13C chemical shifts for 6e and
the calculated chemical shifts for 6f.
1
in 58% yield. The H, 11B, and 13C NMR spectra of 6e are
likely be changed, the syntheses give promise of being
general. We are presently attempting to extend the RCM
procedure for the syntheses of other heteroaromatic com-
pounds.
consistent with our expectation that it is weakly aromatic.
In fact, Clark and Kranz have calculated the 11B and 13C
NMR chemical shift values for the parent 1,2-azaborine 6f14
from their ab initio MO calculations using the IGLO
method.21 Comparison of their calculated chemical shift
values for 6f with our experimental values for 6e show a
modest level of agreement (see Figure 1). The fact that this
prediction preceded experiment enhances our confidence in
their MO treatment.
Acknowledgment. This work was financially supported
by the NSF (CHE96-16889).
Supporting Information Available: Experimental pro-
cedures and characterization data for all new compounds.
This material is available free of charge via the Internet at
http://pubs.acs.org.
In summary, we have shown that the Grubbs RCM
procedure allows efficient synthesis of 1,2-azaborolide 5c
and 1,2-azaborine 6e. Since the B- and N-substituents can
(21) Kutzelnigg, W. Isr. J. Chem. 1980, 19, 193; Schindler, M.;
Kutzelnigg, W. J. Chem. Phys. 1982, 76, 1919.
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