New heteroaromatic π-ligands containing boron

Article abstract of DOI:10.1080/10426500490426610

The heteroaromatic anions 2-(diisopropylamino)-1,2-thiaborolide(2a), 1-ethyl-2-phenyl-1,2-azaborolide(2b), and 2-(diisopropylamino)-1,2- oxaborolide(2c) have been prepared by a multi-step synthesis using the Grubbs ring-closing metathesis. Reactions of 2a

Full text of DOI:10.1080/10426500490426610

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Phosphorus, Sulfur, and Silicon and the Related
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NEW HETEROAROMATIC π -LIGANDS CONTAINING BORON
Arthur J. Ashe a , Zoltan Bajko a , Xiangdong Fang a , Jeff W. Kampf a & Hong Yang a
a University of Michigan , Ann Arbor, Michigan, USA
Published online: 12 Aug 2010.
To cite this article: Arthur J. Ashe , Zoltan Bajko , Xiangdong Fang , Jeff W. Kampf & Hong Yang (2004) NEW
HETEROAROMATIC π -LIGANDS CONTAINING BORON, Phosphorus, Sulfur, and Silicon and the Related Elements, 179:4-5,
711-714, DOI: 10.1080/10426500490426610
To link to this article: http://dx.doi.org/10.1080/10426500490426610
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Phosphorus, Sulfur, and Silicon, 179:711–714, 2004
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Copyright Taylor & Francis Inc.
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500490426610
NEW HETEROAROMATIC π-LIGANDS
CONTAINING BORON
Arthur J. Ashe, III, Zoltan Bajko, Xiangdong Fang,
Jeff W. Kampf, and Hong Yang
University of Michigan, Ann Arbor, Michigan, USA
(Received August 17, 2003; accepted October 3, 2003)
The heteroaromatic anions 2-(diisopropylamino)-1,2-thiaborolide(2a),
1-ethyl-2-phenyl-1,2-azaborolide(2b), and 2-(diisopropylamino)-1,2-
oxaborolide(2c) have been prepared by a multi-step synthesis using
the Grubbs ring-closing metathesis. Reactions of 2a, 2b, and 2c with
Cp∗ZrCl3 afforded the corresponding Zr(IV) complexes 3.
Keywords: Boron heterocycles; polymerization; zirconium complexes
Zirconocene dichlorides (1) have important uses as catalysts for the
Ziegler-Natta polymerization of olefins.1 We have been interested in
preparing new derivatives of 1 in which the Cp groups are replaced
by boron containing heterocycles.2,3 Our goal is to adjust the electronic
environment about the zirconium atom so that the reactivity of the
catalyst can be tuned.
The 1,2-heteroborolides 2a,4 2b,5−7 and 2c are isoelectronic with Cp,
but their electronic character should depend on the identity of the ring
heteroatoms as well as the B-pendant group.2 Extensive prior work by
Schmid and coworkers5 has demonstrated that 1,2-azaborolides 2b can
form Cp-like complexes with a variety of transition metals. However,
previous to our work, 1,2-thiaborolide (2a)4 and 1,2-oxaborolides were
unknown. We report here on a general preparation of 2 and on their
conversion to zirconium (IV) complexes 3.
RESULTS AND DISCUSSION
The synthesis of the 1,2-heteroborolides is outlined in Scheme 1.4,6
(Heteroallyl)-vinylboranes 6 are easily prepared in quantity from the
The work was supported by the National Science Foundation and the Dow Chemical
Company. We are grateful to the Boulder Scientific Company for a generous gift of the
Grubbs catalyst.
Address correspondence to Arthur J. Ashe, III, Department of Chemistry, University
of Michigan, Ann Arbor, MI 48109. E-mail: ajashe@umich.edu
711

712
A. J. Ashe, III et al.
SCHEME 1 Synthesis Key: (i), PhBCl3 or BCl3 followed by HN(i-Pr)2;
(ii) C3H7EH, base; (iii) Grubbs catalyst (RuCl2(CHPh)[P(C6H11)3]2); (iv) LDA
or t-Buli; (v) Cp∗ZrCl3.
commercially available tributylvinylstannane (4). The key step in the
synthesis involves a Grubbs ring-closing metathesis of 6 to 7, which
takes place in 80–95% yield using 1–5 mmol percent catalyst. Depro-
tonation of 7 with base affords the 1,2-heteroborolide. The NMR spec-
tra of 2 are consistent with π-delocalized structures. The reaction of
2 with Cp∗ZrCl3 affords the 1,2-diheterozirconocene dichlorides 3 as
crystalline solids (Scheme 2).
The x-ray structure of 3b and 3c are illustrated in Figures 1 and 2 re-
spectively. The structure of a Me2Si-bridged derivative of 3a previously
has been reported.4 The structure of 3b shows that the azaborolyl ring is
5
˚
η -coordinated to Zr, although the Zr–B distance (2.68 A) is longer than
˚
the Zr–C and Zr–N distances (2.46–2.50 A). In 3c the differentiation
SCHEME 2

New Heteroaromatic π-Ligands Containing Boron
713
˚
FIGURE 1 The molecular structure of 3b. Selected distances (A) to Zr: B,
2.681(2); N, 2.531(1); C(1), 2.479(2); C(2), 2.464(2); C(3), 2.491(2); C(Cp∗), 2.486–
2.524.
❛
FIGURE 2 The molecular structure of 3c. Selected distances (A) to Zr: B,
2.802(3); O, 2.516(2); C(1), 2.402(1); C(2), 2.500(2); C3, 2.525(2); C(Cp∗), 2.486–
2.527.

714
A. J. Ashe, III et al.
˚
between the Zr–B distance (2.80 A) and the Zr–C and Zr–O distances
4
˚
(2.40–2.53 A) is so great that the coordination to Zr approaches η. In
both cases, the metal slippage away from boron is probably due to the
high electron demand of Zr(IV), which prefers to coordinate to the more
electron rich ring atoms.2
On activation by excess methylaluminoxane 3a and 3b are active
catalysts for the polymerization of ethylene. Under typical conditions9
the activities of 3a and 3b were found to be 6.0 × 104 and 2.1 ×
106 (g of polymer)/(mol of Zr) atm respectively. Although polymer-
izations have not been run with 3c, yet, it is clear that the 1,2-
heteroborolyl ligands can serve as a replacement ligand for Cp in useful
catalysts.
REFERENCES
[1] J. A. Gladysz, ed., Chem. Rev., 100, 1167 (2000).
[2] G. C. Bazan, G. Rodriguez, A. J. Ashe III, et al., J. Am. Chem. Soc., 118, 2291 (1996);
b) G. C. Bazan, G. Rodriguez, A. J. Ashe III, et al., Organometallics, 16, 2492 (1997).
[3] A. J. Ashe III, Z. Bajko, M. D. Carr, and J. W. Kampf, Organometallics, 22, 910 (2003).
[4] A. J. Ashe III, X. Fang, and J. W. Kampf, Organometallics, 19, 4935 (2000).
[5] G. Schmid, in Comprehensive Heterocyclic Chemistry II, edited by I. Shinkai
(Pergamon, Oxford, 1966), vol. 3, p. 739.
[6] A. J. Ashe III and X. Fang, Org. Lett., 2, 2089 (2000).
[7] a) S.-Y. Liu, M. M.-C. Lo, and G. C. Fu, Angew. Chem. Int. Ed. Engl., 41, 174 (2002);
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