The first general method for the synthesis of transition-metal π complexes of an electronically diverse family of 1,2-azaborolyls

Article abstract of DOI:10.1021/om020533b

The first general method for the synthesis of transition-metal π complexes of an electronically diverse family of 1,2-azaborolyls was studied. It was demonstrated that the azaborolides react with a variety of transition-metal electrophiles to produce an unprecedented array of η⁵-azaborolyl adducts. These adducts were then investigated crystallographically and spectroscopically.

Full text of DOI:10.1021/om020533b

Organometallics 2002, 21, 4323-4325
4323
Th e F ir st Gen er a l Meth od for th e Syn th esis of
Tr a n sition -Meta l π Com p lexes of a n Electr on ica lly
Diver se F a m ily of 1,2-Aza bor olyls
Shih-Yuan Liu, Ivory D. Hills,1 and Gregory C. Fu*
Department of Chemistry, Massachusetts Institute of Technology,
Cambridge, Massachusetts 02139
Received J uly 8, 2002
Sch em e 1. Ap p r oa ch es to th e Syn th esis of
Tr a n sition -Meta l Com p lexes of η5-1,2-Aza bor olyls
Summary: A general method for the synthesis of new
families of transition-metal-free 1,2-azaborolides (3) has
been developed, based on a readily available precursor
(1). These azaborolides react with a variety of transition-
metal electrophiles to produce an unprecedented array
of η5-azaborolyl adducts (4) (substituents bound to
boron: nitrogen, oxygen, hydrogen, carbon, and phos-
phorus).
We have initiated a program focused on developing
the chemistry of η5-1,2-azaborolyls,2-4 ligands that are
of particular interest due to their isoelectronic relation-
ship with the ubiquitous cyclopentadienyl group.5 Thus,
ready access to an electronically diverse set of azaborolyl
complexes may enhance our fundamental understand-
ing of metal-based reactivity as well as provide more
active and/or selective catalysts.
with a transition-metal electrophile should provide a
more versatile pathway (Scheme 1, bottom).
In this communication, we describe the synthesis of
a wide range of B-heteroatom-substituted 1,2-aza-
borolides 3 (Scheme 1; e.g., Nu ) NR2, OR, H, PR2), the
first examples of transition-metal-free azaborolides
which bear substituents that are not carbon-based. In
addition, we demonstrate that these isolable reagents
react with a spectrum of transition-metal (zirconium,
chromium, and rhodium) electrophiles to generate (η5-
azaborolyl)metal complexes, which we have investigated
crystallographically and spectroscopically.
The chloride of heterocycle 16 can be displaced by any
of an array of nucleophiles, thereby producing B-
substituted derivatives 2 in generally excellent yield
(Table 1, step A). Deprotonation of 2 with sterically
demanding lithium 2,2,6,6-tetramethylpiperidide (Li-
TMP) furnishes the desired lithium 1,2-azaborolide (step
B), wherein the boron bears a nitrogen, oxygen, hydro-
gen, carbon, or phosphorus substituent.
During the 1980s, Schmid pioneered the development
of 1,2-azaborolyl/transition-metal chemistry, focusing
exclusively on adducts in which the boron substituent
is carbon-based (i.e., methyl or phenyl).3 Recently, we
described the first azaborolyl complexes in which the
group on boron is hydrogen, nitrogen, oxygen, fluorine,
phosphorus, or sulfur.2 Furthermore, we established
that the electronic character of the boron-bound sub-
stituent is transmitted to the metal.
Unfortunately, the route that we employed to produce
1,2-azaborolyl adducts of iron is not readily generaliz-
able to the synthesis of complexes with other transition
metals (Scheme 1, top). In analogy with cyclopentadi-
enyl (as well as Schmid’s azaborolyl) chemistry, an
approach that exploits the reaction of an azaborolide
With these new families of 1,2-azaborolides in hand,
it was incumbent upon us to demonstrate that they are
indeed suitable, versatile precursors to azaborolylmetal
complexes. We therefore reacted them with a range of
transition-metal electrophiles.
With respect to early-transition-metal chemistry, we
decided to focus our attention on the synthesis of
zirconocene derivatives, since B-methyl and B-phenyl
1,2-azaborolylzirconium complexes serve as active cata-
lysts for Ziegler-Natta polymerizations.7,8 Furthermore,
electronic tuning of the zirconium adducts of another
(1) To whom correspondence concerning X-ray crystallography
should be directed.
(2) Liu, S.-Y.; Lo, M. M.-C.; Fu, G. C. Angew. Chem., Int. Ed. 2002,
41, 174-176.
(3) For overviews of 1,2-azaborolyl chemistry, see: (a) Schmid, G.
In Comprehensive Heterocyclic Chemistry II; Shinkai, I., Ed.; Else-
vier: Oxford, U.K., 1996; Vol. 3, Chapter 3.17. (b) Schmid, G.
Comments Inorg. Chem. 1985, 4, 17-32.
(4) For two recent contributions to 1,2-azaborolyl chemistry, see: (a)
Ashe, A. J ., III; Fang, X. Org. Lett. 2000, 2, 2089-2091. (b) Ashe, A.
J ., III; Fang, X.; Kampf, J . W. Organometallics 2001, 20, 5413-5418.
(5) (a) Metallocenes; Togni, A., Halterman, R. L., Eds.; Wiley: New
York, 1998, Vols. 1-2. (b) Ferrocenes; Togni, A., Hayashi, T., Eds.;
VCH: New York, 1995.
(6) Heterocycle 1 is available on a multigram scale in three steps
from commercially available compounds.2
(7) For an example of the application of 1,2-azaborolyl complexes
to olefin polymerization, see: Nagy, S.; Krishnamurti, R.; Etherton,
B. P. U.S. Patent 5 902 866, May 11, 1999.
10.1021/om020533b CCC: $22.00 © 2002 American Chemical Society
Publication on Web 09/20/2002

4324 Organometallics, Vol. 21, No. 21, 2002
Communications
Ta ble 1. Syn th esis of a Diver se Ar r a y of Lith iu m
1,2-Aza bor olid es via Nu cleop h ilic Su bstitu tion
F ollow ed by Dep r oton a tion
Ta ble 2. Syn th esis of 1,2-Aza bor olylzir con iu m
Com p lexes w ith a Ra n ge of Bor on Su bstitu en ts
a
b
Average of two runs; ∼95% purity by NMR. ∼90% purity by
NMR.
a
Isolated yield; average of two runs.
class of boron-based Cp analogues, boratabenzenes, has
been shown to provide excellent control of polymeriza-
tion/oligomerization activity.9-11 Thus, the capacity to
generate an electronically diverse set of azaborolyl
complexes should open the door to new avenues of
investigation in metallocene-catalyzed processes. We
were pleased to determine that ZrCpCl3 reacts with the
entire spectrum of lithium 1,2-azaborolides (3) to pro-
duce the desired zirconocene derivatives in good yields
(Table 2).
We have established that halide complexes of late
transition metals also undergo substitution by 1,2-
azaborolides to provide η5-azaborolyl complexes. For
example, the reaction of [Rh(cod)Cl]2 with the NiPr2-
substituted heterocycle proceeds smoothly to furnish the
desired rhodium adduct in 68% yield (eq 1).12 The
F igu r e 1. ORTEP illustration, with thermal ellipsoids
drawn at the 35% probability level, of the 1,2-azaborolyl-
rhodium complex 4.
Ta ble 3. Syn th esis a n d CO Str etch in g F r equ en cies
of 1,2-Aza bor olylch r om iu m Com p lexes w ith a
Ra n ge of Bor on Su bstitu en ts
crystal structure of this complex (Figure 1) is consistent
with donation by the nitrogen “lone pair” to the het-
eroaromatic ring. Thus, the nitrogen is sp2 hybridized
(sum of bond angles 360.0°), the NiPr2 group adopts a
(8) For recent reviews of Ziegler-Natta polymerization, see: (a)
Metallocene-Based Polyolefins; Scheirs, J ., Kaminsky, W., Eds.;
Wiley: New York, 2000. (b) Kaminsky, W.; Arndt, M. In Applied
Homogeneous Catalysis with Organometallic Compounds; Cornils, B.,
Hermann, W. A., Eds.; VCH: New York, 1996. (c) Ziegler Catalysis;
Fink, G., Mu¨lhaupt, R., Brintzinger, H. H., Eds.; Springer: Berlin,
1995. (d) Brintzinger, H. H.; Fischer, D.; Mu¨lhaupt, R.; Rieger, B.;
Waymouth, R. M. Angew. Chem., Int. Ed. Engl. 1995, 34, 1143-1170.
(9) (a) Bazan, G. C.; Rodriguez, G.; Ashe, A. J ., III; Al-Ahmad, S.;
Mu¨ller, C. J . Am. Chem. Soc. 1996, 118, 2291-2292. (b) Rogers, J . S.;
Bazan, G. C.; Sperry, C. K. J . Am. Chem. Soc. 1997, 119, 9305-9306.
(c) Lee, R. A.; Lachicotte, R. J .; Bazan, G. C. J . Am. Chem. Soc. 1998,
120, 6037-6046. (d) Rogers, J . S.; Lachicotte, R. J .; Bazan, G. C. J .
Am. Chem. Soc. 1999, 121, 1288-1298. (e) Bazan, G. C.; Cotter, W.
D.; Komon, Z. J . A.; Lee, R. A.; Lachicotte, R. J . J . Am. Chem. Soc.
2000, 122, 1371-1380.
a
b
Isolated yield; average of two runs. Product slowly decom-
poses in solution; ∼95% purity by NMR.
conformation that permits a π interaction between
nitrogen and boron ( N1-B1-N2-C10 ) 9.8(4)°), and
the B-N bond distance is short (1.427(2) Å; sum of
covalent radii 1.51 Å).
We were also interested in demonstrating that 1,2-
azaborolides can react with non-halide-based transition-
metal electrophiles to generate η5-azaborolyl complexes.
We chose to investigate reactions with Cr(CO)3(CH3-
(10) For reviews on boratabenzene chemistry, see: (a) Herberich,
G. E.; Ohst, H. Adv. Organomet. Chem. 1986, 25, 199-236. (b) Fu, G.
C. Adv. Organomet. Chem. 2001, 47, 101-119.
(11) For a review of the influence of cyclopentadienyl-ring substit-
uents on Ziegler-Natta catalysts based on group 4 metallocenes, see:
Mo¨hring, P. C.; Coville, N. J . J . Organomet. Chem. 1994, 479, 1-29.
(12) In preliminary studies, other azaborolides (3) have reacted
analogously.

Communications
Organometallics, Vol. 21, No. 21, 2002 4325
CN)3, since, if the complexations were successful, the
CO stretching frequencies of the products would furnish
additional insight into how the substituent on boron
impacts the electronic nature of the metal.13 We were
pleased to determine that this approach to η5-azaborolyl
adducts is also effective. Thus, treatment of Cr(CO)3-
(CH3CN)3 with an azaborolide, followed by stannylation
of the resulting anion with Me3SnCl, provides the
desired complexes (Table 3). The CO stretching frequen-
cies of the azaborolylchromium adducts are consistent
with effective transmission of electronic effects from the
boron-bound NiPr2 and OtBu substituents to chromium.
In summary, we have developed a general method for
the synthesis of new families of transition-metal-free
1,2-azaborolides (3) from a readily available precursor
(1), and we have demonstrated that these azaborolides
react with a variety of transition-metal electrophiles to
produce η5-azaborolyl adducts with an unprecedented
diversity of substituents on boron (nitrogen, oxygen,
hydrogen, carbon, and phosphorus). These advances
open the door to a wide range of studies of azaborolyl-
metal complexes that, in view of the richness of the
chemistry of the isoelectronic cyclopentadienylmetal
complexes, will doubtlessly lead to the discovery of
interesting new reactivity.
Ack n ow led gm en t. Support has been provided by
Bristol-Myers Squibb and Novartis. Funding for the
MIT Department of Chemistry Instrumentation Facility
has been furnished in part by NSF Grant No. CHE-
9808061 and NSF Grant No. DBI-9729592.
Su p p or tin g In for m a tion Ava ila ble: Text giving experi-
mental details and tables giving crystallographic data for 4.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(13) Crabtree, R. H. The Organometallic Chemistry of the Transition
Metals; Wiley: New York, 2001.
OM020533B