Significance of borane tuning in titanium-catalyzed borylation chemistry

Full text of DOI:10.1021/ja961111u

J. Am. Chem. Soc. 1997, 119, 2743-2744
2743
tempted catalytic synthesis of CH2dC(H)BCat from ethylene
and HBCat gave CH3CH2BCat instead of CH2dC(H)BCat.
Hence, the selectivity for the stoichiometric reaction is lost under
catalytic conditions. This paper reports the effects of a simple
modification in the borane reagent that proves to be critical for
catalytic control in this system.
Significance of Borane Tuning in
Titanium-Catalyzed Borylation Chemistry†
Douglas H. Motry, Aimee G. Brazil, and
Milton R. Smith, III*
In the reaction between ethylene and HBCat, catalyzed by 1,
a complex mixture of Ti species results from indiscriminate
attack of HBCat. Reaction of the catecholate oxygen atoms in
HBCat with the Lewis acidic Ti center could account for catalyst
decomposition in this system.11 Alternatively, subsequent
borylations of compound 2 could generate other active species
that account for the normal hydroboration product, CH3CH2-
BCat. Clearly, catalyst integrity must be maintained if the
stoichiometric selectivities are to be preserved, and rates for
catalyst degradation must be significantly slower than that for
regeneration of the ethylene compound, 1. The desired balance
can be achieved by suppressing side reactions between the
borane reagent and Ti species and/or by accelerating the
displacement of the vinylborane by ethylene.
We chose to examine the reactivity of benzo-1,3,2-diazaboro-
lane (HBOp),12 the borane derived from BH3 and o-phenylene-
diamine, for the following reasons. First, there is literature
precedence for B-N linkages being more robust than B-O
frameworks in isoelectronic compounds.13 Hence, HBOp should
be less susceptible to metal-mediated disproportionation. Sec-
ond, borylation of the coordinated vinylborane ligand could be
suppressed to the extent that regeneration of the catalyst 1
competes in the catalytic cycle.14-18 Last, solutions to catalyst
degradation could prove useful for harnessing reactivity of other
Cp′2Ti derivatives.
Department of Chemistry
Michigan State UniVersity
East Lansing, Michigan 48824
ReceiVed April 4, 1996
The potential for transition metal promoted boron-carbon
bond formation is reflected by recent design of catalysts that
accelerate hydroboration reactions1 and mediate transformations
that are prohibitively disfavored on kinetic grounds.2,3 Although
considerable effort has been focused on late metal systems,4
we5 and others6 have been interested in borylation chemistry
effected by lanthanide and early transition element complexes.
In this vein, we recently reported that Cp*2Ti(η2-CH2dCH2)
(1)7 and catecholborane (HBCat) afford the vinylboronate ester
complex, Cp*2Ti(η2-CH2dC(H)BCat) (2).5 A mechanism was
proposed where C-B bond formation proceeds by ring-opening
σ-bond metathesis, and â-hydrogen elimination accounts for
retention of the CdC bond.8-10 A catalytic circuit could be
closed by a series of stoichiometric reactions; however, at-
† A portion of this work was presented at the 210th ACS National
Meeting, Chicago, 1995, INOR 091.
(1) For a review of transition-metal catalyzed hydroboration, see:
Burgess, K.; Ohlmeyer, M. J. Chem. ReV. 1991, 91, 1179-1191.
(2) For catalytic diborylation of olefins, see: Baker, R. T.; Nguyen, P.;
Marder, T. B.; Westscott, S. A. Angew. Chem., Int. Ed. Engl. 1995, 34,
1336-1337.
(3) For catalytic addition of B-B bonds to alkynes, see: Ishiyama, T.;
Matsuda, N.; Murata, M.; Ozawa, F.; Suzuki, A.; Miyaura, N. Organome-
tallics 1996, 15, 713-720.
Solutions of 1 catalyze the reaction between ethylene and
HBOp, and CH2dC(H)BOp can be isolated in reasonable yield
(58% isolated yield based on HBOp) at low catalyst loading
(eq 1).
(4) (a) Hewes, J. D.; Kreimendahl, C. W.; Marder, T. B.; Hawthorne,
M. F. J. Am. Chem. Soc. 1984, 106, 5757-5759. (b) Manning, D.; No¨th,
H. Angew. Chem., Int. Ed. Engl. 1985, 24, 878-879. (c) Mirabelli, M. G.
L.; Sneddon, L. G. J. Am. Chem. Soc. 1989, 111, 592-597. (d) Evans, D.
A.; Fu, G. C.; Hoveyda, A. H. J. Am. Chem. Soc. 1988, 110, 6917-6918.
(e) Burgess, K.; Ohlmeyer, M. J. J. Org. Chem. 1988, 53, 5178-5179. (f)
Hayashi, T.; Matsumoto, Y.; Ito, Y. J. Am. Chem. Soc. 1989, 111, 3426-
3428. (g) Evans, D. A.; Fu, G. C.; Hoveyda, A. H. J. Am. Chem. Soc. 1992,
114, 6671-6679. (h) Evans, D. A.; Fu, G. C.; Anderson, B. A. J. Am.
Chem. Soc. 1992, 114, 6679-6685. (i) Westcott, S. A.; Blom, H. P.; Marder,
T. B.; Baker, R. T. J. Am. Chem. Soc. 1992, 114, 8863-8869.(j) Burgess,
K.; van der Donk, W. A.; Westcott, S. A.; Marder, T. B.; Baker, R. T.;
Calabrese, J. C. J. Am. Chem. Soc. 1992, 114, 9350-9359. (k) Westcott,
S. A.; Marder, T. B.; Baker, R. T. Organometallics 1993, 12, 975-979.
(1) Brown, J. M.; Hulmes, D. I.; Layzell, T. P. J. Chem. Soc., Chem.
Commun. 1993, 1673-1674. (m) Brown, J. M.; Lloyd-Jones, G. C. J. Am.
Chem. Soc. 1994, 116, 866-878.
2CH2dCH2 + HBOp 1 (3 mol%)8 C2H6 + CH2dC(H)BOp
(1)
When the reaction is monitored by NMR, spectra indicate
clean conversion of ethylene and HBOp to CH2dC(H)BOp and
ethane. Under these conditions, 1 is the major Ti-containing
compound in solution, and the vinylboronate amide complex,
Cp*2Ti(η2-CH2dC(H)BOp) (3), is not observed. The equilib-
rium between 1 and ethylene generates small quantities of the
(11) In early metal systems, Burgess and co-workers have observed rapid
redistribution for the catecholate ligands of HBCat. They suggest that
interactions between the catecholate oxygen and the acidic metal center
are responsible for the exchange.6c,e Similar promiscuity for catecholate
ligands in HBCat has also been suggested as a mechanistic possibility in
lanthanide-catalyzed hydroborations.6d
(5) Motry, D. H.; Smith, M. R., III. J. Am. Chem. Soc. 1995, 117, 6615-
6616.
(6) (a) Harrison, K. M.; Marks., T. J. J. Am. Chem. Soc. 1992, 114, 9220-
9221. (b) Erker, G.; Noe, R.; Wingbermuhle, D.; Petersen, J. L. Angew.
Chem., Int. Ed. Engl. 1993, 32, 1213-1215. (c) Burgess, K.; Jaspars, M.
Tetrahedron Lett. 1993, 34, 6813-6826. (d) Evans, D. A.; Muci, A. R. J.
Org. Chem. 1993, 58, 5307-5309. (e) Burgess, K.; van der Donk, W. A.
Organometallics 1994, 13, 3616-3620. (f) Burgess, K.; van der Donk, W.
A. J. Am. Chem. Soc. 1994, 116, 6561-6569. (g) Bijpost, E. A.; Duchateau,
R.; Teuben, J. H. J. Mol. Catal., A: Chem. 1995, 95, 121-128. (h) Pereira,
S.; Srebnik, M. Organometallics 1995, 14, 3127-3128. (i) Pereira, S.;
Srebnik, M. J. Am. Chem. Soc. 1996, 118, 909-910. (j) He, X.; Hartwig,
J. F. J. Am. Chem. Soc. 1996, 118, 1696-1702. (k) Sun, Y.; Piers, W. E.;
Rettig, S. J. Organometallics 1996, 15, 4110-4112.
(12) (a) Morales, H. R.; Tlahuext, H.; Santiesteban, F.; Contreras, R.
Spectrochim. Acta 1984, 40A, 855-862. (b) Camacho, C.; Paz-Sandoval,
M. A.; Contreras, R. Polyhedron 1986, 5, 1723-1732.
(13) Compared to borazine, boroxine (H3BO3) is kinetically unstable and
readily disproportionates to B2O3 and B2H6 at room temperature: Porter,
R. J.; Gupta, S. K. J. Phys. Chem. 1964, 68, 280-289.
(14) Pelter, A.; Smart, K.; Brown, H. C. Borane Reagents; Academic
Press: London, 1988.
(15) For substituted boranes, ab initio calculations predict the follow
ordering of activation energies for hydroboration of ethylene: BH3 < BH2-
CH3 < BHFCH3. This agrees with experimentally observed trends: Wang,
X.; Li, Y.; Wu, Y.-D.; Paddon-Row, M. N.; Rondan, N. G.; Houk, K. N.
J. Org. Chem. 1990, 55, 2601-2609.
(7) Cohen, S. A.; Auburn, P. R.; Bercaw, J. E. J. Am. Chem. Soc. 1983,
105, 1136-1143.
(8) A similar mechanism was proposed by Erker to account for reactivity
between zirconocene and hafnocene butadiene complexes and 9-BBN.6b
More recent reports invoke similar mechanisms for reactions of boranes
with olefin complexes.6j,k
(9) Rhodium systems mediate related dehydrogenative borylation for aryl
substituted olefins, and reasonable yields of vinylboranes have been
produced when oxazaborolidine derivatives from ephedrine are used as the
borane source.4m However, dehydrogenative borylations for unactivated
olefins are rare4k,6b,10 and mixtures of vinylboranes and normal hydroboration
products are usually observed in these cases.
(16) The decrease in rate for hydroboration relative to dialkylboranes
has been attributed to O-B π-donation. The exceedingly slow rate for
uncatalyzed hydroboration by HBOp17 is consistent with this notion: Brown,
H. C.; Chandrasekharan, J. J. Org. Chem. 1983, 48, 5080-5082.
(17) In uncatalyzed reactions, no hydroboration products are evident after
heating HBOp with 1-hexene for 1 week at 110 °C.
(18) If borylation of the vinylborane complex triggers catalyst decom-
position, a nitrogen-substituted borane should better preserve catalyst
integrity, assuming that heteroatom substitution at boron has similar effects
on borylation and hydroboration rates.
(10) Davan, T.; Corcoran, E. W.; Sneddon, L. G. Organometallics 1983,
2, 1693-1694.
S0002-7863(96)01111-0 CCC: $14.00 © 1997 American Chemical Society

2744 J. Am. Chem. Soc., Vol. 119, No. 11, 1997
Communications to the Editor
Scheme 1
2 + CH2dCH2 h 1 + CH2dC(H)BCat;
∆G°rxn ) 0.09(1) kcal/mol (2)
3 + CH2dCH2 h 1 + CH2dC(H)BOp;
∆G°rxn < -4.5 kcal/mol (3)
reactivity were consistent with observations from the catalytic
reaction as the kinetic and thermodynamic parameters governing
the reactivity of 3 preclude its detection when excess ethylene
is present. Although the differences in reactivity between O-
and N-substituted vinylborane complexes are pronounced, the
factors responsible for this disparity are not obvious.20
The enhanced rate for displacement of the coordinated
vinylborane ligand and the suppression of borane-promoted
catalyst degradation both favor catalytic viability when HBOp
is used as the borane reagent. The influence of the borane
substituent on rates for vinylborane displacement was unex-
pected. The observed rate acceleration may be significant for
dehydrogenative pathways, but the catalyst compatibility en-
gendered by HBOp has more general applications in other
titanocene systems. While Hartwig and co-workers recently
reported that Cp2TiMe2 can serve as a precatalyst for addition
of HBCat to olefins,6j,21 preliminary results suggest that the
utility of HBCat, and other oxygen-substituted borane reagents,
may be limited to the parent titanocene system.22 For titanocene
derivatives with alkyl-substituted cyclopentadienyl rings, we
observe rapid catalyst decomposition when hydroborations are
attempted with HBCat. Although the parent titanocene system
failed to catalyze additions of HBOp to olefins,23 catalytic olefin
hydroboration by HBOp proceeds smoothly for other Cp′2Ti
derivatives and TiIII species are not observed. The results from
these investigations will be published in due course.
Scheme 2
Acknowledgment. The NMR equipment was provided by the
National Science Foundation (CHE-8800770) and the National Institutes
of Health (1-S10-RR04750-01). We thank the administrators of the
ACS Petroleum Research Fund, the Center for Fundamental Materials
Research at Michigan State University, the Exxon Educational Founda-
tion, the NSF REU program (A.G.B.), and the National Science
Foundation (CHE-9520176) for support of this work.
metallacyclopentane complex, Cp*2Ti(CH2)4; however, there is
no evidence for participation of this species in borylation
processes since C4 borylation products are not observed. The
catalytic cycle for this dehydrogenative borylation is depicted
in Scheme 1.
The catalytic process in Scheme 1 was dissected to assess
the effects of borane modification on the discrete steps that
comprise the catalytic pathway. As was observed for HBCat,
the stoichiometric reaction between HBOp and 1 gives ethane
and 3, the nitrogen analog of compound 2. However, borylation
of 1 by HBOp proceeds at a substantially diminished rate relative
to HBCat, as shown in Scheme 2. Hence, boron substituent
effects on rates for borylation of the coordinated ethylene ligand
in 1 parallel those observed for olefin hydroboration reactions.
Significantly, selectivity for ethylene borylation is maintained,
since diborylation products resulting from borane attack on 3
are not detected.
The kinetics and thermodynamics governing the displacement
of the coordinated vinylborane ligands in compounds 2 and 3
differ markedly. For compound 2, vinylborane displacement
is slow, and an approximately thermoneutral equilibrium is
reached after 72 h at ambient temperature. In contrast,
vinylborane displacement from compound 3 is rapid, and the
equilibrium strongly favors the ethylene complex (1) and
CH2dC(H)BOp. Competitive binding experiments afforded the
relative energies shown in eqs 2 and 3.19 Activation parameters
for the displacement reaction could not be extracted because
the reaction rates for 3 and ethylene (-80 °C) were too fast for
reliable analysis. The effects of borane modification on
Supporting Information Available: Synthetic details, as well as
spectroscopic and analytical data, for the compounds in this paper (3
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JA961111U
(19) The greatest error in this measurement is integration of the ethylene
resonance which approaches the threshold for detection by NMR. A lower
limit for the equilibrium constant was estimated by assuming that the
ethylene resonance could have an integrated intensity as large as 2% of
that of the most intense resonance.
(20) Given that the Cp2*Ti fragment generally will not accommodate
substituted olefins,7 it can be argued that the stability of the borylated olefin
complexes likely results from overriding electronic effects. The differences
in reactivity between 3 and 2 could result from ground state destabilization
of 3 relative to 2 or stabilization of CH2dC(H)BOp relative to CH2dC-
(H)BCat. Although a lower activation energy for displacement of CH2)C-
(H)BOp from 3 would be consistent with significant ground-state destabi-
lization for 3, this observation is not sufficient proof.
(21) Catalysis in this system is efficient; however, the catalytic activity
for the TiIII compound, Cp2TiBCat2, that forms during the course of the
reaction is diminished relative to TiIV/TiII species.
(22) Stoudt, S. J.; Motry, D. H.; Brazil, A. G.; Smith, M. R., III.
Manuscripts in preparation.
(23) Compared to the thermal decomposition rate for Cp2TiMe2, the
B-H/Ti-Me metathesis that initiates catalysis for HBCat6j is prohibitively
slow for HBOp.