Mechanistic Studies of Titanocene-Catalyzed Alkene and Alkyne Hydroboration: Borane Complexes as Catalytic Intermediates

Article abstract of DOI:10.1021/om990507m

The bis(borane) complex Cp₂Ti(HBcat′)₂ is a highly active catalyst for the hydroboration of vinylarenes. We provide a detailed mechanistic analysis of this hydroboration process and of the hydroboration of alkynes catalyzed by titanocene dicarbonyl. The hydroboration of alkynes showed a reaction rate that was first order in the concentration of alkyne, inverse second order in the concentration of carbon monoxide, and first order in the concentration of borane. These data are consistent with a mechanism in which two equilibria involving Cp₂Ti(CO)(PhCCPh) generate the alkyne borane complex Cp₂Ti(HBcat′)(PhCCPh), which forms the hydroboration product. The hydroboration of alkenes catalyzed by the bis(borane) complex Cp₂Ti(HBcat′)₂ involves the similar intermediate Cp₂Ti(CO)(RCH=CCH₂), containing a coordinated alkene, rather than alkyne. The catalytic process for alkene hydroboration is inverse first order in borane and first order in alkene, indicating that the reaction occurs by reversible dissociation of borane and coordination of alkene to form an alkene borane complex that undergoes elimination of alkylboronate ester. Reactions of this complex that compete with the production of alkylboronate ester include the formation of vinylboronate esters, presumably by β-hydrogen elimination of the intermediate complex formed by addition of alkene to coordinated borane. This β-hydrogen elimination pathway was suppressed by using excess catecholborane.

Full text of DOI:10.1021/om990507m

30
Organometallics 2000, 19, 30-38
Mech a n istic Stu d ies of Tita n ocen e-Ca ta lyzed Alk en e a n d
Alk yn e Hyd r obor a tion : Bor a n e Com p lexes a s Ca ta lytic
In ter m ed ia tes
J ohn F. Hartwig* and Clare N. Muhoro
Department of Chemistry, Yale University, P.O. Box 208107,
New Haven, Connecticut 06520-8107
Received J uly 1, 1999
The bis(borane) complex Cp2Ti(HBcat′)2 is a highly active catalyst for the hydroboration
of vinylarenes. We provide a detailed mechanistic analysis of this hydroboration process
and of the hydroboration of alkynes catalyzed by titanocene dicarbonyl. The hydroboration
of alkynes showed a reaction rate that was first order in the concentration of alkyne, inverse
second order in the concentration of carbon monoxide, and first order in the concentration
of borane. These data are consistent with a mechanism in which two equilibria involving
Cp2Ti(CO)(PhCCPh) generate the alkyne borane complex Cp2Ti(HBcat′)(PhCCPh), which
forms the hydroboration product. The hydroboration of alkenes catalyzed by the bis(borane)
complex Cp2Ti(HBcat′)2 involves the similar intermediate Cp2Ti(CO)(RCHdCCH2), containing
a coordinated alkene, rather than alkyne. The catalytic process for alkene hydroboration is
inverse first order in borane and first order in alkene, indicating that the reaction occurs by
reversible dissociation of borane and coordination of alkene to form an alkene borane complex
that undergoes elimination of alkylboronate ester. Reactions of this complex that compete
with the production of alkylboronate ester include the formation of vinylboronate esters,
presumably by â-hydrogen elimination of the intermediate complex formed by addition of
alkene to coordinated borane. This â-hydrogen elimination pathway was suppressed by using
excess catecholborane.
In tr od u ction
addition of catecholborane to alkenes by a pathway
involving initial oxidative addition of catecholborane to
rhodium, followed by alkene migratory insertion and
reductive elimination of the hydroborated product.22,23
These borane additions are often complicated by side
reactions such as â-hydride elimination to give vinyl-
boronate esters and decomposition of catecholborane to
diborane and hydrogen. The diborane formed by the
latter process produces trialkylborane products, with
selectivity that is independent of the catalyst.24-27
Early-transition-metal-catalyzed hydroborations have
been less well studied. Lanthanide-catalyzed hydro-
boration of alkenes has been reported,28-30 and a mech-
anism that is distinct from those of Rh(I) complexes has
Transition-metal complexes catalyze the addition of
catecholborane to alkenes and alkynes with enhanced
rates and altered chemo-1, regio-,2-6 diastereo-,5-10 and
enantioselectivities4,11-14 relative to the uncatalyzed
transformations.15,16 Since Wilkinson’s catalyst was first
reported by Ma¨nnig and No¨th1 to promote the addition
of catecholborane to alkenes, other late-transition-metal
systems have been investigated.2,12,13,17-21 Mechanistic
studies have shown that Rh(I) catalysts promote the
(1) Mannig, D.; No¨th, H. Angew. Chem., Int. Ed. Engl. 1985, 24,
878.
(2) Westcott, S. A.; Blom, H. P.; Marder, T. B.; Baker, R. T. J . Am.
Chem. Soc. 1992, 114, 8863.
(3) Brands, K. M.; Kende, A. S. Tetrahedron Lett. 1992, 33, 5887.
(4) Zhang, J .; Lou, B.; Guo, G.; Dai, L. J . Org. Chem. 1991, 56, 1670.
(5) Evans, D. A.; Fu, G. C.; Hoveyda, A. H. J . Am. Chem. Soc. 1988,
110, 6917.
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(7) Burgess, K.; Cassidy, J .; Ohlmeyer, M. J . J . Org. Chem. 1991,
56, 1020.
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(10) Burgess, K.; Ohlmeyer, M. J . Tetrahedron Lett. 1989, 30, 395.
(11) Schnyder, A.; Hintermann, L.; Togni, A. Angew. Chem., Int. Ed.
Engl. 1995, 34, 931.
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3426.
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111, 3426.
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dron: Asymmetry 1991, 2, 613.
(15) Brown, H. C.; Gupta, S. K. J . Am. Chem. Soc. 1971, 93, 1816.
(16) Atwood, J . D. Inorganic and Organometallic Reaction Mecha-
nisms; Cole Publishing: Monterey, CA, 1985; p 17.
(17) Knorr, J . R.; Merola, J . S. Organometallics 1990, 9, 3008.
(18) Gridnev, I. D.; Miyaura, N.; Suzuki, A. Organometallics 1993,
12, 589.
(19) Pereira, S.; Srebnik, M. Tetrahedron Lett. 1996, 37, 3283.
(20) Kabalka, G. W.; Narayan, C.; Reddy, N. K. Synth. Commun.
1994, 24, 1019.
(21) Beletskaya, I.; Pelter, A. Tetrahedron 1997, 53, 4957.
(22) Evans, D. A.; Fu, G. C.; Anderson, B. A. J . Am. Chem. Soc. 1992,
114, 6679.
(23) Burgess, K.; Ohlmeyer, M. J . Chem. Rev. 1991, 91, 1179.
(24) Burgess, K.; J aspars, M. Tetrahedron Lett. 1993, 34, 6813.
(25) Burgess, K.; van der Donk, W. A. Organometallics 1994, 13,
3616.
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6561.
(27) Westcott, S. A.; Blom, H. P.; Marder, T. B.; Baker, R. T.;
Calabrese, J . C. Inorg. Chem. 1993, 32, 2175.
(28) Harrison, K. N.; Marks, T. J . J . Am. Chem. Soc. 1992, 114, 0.
(29) Evans, D. A.; Muci, A. R.; Stu¨rmer, R. J . Org. Chem. 1993, 58,
5307.
(30) Bijpost, E. A.; Duchateau, R.; Teuben, J . H. J . Mol. Catal. A
1995, 95, 121.
10.1021/om990507m CCC: $19.00 © 2000 American Chemical Society
Publication on Web 12/09/1999

Titanocene-Catalyzed Hydroboration
Organometallics, Vol. 19, No. 1, 2000 31
been proposed.28 Olefin insertion into the lanthanide-
hydride bond is well known, and this step was suggested
to be followed by a σ-bond metathesis to provide anti-
Markovnikov alkylboronate ester products. Our group
recently reported that titanocene dimethyl is an efficient
catalyst for the hydroboration of alkenes and that
titanocene dicarbonyl catalyzes the hydroboration of
alkynes.31 Sneddon32 has applied these results to the
development of a process to form monoalkyldecaboranes.
Ta ble 1. Hyd r obor a tion of Alk en es by
4-Meth ylca tech olbor a n e Ca ta lyzed by Bis(bor a n e)
Com p lex 2a
Extensive studies by Burgess et al. have shown that
previous titanium systems promoted hydroboration by
decomposing catecholborane to diborane, which was
responsible for hydroboration.24 Some hydroboration
catalysis with group IV complexes has been reported
by Teuben et al., but low activity and borane decompo-
sition were observed.30 Subsequent studies31 with pure
dimethyltitanocene, however, showed that no trialkyl-
boronate esters were formed, indicating that titanocene
dimethyl did catalyze hydroboration and did not simply
catalyze decomposition of catecholborane to diborane.
The catalytic hydroboration of alkynes with cat-
echolborane has received less attention than the addi-
tion of this borane to alkenes21,23 or the addition of
polyboranes to alkynes,33-36 even though the metal-
catalyzed chemistry can induce mild and selective
monoadditions to alkynes to produce vinylboronate
esters that are useful in Suzuki cross-coupling chemis-
try.37 Few alkyne hydroboration catalysts have been
reported, and most of them are based on late-transition-
metal systems.18-20,38,39 Recently, we reported that Cp2-
Ti(CO)2 (1) catalyzes the hydroboration of alkynes
without borane decomposition.31 Srebnik et al. also
reported an early-transition-metal system in which
Schwartz’s reagent (Cp2ZrHCl) catalyzes the hydro-
boration of alkynes by pinacolborane with excellent
yields and selectivities.40 Srebnik’s system presumably
operates by alkyne insertion and σ-bond metathesis, the
same pathway as the lanthanide-catalyzed alkene hy-
droboration.
a
10 mol % catalyst used. Reactions were run in C6D6 solvent,
and yields were determined by 1H NMR spectroscopy with an
internal standard.
faster than those catalyzed by titanocene dimethyl.
Further, kinetic studies support the bis(borane) complex
as the active catalyst. In addition to a brief presentation
of the catalytic chemistry of Cp2Ti(HBcat′)2 (2), we
report an extensive kinetic study of the catalysis by both
1 and 2 that supports reaction mechanisms that are
distinct from those of either Rh(I) or other early-
transition-metal-catalyzed hydroborations. Several types
of monoborane Ti(II) complexes are intermediates.
Resu lts
Hyd r obor a tion s Ca ta lyzed by Cp 2Ti(HBca t-4-t-
Bu )2 (2). A comparison of the activity of 2 and Cp2TiMe2
is summarized in Table 1. Indene and p-methoxystyrene
gave a 95-100% yield of hydroborated product in only
10 min at room temperaturesa rate that is much faster
than those of the 10 h reactions catalyzed by Cp2TiMe2.
Anti-Markovnikov products were formed exclusively in
the reactions with indene and styrenes. The regiochem-
istry with vinylarenes is in contrast with that of most
Rh(I)-catalyzed vinylarene hydroborations.1
Mech a n istic Stu d ies on th e Hyd r obor a tion of
Alk en es Ca ta lyzed by 2. Product compositions of
reactions run with stoichiometric and catalytic quanti-
ties of 2 were different, and the higher ratio of HBcat′
to catalyst seemed to be responsible for the different
selectivity. We were initially surprised to observe that
the reaction of complex 2 with p-methoxystyrene in the
absence of HBcat′ (HBcat′ ) HBO2C6H4-4-t-Bu) gave
arylboronate ester in only 30% yield, p-(methoxyethyl)-
benzene in 30% yield, and the titanocene vinylboronate
ester complex Cp2Ti(cat′BCHdCHAr) in 16% yield (eq
1). The unsubstituted vinylboronate ester complex was
Preliminary mechanistic investigations on the cata-
lytic hydroboration by Cp2TiMe2 led to the discovery of
the titanocene bis(borane) σ complex Cp2Ti(HBcat)2.41
This compound was formed from the reaction of ti-
tanocene dimethyl with excess catecholborane. Because
no reaction was observed between titanocene dimethyl
and alkenes at the temperature of the catalysis, we
suspected that the bis(borane) complex was the active
catalyst in the hydroboration of alkenes by titanocene
dimethyl. Indeed, we report that hydroboration reac-
tions catalyzed by titanocene bis(borane) complexes are
(31) He, X.; Hartwig, J . F. J . Am. Chem. Soc. 1996, 118, 1696.
(32) Pender, M. J .; Wideman, T.; Carroll, P. J .; Sneddon, L. G. J .
Am. Chem. Soc. 1998, 120, 9108.
(33) For earlier examples of alkyne hydroboration with polyboranes
see ref 34-36.
(34) Wilczynski, R.; Sneddon, L. G. Inorg. Chem. 1982, 21, 506-
514.
(35) Wilczynski, R.; Sneddon, L. G. Inorg. Chem. 1981, 20, 3955-
3962.
(36) Wilczynski, R.; Sneddon, L. G. J . Am. Chem. Soc. 1980, 102, 2,
2857-2858.
(37) Miyaura, N.; Suzuki, A. Chem. Rev. 1995, 95, 2457-2483.
(38) Lynch, A. T.; Sneddon, L. G. J . Am. Chem. Soc. 1987, 109, 5867.
(39) 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.
(40) Pereira, S.; Srebnik, M. Organometallics 1995, 14, 3127.
(41) Hartwig, J . F.; Muhoro, C. N.; He, X.; Eisenstein, O.; Bosque,
R.; Maseras, F. J . Am. Chem. Soc. 1996, 118, 10936.

32 Organometallics, Vol. 19, No. 1, 2000
Hartwig and Muhoro
Ta ble 2. In itia l Ra tes of Alk en e Hyd r obor a tion
Ca ta lyzed by 1 a t Va r yin g Con cen tr a tion s of
p-Meth oxystyr en e a n d HBca t′ a
independently prepared and fully characterized by the
reaction of Cp2Ti(HBcat)2 with phenylacetylene. The
reaction of equimolar quantities of 2 and p-methoxy-
styrene in the presence of HBcat′ formed the arylbor-
onate ester in a higher 72% yield, vinylboronate ester
in 15% yield, and p-(methoxyethyl)benzene in 5% yield
(eq 2). In contrast, the catalytic reactions produced only
[p-methoxystyrene], M
[HBcat′], M
initial rate, 106 M s-1
0.0695
0.0695
0.0695
0.0695
0.0695
0.0174
0.0348
0.104
0.209
0.348
0.695
1.04
-8.01
-4.52
-2.96
-2.13
-1.36
-1.18
-2.67
-6.44
-11.5
1.39
0.348
0.348
0.348
0.348
0.139
Rates were measured by 1H NMR spectroscopy at -20 °C over
the first 1000 s of reaction.
evidence that 2 is converted to a catalytically inactive
species, two Ti(III) complexes, Cp2Ti(H2Bcat′) (3) and
fulvalene complex [(C10H8)Cp2Ti2H2] (4),42 which are
known to be decomposition products of the titanocene
bis(borane) complexes,31 were tested for catalytic activ-
ity. Borohydride 3 promoted alkene hydroboration, but
at a rate that was slower than reactions catalyzed by 2
(32% yield for 3 vs 95% yield for 2 after 1 h at -5 °C).
Fulvalene 4 showed no activity as a hydroboration
catalyst. Thus, complex 2 is most likely responsible for
the catalytic chemistry.
trace amounts of hydrogenated product when equimolar
proportions of alkene and HBcat′ were used. For ex-
ample, reaction of p-methoxystyrene with HBcat′ in the
presence of 10 mol % of complex 2 gave the desired
hydroborated product in 95% yield along with 5%
p-methoxyethylbenzene (eq 2). Catalytic reaction mix-
tures containing excess borane even suppressed forma-
tion of the small amounts of vinylboronate ester and
alkylarene product obtained in the absence of the excess
borane (eq 3).
Kinetic studies were conducted to determine how 2
catalyzes the reaction. Catalyst decomposition occurred
in competition with the hydroboration chemistry; there-
fore, the method of initial rates was most appropriate
for our kinetic studies.43 The p-methoxystyrene concen-
tration was monitored by 1H NMR spectroscopy over the
first 1000 s of reaction at -20 °C. Over this time period,
the catalyst concentration decreased by less than 5%,
and good linear fits for the decay of alkene over this
time period were obtained. Excess HBcat′ was used in
all experiments to prevent competing hydrogenation.
The values of the initial rates at different alkene and
HBcat′ concentrations are presented in Table 2.
The order of the reaction in alkene concentration was
determined for reactions with concentrations of p-
methoxystyrene between 0.0695 and 0.139 M. The
concentrations of HBcat′ and catalyst were constant at
0.348 M and 6.95 mM, and rate measurements were
made by monitoring the disappearance of alkene. The
ln[alkene] vs ln(initial rate) plot provided a slope of 1.0,
demonstrating a first-order rate dependence on alkene
concentration. The order of the reaction in HBcat′
concentration was determined by obtaining initial rates
for reactions with concentrations of HBcat′ between
0.209 and 1.39 M. Rate measurements were made by
monitoring the disappearance of alkene. In each run,
the initial alkene and catalyst concentrations were 69.5
and 6.95 mM. The ln[HBcat′] vs ln(initial rate) plot
provided a slope of -0.87, indicating a rate dependence
that is near inverse first order. This inverse order in
borane concentration is unusual for a reactant in a
catalytic process.
Mechanistic studies were conducted on the titanium-
catalyzed reaction between p-methoxystyrene and ex-
cess HBcat′ involving bis(borane) 2. All studies were
performed at temperatures below -5 °C. Initially, we
sought to identify potential reaction intermediates and
the catalyst resting state. A toluene-d8 solution of
p-methoxystyrene and HBcat′ was added to complex 2
at -78 °C. When the temperature was raised to -20
1
°C (at which point the catalytic reaction occurred), H
NMR spectroscopy showed complex 2 to decay slowly.
No new titanium species were observed by NMR spec-
troscopy.
To investigate whether 2 was converted to a catalyti-
cally active paramagnetic species, we probed the early
stages of reaction for an induction period. No induction
period was observed, and the disappearance of 2 oc-
curred on the same time scale as the catalytic reaction.
The latter result rules out the possibility that the
induction period is too rapid to detect. To provide further
Mech a n istic Stu d ies of Alk yn e Hyd r obor a tion
Ca ta lyzed by 1. Low-temperature spectroscopic studies
(42) Watt, G. W.; Baye, L. J .; Drummond, F. O. J . Am. Chem. Soc.
1966, 88, 1138.
(43) For a review of initial rate method see: Wilkins, R. G. Kinetics
and Mechanism of Reactions of Transition Metal Complexes; VCH:
Weinheim, Germany, 1991; p 465.

Titanocene-Catalyzed Hydroboration
Organometallics, Vol. 19, No. 1, 2000 33
Sch em e 1
were conducted on reactions of alkynes and borane
reagents with 1 at -20 °C to identify which species
reacted with 1 to initiate the catalytic reaction in eq 4
F igu r e 1. Concentrations of Cp2Ti(CO)2 (1), Cp2Ti(CO)-
(PhCCPh) (5), Cp2Ti(HBcat′)(PhCCPh) (6), and vinylbor-
onate ester for the reaction of 5 and HBcat′ at -20 °C.
and Scheme 1. Addition of a toluene-d8 solution of
PhCCPh to 1 resulted in the formation of Cp2Ti(CO)-
(PhCCPh) (5) after 2.5 h at -20 °C. Similar reactions
have been conducted at room temperature.44 Most
important for the catalytic studies, the reaction of
equimolar quantities of HBcat′, PhCCPh, and 1 pro-
duced alkyne carbonyl complex 5 in addition to unre-
acted 1. The concentration of 5 steadily increased over
24 h at -30 °C but never exceeded that of 1. The
hydroboration product Ph(H)CdC(Ph)(Bcat′) was gener-
ated slowly over this time period, but only after the
appearance of compound 5. These observations sug-
gested that the formation of 5 from reaction of 1 with
PhCCPh initiates hydroboration of PhCCPh.
Sch em e 2
5 to give alkyne borane complex 6 and titanocene
dicarbonyl rapidly at -30 °C (eq 6). Previously we have
The alkyne carbonyl complex 5 was reacted with
HBcat′ for the purpose of observing the formation of
intermediates en route to B-C bond formation. At room
temperature, this reaction rapidly gave vinylboronate
ester. However, addition of 1 equiv of HBcat′ to a
toluene-d8 solution of complex 5 at -30 °C resulted in
the formation of Cp2Ti(HBcat′)(PhCCPh) (6) after 3 min,
as determined by 1H NMR spectroscopy (eq 5). It is
important to note that the vinylboronate ester product
was generated slowly, after the initial appearance of
compound 6.
shown that bis(borane) 2 and dicarbonyl 1 are favored
thermodynamically over Cp2Ti(CO)(HBcat). Thus, the
kinetic behavior of catalytic systems involving alkyne
complexes as resting states would be complex.
The reversibility of the formation of intermediate
complexes 5 and 6 was assessed. The first step is known
to be reversible, since Cp2Ti(CO)(PhCCPh) reacts with
CO to form Cp2Ti(CO)2.44 The reversibility of the forma-
tion of complex 6 was determined by reacting it with
1
CO and monitoring the reaction by low-temperature H
NMR spectroscopy. As shown in Scheme 2, addition of
610 Torr of CO pressure to complex 6 at -78 °C resulted
in displacement of HBcat′ and formation of alkyne
carbonyl 5. No vinylboronate ester was generated. After
the temperature was increased from -80 to 40 °C,
dicarbonyl 1, free alkyne, and free borane were formed.
An identical experiment that used a lower pressure of
CO (200 Torr) led to formation of free alkyne and free
borane in competition with vinylboronate ester and
titanocene dicarbonyl. Thus, the formation of vinyl-
boronate ester from complex 6 competes with regenera-
tion of 1, and the relative rates depend on the pressure
of CO.
The reaction between 5 and HBcat′ was conducted
with 3 equiv of HBcat′ to ensure that the Cp2Ti
fragment would be trapped by excess borane. Dicarbonyl
complex 1 and bis(borane) complex 2 were formed after
warming to -20 °C. The formation of the products as a
function of time is shown in Figure 1. The convex trace
for the decay of 5 and the concave trace for the
appearance of vinylboronate ester suggest autocatalytic
behavior. In this case, one of the products of the reaction
should react with 5. Indeed, bis(borane) 2 reacted with
(44) Fachinetti, G.; Floriani, C.; Marchetti, F.; Mellini, M. J . Chem.
Soc., Dalton Trans. 1978, 1398.

34 Organometallics, Vol. 19, No. 1, 2000
Hartwig and Muhoro
Ta ble 4. Va lu es of kobs a t Va r yin g CO P r essu r esa
PCO, Torr
kobs, 105 s-1
PCO, Torr
kobs, 105 s-1
100
110
132
130
113
64.8
200
300
600
29.0
12.5
4.23
a
Conditions: [PhCCPh] ) 0.0625 M, [HBcat′] ) 0.313 M, [1]
) 0.00625 M.
Ta ble 5. P r od u ct Ra tios for Hyd r obor a tion of
1-Hexyn e w ith Differ en t Ca ta lysts (4 m ol %)
F igu r e 2. Natural logarithm of the decay of PhCCPh as
a function of time ([PhCCPh] ) 0.0625 M, [HBcat] ) 0.310
M, [1] ) 6.25 mM, PCO ) 122 Torr).
The reaction order in added CO was evaluated from
observed rate constants for the decay of alkyne (0.0625
M initial concentration) in the presence of 1.25 M HBcat′
and 6.25 mM 1 at 40 °C. The values of kobs were obtained
for CO pressures ranging from 63 Torr to 400 Torr. A
plot of ln(kobs) vs ln(PCO) provided a rate dependence
that was less than inverse second order (-1.7). The
dependence of rate on CO pressure was also determined
by monitoring the disappearance of PhCCPh in the
presence of a lower 0.313 M concentration of borane at
CO pressures ranging from 100 to 600 Torr. The values
of kobs at different CO pressures are presented in Table
4. In this case, the rate dependence on the pressure of
added CO was within experimental error of inverse
second order (-1.9).
To determine if different titanocene hydroboration
catalysts generate the same reactive intermediates, the
selectivities of several catalysts were compared. A
solution of 1-hexyne and HBcat′ in ether was added to
catalytic quantities of titanocene dimethyl, bis(borane)
complex 1, and dicarbonyl 2 at room temperature. Table
5 shows that Cp2TiMe2 and Cp2Ti(HBcat)2 yielded
similar mixtures of products. The major hydroboration
product was the anti-Markovnikov vinylboronate ester,
but the Markovnikov product was also formed in small
quantities. In addition, both systems yielded products
of hydrogenation. In agreement with earlier experi-
ments, titanocene dicarbonyl gave exclusively anti-
Markovnikov hydroboration products, with only trace
amounts of hydrogenation products detected.
Ta ble 3. Va lu es of kobs a t Va r yin g Con cen tr a tion s
a
of HBca t′
[HBcat′], M
kobs, 104 s-1
[HBcat′], M
kobs, 104 s-1
0.313
0.625
1.4 ( 0.3
2.2 ( 0.1
0.938
1.25
1.8
2.1
a
Conditions: [tolCCtol] ) 0.0625 M, [1] ) 6.25 MM, PCO ) 200
Torr.
Because observed species often do not lie on the
catalytic cycle, we conducted kinetic studies to investi-
gate whether the rate law was consistent with the
intermediacy of complexes 5 and 6. The experiments
were performed using either PhCCPh or p-tolCC-p-tol
as alkyne and HBcat, HBcat-4-t-Bu (HBcat′), or HBcat-
4-Me as borane. The dependence of reaction rate on the
concentration of alkyne was obtained by monitoring the
disappearance of PhCCPh or p-tolCC-p-tol in the pres-
ence of excess of the borane and added CO. The reaction
rate was measured at 40 °C with a 0.0625 M initial
concentration of PhCCPh, a concentration of HBcat′ of
0.313 M, and a concentration of Cp2Ti(CO)2 of 6.25 mM.
To create a single resting state and to avoid compound
5 with its accompanying complex autocatalytic chem-
istry (vide supra) as resting state, we conducted reac-
tions under 122 Torr of carbon monoxide to ensure that
the resting state was 1. The reaction rate showed a first-
order dependence on the concentration of PhCCPh, as
shown in Figure 2.
Discu ssion
The dependence of the rate of reaction on borane
concentration was determined by two methods. The first
involved obtaining values of kobs in the presence of 200
Torr of added CO at various concentrations of excess
borane. The kobs values in Table 3 show that the reaction
became zero order in borane as borane concentration
was increased. Because the dependence of borane at low
concentrations was ambiguous, a second method was
used to evaluate the reaction order at these low con-
centrations. This method involved monitoring the decay
of a 0.0625 M solution of HBcat′ in the presence of
excess (0.375 M) of PhCCPh, 700 Torr of added CO, and
10 mol % of catalyst at 40 °C. A large CO pressure was
required to prevent the accumulation of Cp2Ti(CO)-
(PhCCPh) (2) in solution at this high alkyne concentra-
tion. Under these reaction conditions, the decay of
borane was first order.
Our mechanistic studies were directed at identifying
intermediates in the catalytic cycles of alkene and
alkyne hydroboration and determining the sequence of
ligand dissociation and association involved in the
formation of these intermediates. We were interested
in determining whether borane complexes that we had
previously isolated and characterized were intermedi-
ates in the hydroboration and, in particular, how the
bis(borane) complexes catalyzed the hydroboration of
alkenes.
Our studies on stoichiometric reactions of the catalytic
cycle for alkyne hydroboration catalyzed by dicarbonyl
complex 1 suggested the pathway shown in Scheme 3
that involves alkyne carbonyl 5 and alkyne borane
complex 6 on the catalytic cycle. In addition, our
stoichiometric studies showed that the formation of the

Titanocene-Catalyzed Hydroboration
Organometallics, Vol. 19, No. 1, 2000 35
Sch em e 3
F igu r e 3. Proposed structure of the alkene borane inter-
mediate.
Sch em e 4
alkyne borane complex 6 is highly reversible when 600
Torr of CO is present and partially reversible when 200
Torr of CO is present. Because the formation of 5 and 6
is reversible in the presence of CO,44 it was likely that
these complexes could regenerate dicarbonyl 1, which
could then catalyze the reaction through a different set
of intermediates. Kinetic studies were, therefore, con-
ducted to determine whether 5 and 6 are true interme-
diates.
Equation 7 is a rate expression for the pathway in
Scheme 3 that assumes the two preequilibria K1 and
K2. According to this expression, the rate of the reaction
tions of low PCO and high concentrations of alkyne,
making the resting state a combination of both 1 and 5
under these conditions.
k3K1K2[1][PhCCPh][HBcat′]
rate )
(7)
Mech a n istic Stu d ies of th e Alk en e Hyd r obor a -
tion Ca ta lyzed by 2. Mechanistic studies were per-
formed to determine whether complexes related to 6
were responsible for the hydroboration of alkenes
catalyzed by bis(borane) complex 2. Previous reports
contradicted our observation of clean hydroboration of
alkenes catalyzed by Cp2TiMe2 via complex 2;30 thus,
our study was also directed at confirming that complex
2 behaved as a true hydroboration catalyst. A mecha-
nism that involves 2 and that fits the experimental data
presented below is shown in Scheme 4.
Our studies showed that the titanocene bis(borane)
complex decayed during catalytic hydroboration but that
this decay was like any common catalyst decomposition.
It did not generate a second species that was the true
catalyst. The absence of an induction period, coupled
with the direct observation of catalyst decay on the time
scale of product formation, demonstrates that 2 was
responsible for the catalytic chemistry. Further, two
paramagnetic Ti(III) species that are known decomposi-
tion products of 2, borohydride 3 and fulvalene 4,
catalyzed hydroboration more slowly than did complex
2.
The mechanism in Scheme 4 involves initial dissocia-
tion of coordinated borane to generate a monoborane
intermediate. Coordination of alkene would generate the
alkene borane complex 7, which is similar to a species
isolated by Erker.45 A â-borylalkyl hydride structure
with B‚‚‚H stabilization (Figure 3) is certainly an
important resonance structure of 7. An intramolecular
reaction would extrude the alkylboronate ester product,
and coordination of HBcat′ would regenerate the monob-
orane intermediate. The rate law for the proposed
mechanism derived by a steady-state approximation for
[CO]2
would be first order in the concentration of PhCCPh and
HBcat′ and inverse second order in added CO. Our
kinetic experiments at 0.0625 M or less of borane
showed a first-order dependence on the concentration
of alkyne and borane and an inverse second-order
dependence on added CO. Thus, our kinetic results are
consistent with the proposed mechanism.
However, the dependence of the reaction rate on
borane concentration appeared to vary with the con-
centration of borane. The values of kobs given in Table
3 suggest that the reaction rate has a positive depen-
dence on the concentration of HBcat′ at borane concen-
trations below 0.0625 M (10 equiv vs alkyne), and this
behavior is confirmed by separate experiments in which
the decay of borane concentration is monitored and is
shown to be first order. However, the reaction rate
began to show no dependence on borane concentration
above this concentration. Thus, the value of k2[HBcat′]
appears to approach that of k-1[CO] in Scheme 3 when
[HBcat′] is higher. This relative rate would cause the
second step in the proposed mechanism to become only
partially reversible. The rate expression would then
show an order in borane that is less than 1 and a
dependence on added CO that is between inverse first
and inverse second order. Our data suggest that we have
established this kinetic situation. The order in borane
under these conditions is certainly less than 1, and the
plot of ln(PCO) vs ln(kobs) provided a slope of -1.7,
indicating an order in CO that is between inverse first
and inverse second. Considering the absence of added
CO under standard synthetic conditions that would
employ catalyst 1, the reaction of 6 to give product
would be faster than reversion to 5 or 1 in this case.
Moreover, alkyne carbonyl 5 accumulates under condi-
(45) Binger, P.; Sandmeyer, F.; Kru¨ger, C.; Kuhnigk, J .; Goddard,
R.; Erker, G. Angew. Chem., Int. Ed. Engl. 1994, 33, 197.

36 Organometallics, Vol. 19, No. 1, 2000
Hartwig and Muhoro
Sch em e 5
the monoborane intermediate and irreversible formation
of the alkene borane complex is shown in eq 8. Excess
borane was used in our experiments, and under these
conditions the rate law may simplify to eq 9. Our
k1k2[1][alkene]
rate )
(8)
(9)
k-1[HBcat′] + k2[alkene]
kobs[1][alkene]
rate )
[HBcat′]
mechanistic studies provided a first-order rate depen-
dence on the concentration of alkene and an inverse
first-order rate dependence on the concentration of
borane. These data agree with the proposed mechanism
in Scheme 4.
lower yields for formation of alkylboronate ester from
stoichiometric reactions of isolated 2 with styrenes in
the absence of borane. Perhaps the initial metal product
titanocene reacts with 2 in this case to form a complex
set of reactive intermediates and Ti(III) complexes.
Under these conditions, a high concentration of cat-
echolborane would lead to efficient trapping of ti-
tanocene and prohibit its interference in the reactions
of 2 with alkenes.
Or igin of Regioselectivity. Anti-Markovnikov prod-
ucts were the major isomers formed by this catalytic
hydroboration. Possible geometries of the alkene borane
intermediate are shown in Scheme 5. The R group of
structure A is positioned away from the Cp rings but
adjacent to the catecholate substituent. The perpen-
dicular alignment of the catecholate may cause steric
interactions between the R group and the catecholbo-
rane that are more severe than the interactions of R
with the Cp ligands. Perhaps more important, the
electron-rich metal-bound carbon in structure B is
stabilized by the electron-withdrawing aryl group. The
reduced regioselectivity with alkenes reported previ-
ously31 may reflect the decreased electronic preference
for structure B.
Con clu sion . Our mechanistic studies have revealed
the intermediacy of several borane complexes in cata-
lytic hydroboration processes involving titanocene. First,
bis(borane) complex 2 appears to be the active catalyst
for the hydroboration of alkenes originally catalyzed by
titanocene dimethyl. This catalyst dissociates borane to
generate a monoborane intermediate. This intermediate
coordinates alkene to give a complex that is likely to be
a resonance hybrid between an alkene borane complex
and a â-borylalkyl hydride. The hydroboration of alkynes
involves a similar complex that we have characterized
previously and that is best described as a resonance
hybrid between an alkyne borane complex and a â-bor-
ylvinyl hydride. The greater stability of the dicarbonyl
catalyst for alkyne hydroboration compared to that of
the bis(borane) catalyst for alkene hydroboration may
result from a more efficient trapping of titanocene by
CO rather than by the unusual and weakly bound
ligand catecholborane. The dual purpose of catechol-
borane as substrate and ligand provides the inverse
reaction order in borane, as is often seen in carbonyla-
tion chemistry when CO acts as a substrate and ligand.
The inverse order in borane was unexpected because
borane is a reagent for the catalytic process. However,
the isolation of 2 showed that borane was not only a
reagent but also a ligand on the resting state of the
catalyst. Borane behaves in this process like carbon
monoxide in catalytic carbonylations. In many carbo-
nylation processes, carbon monoxide acts as a ligand
and as a reagent. Thus, some reaction conditions and
catalysts provide a first-order dependence on CO con-
centration, while others show an inverse dependence of
rate on CO.46 Of course, this inverse dependence arises
because reversible CO dissociation creates a site for
alkene coordination. The same kinetic behavior occurs
with 2, except it is dissociation of the uncommon ligand
catecholborane that allows for alkene coordination.
P a th w a ys for Ca ta lyst Decom p osition . A require-
ment for a true catalyst is that decomposition occurs
by a pathway separate from the catalytic cycle. One
possibility for catalyst decomposition is â-hydrogen
elimination rather than reductive elimination from the
alkene borane intermediate to generate titanium hy-
drides. The dihydride product from â-hydrogen elimina-
tion may react with alkene to give alkylarene, thereby
regenerating Cp2Ti and ultimately 2. Titanocene hy-
dride complexes are also known to undergo dimerization
to form stable compounds.47 A second path for catalyst
decomposition is conversion of the titanocene intermedi-
ate to inactive complexes by reaction chemistry that
competes with coordination of borane to re-form 2.
We offer two possible beneficial effects of excess
borane. First, the reductive elimination step may be
induced by free borane, thereby making this step
favored over â-hydrogen elimination at high borane
concentrations. However, we previously studied the
extrusion of vinylboronate ester from alkyne borane
complex 6 and showed that this reaction was zero order
in added borane.48 Thus, borane-induced reductive
elimination from alkene borane complex 7 is unlikely.
Instead, efficient trapping of the titanocene by borane
to regenerate the catalyst resting state 2 may account
for the higher yields in the presence of free borane. We
favor this proposal, but one must also rationalize the
(46) Ikatchenko, I. In Comprehensive Organometallic Chemistry;
Wilkinson, G., Stone, F. G. A., Abel, E., Ed.; Pergamon: New York,
1982; Vol. 8, p 140.
(47) Brintzinger, H. H.; Bercaw, J . E. J . Am. Chem. Soc. 1970, 92,
6182.
(48) Muhoro, C. N.; He, X.; Hartwig, J . F. J . Am. Chem. Soc. 1999,
121, 5033-5046.
Exp er im en ta l Section
Gen er a l Con sid er a tion s. Unless otherwise noted, all
manipulations were conducted using standard Schlenk tech-

Titanocene-Catalyzed Hydroboration
Organometallics, Vol. 19, No. 1, 2000 37
niques or an inert-atmosphere glovebox. 1H NMR spectra were
obtained on a GE QE 300 MHz, GE Ω 300 MHz, or a GE Ω
500 MHz Fourier transform spectrometer and were recorded
relative to residual protiated solvent. 11B NMR spectra were
obtained on the GE QE 300 MHz spectrometer operating at
96.38 MHz. 11B NMR chemical shifts are reported in units of
parts per million downfield from BF3‚OEt2 as an external
standard. GC measurements were made on a Hewlett-Packard
5890 Series II gas chromatograph.
Rea ction of p-Meth oxystyr en e, HBca t′, a n d Stoich io-
m etr ic Am ou n ts of 2. A solution of p-methoxystyrene (1.3
µL, 0.0095 mmol) and HBcat′ (1.7 mg, 0.0095 mmol) in C6D6
(0.6 mL) was added to a vial containing 2 (5.0 mg, 0.0095
mmol). The resulting solution was analyzed by 1H NMR
spectroscopy after 10 min.
Rea ction of Dip h en yla cetylen e, HBca t′, a n d Stoich io-
m etr ic Am ou n ts of 1. A vial was charged with PhCCPh (4.50
mg, 0.0250 mmol), and the solid was dissolved in 0.2 mL of
toluene-d8. The solution was transferred to an NMR tube, and
the tube was sealed with a septum. The tube was then cooled
to -78 °C and was charged with a toluene-d8 solution (0.4 mL)
of HBcat′ (4.40 mg, 0.0250 mmol) and 1 (5.9 mg, 0.0250 mmol)
by syringe. The NMR tube was shaken without allowing it to
warm and was quickly inserted into the precooled probe (-30
°C) of the NMR spectrometer for analysis.
Unless otherwise specified, all reagents were purchased
from commercial suppliers and used without further purifica-
tion. Cp2Ti(PhCCPh)(CO) (5),44 Cp2Ti(PhCCPh),49 tolCCtol,50
4-tert-butylcatecholborane (HBcat′),48 Cp2Ti(HBcat′)2 (2),48 Cp2-
Ti(HBcat)2,48 Cp2Ti(H2Bcat′) (3),31 [(C10H8)Cp2Ti2H2] (4),42 and
51
Cp2TiMe2 were prepared by published procedures. All pro-
tiated solvents were dried over purple sodium benzophenone
ketyl and were obtained by distillation under nitrogen. The
deuterated solvents were dried as their protiated analogues,
but were obtained by vacuum distillation from the drying
agent.
Rea ction of 1 w ith HBca t′. A vial was charged with 1 (10
mg, 0.0427 mmol), and the solid was dissolved in 0.6 mL of
toluene-d8. HBcat′ was added (37.6 mL, 0.214 mmol), and the
mixture was transferred to an NMR tube. The sample was
1
cooled to -10 °C and analyzed by H NMR spectroscopy after
Rea ction s of Alk en es w ith HBca t′ Ca ta lyzed by 2-4.
An NMR tube was charged with 2 (2.2 mg, 0.0042 mmol), and
the tube was sealed with a septum. A separate vial was
charged with alkene (0.042 mmol), HBcat′ (7.3 mg, 0.042
mmol), and ferrocene internal standard in C7D8 (0.6 mL). An
initial NMR spectrum was obtained on this sample. This
solution was then added to the catalyst at room temperature
or at -78 °C for reactions employing catalysts 2-4 at -5 °C.
The sample was shaken to dissolve all the solid and kept at
room temperature for 10 min and at -5 °C for 5 h, at which
30 min.
Rea ction of 5 w ith HBca t′. A vial was charged with
alkyne carbonyl 5 (5.6 mg, 0.0140 mmol), and the solid was
dissolved in 0.6 mL of toluene-d8. The solution was transferred
to an NMR tube, and the tube was sealed with a septum. The
tube was then cooled to -78 °C, and HBcat′ (2.5 µL, 0.0140
mmol) was added to the NMR tube by syringe. The NMR tube
was shaken without allowing it to warm and was quickly
inserted into the precooled probe (-30 °C) of the NMR
spectrometer for analysis.
1
time it was analyzed by H NMR spectrometry. 4-MeOC4H4-
CH2CH2BO2C6H3-4-t-Bu: 1H NMR (C6D6) δ 1.16 (s, 9H), 1.47
(t, J ) 8.0 Hz, 2H), 2.85 (d, J ) 8.0 Hz, 2H), 3.28 (s, 3H), 6.75
(d, J ) 8.6 Hz, 2H), 6.89 (dd, J ) 8.4, 1.8 Hz, 1H), 7.03 (m,
3H), 7.25 (d, J ) 1.6 Hz, 1H); 11B NMR (C6D6) δ 35 (s).
2-Ethylindenyl-Bcat′: 1H NMR (C6D6) δ 1.17 (s, 9H), 2.10
(pentet, J ) 9.1 Hz, 1H), 3.05 (dd, J ) 15.5, 9.1 Hz, 2H), 3.17
(dd, J ) 15.5, 9.1, Hz, 2H), 6.88 (dd, J ) 8.3, 1.4, 1H), 6.98 (d,
J ) 8.3 Hz, 1H), 7.05-7.12 (m, 4H), 7.20 (d, J ) 1.4 Hz, 1H);
11B NMR (C6D6) δ 35.7 (s).
Rea ction of Alk yn es w ith HBca t′ Ca ta lyzed by 1. A vial
was charged with alkyne (0.0379 or 0.0910 mmol), either
HBcat′ (6.6 mg, 0.0375 mmol) or catecholborane (109 mg, 0.910
mmol), ferrocene internal standard, and 1 (2.0 mg, 0.0085
mmol or 8.5 mg, 0.036 mmol). The NMR tube was left at room
temperature for 2-3 h. The solution was then analyzed by 1H
NMR spectroscopy or gas chromatography. Identical proce-
dures were used for the reaction of 1-hexyne and HBcat
catalyzed by titanocene dimethyl. p-tolCHC(Bcat′)-p-tol: 1H
NMR (C6D6) δ 1.18 (s, 9H), 1.94 (s, 3H), 2.14 (s, 3H), 6.75 (d,
J ) 8.0 Hz, 2H), 6.89 (dd, J ) 8.3 Hz, 1.9 Hz, 1H), 7.02 (d, J
) 8.4 Hz, 1H), 7.07 (d, J ) 7.8 Hz, 2H), 7.20 (m, 3H), 7.32 (d,
J ) 7.8 Hz, 2H), 7.99 (s, 1H); 11B NMR (C6D6) δ 32; HRMS
(EI) calcd 382.2104, found 382.2099. PhCHC(Bcat′)Ph: 1H
NMR (C6D6) δ 1.18 (s, 9H), 6.8-7.22 (m, 11H), 7.32 (d, J )
7.3 Hz, 2H), 7.95 (s, 1H); 11B NMR (C6D6) δ 32.
Rea ction of 2 w ith 5. An NMR tube was charged with 2
(2.3 mg, 0.0057 mmol) and 5 (3.0 mg, 0.0057 mmol), and the
tube was sealed with a septum. The tube was then cooled to
-78 °C, and 0.6 mL toluene-d8 was added by syringe. The
NMR tube was shaken without allowing it to warm and was
quickly inserted into the precooled probe (-20 °C) of the NMR
spectrometer for analysis.
Rea ction of 6 w ith CO. A vial was charged with Cp2Ti-
(PhCCPh) (2.4 mg, 0.00674 mmol), and the solid was dissolved
in toluene-d8 (0.6 mL). The solution was transferred by syringe
into a medium-walled Young NMR tube. HBcat′ was added to
the tube (1.2 µL, 0.0067 mmol) by syringe onto the wall of the
NMR tube, with care taken to prevent mixing with the
solution. The tube was closed and quickly cooled to -190 °C.
The tube was evacuated at this temperature and was then
charged with 610 Torr of CO at -78 °C. The tube was sealed,
shaken without allowing it to warm, and quickly inserted into
the precooled probe (-30 °C) of the NMR spectrometer for
analysis. The experiment was repeated using 133 Torr of CO
added at -78 °C.
Kin etic Stu d ies on th e Ad d ition of Ca tech olbor a n es
to Alk yn es Ca ta lyzed by 1. Studies were performed using
neat HBcat′ and stock solutions of p-tolCC-p-tol (0.3636 M)
and Cp2Ti(CO)2 (0.214 M) in C6D6. Reaction rates were
measured by 1H NMR spectroscopy in the spectrometer probe
using single-pulse experiments performed every 90 s, using
an automated program for at least 3 half-lives. A vial was
charged with HBcat (20.0 µL, 0.188 mmol) and aliquots of the
stock solutions of tolCCtol (103 µL, 0.0375 mmol) and Cp2Ti-
(CO)2 (1; 17.6 µL, 0.00375 mmol). Benzene-d6 (0.46 mL) was
added to the vial, and the mixture was transferred by pipet
into an NMR tube. The solution was degassed, the tube was
charged with 200 Torr of CO pressure at room temperature,
and the tube was flame-sealed. The NMR tube was shaken
and inserted into the probe of the NMR spectrometer for
analysis. Rate measurements were performed at 40 °C by
measuring the integral of the p-tolyl resonance of the alkyne.
The experiment was repeated using 110, 132, 200, 300, and
600 Torr of CO pressure to determine the order in CO. To
determine the order in borane, the experiment was repeated
R ea ct ion of 2 w it h p -Met h oxyst yr en e. An NMR tube
was charged with 2 (5.0 mg, 0.0095 mmol), and the tube was
sealed with a septum. The tube was cooled to -78 °C, and a
solution of methoxystyrene (3.8 µL, 0.029 mmol) in C7D8 (0.6
mL) was added by syringe. The sample was shaken to dissolve
all the solid without allowing it to warm. The sample was
maintained at -30 °C for 4 h and then quickly immersed into
the precooled probe (-30 °C) of the NMR spectrometer for
analysis.
(49) Shur, V. B.; Burlakov, V. V.; Vol’pin, M. E. J . Organomet. Chem.
1988, 347, 77.
(50) Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron Lett.
1975, 50, 4467.
(51) Erskine, G. H.; Wilson, D. A.; McCowan, J . D. J . Organomet.
Chem. 1976, 114, 119.

38 Organometallics, Vol. 19, No. 1, 2000
Hartwig and Muhoro
using 40.0, 60.0, and 80.0 µL of added HBcat. Alternatively, a
vial was charged with PhCCPh (40.0 mg, 0.225 mmol),
HBO2C6H3-4-Me (5.01 mg, 0.0375 mmol), and Cp2Ti(CO)2 (1;
17.6 µL, 0.003 75 mmol). Benzene-d6 (0.6 mL) was added to
the vial, and the mixture was transferred by pipet into an
NMR tube. The solution was degassed, and the tube was
charged with 436 Torr of CO pressure at -78 °C and flame-
sealed. The NMR tube was shaken and inserted into the probe
of the NMR spectrometer for analysis. Rate measurements
were performed by measuring the integrals of the methyl
resonance of the borane. The NMR tube was shaken and
inserted into the probe of the NMR spectrometer for analysis
at 40 °C. Rate measurements were performed by measuring
the integral of the p-tolyl peak of the alkyne.
P r ofile of th e Rea ction betw een 5 a n d 3 Equ iv of
HBca t′. A vial was charged with 5 (5.6 mg, 0.014 mmol), and
the solid was dissolved in toluene-d8 (0.6 mL). The solution
was transferred into an NMR tube, and the tube was sealed
with a septum. The tube was then cooled to -78 °C, and HBcat′
(7.5 µL, 0.042 mmol) was added to the NMR tube by syringe.
The NMR tube was shaken without allowing it to warm and
was quickly inserted into the precooled probe (-20 °C) of the
NMR spectrometer for analysis. Single-pulse experiments were
performed every 60 s using an automated program. Concen-
trations were determined from the integrals of the Cp peaks
of 1, 2, and 5, as well as the vinyl peak of (Ph)(H)CdC(Bcat′)-
(Ph).
Cp 2Ti(ca tBCHdCHP h ). A vial containing Cp2Ti(HBcat)2
(800 mg, 1.91 mmol) was charged with a solution of pheny-
lacetylene (390 mg, 3.82 mmol) in 15 mL of toluene, and the
mixture was stirred at room temperature. After 2 h, the solvent
was evaporated under vacuum, leaving a dark brown paste.
This material was rinsed with small quantities of pentane to
remove unreacted alkyne. The residual dark brown solid was
dissolved in ether, and the resulting solution was stored at
-30 °C for 3 days. The dark red-brown product precipitated,
and the supernatant was removed by pipet. Analytically pure
red-brown solid (101 mg, 13% yield) was obtained by recrys-
tallization from ether. 1H NMR (C6D6): δ 7.11 (m, 5H), 6.86
(m, 4H), 6.35 (s, 5H), 6.03 (s, 5H), 5.65 (d, J ) 14.5 Hz, 1H),
4.16 (d, J ) 14.0 Hz, 1H). 13C NMR (C6D6): δ 150.87, 148.87,
127.56, 127.26, 123.08, 123.00, 121.85, 119.05 (Cp), 117.93
(Cp), 111.54. 11B NMR (C6D6): δ 33.4. Anal. Calcd for C24H21
-
BO2Ti: C, 72.07; H, 5.26. Found: C, 71.84; H, 5.58.
Cp 2TiAr (H)CdC(H)Bca t′ (Ar ) p-OMe-C6H4). A vial was
charged with 2 (39 mg, 0.074 mmol) and cooled to -30 °C. A
cold solution of p-methoxystyrene (40 mg, 0.030 mmol) in 10
mL of toluene was added to 2, and the resulting clear burgundy
solution was kept at -30 °C for 1.5 h. The solution was
warmed to room temperature and was concentrated by re-
moval of solvent under reduced pressure. The resulting
concentrated solution was layered with pentane and kept at
-30 °C for 8 h. A red solid precipitated and was collected by
removing the supernatant by pipet. The solid was washed
twice with cold pentane and dried under reduced pressure
1
(53%, 19 mg). H NMR (C7D8): δ 6.70 (m, 4 H), 6.42 (s, 5H),
6.12 (s, 5H), 5.60 (d, J ) 4.1 Hz, 1H), 4.09 (d, J ) 4.1 Hz, 1H),
3.44 (s, 3H), 1.26 (s, 9H).
Kin etic Stu d ies of Alk en e Hyd r obor a tion s Ca ta lyzed
by 2. Reaction rates were measured by NMR spectroscopy
using single-pulse experiments performed every 60 s for 1000
s using an automated program. Rates were determined by
integrating the vinyl peaks of p-methoxystyrene. To determine
the reaction order in alkene, an NMR tube was charged with
2 (2.2 mg, 0.0042 mmol), and the tube was sealed with a
septum. The sample was cooled to -78 °C, and a solution of
p-methoxystyrene (5.5 µL, 0.042 mmol) and HBcat′ (37 mg,
0.21 mmol) in 0.6 mL of C6D6 was added to the NMR tube by
syringe. The tube was shaken to dissolve all the solid without
allowing it to warm and was then quickly immersed into the
precooled probe (-20 °C) of the NMR spectrometer for analysis.
The experiment was repeated with 0.21 mmol (5 equiv) of
added HBcat′ and 0.010, 0.021, 0.063, and 0.083 mmol of added
p-methoxystyrene. To determine the reaction order in cat-
echolborane, reactions were performed with p-methoxystyrene
(5.5 µL, 0.042 mmol), 2 (2.2 mg, 0.0042 mmol), and 0.13, 0.27,
0.42, 0.63, and 0.83 mmol of added HBcat′.
Ack n ow led gm en t. We thank the NSF (Grant No.
CHE-9617171) for support of this work.
OM990507M