Mechanistic investigation of stoichiometric alkyne insertion into Pt-B bonds and related chemistry bearing on the catalytic diborylation of alkynes mediated by platinum(II) diboryl complexes

Article abstract of DOI:10.1021/om960123l

The insertion reactivity of alkynes with the diboryl complex (Ph₃P)₂Pt(BCat)₂ (1, Cat = (C₆H₄O₂}^2-) has been investigated. Under stoichiometric conditions 1 mediates cisdiborylat

Full text of DOI:10.1021/om960123l

Organometallics 1996, 15, 5155-5165
5155
Mech a n istic In vestiga tion of Stoich iom etr ic Alk yn e
In ser tion in to P t-B Bon d s a n d Rela ted Ch em istr y
Bea r in g on th e Ca ta lytic Dibor yla tion of Alk yn es
Med ia ted by P la tin u m (II) Dibor yl Com p lexes†
Carl N. Iverson and Milton R. Smith, III*
Department of Chemistry, Michigan State University, East Lansing, Michigan 48824
Received February 20, 1996X
The insertion reactivity of alkynes with the diboryl complex (Ph3P)2Pt(BCat)2 (1, Cat ≡
{C6H4O2}2-) has been investigated. Under stoichiometric conditions 1 mediates cis-
diborylation of alkynes and the (PPh3)2Pt fragment is trapped by alkyne to give the
corresponding Pt-alkyne complexes. Kinetic studies under pseudo first-order conditions of
alkyne indicate that the reaction is first order in 1. In the absence of added phosphine, no
alkyne dependence is observed. The stoichiometric reaction is inhibited by phosphine
addition, and under these conditions, a first-order dependence on alkyne concentration is
observed for the disappearance of 1. The stoichiometric results exclude simple, bimolecular
insertion of an alkyne into Pt-B bonds of 1, and the observed dependence on phosphine
and alkyne strongly favors a mechanism where phosphine dissociation generates a three-
coordinate intermediate that mediates alkyne insertion. Activation parameters for the
stoichiometric alkyne insertion were derived from the temperature dependence of kobs (70-
110 °C). An Eyring plot yielded the following: ∆Hq ) 25.9(7) kcal/mol and ∆Sq ) 4(2) eu.
The rates of alkyne diborylation are also sensitive to the nature of the alkyne as 4-octyne
reacts much more readily than diphenylacetylene. For para-substituted diarylacetylenes,
the rate for the bis(p-trifluoromethyl) derivative is accelerated and the rate for the bis(p-
methoxy) derivative is retarded relative to diphenylacetylene. The reactivity of the related
diboryl complex, (PPh3)2Pt(BPin)2 (9, Pin ≡ {(CH3)2CO-CO(CH3)2}2-), is much more complex
as reductive elimination of PinB-BPin is observed before the onset of the diborylation
reaction. This appears to be a general feature for this compound as elimination is promoted
by various reagents (e.g., CO, PPh3, Me3Sn-SnMe3, and CatB-BCat). The catalytic
diborylation of alkynes mediated by 1 (in the presence of added triphenylphosphine) was
investigated. Kinetics experiments revealed many similarities to the stoichiometric reaction
as an inverse dependence on [PPh3] and first-order dependence on [alkyne] and [1] were
observed. Expressions that directly relate the catalytic and stoichiometric observed rate
constants were derived, and the measured values for these two systems were identical within
experimental error. Thus, the data are consistent with a catalytic manifold that is identical
to that observed in the stoichiometric reaction. Under catalytic conditions, the rate of alkyne
diborylation exhibited no dependence on [CatB-BCat].
In tr od u ction
however, lanthanide11 and early transition metal
systems12-16 offer some interesting possibilities with
regard to controlling chemo-, stereo-, and regioselectiv-
Transition metal complexes have shown considerable
promise in promoting borylation reactions of unsatur-
ated substrates. Hydroboration is one of the few reduc-
tive transformations where chemical functionality is
retained, as the B-C bonds that result from borylation
can be converted to a wide variety of C-E functional
groups with retention of regio- and stereochemistry at
carbon.1 Due in large part to the general utility of the
hydroboration reaction, transition-metal-catalyzed
borylation chemistry has emerged in the past decade.2
Catalysis by late transition metals is most prevalent;3-10
(3) For a recent review see: Burgess, K.; Ohlmeyer, M. J . Chem.
Rev. 1991, 91, 1179-1191.
(4) An excellent review of Pd-catalyzed cross-couplings of boronic
acids has recently appeared: Miyaura, N.; Suzuki, A. Chem. Rev. 1995,
95, 2457-2483.
(5) Evans, D. A.; Fu, G. C.; Hoveyda, A. H. J . Am. Chem. Soc. 1992,
114, 6671-6679.
(6) Evans, D. A.; Fu, G. C.; Anderson, B. A. J . Am. Chem. Soc. 1992,
114, 6679-6685.
(7) Burgess, K.; van der Donk, W. A.; Kook, A. M. J . Org. Chem.
1991, 56, 2949-2951.
(8) 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.
† A portion of this work was presented at the 210th National Meeting
of the American Chemical Society, Chicago, IL, Aug 1995; Abstract
INOR 690.
(9) Westcott, S. A.; Taylor, N. J .; Marder, T. B.; Baker, R. T.; J ones,
N. L.; Calabrese, J . C. J . Chem. Soc., Chem. Commun. 1991, 304-
305.
X Abstract published in Advance ACS Abstracts, November 1, 1996.
(1) Brown, H. C.; Singaram, B. Pure Appl. Chem. 1987, 59, 879-
894.
(10) Westcott, S. A.; Blom, H. P.; Marder, T. B.; Baker, R. T. J . Am.
Chem. Soc. 1992, 114, 8863-8869.
(11) Harrison, K. M.; Marks., T. J . J . Am. Chem. Soc. 1992, 114,
9220-9221.
(2) For the first report of transition-metal-catalyzed hydroboration
see: Manning, D.; No¨th, H. Angew. Chem., Int. Ed. Engl. 1985, 24,
878-879.
(12) Motry, D. H.; Smith, M. R. I. J . Am. Chem. Soc. 1995, 117,
6615-6616.
S0276-7333(96)00123-9 CCC: $12.00 © 1996 American Chemical Society

5156 Organometallics, Vol. 15, No. 24, 1996
Iverson and Smith
Sch em e 1
ity. While the modes of interaction in early metal
systems are diverse, metal boryl complexes are the
principal actors in late metal systems.
Although diboron tetrahalides react with various
unsaturated organic substrates,17 their difficult prepa-
ration dramatically limits their synthetic utility. When
the boron-boron bond is supported by amide or alkoxide
linkages, the stability of the B-B species is dramatically
enhanced at the expense of reactivity. Simple molecular
orbital arguments suggest topological similarities be-
tween diboron tetrahalides, and their derivatives, with
carbon based fragments, as well as dihydrogen. While
comparisons to olefinic fragments are experimentally
supported by the observation that the B-B bond
distance decreases when Ar2B-BAr2 derivatives are
reduced by two electrons,18,19 the comparison to H2 is
more tenuous. Nonetheless, preliminary reports indi-
cate that B-B bonds oxidatively add to metal centers
with B-B bond cleavage.20,21 Thus, catalyzed transfer
of B-B bonds to unsaturated organic substrates might
be expected to parallel metal-mediated reactivity of H2
and other E-X transfer reactions. Recent reports are
consistent with this notion as metal mediated addition
of B-B bonds to CtC,22,23 CdC,24,25 and M-C bonds
have appeared.21,26
Sch em e 2
Detailed mechanistic information regarding reactivity
of M-B bonds is sparse with the most significant work
involving B-H activation related to metal-mediated
hydroboration.27,28 Most of these investigations have
focused on reactivity of Wilkinson’s catalyst where the
manifold of accessible intermediates is complex.6,8 La-
beling studies have provided useful information par-
ticularly with regard to the reaction pathways favored
in this system.6 Examples where insertions into metal
boron bonds proceed cleanly are rare, and factors which
determine product selectivities are not well under-
stood.29-31
We recently reported oxidative and metathetic reac-
tivity of simple B-B bonds with olefin, alkyne, and
organometallacyclic bis(triphenylphosphine)platinum
complexes.21 In each case CatB-BCat reacted smoothly
to generate a common diboryl complex, (Ph3P)2Pt(BCat)2
(1, Cat ≡ {C6H4O2}2-), that has been implicated as the
active catalyst in the catalytic diborylation of alkynes
mediated by Pt(PPh3)4. Preliminary experiments showed
that the pure diboryl complex catalyzed addition of
CatB-BCat to alkynes, consistent with the earlier
supposition by Miyaura and Suzuki.22 Under catalytic
conditions, 1 was the sole platinum containing species
observed in solution. Since the reactions in Scheme 1
proceeded cleanly and quantitatively as judged by NMR
spectroscopy, this system seemed ideally suited for
addressing fundamental issues of B-C bond formation
in this system. This paper addresses the mechanism
for alkyne insertion into Pt-B bonds in bis(phosphine)-
platinum systems.
(13) Erker, G.; Noe, R.; Wingbermuhle, D.; Petersen, J . L. Angew.
Chem., Int. Ed. Engl. 1993, 32, 1213-1215.
(14) Deloux, L.; Skrzypczak-J ankun, E.; Cheesman, B. V.; Srebnik,
M.; Sabat, M. J . Am. Chem. Soc. 1994, 116, 10302-10303.
(15) Pereira, S.; Srebnik, M. Organometallics 1995, 14, 3127-3128.
(16) He, X.; Hartwig, J . F. J . Am. Chem. Soc. 1996, 118, 1696-1702.
(17) For an excellent review see: Massey, A. G. Adv. Inorg. Chem.
Radiochem. 1983, 26, 1-54.
(18) Moezzi, A.; Olmstead, M. M.; Power, P. P. J . Am. Chem. Soc.
1992, 114, 2715-2717.
(19) Moezzi, A.; Olmstead, M. M.; Power, P. P. J . Chem. Soc., Dalton
Trans. 1992, 2429-2434.
(20) Nguyen, P.; Lesley, G.; Taylor, N. J .; Marder, T. B.; Pickett, N.
L.; Clegg, W.; Elsegood, M. R. J .; Norman, N. C. Inorg. Chem. 1994,
33, 4623-4624.
(21) Iverson, C. N.; Smith, M. R. I. J . Am. Chem. Soc. 1995, 117,
4403-4404.
(22) Ishiyama, T.; Matsuda, N.; Miyaura, N.; Suzuki, A. J . Am.
Chem. Soc. 1993, 115, 11018-11019.
(23) Ishiyama, T.; Matsuda, N.; Murata, M.; Ozawa, F.; Suzuki, A.;
Miyaura, N. Organometallics 1996, 15, 713-720.
(24) Baker, R. T.; Nguyen, P.; Marder, T. B.; Westcott, S. A. Angew.
Chem., Int. Ed. Engl. 1995, 34, 1336-7.
(25) In ref 21 borylation of norbornene ligands in Pt(NBE)3 by
CatB-BCat was reported.
(26) Related B-C bond forming reactions of metal boryls have been
reported: Waltz, K. M.; He, X.; Muhoro, C.; Hartwig, J . F. J . Am. Chem.
Soc. 1995, 117, 11357-11358.
(27) For the first report of alkene insertion into a metal-boron
linkage see: Baker, R. T.; Calabrese, J . C.; Westcott, S. A.; Nguyen,
P.; Marder, T. B. J . Am. Chem. Soc. 1993, 115, 4367-8.
(28) Cleary, B. P.; Eisenberg, R. Organometallics 1995, 14, 4525-
4534.
(29) Musaev, D. G.; Mebal, A. M.; Morokuma, K. J . Am. Chem. Soc.
1994, 10693-10792.
(30) Dorigo, A. E.; Schleyer, P. v. R. Angew. Chem., Int. Ed. Engl.
1995, 34, 115-117.
Resu lts a n d Discu ssion
Stoich iom etr ic Alk yn e Dibor yla tion . A mecha-
nism has been proposed for the diborylation of alkynes
(31) Calculations have yielded disparate results regarding olefin
insertion into the Rh-B29 vs Rh-H30 bonds in the boryl-hydride
complex derived from oxidative addition of catecholborane to Wilkin-
son’s catalyst. Alkyne insertion into the Ir-H bond of an iridium
hydridoboryl complex: Knorr, J . R.; Merola, J . S. Organometallics
1990, 9, 3008-3009.

Stoichiometric Alkyne Insertion into Pt-B Bonds
Organometallics, Vol. 15, No. 24, 1996 5157
Sch em e 3
alkyne insertion is presumed to be rate limiting. Both
rate laws require a first-order dependence in [1] and
[RCtCR], and although these two pathways are mecha-
nistically distinct, they are indistinguishable if inter-
mediate 2 cannot be detected.
When the reaction in eq 1 was monitored under
pseudo-first-order conditions with regard to alkyne
concentration, plots of ln [1] vs time were linear over 3
half-lives. This suggested that the reaction in eq 1 is
first order in [1]. Although the limited solubility of [1]
prevented determination over a broad concentration
range, kobs appears to be invariant with respect to [1].
Significantly, kobs exhibited no alkyne dependence over
a wide range of alkyne concentration. Thus, mecha-
nisms corresponding to the rate laws in eqs 2 and 3 can
be excluded as the observed kinetics are consistent with
the rate law in eq 4.
k1k2
-d[1]
dt
)
[1][RCtCR]
(2)
(k-1 + k2)
-d[1]
dt
) k3[1][RCtCR]
(3)
(4)
-d[1]
) kobs[1]
dt
The reactivity of square-planar, d8 systems has re-
ceived a great deal of attention in the chemical litera-
ture, and mechanistic studies have revealed that alkylbis-
(phosphine)M(II) systems (M ) Pd, Pt) often react by
dissociation of phosphine to give three-coordinate reac-
tive intermediates.35-39 Although boryl ligands are
topologically distinct from alkyl or aryl counterparts,40
the exclusion of the mechanisms in Scheme 3 suggested
dissociative pathways involving three-coordinate inter-
mediates as possible alternatives. To test these pos-
sibilities the reaction in eq 1 was monitored at various
phosphine concentrations. In the stoichiometric reac-
tion, phosphine addition clearly suppressed the reaction
rate, and plots of 1/kobs vs [PPh3] were linear (Figure
1). Two mechanisms that involve dissociative first steps
are shown in Schemes 4 and 5. In Scheme 4 phosphine
dissociation is presumed to be rate-limiting, while the
subsequent coordination and insertion steps are rapid.
In Scheme 5, alkyne insertion is presumed to be the
rate-limiting step, whereas 1, 4, and 5 are in rapid
equilibrium. Alkenylborane elimination is assumed to
be irreversible in both instances.41 Although we have
not been able to probe reversibility of alkyne coordina-
tion or insertion, crossover experiments do show that
in the presence of (PPh3)4Pt.22,23 In this mechanism,
shown in Scheme 2, a species related to 1 is proposed
to be the active catalyst, and the initial B-C forming
reaction is insertion of the alkyne into the Pt-B bond
of cis-(PPh3)2Pt(BPin)2 (Pin
≡
{(CH3)2CO-CO-
(CH3)2}2-).32,33 Reductive elimination from an alkenyl-
boryl species generates “(PPh3)2Pt” and the diborylated
alkene product, and oxidative addition of PinB-BPin
to “(PPh3)2Pt” completes the catalytic cycle.
While NMR data indicate that cis-(PPh3)2Pt(BPin)2
is present under reaction conditions, no experimental
evidence has been offered to support or exclude steps
in the postulated mechanism. Certainly, a deeper
understanding of the steps preceding B-C bond forma-
tion could prove useful in designing systems that
diborylate other unsaturated substrates. Since direct
study of the catalytic reaction is potentially more
complex, we attempted to unravel the mechanism by
applying stoichiometric constraints to diborylation me-
diated by 1 (eq 1).
(PPh3)2Pt(BCat)2 + 2RCtCR ssf
(PPh3)2Pt(η2-RCtCR) + (R)(BCat)CdC(R)(BCat)
(35) Phosphine dissociation in Pd(II) alkyl comlexes precedes alkyne
insertion into Pd-C bonds: Samsel, E. G.; Norton, J . R. J . Am. Chem.
Soc. 1984, 106, 5505-5512.
(1)
(36) Calculations predict that barriers for reductive elimination are
lower for three-coordinate, 14 electron dialkyl complexes as opposed
to square-planar 16 electron species. This has been confirmed
experimentally.: Kominya, S.; Albright, T. A.; Hoffmann, R.; Kochi,
J . K. J . Am. Chem. Soc. 1977, 99, 8440.
In the absence of added diboron reagent, a clean
reaction is observed when excess alkyne is present to
trap “(PPh3)2Pt” as (PPh3)2Pt(η2-alkyne).34 Two distinct
pathways that involve bimolecular reactions between
1 and alkyne are shown in Scheme 3. In both cases,
(37) McDermott, J . X.; White, J . F.; Whitesides, G. M. J . Am. Chem.
Soc. 1976, 98, 6521-6536.
(38) Reamey, R. H.; Whitesides, G. M. J . Am. Chem. Soc. 1984, 106,
81-85.
(32) Associative pathways for alkyne insertion and alkene exchange
have been proposed for Ni(II) and Pd(II) square-planar complexes:
Gridnev, I. D.; Miyaura, N.; Suzuki, A. Organometallics 1993, 12, 589-
592.
(39) Gillie, A.; Stille, J . K. J . Am. Chem. Soc. 1980, 102, 4933-4941.
(40) Boryl ligands are isolobal to carbene radical cations.
(41) Experimental and theoretical work suggest that reductive
(33) The evidence for associative pathways for alkene exchange is
convincing: J ohnson, L. K.; Killian, C. M.; Brookhart, M. J . Am. Chem.
Soc. 1995, 117, 6414-6415.
elimination from a three-coordinate intermediate, such as 6, is
reasonable. However, reductive elimination from square-planar Pt(II)
complexes has been observed: Sen, A.; Halpern, J . Inorg. Chem. 1980,
19, 1073-1075.
(34) Wittig, G.; Fischer, S. Chem. Ber. 1972, 105, 3542-3552.

5158 Organometallics, Vol. 15, No. 24, 1996
Iverson and Smith
Sch em e 4
Sch em e 5
Simplified rate expressions can be derived by applying
steady-state or equilibrium approximations43 to Schemes
4 and 5, respectively.44 Application of the steady-state
approximation to Scheme 4 gives the rate law and kobs
expression shown in eqs 5 and 6, while the equilibrium
treatment for Scheme 5 gives the expressions in eqs 7
and 8.
d[1]
dt
-
) kobs[1]
(5)
(6)
k4k5k6[RCtCR]
kobs
)
k-4(k-5 + k6)[PPh3] + k5k6[RCtCR]
d[1 + 4 + 5]
) kobs[1 + 4 + 5]
dt
(7)
(8)
-k6K1K2[RCtCR]
kobs
)
K1 + [PPh3] + K1K2[RCtCR]
An inverse dependence on phosphine concentration
is consistent with either mechanism, as eqs 6 and 8
reduce to eqs 9 and 10 when phosphine terms dominate
the denominator. When [PPh3] ≈ 0, eq 6 reduces to kobs
) k4, while eq 8 yields eq 11. In principle, the two
mechanisms are distinguishable in the absence of phos-
F igu r e 1. Plot of 1/kobs vs PPh3 concentration for the
reaction of 1 with 4-octyne at 90 °C in C6D6 ([1]0 ) 8.7 mM,
[4-octyne]0 ) 52 mM).
(42) 4-Octyne was diborylated under catalytic conditions. Once the
reaction was complete, 3-hexyne was added to the reaction mixture
and the solution was heated for 5 h at 80 °C. Diborylation of hexyne
was not observed.
(43) Pyun, C. W. J . Chem. Educ. 1971, 48, 194-196.
(44) See the Experimental Section for details regarding rate law
derivation.
elimination of the diborylated alkene is irreversible.42
Obviously, the failure to observe intermediates has no
bearing on the choice of mechanism, since 1 can be
thermodynamically favored to the extent that 4 and 5
are not detectable.

Stoichiometric Alkyne Insertion into Pt-B Bonds
Organometallics, Vol. 15, No. 24, 1996 5159
F igu r e 3. Eyring plot for diborylation of 4-octyne by 1 in
the presence of PPh3-d15 (C6D6, T ) 343-383 K, ∆Hq
)
25.9(7) kcal/mol, ∆Sq ) 4(2) eu, [1]0 ) 8.7 mM, [4-octyne]0
) 52 mM, [PPh3-d15] ) 8.7 mM).
F igu r e 2. Plot of ln{[1]t/[1]0} vs time for the reaction of 1
with 4-octyne in the presence of PPh3-d15 ([1]0 ) 8.7 mM,
[PPh3-d15] ) 70 mM) at 105 °C in C6D6 (O, [4-octyne]0 )
27 mM], kobs ) 9.8(2) × 10-3 s-1: [, [4-octyne]0 ) 52 mM,
as the rate-limiting step;45 nonetheless, rate-limiting
dissociation is not categorically excluded by a small ∆Sq
value.46 At first glance, a negative value for ∆Sq might
be expected in a pathway involving rate-limiting alkyne
insertion into the Pt-B bond; however, investigations
of CtC and CdC insertions into M-X bonds indicate
that the magnitude of ∆Sq can be small, and its sign
can be either negative or positive.35,47
In the absence of phosphine, kobs reduces to the rate
constant for the rate-determining step in the proposed
mechanisms. If phosphine dissociation is truly rate
limiting, borylation rates for different alkynes should
be invariant. Conversely, if alkyne insertion is rate
limiting, then it might be expected that variations in
the steric or electronic requirements of the alkyne could
kobs ) 18.9(3) × 10-3 s-1; +, [4-octyne]0 ) 105 mM, kobs
)
40(2) × 10-3 s-1; ×, [4-octyne]0 ) 208 mM, kobs ) 82(2) ×
10-3 s-1); +, [4-octyne]0 ) 260 mM, kobs ) 98(4) × 10-3 s-1).
phine if K1 . K1K2[RCtCR]. In this regime the
k4k5k6[RCtCR]
kobs
)
(9)
(10)
(11)
k-4(k-5 + k6)[PPh3]
-k6K1K2[RCtCR]
kobs
)
[PPh3]
effect kobs
. We attempted to compare rates for the
-k6K1K2[RCtCR]
K1 + K1K2[RCtCR]
stoichiometric reactions between 1 and 1-hexyne, phen-
ylacetylene, and diphenylacetylene. For 1-hexyne and
phenylacetylene, 31P-NMR indicated that several metal-
containing products form which unfortunately prevents
comparisons between internal and external alkynes.
Since the acetylene proton seemed the likely culprit, the
reaction between 1 and diphenylacetylene was exam-
ined. In this case borylation proceeded smoothly,
though at a severely retarded rate as compared to
dialkyl-substituted alkynes. Thus, kobs is effected by the
nature of the alkyne, which would not be expected if
the phosphine dissociation were slower than coordina-
tion and insertion steps.48
kobs
)
mechanism in Scheme 5 requires an alkyne dependence.
We do not see an alkyne dependence at any concentra-
tion of alkyne. Thus if the mechanism in Scheme 5
operates, K1 , K1K2[RCtCR] and kobs ≈ k6. Both eqs
9 and 10 predict that an alkyne dependence should be
detectable for either mechanism at sufficiently high
phosphine concentrations (k-4[PPh3] . (k5k6/k5 + k6)-
[RCtCR] or [PPh3] . K1 + K1K2[RCtCR]). This is
indeed the case, as kobs exhibits a first-order dependence
on [RCtCR] at modest phosphine concentrations (Fig-
ure 2). Thus, the kinetic data are consistent with either
proposal.
The sensitivity of borylation to the electronic proper-
ties of the alkyne substrate was probed by comparing
the rates of borylation for 4,4′-substituted diarylacety-
In the absence of phosphine, reaction rates were too
rapid for deriving reliable activation parameters from
the temperature dependence of kobs. This problem was
circumvented by suppressing the reaction rate by ad-
dition of phosphine, and kobs was measured from 70 to
110 °C. The Eyring plot in Figure 3 yielded the
following activation parameters: ∆Hq ) 25.9(7) kcal/
mol and ∆Sq ) 4(2) eu. The magnitude of ∆Sq seems to
disfavor a mechanism involving phosphine dissociation
(45) McCarthy, T. J .; Nuzzo, R. G.; Whitesides, G. M. J . Am. Chem.
Soc. 1981, 103, 3396.
(46) Wilkins, R. G. Kinetics and Mechanisms of Transition Metal
Complexes; Wilkins, R. G., Ed.; VCH: Weiheim, Germany, 1991, p 109.
(47) Burger, B. J .; Santarsiero, B. D.; Trimmer, M. S.; Bercaw, J .
E. J . Am. Chem. Soc. 1988, 110, 3134-3146.
(48) It could be argued that disparities in the equilibrium constants
for trapping of 4 with alkyne could account for the discrepancy in rates.
We believe that this is unlikely since we observe preferential binding
of diphenylacetylene vs 4-octyne to the “(PPh3)2Pt” fragment.

5160 Organometallics, Vol. 15, No. 24, 1996
Iverson and Smith
Ta ble 1. Rela tive Ra tes for Stoich iom etr ic
Bor yla tion s of Alk yn es by 1
Sch em e 6
alkyne
kobs(s-1
)
octyne
diphenylacetylene
bis(p-anisyl)acetylene
bis[p-(trifluoromethyl)phenyl]acetylene
0.0137(1)
0.0064(2)
0.0047(1)
fast
lenes. Borylation of 1,2-di-p-anisylacetylene proceeded
at a slower rate as compared to diphenylacetylene (70
°C). Unfortunately, borylation of 1,2-bis[p-(trifluoro-
methyl)phenyl]acetylene was too rapid for rigorous
kinetic analysis,49 and other substituted diarylacety-
lenes did not yield tractable data. The rapid reaction
rate for 1,2-bis[p-(trifluoromethyl)phenyl]acetylene is
puzzling as an approximate doubling of the reaction rate
was expected from linear free-energy relationships and
tabulated σpara values.50 In this case, rates for substi-
tuted diphenylacetylenes appear to offer little informa-
tion regarding the borylation mechanism; however, the
qualitative observations for substituted diarylacetylenes
show that gross borylation rates are sensitive to elec-
tronic perturbations in the alkyne substrate. The rate
data for borylation of alkyne substrates in this work are
included in Table 1.
From this stoichiometric investigation emerges an
obvious feature concerning reactivity in this system.
Specifically, if 1 serves as the active catalyst under
catalytic conditions, catalysis by 1 should be more
efficient than catalysis mediated by the precatalyst,
(PPh3)4Pt, since conversion to 1 liberates PPh3. This is
indeed the case, as alkynes which were not effectively
diborylated by catalysis originating from (PPh3)4Pt
undergo smooth diborylation when pure 1 is used as the
catalyst.
While the mechanism for alkyne diborylation pro-
posed in this work differs from that suggested by
Miyaura and Suzuki,22 our results argue that the
original assertion for the diboryl complex, cis-(PPh3)2-
Pt(BPin)2 (9), being the active catalyst was correct.22
Since cis-(PPh3)2Pt(BPin)2 can be readily prepared by
the route analogous to that for 1, comparison of the
stoichiometric borylation reaction should be straight-
forward. However, in contrast to the catecholate boryl
chemistry, spectra recorded under ambient conditions
prior to initiation of the diborylation reaction indicated
chemical equilibrium between 9, (PPh3)2Pt(η2-4-octyne)
(8), 4-octyne, and PinB-BPin (eq 12). To test for
confirmed the reversibility of the elimination. Reductive
elimination of PinB-BPin is observed when 9 is reacted
with PPh3, Me3Sn-SnMe3, CO, and CatB-BCat (Scheme
6).51 In contrast, elimination of CatB-BCat from 1 is
observed only for CO addition. Other qualitative ob-
servations suggest that 9 is intrinsically less stable than
1. For example, when 9 is dissolved in chloroform-d1
under ambient conditions, cis-(PPh3)2PtCl2 forms readily
while 1 exhibits moderate stability under these condi-
tions. Qualitative disparities between rates for oxida-
tive addition of CatB-BCat and PinB-BPin to Wilkin-
son’s catalyst have been reported;20 however, the
conversion of 9 to 1 obviously has thermodynamic
origins. Although we are not yet certain that Pt-B
bond cleavage occurs in the reaction between 9 and
CatB-BCat, this observation clearly indicates that
formation of 1 + PinB-BPin is thermodynamically
favored over 9 + CatB-BCat. It is tempting to argue
that the equilibrium is driven by formation of stronger
Pt-B bonds in 1; however, a stronger B-B bond in
PinB-BPin relative to CatB-BCat would also be con-
sistent with the experimental observations.52 The ease
with which the elimination reactions proceed is in
marked contrast to reductive eliminations of alkanes
from cis-dialkylplatinum bis(phosphine) complexes.37-39
The observed equilibria between 9, PPh3, and Pin-
B-BPin certainly complicates comparisons between the
relative rates of borylation for catecholate and pinaco-
late (RO)2B-B(OR)2 species under “stoichiometric” con-
ditions. Since the diborylation reaction precludes evalu-
ation of the equilibrium between 9 and alkynes under
reaction conditions, the equilibrium constant at elevated
temperature was determined by extrapolating data from
a van’t Hoff plot for the equilibrium in eq 12 (Figure 4).
From these data the equilibrium constant at 50 °C was
calculated to be 12.9. Thus, under typical conditions
for catalytic borylation of 4-octyne (in the absence of
added phosphine), 9 is the principal Pt-containing
species. Not surprisingly, the equilibrium is sensitive
(PPh3)2Pt(BPin)2
+ RCtCR h
9
2
+ PinB-BPin (12)
(PPh3)2Pt(η -RCtCR)
8
reversibility of this reaction, the analogous experiment
was performed using the volatile alkyne 2-butyne. As
was the case for 4-octyne, the alkyne complex formed
readily with concurrent elimination of PinB-BPin.
Removal of 2-butyne, in vacuo, regenerated 9 and
(49) In all kinetic runs, the NMR tubes were heated briefly at 50
°C to dissolve the substrate. The integrity of the sample was confirmed
by NMR prior to kinetic runs at 70 °C. In the case of 1,2-bis[p-
(trifluoromethyl)phenyl]acetylene, approximately 50% of the alkyne
was diborylated during dissolution at 50 °C.
(50) Lowry, T. H.; Richardson, K. S. Mechanism and Theory in
Organic Chemistry, 2nd ed.; Lowry, T. H., Richardson, K. S., Eds.;
Harper and Row: New York, 1981; p 131.
(51) The compounds in Scheme 6 were identified by their 31P{1H}
NMR spectra, which corresponded to the reported literature values.
(52) The B-B bond in PinB-BPin measures 1.711(6) Å and is
significantly longer than the B-B bond in related catecholate deriva-
tives where the B-B distances are 1.68 Å: No¨th, H. Z. Naturforsch.
1984, 39b, 1463-1466.

Stoichiometric Alkyne Insertion into Pt-B Bonds
Organometallics, Vol. 15, No. 24, 1996 5161
Sch em e 7
tivity of 1 could be made, the kinetics of the diborylation
of 4-octyne, catalyzed by 1, were investigated.
A potential mechanism for this catalysis is illustrated
in Scheme 7. Similarities to the mechanism proposed
for the stoichiometric reactivity of 1 with alkyne
(Schemes 4 and 5) are readily apparent; however, in
contrast to the stoichiometric reaction, [1] remains
constant throughout the course of the catalysis. Scheme
7 represents the case where rates for equilibration are
rapid compared to the rate for alkyne insertion. Again,
application of either steady-state or equilibrium ap-
proximations gives rate laws for the consumption of
alkyne.
F igu r e 4. van’t Hoff plot for the equilibrium in eq 12 ([9]0
) 8.7 mM, [4-octyne]0 ) 52 mM; ∆H ) 0.49 kcal/mol; ∆S
) 6.6 eu).
to the alkyne substrate. For example, when equimolar
quantities of 9 and diphenylacetylene are dissolved in
C6D6, the alkyne complex, (Ph3P)2Pt(η2-PhCCPh), is the
only phosphine-containing species detected by 31P{1H}
NMR.
Although in stoichiometric reactions between 1 and
alkynes neither CatB-BCat nor 8 is detected in solu-
tion, the equilibria observed in the reactions of 9 with
alkynes raise the possibility that alkyne diborylation in
the catecholate system could stem from a bimolecular
reaction between trace concentrations of 8 (and related
alkyne complexes) and CatB-BCat.53 This seems un-
likely as the borylation rates in the stoichiometric
reaction between 1 and alkyne exhibit no alkyne de-
pendence. If borylation proceeded via pathways that
are first (or higher) order in 8, an alkyne dependence
would be expected since equilibrium concentrations of
8, albeit small, depend on alkyne concentration.
-d[RCtCR]
)
dt
k4k5k6[1]
[RCtCR] (13)
(
)
k-4(k5 + k6)[PPh3] + k5k6[RCtCR]
-k6K1K2[1]
-d[RCtCR + 5]
dt
)
[RCtCR + 5]
[PPh3] + K1K2[1]
(14)
The rate laws in eqs 13 and 14 simplify under limiting
conditions where k-4(k5 + k6)[PPh3] . k5k6[RCtCR] (eq
15) or [PPh3] . K1K2[1] (eq 16). Under these conditions
Ca ta lytic Dibor yla tion Ch em istr y. Solutions of
the diboryl complexes 1 and 9 both exhibit catalytic
activity for diborylation of alkynes by their respective
B-B reagents. Addition of CatB-BCat or PinB-BPin
to 4-octyne proceeds at comparable rates when catalyzed
by 1 or 9, respectively. Quantititative comparisons of
rates were not possible due to deviations from pseudo-
first-order consumption of alkyne in the absence of
added phosphine. The cause of these deviations in the
catecholate system is not clear; however, borylation by
other boryl species generated in the presence of excess
CatB-BCat could account for the observations. When
phosphine is present during catalytic borylation medi-
ated by 1, well-behaved, pseudo-first-order consumption
of 4-octyne was observed. Since 1 was the only Pt-
containing complex detected (1H and 31P NMR) when
catalytic diborylations were run in the presence of added
phosphine, and comparisons to the stoichiometric reac-
-d[RCtCR]
) kobs[RCtCR]
(15)
(16)
dt
(k4k5k6)[1]
kobs
)
k-4(k5 + k6)[PPh3]
d[RCtCR + 5]
dt
) -kobs[RCtCR + 5]
-k6K1K2[1]
kobs
)
[PPh3]
both mechanisms require that the reaction is first order
in [1] and [RCtCR] and exhibits an inverse dependence
on phosphine concentration. Even at modest phosphine
concentrations, plots of ln [RCtCR] vs time in Figure
5 are linear through 3 half-lives confirming a first-order
dependence on alkyne and [1], and plots of 1/kobs vs
[PPh3] are linear as required by eqs 15 and 16 (Figure
(53) Alkyne complexes in these systems are known to react via
associative and dissociative pathways: Birk, J . P.; Halpern, J .; Pickard,
A. L. Inorg. Chem. 1968, 7, 2672-2673.

5162 Organometallics, Vol. 15, No. 24, 1996
Iverson and Smith
The dependence of kobs on concentrations of alkyne,
phosphine, and 1 suggests that the mechanisms for
diborylation under stoichiometric and catalytic condi-
tions are essentially identical. Clearly, a direct com-
parison of rate constants between the stoichiometric and
catalytic systems could corroborate this supposition.
Inspection of eqs 10 and 16 immediately reveals that a
direct comparison can be made, as k6K1K2 can be
expressed in terms of kobs and concentrations of 1, PPh3,
and alkyne. The calculated values for k6K1K2 from the
stoichiometric and catalytic measurements are
0.019(3) and 0.024(3) s-1, respectively. Thus, the data
support the notion that the mechanisms for diborylation
in both systems are identical as the values derived from
eqs 17 and 18 are identical within experimental
error.
kobs[PPh3]
k6K1K2 )
k6K1K2 )
(17)
(18)
[RCtCR]
kobs[PPh3]
[1]
F igu r e 5. Plot of ln{[4-octyne]t/[CatB-BCat]0} vs time for
the catalytic diborylation of 4-octyne by CatB-BCat in the
presence of 1 and PPh3 at 105 °C in C7D8 ([, [4-octyne]0 )
42 mM, [CatB-BCat] ) 84 mM; O, [4-octyne]0 ) 42 mM,
[CatB-BCat] ) 42 mM; +, [4-octyne]0 ) 84 mM, [CatB-
BCat] ) 42 mM, [1]0 ) 7.0 mM, [PPh3] ) 55 mM).
Con clu sion s
The results from this detailed investigation of the
diborylation of alkynes by 1 under stoichiometric and
catalytic conditions are most consistent with a mecha-
nism whereby phosphine dissociation from 1 generates
a three-coordinate species which mediates alkyne inser-
tion. Clearly, catalytic activity of 1 is enhanced when
the pure complexes are isolated, instead of generated
from (PPh3)4Pt and the corresponding boron reagents.
The identity of the heteroatom substituents has pro-
nounced effects on the metal-containing species in
stoichiometric and catalytic systems, and the borylation
rate is sensitive to the nature of the alkyne. This is
not surprising if alkyne insertion into the Pt-B bond
is the rate-limiting step in the diborylation reaction. The
requirement for phosphine dissociation has clear im-
plications for design of other catalysts that effect di-
borylation of other unsaturated substrates. This may
account for the empirical observations that Pt(NBE)3,21
and related base-free Pt complexes,54 will effect catalytic
diborylation of olefins, while these same substrates are
not borylated by 1 or 9.
Exp er im en ta l Section
Gen er a l Con sid er a tion s. All manipulations were per-
formed using glovebox, Schlenk, or vacuum-line techniques.
B2Cat2 was prepared by the literature method.55 B2Pin2 was
prepared in analogous fashion, except that trace HCl impuri-
ties were removed by washing the solid with triethylamine
prior to pentane extraction and crystallization of the product.
Diethyl ether, pentane, and toluene were predried over sodium
and distilled from sodium/benzophenone ketyl. Methylene
chloride was predried and then distilled from CaH2. Benzene-
d6 and toluene-d8 were dried over 3 Å sieves, and the distilled
solvents were stored over sodium. Methylene chloride-d2 and
chloroform-d1 were dried over 3 Å sieves and distilled prior to
use. Dinitrogen was purified by passage through a column of
MnO on silica. 1H NMR spectra were recorded on Varian
F igu r e 6. Plot of 1/kobs vs PPh3 concentration for the
catalytic diborylation of 4-octyne by CatB-BCat in the
presence of 1 and PPh3 at 100 °C in C6D6 ([4-octyne]0 )
[CatB-BCat] ) 42 mM, [1] ) 7.0 mM).
6). The rate for alkyne consumption was unaffected by
the concentration of CatB-BCat. Again, this finding,
in conjunction with the facility with which CatB-BCat
oxidative addition to Pt(0) complexes occurs, is consis-
tent with the assumption that regeneration of [1] is
rapid and follows the rate-determining step. The data
from catalytic runs in the presence of phosphine are
reliably reproducible and exhibit dependencies that are
required by the mechanism in Scheme 7.
(54) Iverson, C. N.; Smith, M. R., III. Manuscript in preparation.
(55) Welch, C. N.; Shore, S. G. Inorg. Chem. 1968, 7, 225-230.

Stoichiometric Alkyne Insertion into Pt-B Bonds
Organometallics, Vol. 15, No. 24, 1996 5163
Gemini-300 (300.1 MHz) and Varian VXR-300 (300.0 MHz)
spectrometers and referenced to residual proton solvent sig-
nals. 11B and 31P spectra were recorded using a Varian VXR-
300 spectrometer operating at 96.234 and 121.994 MHz,
respectively. Boron chemical shifts are referenced to a BF3‚
Et2O (15% v/v in CDCl3) external standard. Phosphorous
chemical shifts are referenced to a 85% phosphoric acid
external standard. Mass spectroscopic data were obtained on
ferred to a sealable NMR tube. After sealing, the tube was
thawed, and traces of a white precipitate indicated cis-(PPh3)2-
Pt(BCat)2 deposition. 1H NMR indicated that cis-(PPh3)2Pt-
(BCat)2, 4-octyne, and B2Cat2 were the sole soluble species.
The tube was heated for 4 h at 80 °C to yield cis-(PPh3)2Pt-
(BCat)2 (by 1H and 31P NMR) as the free alkyne was converted
to the cis-4,5-diboryl-4-octene. 1H NMR (C6D6): δ 0.90 (tr, 6H,
-CH2CH2CH3), δ 1.53 (sextet, 4H, -CH2CH2CH3), δ 2.49 (tr,
4H, -CH2CH2CH3), δ 6.73 (m, 4H, BO2C6H4), δ 6.91 (m, 4 H,
BO2C6H4). 11B NMR (C6D6): δ 32.7.
Rea ction of cis-(P P h 3)2P t(BP in )2 w ith Me6Sn 2. (PPh3)2-
Pt(BPin)2 (10 mg, 0.010 mmol) was dissolved in 600 µL of C6D6.
Addition of Me6Sn2 (2.0 µL, 0.010 mmol) via syringe im-
mediately generated a bright yellow solution. 1H, 31P, and 11B
NMR spectroscopy all indicated complete conversion to trans-
(PPh3)2Pt(SnMe3)2 and B2Pin2.
Rea ction of cis-(P P h 3)2P t(BP in )2 w ith B2Ca t2. (PPh3)2-
Pt(BPin)2 (12.0 mg, 0.013 mmol) and B2Cat2 (3.0 mg, 0.013
mmol) were placed in a sealable NMR tube, and 600 µL of
C6D6 was added via vacuum transfer. Upon warming of the
sample to room temperature, traces of a white precipitate
indicated (PPh3)2Pt(BCat)2 deposition. 1H, 31P, and 11B NMR
spectroscopy all indicated conversion to (PPh3)2Pt(BCat)2 and
B2Pin2, as cis-(PPh3)2Pt(BPin)2 was not detected.
a
portable Trio-1 VG Masslab Ltd. mass spectrometer.
Elemental analyses were determined by Desert Analytics,
Tuscon, AZ.
cis-(P P h 3)2P t(BCa t)2. (PPh3)2Pt(η2-C2H4)56 (100 mg, 0.134
mmol) and B2Cat2 (35 mg, 0.15mmol) were placed in a flask
and dissolved in 5 mL of toluene. A white precipitate formed
immediately. The solution was stirred for 2 h at room
temperature, and the supernatant was removed via cannula.
Additional product was obtained by removing the solvent
under reduced pressure and washing the residue with toluene
(2 mL). The overall yield was 96 mg (75%). 1H NMR (C6D6):
δ 6.54 (m, 4 H, BO2C6H4), δ 6.77 (m, 4 H, BO2C6H4), δ 6.80
(m, 18 H, P(C6H5)3), δ 7.55 (m, 12 H, P(C6H5)3). 31P NMR
(C6D6): δ 30.19 (1J Pt-P ) 1608 Hz). 11B NMR (C6D6, quartz
NMR tube): δ 47.2 (broad). Mp: 193-195 °C (dec).
cis-(P P h 3)2P t (BP in )2 (9).23 (PPh3)2Pt(η2-C2H4) (100 mg,
0.134 mmol) and B2Pin2 (35 mg, 0.15 mmol) were placed in a
flask and dissolved in 5 mL of toluene. The golden yellow
solution was stirred for 2 h at room temperature, and
subsequent solvent evaporation yielded a light orange-yellow
powder. The residue was recystallized from CH2Cl2/ethanol,
washed with hexanes, and dried in vacuo to give white crystals
(91 mg, 70%). 1H NMR (C6D6): δ 1.02 (s, 24 H, BO2C6H12), δ
6.88 (m, 18 H, P(C6H5)3), δ 7.58 (m, 12 H, P(C6H5)3). 31P NMR
(C6D6): δ 29.77 (1J Pt-P ) 1506 Hz). 11B NMR (C6D6, quartz
NMR tube): δ 46.3 (broad). Mp: 183-187 °C (dec). Anal.
Calcd for C48.5H55B2O4P2ClPt: C, 57.29; H, 5.45. Found: C,
55.28; H, 5.27. Analyses for crystalline samples of 9‚1/2CH2-
Cl2 were consistently low in carbon. Although this compound
has been structurally characterized, the combustion analysis
was not reported.23 Our melting points for crystalline materi-
als were consistently 40 °C higher than the reported value.
tr a n s-(P P h 3)2P t(Sn Me3)2. This preparation is analogous
to that reported in the literature.57 (PPh3)2Pt(η2-C2H4) (100
mg, 0.134 mmol) was placed in a flask and dissolved in 7 mL
of toluene. A bright yellow solution formed on addition of Me6-
Sn2 (28 µL, 0.134 mmol), and within 15 min a bright yellow
precipitate formed. After the solution was stirred for 2 h, the
solvent was removed under reduced pressure to afford a bright
yellow powder. 1H NMR (C6D6): δ 0.40 (s, 18 H, SnMe3, 4J Pt-H
) 8 Hz, 3J Sn-H ) 20 Hz), δ 6.86 (m, 18 H, P(C6H5)3), δ 7.46 (m,
12 H, P(C6H5)3). 31P NMR (C6D6): δ 35.1 (1J Pt-P ) 2621 Hz,
2J Sn-P ) 614, 642 Hz).
Equ ilibr iu m of cis-(P P h 3)2P t(BCa t)2 w ith CO. (PPh3)2-
Pt(BCat)2 (10.0 mg, 0.010 mmol) was dissolved in 600 µL of
CD2Cl2 in a J . Young NMR tube. After the solution was frozen,
the argon atmosphere was evacuated and replaced with CO.
Upon warming of the sample to room temperature conversion
to (PPh3)2Pt(CO)2 had occurred as evidenced by 1H and 31P
NMR spectra, and reductive elimination of B2Cat2 was con-
firmed by 11B NMR. Chemical reversibility was confirmed by
the regeneration of (PPh3)2Pt(BCat)2 upon evacuation of the
CO atmosphere.
Equ ilibr iu m of cis-(P P h 3)2P t(BP in )2 w ith CO. (PPh3)2-
Pt(BPin)2 (10.0 mg, 0.010 mmol) was dissolved in 600 µL of
C7D8 in a J . Young NMR tube. After the solution was frozen,
the argon atmosphere was evacuated and replaced with CO.
Upon warming of the sample to room temperature, 1H and 31P
NMR indicated complete conversion to (PPh3)2Pt(CO)2. 11B
also confirmed reductive elimination of B2Pin2. After evacu-
ation of the CO atmosphere, a trace of (PPh3)2Pt(BPin)2 was
regenerated.
Equ ilibr iu m of cis-(P P h 3)2P t(BP in )2 w ith 2-Bu tyn e.
(PPh3)2Pt(BPin)2 (7.5 mg, 0.0077 mmol) was dissolved in 600
µL of C6D6 in a J . Young NMR tube. The atmosphere above
the frozen solution was evacuated and replaced with ap-
proximately 10 molar equiv of 2-butyne. Upon warming of the
sample to room temperature, 1H and 31P NMR indicated nearly
complete conversion to cis-(PPh3)2Pt(η2-CH3CCCH3)2, and
reductive elimination of B2Pin2 was confirmed by 11B NMR.
Reversibility was demonstrated by regeneration of cis-(PPh3)2-
Pt(BPin)2 upon evacuation of the 2-butyne atmosphere. Evacu-
ation to dryness and dissolution of the residue in C6D6 showed
cis-(PPh3)2Pt(BPin)2 to be the major product (31P NMR);
however, 1H NMR indicated that some diborylation of 2-butyne
had occurred.
Equ ilibr iu m of cis-(P P h 3)2P t(BP in )2 w ith 4-Octyn e.
(PPh3)2Pt(BPin)2 (5.1 mg, 0.0052 mmol) was placed in a
sealable NMR tube, 4.6 µL of 4-octyne was added via syringe,
and toluene-d8 was added to 600 mL. After sealing, 31P NMR
spectra were recorded at 193, 213, 233, 253, and 273 K. The
concentrations of cis-(PPh3)2Pt(BPin)2, (PPh3)2Pt(η2-4-octyne),
B2Pin2, and 4-octyne were determined by the relative intensi-
ties of the Pt-containing species.
Rea ction of (P P h 3)2P t(BCa t)2 w ith Me6Sn 2. cis-(PPh3)2-
Pt(BCat)2 (100 mg, 0.104 mmol) and Me6Sn2 (21.6 µL, 0.208
mmol) were placed in a Schlenk tube and dissolved in 5 mL of
toluene in a glovebox. The solution was stirred at 55 °C for 3
h. The solvent was removed in vacuo and the residue washed
with 10 and 5 mL of pentane to yield 51 mg (49% yield) of a
light tan solid. 1H, 31P, and 11B NMR spectroscopy indicated
formation of cis-(PPh3)2Pt(BCat)(SnMe3). 1H NMR (CDCl3):
4
δ -0.45 (m, 9 H, Sn(CH3)3 (5J P-H ) 0.9 Hz, J Pt-H ) 12 Hz,
3J Sn-H ) 42 Hz), δ 6.72 (m, 2 H, BO2C6H4), δ 6.81 (m, 2 H,
BO2C6H4), δ 6.92-7.37 (m, 30 H, P(C6H5)3). 31P NMR (CDCl3,
-50 °C): δ 26.63 (d, trans SnMe3, 2J P-P ) 23 Hz, 1J Pt-P ) 2371
2
2
Hz, J Sn-P ) 1268 Hz), δ 32.84 (br d, trans BCat, J P-P ) 20
1
2
Hz, J Pt-P ) 1761 Hz, J Sn-P ) 815 Hz).
NMR Tu be Rea ction s: cis-(P P h 3)2P t(η2-4-octyn e) w ith
B2Ca t2. cis-(PPh3)2Pt(η2-4-octyne) (7.2 mg, 0.0087 mmol) and
B2Cat2 (4.1 mg, 0.017 mmol) were mixed in C6D6 and trans-
Rea ction of cis-(P P h 3)2P t(BP in )2 w ith P P h 3. cis-(PPh3)2-
Pt(BPin)2 (10 mg, 0.010 mmol) and PPh3 (30 mg, 0.11 mmol)
were dissolved in 600 µL of CD2Cl2 in an NMR tube. NMR
spectra (1H and 31P at -80 °C) of the yellow solution indicated
reductive elimination of B2Pin2 and formation of tris(tri-
phenylphosphino)platinum(0) and tetrakis(triphenylphos-
phino)platinum(0).
(56) Camalli, M.; Caruso, F.; Chaloupka, S.; Leber, E. M.; Rimml,
H.; Venanzi, L. M. Helv. Chim. Acta 1990, 73, 2263-2274.
(57) 31P data were not available in the original report: Akhtar, M.;
Clark, H. C. J . Organomet. Chem. 1970, 22, 233-240.

5164 Organometallics, Vol. 15, No. 24, 1996
Iverson and Smith
Reaction of cis-(P P h 3)2P t(BCat)2 with P P h 3. cis-(PPh3)2-
Pt(BCat)2 (10 mg, 0.010 mmol) and PPh3 (30 mg, 0.11 mmol)
were placed in an NMR tube and dissolved in 600 µL of CD2-
Cl2 to form a clear, colorless solution. 1H, 31P, and 11B NMR
(-80 and 20 °C) showed 1 to be the exclusive Pt-containing
species.
A typical experimental run for the stoichiometric diboryla-
tion of alkynes by cis-(PPh3)2Pt(BCat)2 is described as fol-
lows: cis-(PPh3)2Pt(BCat)2 (5.0 mg, 0.0052 mmol) and 4-octyne
(4.6 µL, 0.032 mmol) were placed in an NMR tube. In a
drybox, 595 µL of C6D6 was added via syringe. The solution
was frozen in liquid nitrogen, and the tube was flame-sealed.
The tube was heated briefly to dissolve the Pt complex. The
tube was then placed in a constant-temperature oil bath. At
specific intervals the tube was removed from the oil bath and
Cr ossover Exper im en t. cis-(PPh3)2Pt(BCat)2 (4 mg, 0.0042
mmol), B2Cat2 (1 mg, 0.0042 mmol), and 4-octyne (4.6 µL, 0.032
mmol) were placed in a sealable NMR tube and 595 µL of C6D6
was added while in a glovebox. The tube was flame-sealed,
and the mixture was heated at 80 °C in a constant-tempera-
ture oil bath until 1 was consumed. The sealed tube was
opened in the glovebox. The solution was transferred to a new
sealable tube, and 3-hexyne (2 µL, 0.02 mmol) was added. After
flame-sealing, the solution was heated at 80 °C for 5 h. 1H
NMR spectra showed no evidence for diborylation of hexyne.
Dibor yla tion of 1,2-Bis(p-a n isolyl)a cetylen e. 1,2-bis-
(p-anisolyl)acetylene58 (50 mg, 0.21 mmol), B2Cat2 (50 mg, 0.21
mmol), and 2.5 mol % 1 were dissolved in 4 mL of toluene.
The solution was heated at reflux for 4 h, and then the solvent
was removed in vacuo. A 20 mL volume of Et2O was added,
and the mixture was filtered. Evaporation of the filtrate
yielded 74 mg (74%) of the diborylated product. 1H NMR
(CDCl3): δ 3.76 (s, 3 H, OCH3, δ 6.76 (m, 2 H, BO2C6H4), δ
7.05 (m, 4 H, C6H4OMe), δ 7.11 (m, 2 H, BO2C6H4). 11B NMR
(CDCl3): δ 32.3. EI/MS: m/ z ) 476 (M+), I ) 100 (100); m/ z
) 477 ([M + 1]+), I ) 30 (30); m/ z ) 475 ([M - 1]+), I ) 48
(45). Mp; 146-151 °C.
Dib or yla t ion of 1,2-Bis[p -(t r iflu or om et h yl)p h en yl]-
a cetylen e. 1,2-Bis[p-(trifluoromethyl)phenyl]acetylene (57
mg, 0.18 mmol), bis[p-(trifluoromethyl)phenyl]acetylene, B2-
Cat2 (43 mg, 0.18 mmol), and 2.5% 1 were dissolved in 4 mL
of toluene. The solution was heated at reflux for 2 h, and then
the solvent was removed in vacuo. A 5 mL volume of Et2O
was added, and the mixture was filtered. Evaporation of the
filtrate yielded 85 mg (85%) of the diborylated product. 1H
NMR (CDCl3): δ 7.09 (m, 4 H, C6H4CF3), δ 7.27 (m, 2 H,
BO2C6H4), δ 7.49 (d, 2 H, BO2C6H4). 11B NMR (CDCl3): δ 31.5.
EI/MS: m/ z ) 552 (M+), I ) 100 (100); m/ z ) 553 ([M + 1]+),
I ) 31 (30); m/ z ) 551 ([M - 1]+), I ) 51 (45). Mp; 140-145
°C.
1
the reaction was quenched by rapid cooling in an ice bath. H
NMR spectra were then recorded at room temperature. The
progress of the reaction was monitored to 3 half-lives by
measuring the disappearance of the catecholate resonance at
δ 6.54.
A typical experimental run under catalytic conditions is
described as follows: cis-(PPh3)2Pt(BCat)2 (4 mg, 0.0042 mmol),
CatB-Bcat (6 mg, 0.025 mmol), 4-octyne (3.7 µL, 0.025 mmol),
and C6D6 (595 µL) were added to an NMR tube. The tube was
then brought out of the glovebox and flame-sealed. The
mixture was heated to 50 °C in the Varian 300 MHz NMR
spectrometer, and the reaction was monitored to 3 half-lives
completion. A program supplied within the spectrometer’s
software was used to take scans of the mixture at specified
intervals, and the time-dependent alkyne concentration was
calculated by integrating the resonance of the methylene group
adjacent to the alkyne triple bond.
Der iva tion of Ra te Exp r ession s. In Scheme 4, the time-
dependent concentrations for 1, 4, 5, and 8 are governed by
the eqs 19-22. Application of the steady state approximation
d[1]
dt
-
) k4[1] - k-4[4][PPh3]
(19)
d[4]
dt
) k4[1] + k-5[5] - (k-4[PPh3] + k5[RCtCR])[4] (20)
d[5]
) k5[RCtCR][4] - (k-5 + k6)[5]
(21)
dt
d[8]
) k6[5]
dt
(22)
P r oton olysis of Dibor yla ted P h en yla cetylen es.59 A 51
mg amount of the alkene derived from diborylation of 1,2-bis-
(p-anisolyl)acetylene (0.11 mmol) was dissolved in 0.5 mL of
diglyme. Several drops of glacial acetic acid were added, and
the solution was heated at 45 °C for 15 h. The solvent was
removed in vacuo. Formation of the cis-stilbene was confirmed
by comparing the 1H NMR shift of the olefinic protons (δ )
6.43 ppm) to the literature value (δ ) 6.44 ppm).60
to [5] yields 23, and combination of the steady-state expression
k5[RCtCR]
k-5 + k6
d[5]
dt
≈ 0 w [5] )
[4]
(23)
[1] (24)
[1] (25)
(
)
k-5 + k6
d[4]
dt
≈ 0 w [4] )
(
)
k-4(k-5 + k6) + k5k6[RCtCR]
A 50 mg amount of the alkene derived from diborylation of
1,2-bis[p-(trifluoromethyl)phenyl]acetylene (0.091 mmol) was
dissolved in 0.5 mL of diglyme. Several drops of glacial acetic
acid were added, and the solution was heated at reflux for 12
h. The solvent was removed in vacuo. Formation of the cis-
k4k5k6
d[1]
-
)
(
)
dt
k-4(k-5 + k6)[PPh3] + k5k6[RCtCR]
1
for [4] and eq 23 gives eq 24. The first-order rate law in eq 25
follows from substitution of the expression for [4] (eq 24) in
eq 19.
stilbene was confirmed by comparing the H NMR shift of the
olefinic protons (δ ) 6.70 ppm) to the literature value (δ )
6.72 ppm).61
In the application of the equilibrium approximation to
Scheme 5, the assumption in eq 26 is made. From the
Kin etic Exp er im en ts. All kinetic experiments were run
in sealed NMR tubes with a total solution volume of 600 µL.
PPh3 was added to the tubes in the form of 0.2 M solutions of
PPh3 in C6D6 and C7D8. The reactions were heated in constant
temperature oil baths (Cole-Parmer Polystat Constant Tem-
perature Circulator or Tekmar RCT) or in the Varian 300 MHz
spectrometer when the reaction temperature and completion
times were convenient.
d[1 + 4 + 5] d[8]
-
≈
) k6[5]
(26)
dt
dt
equilibrium constants K1 and K2, [1] and [4] can be expressed
in terms of [5] (eqs 27 and 28). Equations 27 and 28 provide
[5]
[5]
(58) Cummins, C. H. Tetrahedron Lett. 1994, 35, 857-860.
(59) Brown, H. C.; Zweifel, G. J . Am. Chem. Soc. 1961, 83, 3834-
3840.
K2 )
w [4] )
(27)
[4][RCtCR]
K2[RCtCR]
(60) For olefinic protons: (a) Engman, L. J . Org. Chem. 1984, 49,
3559-3563. For methoxy protons: (b) Todrez, Z. V.; Ionina, E. A. J .
Organomet. Chem. 1993, 453, 193-195.
(61) Bellucci, G.; Bianchini, R.; Chiappe, C.; Brown, R. S.; Sleboka-
Tilk, H. J . Am. Chem. Soc. 1991, 113, 8012-8016.
[4][PPh3]
[1]
[4][PPh3]
[5][PPh3]
K1K2[RCtCR]
(28)
K1 )
w [1] )
w [1] )
K1

Stoichiometric Alkyne Insertion into Pt-B Bonds
Organometallics, Vol. 15, No. 24, 1996 5165
an expression of [5] in terms of [1 + 4 + 5] (eq 30), and
k4k5k6[RCtCR]
-d[RCtCR]
dt
)
[1]
(
)
k-4(k-5 + k6)[PPh3] + k5k6[RCtCR]
[PPh3]
K2[RCtCR] K1K2[RCtCR]
1
(40)
[1 + 4 + 5] ) 1 +
+
(29)
(
)
the equilibrium approximation to the catalytic reaction in
Scheme 7 gives eq 41. From the equilibrium constants in eqs
K1K2[RCtCR]
[5] )
[1 + 4 + 5] (30)
(
)
K1 + [PPh3] + K1K2[RCtCR]
d[RCtCR + 5] d[8]
-
≈
) k6[5]
(41)
dt
dt
d[1 + 4 + 5]
dt
-
)
42 and 43, [5] can be expressed in terms of [5 + RCtCR]
(eq 45). Substituting this expression for [5] in eq 41 gives the
k6K1K2[RCtCR]
[1 + 4 + 5] (31)
(
)
K1 + [PPh3] + K1K2[RCtCR]
[5]
[5]
substitution for [5] in eq 26 gives the final form of the rate
law in eq 31. Under catalytic conditions the diboryl complex,
1, is regenerated. The rate laws that govern the time-
dependent concentrations of the species in Scheme 7 are given
as follows:
K2 )
w [RCtCR] )
(42)
[4][RCtCR]
K2[4]
[4][PPh3]
[1]
K1[1]
[PPh3][5]
K1 )
w [4] )
w [RCtCR] )
[PPh3]
K1K2[1]
(43)
d[1]
dt
-
) k4[1] - k-4[4][PPh3] - k6[5]
(32)
[PPh3]
[RCtCR + 5] ) 1 +
[5]
(44)
(45)
(
)
K1K2[1]
d[4]
dt
) k4[1] + k-5[5] - (k-4[PPh3] + k5[RCtCR])[4] (33)
d[5]
K1K2[1]
[5] )
[RCtCR + 5]
(
)
) k5[RCtCR][4] - (k-5 + k6)[5]
(34)
[PPh3] + K1K2[1]
dt
rate law in eq 46. This is identical to the rate equation in eq
14.
d[RCtCR]
dt
) -k6[5] + k-5[5] - k5[RCtCR][4] (35)
With application of the steady-state approximation to [4]
and [5], eqs 33 and 34 yield the expressions in eqs 36 and
37. Application of the steady-state approximation to [1] in eq
k6K1K2[1]
d[RCtCR + 5]
dt
-
)
[RCtCR + 5] (46)
(
)
[PPh3] + K1K2[1]
Ack n ow led gm en t. We thank the administrators of
the ACS Petroleum Research Fund, the Center for
Fundamental Materials Research, and the National
Science Foundation (Grant CHE-9520176) for support
of this work. The NMR equipment was provided by the
National Science Foundation (Grant CHE-8800770) and
the National Institutes of Health (Grant 1-S10-RR04750-
01). We thank Professor Todd B. Marder for disclosing
results prior to publication. We also thank Professor
k4[1] + k-5[5]
[4] )
(36)
(37)
k-4[PPh3] + k5[RCtCR]
k-5 + k6
[4] )
[5]
(
)
k5[RCtCR]
31, with substitution of the value for [4] in eq 36, yields
eq 38. Similarly, substitution of the expression for [4] in eq
Gregory L. Hillhouse for
phenylphosphine-d15 that had previously been prepared
in his laboratories.
a generous gift of tri-
k4k5[RCtCR]
[5] )
[1] (38)
(
)
k-4(k-5 + k6)[PPh3] + k5k6[RCtCR]
d[RCtCR]
) -k6[5]
dt
Su p p or tin g In for m a tion Ava ila ble: Text and tables for
kinetic studies (3 pages). Ordering information is given on
any current masthead page.
(39)
37 in eq 35 gives eq 39. Combining eqs 38 and 39 gives the
rate law in eq 40. This is identical to that in eq 13. Applying
OM960123L