Solvent-and ligand-free diboration of alkynes and alkenes catalyzed by platinum nanoparticles on titania

Article abstract of DOI:10.1002/cctc.201300946

Platinum nanoparticles supported on titania efficiently catalyzed the diboration of alkynes and alkenes under solvent-and ligand-free conditions in air. The cis-1,2-diborylalkenes and 1,2-diborylalkanes were obtained in moderate to excellent yields following, in most cases, a simple filtration workup protocol. The versatility of the cis-1,2-diboronvinyl compounds was demonstrated in a series of organic transformations, including the Suzuki-Miyaura cross coupling and the boron-halogen exchange. Click diboration: The diboration of alkynes and alkenes catalyzed by platinum nanoparticles on titania is shown to take place under solvent-and ligand-free conditions in air. The 1,2-diboronvinyl compounds are obtained with exclusive cis stereochemistry and are subjected to different useful chemical transformations.

Full text of DOI:10.1002/cctc.201300946

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DOI: 10.1002/cctc.201300946
Solvent- and Ligand-free Diboration of Alkynes and
Alkenes Catalyzed by Platinum Nanoparticles on Titania
Francisco Alonso,*[a] Yanina Moglie,[a] Laura Pastor-Pꢀrez,[b] and Antonio Sepffllveda-
Escribano*[b]
Platinum nanoparticles supported on titania efficiently cata-
lyzed the diboration of alkynes and alkenes under solvent- and
ligand-free conditions in air. The cis-1,2-diborylalkenes and 1,2-
diborylalkanes were obtained in moderate to excellent yields
following, in most cases, a simple filtration workup protocol.
The versatility of the cis-1,2-diboronvinyl compounds was dem-
onstrated in a series of organic transformations, including the
Suzuki–Miyaura cross coupling and the boron–halogen ex-
change.
Introduction
Boronic acids and their derivatives are versatile and useful
starting materials for the formation of carbonÀcarbon and
carbonÀheteroatom bonds.[1] Their pivotal role in modern syn-
thetic chemistry has led to the development of new and more
efficient methodologies for the preparation of this type of
compounds. Among them, the transition-metal-catalyzed dibo-
ration of carbonÀcarbon multiple bonds represents an straight-
forward and atom-economic strategy for this purpose.[2] Partic-
ularly interesting is the platinum-catalyzed [Pt(PPh3)4] addition
of bis(pinacolato)diboron (B2pin2) to alkynes, pioneered by
Miyaura et al., which leads to 1,2-diboronvinyl compounds in
a cis selective manner.[3] The utility of the latter was elegantly
exemplified by Armstrong et al. in the synthesis of tamoxifen
derivatives.[4] Some other methodologies, also involving phos-
phine-based platinum complexes, were later reported by the
groups of Marder[5] and Smith III.[6] Although the catalytic dibo-
ration of alkynes is clearly dominated by platinum homogene-
ous catalysis,[2] the heterogeneous version of this reaction has
been mainly studied with other metals, namely: a) the Pd/C-
catalyzed alkyne diboration with [2]borametalloarenophanes to
form ansa-bis(boryl)alkenes[7] and b) the alkyne diboration cat-
alyzed by nanoporous gold.[8]
To the best of our knowledge, only Corma and García et al.
have studied the heterogeneous platinum-catalyzed diboration
of alkynes with B2pin2, using for this purpose magnesia-sup-
ported platinum nanoparticles.[9] However, some of the afore-
mentioned methodologies suffer from disadvantages such as
long reaction times or the necessity of inert atmospheres,
phosphane ligands, and organic solvents such as DMF, toluene,
or benzene. The Pt/MgO catalyst could work in some cases in
the absence of PPh3; however, elevated temperatures were re-
quired (130–1608C) and the substrate scope covered was
rather limited (four alkynes bearing no functional group).[9] An-
other common feature of some the above commented cata-
lysts, such as Pt(PPh3)4[3] and Pt/MgO,[9] is that they are not suit-
able for alkene diboration.
The first platinum-catalyzed diboration of aliphatic and aro-
matic terminal alkenes, and some cyclic alkenes was independ-
ently achieved by the groups of Miyaura and Smith III, by
using Pt(dba)2 and Pt(COD)2 (or Pt(NBE)3) (dba=dibenzylide-
neacetone; COD=cycloocta-1,5-diene; NBE=norbornene), re-
spectively, as catalysts.[10] Soon after, Marder et al. developed
an asymmetric version of this reaction with chiral diborane
compounds and Pt(dba)2.[11a] Alkene diboration has also been
accomplished with metals other than platinum, such as
gold,[12] rhodium,[13] palladium,[14] and copper.[15] The group of
Fernµndez introduced the diboration of alkenes catalyzed by
gold(0) nanoparticles[16] and, more recently, an organocatalytic
version of this reaction.[17] However, the heterogeneous nature
of the gold nanoparticle process was not clearly ascertained
because of the presence of 2,2’-bis(diphenylphosphino)-1,1’-bi-
naphthyl as a ligand. In view of the above information, we can
state that the heterogeneous ligand-free catalytic diboration of
alkenes has not been described hitherto. Therefore, we can
conclude that the information on the heterogeneous plati-
num-catalyzed diboration of alkynes and alkenes is sparse and
that new platinum catalysts which can upgrade the catalytic
behavior of those already reported are welcome.
[a] Dr. F. Alonso, Dr. Y. Moglie
Departamento de Quꢀmica Orgꢁnica
Facultad de Ciencias, and Instituto de Sꢀntesis Orgꢁnica (ISO)
Universidad de Alicante
Apdo. 99, 03080 Alicante (Spain)
Fax: (+34)965903549
E-mail: falonso@ua.es
[b] L. Pastor-Pꢂrez, Prof. A. Sepffllveda-Escribano
Laboratorio de Materiales Avanzados
Departamento de Quꢀmica Inorgꢁnica—Instituto Universitario de Materi-
ales de Alicante (IUMA)
Universidad de Alicante
Apdo. 99, 03080 Alicante (Spain)
Fax: (+34)965903454
E-mail: asepul@ua.es
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201300946.
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Owing to our dedication to studying and understanding the
reactivity of active metals and nanoparticles,[18] we have report-
ed the hydrogen-transfer reduction of ketones catalyzed by
highly reusable platinum nanoparticles on carbon.[19] More re-
cently, we demonstrated that platinum nanoparticles on titania
catalyzed the hydrosilylation of terminal and internal al-
kynes,[20a] as well as that of diynes.[20b] As part of our research
on organic reactions promoted by platinum nanoparticles, we
herein present the results attained on the selective diboration
of alkynes and alkenes catalyzed by Pt/TiO2 under solvent- and
ligand-free conditions.
lysts, such as Pt/CeO2 and Pt/C, exhibited a moderate activity
under air and solvent-free conditions (entries 8 and 9), with
Pt/Al2O3 performing significantly better under air than under
argon atmospheres (entries 10 and 11). In contrast, rather poor
results were obtained with Pt/zeolite-Y catalysts (entry 12).
Then, we tested bimetallic Pt–Pd and Pt–Sn catalysts support-
ed on titania; in general, the 1:1 Pt–Sn catalysts performed
better that the 1:1 Pt–Pd counterparts, especially if the pro-
cesses were performed in the absence of solvent in air with
lower catalyst loading (compare entries 13–16 with 17 and 18).
A high conversion, comparable to that of Pt/Al2O3, was
reached with Pt–Sn/TiO2 in toluene under argon (entry 19). Un-
fortunately, catalyst recycling was troublesome irrespective of
the catalyst tested (entries 7, 11, and 19). In view of the above
results and as solvent-free organic synthesis is one of the most
promising steps toward waste prevention and environmental
protection,[21] Pt/TiO2 (1 wt%) was considered as the catalyst of
choice in the absence of solvent and in air (entry 7).
Results and Discussion
Screening of the catalysts
The platinum catalysts were prepared by the impregnation
method (see the Experimental Section). The diboration of phe-
nylacetylene (1a) with bis(pinacolato)diboron (B2(bpin)2) was
used as the model reaction at 708C (Table 1). Initial experi-
ments with Pt/TiO2 (1 wt%) in different solvents under argon
were unsuccessful (entries 1–4). However, the reaction in tolu-
ene under air gave the product 2a in 84% conversion
(entry 5). It is noteworthy that an excellent conversion was re-
corded in the absence of solvent under air, whereas the inert
atmosphere exerted a deleterious effect on the conversion
(compare entries 6 and 7). Other supported monometallic cata-
Characterization of the Pt/TiO2 catalyst
The Pt/TiO2 catalyst was calcined in air at 4008C for 2 h after
preparation (see the Experimental Section). TEM analysis[20b] re-
vealed that it is composed of very small (1–2 nm) platinum
clusters with a narrow size distribution, homogeneously dis-
persed on the surface of the titania crystals (Supporting Infor-
mation). The X-ray photoelectron spectrum in the Pt4f region
for this catalyst is shown in Figure 1. It can be deconvoluted
Table 1. Catalyst screening in the diboration of phenylacetylene.[a]
Entry
Catalyst (wt% Pt)
Solvent
Atmosphere
Conversion [%][b]
1
2
3
4
5
6
7
8
Pt/TiO2 (1)
Pt/TiO2 (1)
Pt/TiO2 (1)
Pt/TiO2 (1)
Pt/TiO2 (1)
Pt/TiO2 (1)
Pt/TiO2 (1)
Pt/CeO2 (1)
Pt/C (1.5)
Pt/Al2O3 (1)
Pt/Al2O3 (1)
Pt/zeolite-Y (1)
Pt–Pd/TiO2 (1)
Pt–Pd/TiO2 (1)[e]
Pt–Sn/TiO2 (2)
Pt–Sn/TiO2 (2)
Pt–Sn/TiO2 (1)
Pt–Sn/TiO2 (1.5)
Pt–Sn/TiO2 (2)[f]
THF
Ar
Ar
Ar
Ar
air
Ar
air
air
air
Ar
air
air
air
air
Ar
air
air
air
Ar
3
–
3
11
84
13
94 (11)[c]
67
60
53
87 (44)[c]
1 (3)[d]
50
62
14
35
75
CH2Cl2
MeCN
PhMe
PhMe
neat
neat
neat
neat
neat
neat
neat
neat
neat
neat
neat
neat
neat
PhMe
9
10
11
12
13
14
15
16
17
18
19
Figure 1. X-ray photoelectron spectra for the Pt4f region.
into four peaks, two of them (those placed at lower binding
energies) corresponding to the Pt4f7/2 level and the others to
the Pt4f5/2 level. The peaks arising from the Pt4f7/2 level are
centered at 73.0 and 74.8 eV, respectively, which can be as-
signed to oxidized platinum species. An accurate assignment
of the peaks is not easy because the binding energies of the
core electrons are affected not only by the metal oxidation
state but also by the surrounding atoms. The binding energy
of the Pt4f7/2 level for PtIV coordinated to six chlorine atoms
74
88 (0)[c]
[a] Reaction conditions: 1a (0.5 mmol), B2(pin)2 (0.5 mmol), catalyst
(20.0 mg), solvent (2.0 mL) or neat, 708C, 18 h. [b] Conversion determined
by GLC. [c] Conversion in a second cycle. [d] Conversion using the non-
calcined catalyst. [e] A catalyst quantity of 40.0 mg was used. [f] Catalyst
reduced at 2508C.
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(as in the H2PtCl6 precursor compound) is reported to be
75.3 eV (for K2PtCl6), but it decreases to 74.8 eV for PtO2 and to
74.2 eV for Pt(OH)4. Additionally, the Pt4f7/2 level for PtCl2 is ap-
proximately 73.4 eV, whereas that for Pt(OH)2 is approximately
72.4 eV.[22] In this way, the X-ray photoelectron spectrum of the
catalyst reveals that it contains two different kinds of PtII spe-
cies, although the presence of some remaining PtIV cannot be
discarded. This indicates, on one side, that reductive decompo-
sition of the metal precursor has taken place upon impregna-
tion, by which the parent PtIV is reduced to PtII. On the other
hand, this surface decomposition is accompanied by dechlori-
nation to a large extent. Thus, the atomic Cl/Pt ratio found in
the X-ray photoelectron spectra is as low as 0.6 in the catalyst,
whereas it is 6 in the parent precursor. Importantly, X-ray pho-
toelectron spectroscopy (XPS) analysis indicated the absence
of Pt0 species (binding energy at approximately 70–71 eV).
Therefore, we can conclude that the catalyst is mainly com-
posed of PtII in the form of PtO and PtCl2 (the latter in minor
proportion) together with some PtIV, primarily, as PtO2.
With the aim of answering some of the posed questions, the
used catalyst was subjected to a calcination treatment in air, at
4008C for 4 h. The objective was to remove any of the reaction
products that could cover the platinum surface. It can be seen
in Table 2 that the atomic ratios were nearly unmodified. Fur-
thermore, the X-ray photoelectron spectra (Figure 1) demon-
strated that the relative amount of high-valent platinum has
increased, which can be explained by a larger amount of PtO2
species as a result of the calcination treatment. It has to be
noted that this reactivation treatment allowed increasing the
catalytic activity to some extent. Thus, the conversion attained
with the fresh calcined catalyst (94%, Table 1, entry 7) de-
creased up to 11% in a second cycle (without any reactivation
treatment). The conversion increased up to 33% after calcina-
tion in air. Thus, it can be concluded that the reactivation treat-
ment could effectively remove reaction products that were
covering the platinum active sites, but it also decreased the
amount of PtII species, which seems to be important for this re-
action.
In Table 2, the surface atomic ratios for Pt/TiO2 after different
treatments are reported, as determined by XPS. If one com-
Finally, to add some light on which are the active species for
this reaction, the Pt/TiO2 catalyst was also tested after a reduc-
tion treatment. Thus, the calcined Pt/TiO2 catalyst was subject-
ed to a reduction treatment under a H2 flow (50 mLminÀ1) at
2508C for 2 h. TEM images reveal that there is not any clear
modification of the particle size distribution (Supporting Infor-
mation). The X-ray photoelectron spectra (Figure 1) indicate
that platinum is effectively reduced to the metallic state (band
centered at 71.7 eV), although some PtII species are still pres-
ent (72.8 eV). In this case, there is a slight decrease in the Pt/Ti
atomic ratio and also a small dechlorination (Cl/Pt shifts from
0.622 to 0.541, Table 2, entry 2). Reaction with this catalyst was
performed under Ar and compared with that for the calcined
catalyst (Table 1, entry 6). Thus, whereas the reaction with cal-
cined Pt/TiO2 yielded a 13% conversion under these condi-
tions, a conversion degree of only 3% was obtained with the
reduced sample. These results indicate that the PtII species are
the active sites for the reaction instead of the reduced Pt0 spe-
cies, which are commonly considered in the proposed reaction
mechanisms catalyzed either by platinum complexes[3,6,23] or
platinum nanoparticles.[9] In these cases, an oxidative addition
step of the BÀB bond to Pt0 is always suggested in the catalyt-
ic cycle. The absence of metallic platinum and major presence
of PtII in our fresh catalyst, together with the notable amount
of PtIV observed in the used catalysts, points to an unprece-
dented PtII/PtIV mechanism which might operate in this catalyt-
ic system. However, more experimental studies are needed to
confirm this claim.
Table 2. Surface atomic ratios from XPS analysis.
Entry
Catalyst
Pt/Ti
O/Ti
Cl/Pt
1
2
3
4
Pt/TiO2 (calcined)
Pt/TiO2 (reduced)
Pt/TiO2 (used)
0.035
0.029
0.013
0.009
3.706
3.693
4.203
3.829
0.622
0.541
0.007
0.007
Pt/TiO2 (reactivated)
pares the fresh calcined catalyst with the used one, a strong
decrease in the Pt/Ti ratio is clearly observed (entries 1 and 3).
This indicates a loss of surface platinum, which could be
caused by metal sintering. However, the mild reaction condi-
tions applied allow discarding this possibility. This assertion is
also supported by the TEM images of the used catalyst, which
show that no modification has taken place in the particle size
distribution after reaction (Supporting Information). The possi-
bility of platinum leaching was also ruled out because the
amount of platinum found by inductively coupled plasma
mass spectrometry analysis of the reaction filtrate was as low
as 0.75 ppb. The third, most likely possibility, is the coverage of
surface platinum by reaction species.
The distribution of the platinum species clearly changes
during the reaction, as can be deduced by comparison of the
X-ray photoelectron spectra for the Pt4f region of the calcined
and used catalysts (Figure 1). It is clear that the proportion of
high-valent platinum (peaks at higher binding energies) is in-
creased upon reaction. Given the low amount of chlorine in
the used sample (Table 2, entry 3), these species are likely to
correspond to PtO2 and/or to Pt coordinated to electron-with-
drawing species (e.g., organoboron compounds). The notice-
able nearly complete dechlorination of the catalyst upon reac-
tion (Cl/Ti atomic ratio varies from 0.622 to 0.007) would indi-
cate that the chloride ligands are removed during the reaction
(entries 1 and 3).
Diboration of alkynes catalyzed by Pt/TiO2
The optimized catalyst and reaction conditions [Pt/TiO2
(1 wt%) 0.2 mol%, neat, air, 708C] were first applied to the di-
boration of terminal alkynes (Table 3). The kinetic profile for
the diboration of phenylacetylene reveals a polynomial in-
crease of the conversion versus time, with quantitative conver-
sion after 11 h (Figure 2). For this particular reaction, turnover
numbers and frequencies of up to 1935 and 176 hÀ1, respec-
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Table 3. Diboration of alkynes catalyzed by Pt/TiO2.[a]
Entry Alkyne
1
t [h] Product
16
Yield [%][b]
96
2
3
16
16
94
80[c]
4
5
6
16
16
16
88[c]
85[c]
84
Figure 2. Plot displaying the evolution of the diboration of phenylacetylene
catalyzed by Pt/TiO2.
tively were measured. Notably, although the addition of
B2(pin)2 to phenylacetylene catalyzed by Pt(PPh3)4 (DMF, 808C,
24 h)[3a] or Pt(NBE)3 (PPh3, PhMe, ambient temperature, 36 h)[5b]
was inefficient (ꢀ30% conversion), our catalytic system fur-
nished the corresponding product 2a in near quantitative
yield (Table 3, entry 1). The reaction conditions were equally ef-
fective for some other aromatic alkynes bearing electron-neu-
tral, electron-withdrawing, or electron-releasing groups (en-
tries 2–5) as well as for chlorinated aromatics (entries 6 and 7).
We next studied a range of aliphatic terminal alkynes; linear
alkyl (1h), branched (1i) and cycloalkyl-substituted (1j) acety-
lenes were diborated in high yields (entries 8–10). Relatively
lower yield was obtained in the case of the functionalized
chloroalkyl acetylene 1k (entry 11), whereas higher yield was
noted in the case of trimethylsilylacetylene (1l) (entry 12). Not
only terminal but internal alkynes were successfully subjected
to the standard conditions. Diphenylacetylene (1m) was found
to be, comparatively, more reluctant to react, probably be-
cause of the steric hindrance developed in the transition state
to the product 2m (entry 13). Other internal alkynes including
(methyl)phenylacetylene (1n), (trimethylsilyl)phenylacetylene
(1o), or even the aliphatic hex-3-yne (1p) gave rise to the ex-
pected products 2n–2p in good to high isolated yields (en-
tries 14–16). It is worth noting that exclusive cis-diboration was
observed for all the alkynes studied.
7
16
89
8
9
24
16
95
94
10
11
12
13
14
15
16
18
12
24
16
12
91
75
88
62[c]
92
Diboration of alkenes catalyzed by Pt/TiO2
Contrary to the behavior of other platinum catalysts,[3,9] this
methodology could be extended to the diboration of alkenes
(Table 4). Styrene (3a) was found to react much faster than
phenylacetylene (2a), reaching 82% conversion in only 10 min
and almost quantitative conversion after 2 h (Figure 3, Table 4,
entry 1). The halogenated styrenes 3b and 3c were successful-
ly diborated in good to high yields though required longer
heating (entries 2 and 3). The reactions were somewhat slug-
gish in the case of the cyclic olefins 3d and 3e, producing the
cis-diborated compounds 4d and 4e, respectively, in moderate
yields (entries 4 and 5). Nevertheless, the latter results were
much better than those achieved with Pt(dba)2 in toluene for
82[c]
16
18
85
[a] Reaction conditions: 1a (0.5 mmol), B2(pin)2 (0.5 mmol), catalyst
(20.0 mg, 0.2 mol%), in air at 708C. [b] Isolated yield after filtration of the
reaction crude (Celite and alumina) and washing with hexane. [c] Isolated
yield after purification by column chromatography (alumina, hexane/
ethyl acetate).
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smoothly underwent diboration, giving the 1,2-diboron alkane
in good yield (entry 6). In this case, we must highlight that no
isomers were formed resulting from b-hydride elimination pro-
cesses. Two vinyl silanes were subjected to the standard reac-
tion conditions, with triethoxy(vinyl)silane (3h) leading to the
expected product 4h in higher yield than 4g, derived from
trimethyl(vinyl)silane (3g, entries 7 and 8). A different trend
was observed for the a,b-unsaturated ester ethyl cinnamate
(3i) which, as expected, gave the b-monoborated product,
Table 4. Diboration of alkenes catalyzed by Pt/TiO2.[a]
Entry Alkene
1
t [h] Product
Yield [%][b]
92
2
2
3
16
16
81[c]
90
probably following
(entry 9).[24]
a
1,4-addition/hydrolysis sequence
An advantageous feature of this methodology is that, in
most cases of alkyne and alkene diboration, the reaction
crudes were pure enough to make purification by chromato-
graphic methods unnecessary; products could be obtained
after simple filtration and solvent evaporation.
4
5
24
24
46[c]
55
Chemical transformations of the cis-1,2-diborylalkenes
The versatility of the 1,2-diboronvinyl compounds was demon-
strated in a series of organic transformations, including the
Suzuki–Miyaura cross coupling and the boron–halogen ex-
change (Scheme 1). If compound 2a was reacted with iodo-
benzene under standard Suzuki–Miyaura conditions,[4] 1,1,2-tri-
phenylethene (5) was obtained in quantitative yield. Treatment
of 2a with iodine under basic conditions furnished the
a-boron-b-iodo styrene derivative 6 in short reaction time and
excellent yield. The b-chloro (7) and b-bromo (8) analogues
were readily synthesized by reaction of 2a with the corre-
sponding copper(II) halides in aqueous THF. All these a-boron-
b-halo styrenes, which were obtained with retention of the ste-
reochemistry, are new valuable compounds, which might be
further used in coupling reactions. Oxidation of the bromo de-
rivative 8 led to a-bromoacetophenone (9) in good yield.
6
7
24
12
82
65
8
9
12
18
85
73[c]
[a] Reaction conditions: 1a (0.5 mmol), B2(pin)2 (0.5 mmol), catalyst
(20.0 mg, 0.2 mol%), in air at 708C. [b] Isolated yield after filtration of the
reaction crude (Celite and alumina) and washing with hexane. [c] Isolated
yield after purification by column chromatography (alumina, hexane/
ethyl acetate).
Conclusions
A variety of monometallic and bimetallic platinum-based sup-
ported catalysts were tested in the diboration of phenylacety-
lene. The catalyst consisting of platinum nanoparticles on tita-
nia exhibited the best performance under solvent- and ligand-
free conditions in air. This methodology was successfully ap-
plied to the diboration of a wide range of alkynes, including
terminal aryl and alkyl acetylenes as well as internal alkynes, in
all cases with exclusive cis diastereoselectivity. Interestingly,
not only alkynes but alkenes could be diborated under the
same simple reaction conditions. In general, products were ob-
tained in good to excellent isolated yields. X-ray photoelectron
spectroscopy data of different catalyst samples suggested the
participation of PtII/PtIV species in the catalytic cycle. The fact
that reactions proceed in the absence of solvent and ligand
under air, together with the generally applied easy purification
procedure and low catalyst loading, makes this an environ-
mentally benign approach to the synthesis of diboron com-
pounds. Moreover, taking additionally into account that the
processes are highly stereoselective with 100% atom economy,
it would be reasonable to include them within the set of reac-
Figure 3. Plot displaying the evolution of the diboration of styrene catalyzed
by Pt/TiO2.
these cyclic olefins, for which diboration was unsuccessful.[10a]
The lower yield could be attributed to the steric hindrance
arisen through the presence of two Bpin units in a cis disposi-
tion. Unlike cyclic olefins, terminal alkene oct-1-ene (3 f)
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Scheme 1. Chemical transformations of cis-1,2-diborylalkene 2a.
tions belonging to the so-called click chemistry.[25] Finally, we
also presented some interesting chemical transformations of
a cis-1,2-diboronvinyl derivative, including Suzuki–Miyaura and
boron–halogen exchange reactions.
spectrometer; fragment ions in m/z with relative intensities (%) in
parenthesis. HRMS analyses were also performed in the electron
impact mode (EI) at 70 eV on a Finnigan MAT95S spectrometer. Ele-
mental analyses were performed on a Thermo Finnigan Flash 1112
microanalyzer. The purity of volatile compounds and the chroma-
tographic analyses (gas–liquid chromatography, GLC) were deter-
mined with a Hewlett Packard HP-6890 instrument equipped with
a flame ionization detector and a 30 m capillary column (0.32 mm
diameter, 0.25 mm film thickness), using nitrogen (2 mLminÀ1) as
carrier gas, Tinjector =2708C, Tcolumn =608C (3 min) and 60–2708C
(158CminÀ1); retention times (tr) are given in min. Thin layer chro-
matography was performed on silica gel precoated aluminum
sheets (Macherey–Nagel). Column chromatography was performed
by using silica gel 60 of 40–60 microns (hexane/EtOAc as eluent).
Experimental Section
General
Degussa P25 TiO2 (60% anatase, 40% rutile) with a surface area of
50 m2 gÀ1 (N2, À1958C, BET method) was used as a support, after
its calcination in air at 5008C for 5 h. X-ray photoelectron spectra
(XPS) were acquired with a VG-Microtech Multilab 3000 spectrome-
ter equipped with a hemispherical electron analyzer and a MgKa
(h=1253.6 eV, 1 eV=1.602”10À19 J) 300 W X-ray source. Conven-
tional TEM analysis was performed with a Jeol model JEM-210 elec-
tron microscope working at 200 kV and equipped with a INCA
Energy TEM100 analytical system and a SIS Mega View II camera.
Samples for analysis were suspended in methanol and placed on
copper grids with a holey carbon film support.
It is noteworthy that the carbon atoms attached to boron were
not observed in the 13C NMR spectra of the new compounds char-
acterized below. It is often the case that these signals are very
broad and cannot be detected.
[26]
Preparation of Pt/TiO2
All reagents were of the best grade commercially available and
were used without further purification. All reactions were per-
formed without air exclusion on a multireactor apparatus using the
corresponding reactor tubes. Melting points were measured with
a Reichert Thermovar apparatus and are uncorrected. NMR spectra
were recorded on Bruker Avance 300, 400, and 500 spectrometers
(300 and 400 MHz for 1H NMR; 75 and 101 MHz for 13C NMR;
160 MHz for 11B NMR) by using CDCl3 as solvent and TMS as inter-
nal standard. Infrared analysis was performed with a Jasco 4100 LE
(Pike MIRacle ATR) spectrophotometer; wavenumbers are given in
cmÀ1. Mass spectra (EI) were obtained at 70 eV on an Agilent 5973
Typical procedure: The Pt/TiO2 catalyst was prepared by the im-
pregnation method with an aqueous solution of H2PtCl6·6H2O (Al-
drich) of the appropriate concentration to achieve a Pt content of
1 wt%. The slurry (10 mLgÀ1 of TiO2) was stirred for 12 h and the
excess of solvent was removed by heating at 908C under vacuum
in a rotary evaporator. Finally, the catalyst was dried at 1108C for
24 h and calcined at 4008C for 5 h, with a heating rate of
38CminÀ1
.
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(24), 311 (10), 272 (12), 271 (10), 270 (34), 269 (15), 237 (14), 231
(34), 230 (26), 229 (100), 228 (46), 107 (11), 85 (13), 84 (69), 83 (77),
81 (12), 69 (23), 59 (19), 55 (21). Elemental analysis calcd. for
C18H33B2ClO4: C 58.35, H 8.98; Found: C 58.48, H 9.12.
Diboration reaction
General procedure: The alkyne or alkene (0.5 mmol), B2(pin)2
(0.5 mmol, 127.0 mg) and the catalyst (Pt/TiO2, 20.0 mg, 0.2 mol%)
were stirred during the specified time at 708C (see Tables 3 and 4).
The progress of the reaction was monitored by TLC and GLC until
total or steady conversion was achieved. The resulting mixture was
dissolved in hexane (20.0 mL) followed by the addition of basic
alumina and removal of the solvent in vacuo. The reaction crude
absorbed on alumina was subjected to column chromatography
(basic alumina, hexane/EtOAc) to give the pure borylated com-
pounds 2 or 4. In the cases in which the reaction was complete
(see Tables 2 and 3), the mixture was filtered through a pad con-
taining Celite and alumina and washed with hexane giving, after
solvent evaporation, the expected products without any further
purification. Compounds 2a,[3b] 2b,[27] 2d,[28] 2e,[5b] 2h,[3a] 2i,[27]
2j,[3a] 2m,[3a] 2n,[4] 2p,[4] 4a,[5b] 4d,[17] 4e,[13b] 4 f,[17] and 4i[29] were
characterized by comparison of their physical and spectroscopic
data with those reported in the literature. Data for the new com-
pounds follow:
(E)-[1,2-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)vinyl]trime-
thylsilane (2l): Pale yellow semisolid; tR 13.31; Rf 0.56 (hexane/
EtOAc 9.5:0.5); IR: n˜ =2977, 2927, 1584, 1460, 1368, 1329, 1298,
1
1230, 1138, 970, 934, 857, 837 cmÀ1; H NMR (400 MHz, CDCl3): d=
0.10 (s, 9H), 1.26 (s, 12H), 1.33 (s, 12H), 6.59 ppm (s, 1H); 13C NMR
(101 MHz, CDCl3): d=À1.3, 25.1, 25.4, 83.6, 83.7 ppm; 11B NMR
(160 MHz, CDCl3): d=33.23 ppm; MS m/z 352 (M+, 1), 337 (15), 294
(10), 101 (16), 85 (13), 84 (100), 83 (75), 73 (15), 69 (28), 55 (20). Ele-
mental analysis calcd. for C17H34B2O4Si: C 57.98, H 9.73; Found: C
58.10, H 9.92.
(Z)-Trimethyl[2-phenyl-1,2-bis(4,4,5,5-tetramethyl-1,3,2-dioxaboro-
lan-2-yl)vinyl]silane (2o): White solid; m.p. 121.0–123.98C; tr 12.69;
Rf 0.56 (hexane/EtOAc 9.5:0.5); IR: n˜ =2977, 2926, 1371, 1304, 1292,
1
970, 839, 754, 700 cmÀ1; H NMR (400 MHz, CDCl3): d=0.25 (s, 9H),
0.99 (s, 12H), 1.22 (s, 12H), 7.13–7.18 (m, 1H), 7.20–7.23 ppm (m,
4H); 13C NMR (101 MHz, CDCl3): d=1.1, 25.1, 25.3, 83.4, 83.8, 126.7,
127.6, 127.9, 147.2 ppm; 11B NMR (160 MHz, CDCl3): d=29.85 ppm;
MS m/z 370 (M+, 1), 313 (12), 312 (24), 311 (10), 272 (12), 271 (10),
270 (34), 269 (15), 237 (14), 231 (34), 230 (26), 229 (100), 228 (46),
107 (11), 85 (13), 84 (69), 83 (77), 81 (12), 69 (23), 59 (19), 55 (21).
Elemental analysis calcd. for C23H38B2O4Si: C 64.51, H 8.94; Found:
C 64.72, H 8.82.
(E)-2,2’-[1-(m-Tolyl)ethene-1,2-diyl]bis(4,4,5,5-tetramethyl-1,3,2-diox-
aborolane) (2c): Yellow oil; tr 16.99; Rf 0.88 (hexane/EtOAc 9.5:0.5);
IR: n˜ =2976, 2950, 1600, 1371, 1332, 1332, 1314, 1139, 969, 851,
1
838 cmÀ1; H NMR (300 MHz, CDCl3): d=1.30 (s, 12H), 1.38 (s, 12H),
2.32 (s, 3H), 6.27 (s, 1H), 7.05–7.08 (m, 1H), 7.18–7.21 (m, 2H),
7.23–7.25 ppm (m, 1H); 13C NMR (75 MHz, CDCl3): d=21.6, 25.0,
25.2, 83.7, 84.2, 123.8, 127.5, 128.3, 128.5, 137.9, 143.2 ppm;
11B NMR (160 MHz, CDCl3): d=31.21 ppm; MS m/z 370 (M+, 0.1),
281 (29), 253 (10), 228 (12), 209 (10), 208 (18), 207 (100), 96 (11), 84
(48). Elemental analysis calcd. for C21H32B2O4: C 68.15, H 8.72;
Found: C 68.48, H 8.92.
2,2’-[1-(4-Bromophenyl)ethane-1,2-diyl]bis(4,4,5,5-tetramethyl-1,3,2-
dioxaborolane) (4b): White solid; m.p. 82.5–85.78C; tr 17.23; Rf 0.68
(hexane/EtOAc 9.5:0.5); IR: n˜ =2977, 2927, 1483, 1370, 1354, 1320,
1
1138, 1006, 966, 841, 820, 690 cmÀ1; H NMR (300 MHz, CDCl3): d=
(E)-2,2’-[1-(4-Chlorophenyl)ethene-1,2-diyl]bis(4,4,5,5-tetramethyl-
1,3,2-dioxaborolane) (2 f): Yellow oil; tr 17.69; Rf 0.73 (hexane/EtOAc
9.5:0.5); IR: n˜ =3040, 2977, 2932, 1596, 1488, 1378, 1372, 1332,
1.08 (dd, J=16.0, 5.9 Hz, 1H), 1.17 (s, 6H), 1.18 (s, 6H), 1.20 (s,
12H), 1.34 (dd, J=16.0, 10.7 Hz, 1H), 2.48 (dd, J=10.6, 5.9 Hz, 1H),
7.07–7.12 (m, 2H), 7.31–7.36 ppm (m, 2H); 13C NMR (75 MHz,
CDCl3): d=24.6, 24.8, 25.1, 83.3, 83.5, 118.7, 129.9, 131.3,
144.6 ppm; 11B NMR (160 MHz, CDCl3): d=23.67 ppm; MS m/z 436
(M+, 1), 297 (21), 296 (13), 295 (23), 294 (11), 84 (100), 83 (24), 69
(13). Elemental analysis calcd. for C20H31B2BrO4: C 54.97, H 7.15;
Found: C 55.12, H 7.36.
1
1316, 1214, 1138, 1091, 968, 849, 834, 822 cmÀ1; H NMR (300 MHz,
CDCl3): d=1.31 (s, 12H), 1.37 (s, 12H), 6.27 (s, 1H), 7.27–7.29 (m,
2H), 7.34–7.37 ppm (m, 2H); 13C NMR (75 MHz, CDCl3): d=25.0,
25.2, 83.9, 84.4, 128.1, 128.6, 133.6, 141.7 ppm; 11B NMR (160 MHz,
CDCl3): d=30.97 ppm; MS m/z 390 (M+, 6), 334 (12), 333 (11), 332
(14), 251 (36), 250 (29), 249 (100), 248 (47), 209 (15), 191 (20), 84
(95), 83 (16), 69 (18), 55 (10). Elemental analysis calcd. for
C20H29B2ClO4: C 61.51, H 7.49; Found: C 61.82, H 7.79.
2,2’-[1-(4-Chlorophenyl)ethane-1,2-diyl]bis(4,4,5,5-tetramethyl-1,3,2-
dioxaborolane) (4c): White solid; m.p. 77.5–80.58C; tr 17.23; Rf 0.72
(hexane/EtOAc 9.5:0.5); IR: n˜ =2979, 2930, 1486, 1357, 1321, 1263,
;
1211, 966, 840, 822, 705 cmÀ1 1H NMR (300 MHz, CDCl3): d=1.08
(dd, J=16.0, 5.9 Hz, 1H), 1.17 (s, 6H), 1.19 (s, 6H), 1.20 (s, 12H),
1.34 (dd, J=16.0, 10.6 Hz, 1H), 2.50 (dd, J=10.6, 5.9 Hz, 1H), 7.12–
7.21 ppm (m, 4H); 13C NMR (75 MHz, CDCl3): d=24.6, 24.8, 25.1,
83.3, 83.5, 128.3, 129.4, 130.7, 144.1 ppm; 11B NMR (160 MHz,
CDCl3): d=22.27 ppm; MS m/z 392 (M+, 1), 253 (11), 251 (31), 250
(14), 84 (100), 83 (17), 69 (13). Elemental analysis calcd. for
C20H31B2ClO4: C 61.20, H 7.96; Found: C 61.40, H 8.82.
(E)-2,2’-[1-(2-Chlorophenyl)ethene-1,2-diyl]bis(4,4,5,5-tetramethyl-
1,3,2-dioxaborolane) (2g): Yellow oil; tr 16.87; Rf 0.90 (hexane/
EtOAc 9.5:0.5); IR: n˜ =3040, 2977, 2932, 1603, 1467, 1387, 1378,
;
1370, 1319, 1213, 1137, 968, 848, 757, 737, 679 cmÀ1 1H NMR
(300 MHz, CDCl3): d=1.32 (s, 12H), 1.33 (s, 12H), 6.19 (s, 1H), 7.15–
7.21 (m, 2H), 7.22–7.25 (m, 1H), 7.26–7.31 ppm (m, 1H); 13C NMR
(75 MHz, CDCl3): d=25.0, 25.1, 83.8, 84.2, 127.1, 128.4, 129.0, 130.0,
132.2, 144.1 ppm; 11B NMR (160 MHz, CDCl3): d=29.99 ppm; MS m/
z 390 (M+, 0.5), 334 (19), 333 (16), 332 (13), 251 (38), 250 (29), 249
(100), 248 (46), 191 (20), 190 (11), 129 (29), 128 (13), 84 (75), 83
(16). Elemental analysis calcd. for C20H29B2ClO4: C 61.51, H 7.49;
Found: C 61.91, H 7.78.
[1,2-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)ethyl]trimethylsi-
lane (4g): White solid; m.p. 48.6–52.88C; tr 13.03; Rf 0.46 (hexane);
IR: n˜ =2976, 2927, 1377, 1360, 1339, 1304, 1240, 1140, 1164, 855,
1
(E)-2,2’-(6-Chlorohex-1-ene-1,2-diyl)bis(4,4,5,5-tetramethyl-1,3,2-di-
oxaborolane) (2k): Pale yellow oil; tr 15.69; Rf 0.57 (hexane/EtOAc
9.5:0.5); IR: n˜ =2977, 2937, 1618, 1377, 1333, 1306, 1137, 968, 859,
835 cmÀ1; H NMR (400 MHz, CDCl3): d=0.01 (s, 9H), 0.39 (dd, J=
12.1, 3.1 Hz, 1H), 0.77 (dd, J=16.3, 3.1 Hz, 1H), 0.89 (dd, J=16.3,
12.1 Hz, 1H), 1.22 (s, 12H), 1.23 ppm (s, 12H); 13C NMR (101 MHz,
CDCl3): d=À1.8, 24.8, 24.9, 25.2, 82.7, 83.0 ppm; 11B NMR
(160 MHz, CDCl3): d=34.34 ppm; MS m/z 354 (M+, 1), 339 (16), 296
(15), 257 (15), 255 (11), 254 (51), 253 (24), 239 (17), 213 (48), 212
(23), 197 (26), 196 (13), 157 (15), 156 (19), 155 (44), 154 (19), 143
(12), 113 (11), 101 (16), 85 (13), 73 (21), 69 (25), 55 (23). Elemental
1
850 cmÀ1; H NMR (400 MHz, CDCl3): d=1.26 (s, 12H), 1.30 (s, 12H),
1.53–1.62 (m, 2H), 1.73–1.81 (m, 2H), 2.25 (td, J=7.6, 1.3 Hz, 2H),
3.52 (t, J=6.7 Hz, 2H), 5.86 ppm (s, 1H); 13C NMR (101 MHz, CDCl3):
d=25.0, 25.1, 25.9, 32.2, 39.9, 45.2, 83.5, 88.9 ppm; 11B NMR
(160 MHz, CDCl3): d=30.34 ppm; MS m/z 370 (M+, 1), 313 (12), 312
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analysis calcd. for C17H36B2O4Si: C 57.65, H 10.25; Found: C 57.74, H
10.43.
7.22–7.33 ppm (m, 5H); 13C NMR (75 MHz, CDCl3): d=24.9, 84.6,
126.9, 127.4, 127.6, 128.8, 138.9 ppm; 11B NMR (160 MHz, CDCl3):
d=30.68 ppm; MS m/z 266 (M+ +2, 264), 265 (M+ +1, 23), (M+,
100), (33), 263 (25), 249 (18), 229 (53), 228 (18), 206 (13), 193 (19),
192 (11), 191 (58), 190 (15), 187 (25), 186 (15), 185 (17), 166 (21),
165 (14), 164 (59), 163 (15), 149 (10), 147 (26), 144 (21), 143 (48),
129 (51), 128 (22), 103 (40), 102 (33), 85 (10), 77 (27), 59 (11). Ele-
mental analysis calcd. for C14H18BClO2: C 63.56, H 6.86; Found: C
63.96, H 7.06.
[1,2-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)ethyl]triethoxysi-
lane (4h): Colorless oil; tr 14.84; Rf 0.50 (hexane); IR: n˜ =2975,
2926, 2883, 1369, 1343, 1308, 1141, 1103, 1078, 954, 781, 761 cmÀ1
;
1H NMR (400 MHz, CDCl3): d=0.67 (dd, J=10.8, 4.7 Hz, 1H), 1.19–
1.26 (m, 35H), 3.83 ppm (q, J=7.0 Hz, 6H); 13C NMR (101 MHz,
CDCl3): d=18.4, 24.5, 24.9, 25.0, 25.2, 58.6, 82.9, 83.1 ppm; 11B NMR
(160 MHz, CDCl3): d=34.95 ppm; MS m/z 444 (M+, 1), 386 (11), 328
(17), 313 (16), 286 (13), 273 (14), 272 (56), 271 (17), 259 (10), 257
(19), 245 (36), 244 (22), 243 (11), 217 (27), 216 (25), 215 (67), 214
(20), 205 (12), 189 (18), 188 (10), 179 (10) 178 (21), 176 (10), 163
(36), 161 (30), 160 (20), 159 (10), 143 (17), 142 (10), 133 (13), 119
(15), 115 (12), 85 (14), 84 (100), 83 (87), 79 (12), 69 (34), 55 (27). Ele-
mental analysis calcd. for C20H42B2O7Si: C 54.07, H 9.53; Found: C
54.39, H 9.78.
(E)-2-(2-Bromo-1-phenylvinyl)-4,4,5,5-tetramethyl-1,3,2-dioxaboro-
lane (8): Yellow oil; tr 13.69; Rf 0.75 (hexane/EtOAc 9:1); IR: n˜ =
2976, 2927, 1490, 1441, 1364, 1319, 1211, 1138, 969, 851, 758, 727,
1
695 cmÀ1; H NMR (400 MHz, CDCl3): d=1.38 (s, 12H), 6.88 (s, 1H),
7.28–7.30 ppm (m, 5H); 13C NMR (75 MHz, CDCl3): d=24.9, 84.7,
115.3, 126.7, 127.7, 128.8, 140.1 ppm; 11B NMR (160 MHz, CDCl3):
d=30.35 ppm; MS m/z 311 (M+ +3, 12), 310 (M+ +2, 78), 309 (M+
+1, 30), 308 (M+, 79), 293 (13), 237 (12), 235 (12), 229 (32), 210
(32), 209 (13), 208 (32), 187 (50), 186 (18), 185 (33), 171 (34), 170
(11), 144 (13), 143 (100), 130 (11), 129 (62), 128 (29), 128 (29), 103
(42), 102 (27), 85 (11), 77 (32), 59 (16). Elemental analysis calcd. for
C14H18BBrO2: C 54.42, H 5.87; Found: C 54.81, H 5.99.
Suzuki–Miyaura reaction of 2a and iodobenzene
Following a literature procedure,[4] compound 5 was obtained in
quantitative yield. Compound 5 was characterized by comparison
of its physical and spectroscopic data with those of an authentic
commercial sample.
Oxidation of compound 8
Following a literature procedure,[32] compound 9 was obtained in
70% yield. Compound 9 was characterized by comparison of its
physical and spectroscopic data with those of an authentic com-
mercial sample.
Iodination of compound 2a
[30]
Typical procedure:
A solution of NaOH (2.0 mL, 3N) was added
to the diboron compound 2a (0.5 mmol, 178.0 mg) dissolved in
Et2O (2.0 mL). The mixture was cooled to 08C and a solution of
iodine (304.6 mg, 1.2 mmol) in Et2O (4.0 mL) was added. The result-
ing mixture was stirred for 30 min and the reaction crude was
washed with a saturated solution of Na2S2O3 (2”5.0 mL) and water
(10.0 mL). The organic phase was extracted with Et2O (2”15.0 mL),
dried over MgSO4 and filtrated. The solvent was removed under
vacuum giving the product 6 as a yellow oil (169.1 mg, 95%).
Acknowledgements
This work was generously supported by the Spanish Ministerio de
Economꢀa
y Competitividad (MINECO; CTQ2007-65218 and
CTQ2011-24151), the Generalitat Valenciana (GV; PROMETEO/
2009/002 and PROMETEO/2009/039), and FEDER.
(E)-2-(2-Iodo-1-phenylvinyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane
(6): Yellow oil; tr 13.13; Rf 0.75 (hexane/EtOAc 9:1); IR: n˜ =3040,
2981, 2922, 1490, 1441, 1357, 1318, 1209, 1137, 968, 851, 754,
Keywords: alkenes · alkynes · boron · nanoparticles · platinum
1
697 cmÀ1; H NMR (400 MHz, CDCl3): d=1.39 (s, 12H), 6.98 (s, 1H),
7.27–7.30 ppm (m, 5H); 13C NMR (75 MHz, CDCl3): d=25.1, 84.8,
87.3, 126.4, 127.7, 128.7, 142.2 ppm; 11B NMR (160 MHz, CDCl3): d=
30.40 ppm; MS m/z 356 (M+, 100), 357 (M+ +1, 16), 355 (22), 256
(12), 230 (11), 229 (68), 228 (15), 143 (32), 130 (10), 129 (52), 128
(23), 103 (18), 102 (16), 101 (30), 83 (44), 77 (22), 55 (11). Elemental
analysis calcd. for C14H18B2IO2: C 47.23, H 5.10; Found: C 47.48,
5.25.
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Chlorination and bromination of compound 2a
Typical procedure:[31] CuCl2 (0.5 mmol, 67.2 mg) or CuBr2 (0.5 mol,
112.2 mg) was added to a solution of the diboron compound 2a
(0.50 mmol, 178.0 mg) in THF (1.0 mL) and water (1.0 mL). After
stirring for 1 h, the resulting mixture was extracted with EtOAc
(20.0 mL) and the organic layer was dried over MgSO4. After filtra-
tion and solvent evaporation, the residue was purified by column
chromatography (basic alumina, hexane/EtOAc) to give the prod-
ucts 7 (88.4 mg, 62%) and 8 as yellow oils (150.9 mg, 98%).
(E)-2-(2-Chloro-1-phenylvinyl)-4,4,5,5-tetramethyl-1,3,2-dioxaboro-
lane (7): Yellow oil; tr 13.09; Rf 0.76 (hexane/EtOAc 9:1); IR: n˜ =
2982, 2925, 1491, 1444, 1359, 1320, 1209, 1142, 967, 851, 757,
1
698 cmÀ1; H NMR (400 MHz, CDCl3): d=1.37 (s, 12H), 6.73 (s, 1H),
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[6] C. N. Iverson, M. R. Smith III., Organometallics 1996, 15, 5155–5165.
[7] a) H. Braunschweig, T. Kupfer, M. Lutz, K. Radacki, F. Seeler, R. Sigritz,
Angew. Chem. 2006, 118, 8217–8220; Angew. Chem. Int. Ed. 2006, 45,
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Received: November 5, 2013
Revised: December 23, 2013
Published online on February 6, 2014
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