On the scope of the Pt-catalyzed Srebnik diborylation of diazoalkanes. An efficient approach to chiral tertiary boronic esters and alcohols via B-stabilized carbanions

Article abstract of DOI:10.1016/j.tetlet.2014.03.135

Conditions milder than those previously reported are shown to be generally applicable to the Pt-catalyzed insertion of non-carbonyl-stabilized diazoalkanes into B₂pin₂. Selective transformation of one (pinacolato)boryl unit in the products is enabled by rapid, low-temperature deboronation in the presence of a Lewis base and genesis of a highly reactive B-stabilized carbanion.

Full text of DOI:10.1016/j.tetlet.2014.03.135

Tetrahedron Letters 55 (2014) 3149–3152
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Tetrahedron Letters
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On the scope of the Pt-catalyzed Srebnik diborylation
of diazoalkanes. An efficient approach to chiral tertiary
boronic esters and alcohols via B-stabilized carbanions
Andrew J. Wommack a, , Jason S. Kingsbury a,b,
⇑
a Department of Chemistry, Eugene F. Merkert Chemistry Center, Boston College, Chestnut Hill, MA 02467, United States
b Department of Chemistry, Ahmanson Science Center, California Lutheran University, 60 West Olsen Road, Thousand Oaks, CA 91360, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Conditions milder than those previously reported are shown to be generally applicable to the Pt-cata-
lyzed insertion of non-carbonyl-stabilized diazoalkanes into B2pin2. Selective transformation of one
(pinacolato)boryl unit in the products is enabled by rapid, low-temperature deboronation in the presence
of a Lewis base and genesis of a highly reactive B-stabilized carbanion.
Received 28 February 2014
Revised 31 March 2014
Accepted 31 March 2014
Available online 8 April 2014
Ó 2014 Published by Elsevier Ltd.
Keywords:
Diazoalkanes
1,1-Diborylalkanes
Deboronation
B-stabilized carbanions
The CAB bond holds a special status in organic synthesis,
unrivaled as a handle for further elaboration through oxidative,1
cross-coupling,2 or homologation3 strategies. The latter of these
invariably requires introduction of a carbenoid (donor–acceptor)
reagent for complexation with the electrophilic boron atom prior
to 1,2-rearrangement. Along with the highly reactive 1-lithio-1-
halogenoalkanes popularized by Matteson,3 sulfur ylides4 and
R'
B
X
B
R'
X
R1
N2
R1
R2
+
R'
R1 R2
+
N2
B
X
R2
X
X
X
N2
R' = alkyl or BX2
boron ate complex
carbon insertion product
R2B
EtO
O
O
R
Bcat
metalated
a
-chloro sulfoxides4b can be applied to stereoretentive
R
EtO
BR2
R
Si(CH3)3
carbofunctionalization of alkylborons. Hooz, however, was the
pioneer of CAB homologations with diazo compounds.5 With its
reliance on stabilized diazoester and diazoketone nucleophiles, a
protodeborylation,
Aldol/Mannich, etc.
a
(ref. 5)
b
(ref. 6a)
O
B
(alkyl)3B
R1, R2 = H, CO2Et
N2
R1, R2 = H, TMS
Hooz reaction furnishes
a-boryl carbonyls that undergo further
O
alkyl
R1
R2
Aldol, Mannich, or alkylation events, but the process is limited to
trialkylboranes because of their greater electrophilicity relative to
boronic acids or esters (Scheme 1a). Recent work6 has broadened
the reactivity to include reaction of trimethylsilyldiazomethane
c
(ref. 6b)
d
(ref. 11)
R3
pinB
Bpin
R1, R2 = H, Ar, or alkyl
B
O
O
O
O
B R3
pinB
Bpin
R2
Ar
OEt
B
and a-substituted diazoesters with catecholboranes and boroxines
R1
R1, R2 = Ar, CO2Et
R3
R3
(respectively, Scheme 1b and c), findings that hint at generality for
a range of diazoalkanes and boryl acceptors. In our own laboratory,
some experience has been gained in the synthesis7 and solution-
phase handling8 of diverse non-carbonyl-stabilized diazoalkanes,
and catalytic methods for ketone, aldehyde, and formaldehyde
Scheme 1. Varied approaches to CAB and BAB homologation with donor–acceptor
diazo compounds by complexation and 1,2-migration.
homologation have been developed.9 To further expand on the
synthetic utility of these unique carbon nucleophiles, we began
to explore various B-based electrophiles in formal carbon insertion.
⇑
Corresponding author. Tel.: +1 (805) 493 3026.
E-mail address: jkingsbu@callutheran.edu (J.S. Kingsbury).
Current address: Department of Chemistry, High Point University, 833 Montlieu
Avenue, High Point, NC 27262, United States.
http://dx.doi.org/10.1016/j.tetlet.2014.03.135
0040-4039/Ó 2014 Published by Elsevier Ltd.

3150
A. J. Wommack, J. S. Kingsbury / Tetrahedron Letters 55 (2014) 3149–3152
Table 1
Of the available boron reagents to consider for reaction devel-
Doubly C-substituted 1,1-diborons by diazoalkyl insertiona
opment, diboronic esters are among the most intriguing. Formal
diazoalkyl insertion into a BAB bond gives mono- and di-substi-
tuted 1,1-diborylalkanes10,11—molecules that have captured the
interest of multiple research groups.12,13 The common mode of
preparing these building blocks is a double hydroboration14 of ter-
minal alkynes, for which 9-BBN14c,d is the preferred reagent in the
absence of Rh or Cu(I) catalysis.15 However, dihydroboration is not
general; the use of internal alkyne substrates leads to regioisomeric
mixtures, and this has created a gap in reaction scope for doubly
carbon-substituted geminal diborons. Very recently, Wang and
coworkers11a reported a simple and convenient entry to 1,1-
bis(pinacolato)diborylalkanes by merging Bamford–Stevens N-tos-
ylhydrazone cleavage with high-temperature homologation of
B2pin2 (Scheme 1d). A range of base-stable BCB products were pre-
pared, but a limitation was identified in the case of internal,
ketone-derived diazoalkanes: steric hindrance and low solubility
of the intermediate tosylhydrazone sodium salts led to diminished
yields. Therefore, the Pt(0)-catalyzed insertion of diazoalkanes into
B2pin2, a novel transformation first discovered by Srebnik10 with
diazomethane, remains a more efficient approach to fully C-substi-
tuted, sp3 hybridized 1,1-diborons. Herein, we disclose the results
of our own independent study on the scope of the catalytic
Srebnik10 diborylation of ‘non-stabilized’ diazo compounds. Specif-
ically, we have lowered the temperature needed (80 °C) to observe
facile carbon insertion with B2pin2 and further widened prepara-
tive access to the more challenging disubstituted adducts. Encour-
aged by the promise of gem-diborylalkanes as precursors to
valuable quaternary carbon atoms, we further report that the prod-
ucts undergo a clean, high-yield conversion to chiral tertiary alkyl
(pinacolato)borons by Lewis base-induced deboronation and SN2
alkylation of the resulting B-stabilized carbanion (Scheme 2).
We began our studies by screening conditions reported for the
syntheses of 1 and 6 (Table 1, entries 1 and 6), two of the four com-
pounds that had been prepared earlier by Srebnik in 2002.10b
Ni(PCy3)2, Pd(PPh3)4, and Pd2dba3 all failed to provide any of the
desired insertion product, but smooth conversion was observed
with Pt(PPh3)4. At a catalyst loading of 3 mol %, transformation of
pure methyl phenyl diazomethane16 to diboronic ester 1 was rapid
enough to permit a lower reaction temperature (110 ? 80 °C) and
a shorter reaction time (6 h, >98% conversion of B2pin2). Reaction
progress can be monitored by visual inspection due to the brightly
colored appearance of the diazoalkane, but regardless a 6 h dura-
tion was applied to a selection of aryl alkyl diazomethanes in warm
toluene.
O
B
O
B
1.0 equiv B2pin2
N
N
R2
3 mol % Pt(PPh3)4
O
O
+
N2
toluene, 80 °C, 6 h
R2
R1
R1
Entry Diazoalkane
Product
Yieldb (%)
79
N2
pinB Bpin
CH3
1 G = H
G = H
CH3
1
G
G
2
3
4
G = CH3
G = CF3
G = Cl
2 G = CH3
3 G = CF3
4 G = Cl
80
68
61
N2
N2
pinB Bpin
CH3
CH3
5
6
74
78
5
pinB Bpin
6
N2
pinB Bpin
7
8
76
71
7
H3CO
pinB Bpin
H3CO
N2
CH3
CH3
8
N2
pinB Bpin
CH3
9
9
81
75
84
CH3
N2
pinB Bpin
CH3
10c
11c
CH3
10
N2
N2
pinB Bpin
CH3
11
CH3
pinB Bpin
12c
70
H
H
The experimental results are compiled in Table 1. The method
tolerates both electron-donating and -withdrawing substituents
at the para position (entries 2–4, 7). Also of note is chemoselective
reaction with the diboron reagent in the case of p-chlorophenyl
methyl diazomethane (entry 4). The Miyaura borylation17 product
is not observed, an expected result given a slow rate of oxidative
addition of Pt(0) into the arylACl bond. Cyclic diazo compounds
can be employed to prepare diboryl tetrahydronaphthalenes and
indanes (entries 6 and 7). Importantly, reaction efficiency main-
tains in the presence of increased steric congestion, as demon-
strated by the syntheses of benzylic diborylalkanes 5 and 8 in
>70% yield (entries 5 and 8). Additional reactions of internal and
terminal alkyl diazoalkanes16 (entries 9–12) attest to the height-
12
a
Conditions: 1.1 equiv diazoalkane, 1 equiv B2pin2 and 3 mol % Pt(PPh3)4 in dry
toluene (0.1 M, 80 °C, 6 h).
b
Purified yields.
Run at 40 °C for 6 h.
c
ened reactivity that is observed in the absence of resonance delo-
calization. In these cases, complete conversion was achieved in
6 h at just 40 °C. All diboron products proved robust and bench-
stable, and they were typically isolated as colorless crystalline sol-
ids in 60–80% yield by standard flash silica gel chromatography.
At the temperatures reported herein, efficient diborylation is
not observed in the absence of Pt(0). This suggests that a Hooz
pathway involving diazoalkyl addition to B2pin2 and a BAB bond
shift (Scheme 1, top) is not a dominant source of products. Two
other mechanisms differing only in the site of nucleophilic attack
(B vs Pt) are provided in Scheme 3. With di(pinacolato)boryl Pt(II)
complex A well characterized by the work of Ishiyama et al.,18
Srebnik10 postulates that boronate formation and subsequent
3 mol %
1 equiv
pinB
Bpin
pinB
R3
R2
O
N2
MeLi, –78 °C
Pt(PPh3)4
R2
THF; then
R3–Br, 23 °C
R1
R2
R1
R2 B2pin2, 80 °C
toluene, 6 h
R1
R1
purified, titrated
as cold solution
12 examples,
61–84%yield
deboronation,
alkylation (>90%)
Scheme 2. A dual investigation on the scope of Pt-catalyzed Srebnik diazoalkane
diborylation and selective transformation of one B group.

A. J. Wommack, J. S. Kingsbury / Tetrahedron Letters 55 (2014) 3149–3152
3151
pinB
Pt
Bpin
R2
B2pin2
pinB
Ph CH3
CH3
Pt(PPh3)4
CH3
14
(90%)
HO
Ph CH3
R1
PPh3
CH3
boronate
mechanism
13
(88%)
PPh3
R1
R2
Br
pinB Pt PPh3
pinB
A
Br
CH3
Ph3P
pinB
PPh3
pinB Pt PPh3
R1
3M NaOH,
H2O2
pinB
pinB
N2
B
Li
CH3Li
HO
Ph CH3
O
O
R2
O
O
Br
Ph;
N2
R2
B
B
1
Ph3P
PPh3
3M NaOH, H2O2
carbene
mechanism
R1
Ph
CH3
Ph
CH3
R1
R2
pinB CH3
PPh3
PPh3
16
(96%)
pinB Pt
pinB
pinB
pinB
Pt
C
Br
3M NaOH,
Br
H2O2
N2
pinB
HO
15
(92%)
17
(97%) Ph CH3
Scheme 3. Two possible catalytic cycles for formal carbon insertion.
Ph CH3
1,2-migration afford intermediate B. Reductive elimination then
turns over the catalyst. Alternatively, metalation at platinum(II)
could occur after dissociation of a phosphine ligand. Formation of
carbene19 C, followed by migratory insertion and reductive elimi-
nation, would also complete the catalytic cycle.20
1,1-Diborylalkanes are useful synthons marked by a growing
list of strategies for further transformation. Of particular value
are those that chemoselectively divert one boryl unit toward
CAC bond formation. For monosubstituted derivatives with an
Scheme 5. 1,1-Diborons are precursors to tertiary boronic esters and alcohols due
to the ease with which deboronation/substitution occurs.
the same experiment was repeated, this time with benzyl bromide
(1 equiv) added to outcompete the n-butyl bromide formed in situ.
Homobenzylic tertiary alcohol 16 (see Scheme 5) was recovered in
91% purified yield under the conditions of this experiment.
Survey of the literature reveals a number of examples in which
terminal gem-organoborons undergo based-induced deborylation
as part of another transformation. 1,1-diborane intermediates give
only primary alcohols upon direct oxidation due to the rapid rate of
protodeboronation,14b,23 but the use of H2O2 in acidic medium24
furnishes the expected aldehydes in good yield. Treatment with 2
equiv of n-BuLi gives dianionic 1-lithio-1-borato alkanes25 that
can be quenched with carbon dioxide to yield alkylmalonic acids.
Use of only 1 equiv of n-BuLi produces 1-lithio boranes which
can add to aldehydes and ketones to form alkenes.26 A boron
enolate has been prepared from 1,1-diborylhexane by sequential
exposure to methyllithium and methyl benzoate.27 Finally, gem-
diborylalkanes presenting a leaving group at the 3- or 4-position
have given rise to the corresponding cyclopropyl or cyclobutyl
boranes by intramolecular cyclization.14a,b Thus, despite useful
a-CAH, deprotonation with LTMP and nucleophilic addition/
elimination with ketones gives alkenyl boronates that are direct
precursors to tetrasubstituted olefins through Suzuki–Miyaura
cross coupling.21 Shibata and coworkers also identified mild condi-
tions in which terminal pinacol 1,1-diboronic esters participate
outright as cross coupling nucleophiles.22 In this context, enabling
features are an incipient hydroxy boronate that serves to direct
transmetalation and stabilization of the resulting aryl Pd-alkyl
intermediate by the remaining, proximal boronyl. Hall and
coworkers15b achieved similar success with chiral 1,1-diborons
prepared by catalytic asymmetric conjugate borylation in which
chemoselectivity for arylation or vinylation derives from differen-
tiated boron groups.
precedents for forming resonance-delocalized
a-boryl anions by
Judging that severe steric crowding would pose new challenges
for Pd-catalyzed cross coupling, we chose a more classical mode of
reaction to initially probe the utility of the quaternary diboron
products. As shown in Scheme 4, a Matteson homologation was
attempted under the standard conditions for chloromethyl lithium
formation in situ, and a basic peroxide workup was performed to
oxidize any remaining CAB bonds. Naively, we were surprised to
find that none of the homologous quaternary 1,2-diol is formed
under these conditions. Rather, a tertiary alcohol incorporating a
butyl fragment (13, Scheme 4) is produced cleanly in high yield.
It became clear that n-butyl bromide, the byproduct of lithium-
bromine exchange, had served as electrophile in an unforeseen
SN2 alkylation. Instead of rendering the attached CAB bond more
electron-rich toward 1,2-migration, ate complex formation allows
for rapid deboronation. The result is a B-stabilized carbanion that
clearly represents a competent electrofuge. To test this notion,
metalation/deborylation, BACAB structures stay underappreciated
as precursors to tertiary organoborons by a simple substitution
event.28 Moreover, to our knowledge, we are the first to conduct
such a sequence on an internal, fully substituted diboronic ester.
Additional experiments that underscore the remarkable facility
with which quaternary gem-diborons experience net substitution
Table 2
Performance of
a
-boryl anion in boron–Wittig olefinationsa
1 equiv MeLi
CH3
THF, –78 °C, 10 min;
O
O
Ar
B
O
B
then
–78 °C to
23 °C, 3 h
Ar
O
O
H
CH3
Ph
1
H
CN
CH3
CH3
CH3
77%
(E/Z 5.3:1)
81%
(E/Z 4.8:1)
H
18
H
21
OCH3
a) 1 equiv nBuLi,
1 equiv CH2BrCl,
–78 to 23 °C, THF
CH3
Quaternary C
building block:
HO
Ph CH3
O
B
O
B
CH3
CH3
O
O
64%
(E/Z 3.6:1)
19
72%
(E/Z 4.4:1)
13
b) 3M NaOH, H2O2,
0 to 23 °C, 1 h
S
22
Ph CH3
86% isolated yield
1
H
H
E+,
TMS
then [O]
CH3
[O]
– pinBCH2Cl
HO
Li
pinB
CH3
Li
O
OH
O
O
CH3
B
pinB
Cl
Ph CH3
C
H
O
75%
(E/Z 5.1:1)
79%
(E/Z 5.0:1)
B-stabilized
H2
H
4° diol NOT formed
Ph CH3
carbanion
Ph CH3
20
23
a
Conditions stated; yields represent major isomer. E/Z ratios by 1H NMR.
Scheme 4. Attempt to produce quaternary diol gives tertiary alcohol.

3152
A. J. Wommack, J. S. Kingsbury / Tetrahedron Letters 55 (2014) 3149–3152
5. (a) Hooz, J.; Linke, S. J. Am. Chem. Soc. 1968, 90, 5936; (b) Hooz, J.; Linke, S. J. Am.
Chem. Soc. 1968, 90, 6891; (c) Hooz, J.; Gunn, D. M. Chem. Commun. 1969, 139;
(d) Hooz, J.; Morrison, G. F. Can. J. Chem. 1970, 48, 868; (e) Hooz, J.; Bridson, J.
N.; Calzada, J. G.; Brown, H. C.; Midland, M. M.; Levy, A. B. J. Org. Chem. 1973, 38,
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upon metalation are illustrated in Scheme 5. In these cases, the
acetophenone-derived product 1 was treated with a commercial
solution of methyllithium (1 equiv, THF, À78 °C) followed by an
alkyl bromide with warming. Chiral tertiary (pinacolato)borons
14 and 15, derived from alkylation with ethyl and allyl bromide,
are isolated in 90–92% yields. If desired, the sequence can also be
extended to include oxidation of the second CAB bond. Boronate
formation, as before, followed by reaction with n-butyl, benzyl,
or allyl bromide and sodium hydroxide/hydrogen peroxide upon
workup affords tertiary alcohols 13, 16, and 17 in excellent yield.
As shown in Table 2, the efficient production of trisubstituted
alkenes from various aromatic aldehydes lends added support to
a boryl-ylide intermediate. Boron–Wittig reagents are commonly
accessed by deprotonating secondary dimesitylboranes, wherein
a base is sterically prevented from adding to the vacant p orbital
on boron.29 Deprotonation is irrelevant here starting with the
ketone-derived insertion product 1. Nonetheless, these examples
serve to complement other known alkylidenations of B-stabilized
anions and confirm the applicability of a user-friendly pinacolato
ester.30 Selected functional groups (ether, ester, nitrile, or thio-
phene) are well tolerated, and E/Z selectivity approaching >5:1 is
possible in certain cases (18, 20, and 23). Also noteworthy is the
fact that an alkynyl trimethylsilyl function (in 20) remains intact
during the nucleophilic addition/elimination. The presence of any
unreacted methyllithium at the time an electrophile is introduced
is unlikely given the high rate of initial metalation/deboronation.
In summary, we report an expanded scope for the Pt-catalyzed
diazoalkane diborylation reaction, particularly for disubstituted
(pinacolato)diborons. An opening attempt at desymmetrizing the
products led to the precedented discovery that 1,1-diboroalkanes
undergo bond-selective substitution upon treatment with a Lewis
base and alkyl halide. The underlying deboronation mechanism is
now advanced as a convenient and practical route toward chiral
tertiary boronic esters. Although highly enantioselective methods
have been achieved for this class of compounds,31 the universal
importance of organoborons as nucleophiles for Suzuki–Miyaura
cross coupling32 and Rh-catalyzed 1,4-/1,2-addition33 renders the
innate modularity and generality of our findings very compelling.
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a
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to carbene formation) is also possible. Additional studies are needed to identify
which of the various pathways is the most significant.
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We are grateful to the donors of the ACS Petroleum Research
Fund (#5001009) and to Boston College and California Lutheran
University for substantial startup grants. The mass spectrometry
facility at B.C. is supported by NSF instrumentation funds (DBI-
0619576). Dr. A. J. W. was a B.C. LaMattina graduate fellow.
Supplementary data
28. For one case of deboronation/intermolecular alkylation involving synthesis of
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Supplementary data (experimental procedures, characteriza-
tion data, and copies of 1H and 13C NMR spectra) associated with
this article can be found, in the online version, at http://
dx.doi.org/10.1016/j.tetlet.2014.03.135.
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