Amphoteric α-boryl aldehydes

Article abstract of DOI:10.1021/ja205910d

A new class of stable molecules, α-boryl aldehydes, has been prepared from oxiranyl N-methyliminodiacetyl boronates by a 1,2-boryl migration with concomitant epoxide scission. A range of boryl imines, alkenes, alcohols, acids, enol ethers, enamides, and other functionalized boronic acid derivatives that are difficult or impossible to prepare using established protocols can be accessed from α-boryl aldehydes. The chemoselective transformations of these building blocks, including the facile synthesis of functionalized unnatural amino acids from silyloxy and amido vinyl boronates, attest to the potential of α-boryl aldehydes in chemical synthesis.

Full text of DOI:10.1021/ja205910d

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Amphoteric r-Boryl Aldehydes
Zhi He and Andrei K. Yudin*
Davenport Research Laboratories, Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario,
Canada M5S 3H6
S Supporting Information
b
Scheme 1. Equilibrium between C- and O-Enolates
ABSTRACT: A new class of stable molecules, R-boryl
aldehydes, has been prepared from oxiranyl N-methylimi-
nodiacetyl boronates by a 1,2-boryl migration with conco-
mitant epoxide scission. A range of boryl imines, alkenes,
alcohols, acids, enol ethers, enamides, and other function-
alized boronic acid derivatives that are difficult or impossible
to prepare using established protocols can be accessed from
R-boryl aldehydes. The chemoselective transformations of
these building blocks, including the facile synthesis of
functionalized unnatural amino acids from silyloxy and
amido vinyl boronates, attest to the potential of R-boryl
aldehydes in chemical synthesis.
Scheme 2
nols and enolates are among the most widely used inter-
E
mediates in chemical synthesis. In comparison with their
ambident O-bound tautomers, the amphoteric C-bound metal
and metalloid enolates are thermodynamically unstable (Scheme 1).
Direct evidence in support of C-bound enolate species is sparse,
despite the fact that R-boryl carbonyl compounds have been
proposed as reactive intermediates in several transformations.1,2
The well-known affinity of sp2-hybridized boron to oxygen
provides the driving force for the isomerization of the C-bound
enolate into the corresponding O-bound isomer. Here we show
that the quaternary nature of boron in N-methyliminodiacetyl
(MIDA) boronates enables the isolation and structural charac-
terization of stable R-boryl aldehydes. As part of this study, we
report access to a range of densely functionalized organoboron
derivatives and their chemoselective transformations.
silica gel chromatography, and storage in the solid form at room
temperature under air.
The preparation of other R-boryl aldehydes was tested in the
same manner (Table 1). The reaction was found to work well not
only with electron-neutral (1a) and electron-rich (1b) arylsubstrates
but also with electron-deficient (1c) aryl derivatives. Primary- and
secondary-alkyl-substituted oxiranyl MIDA boronates (1d and 1e)
cleanly afforded the desired aldehyde products, but the tert-butyl
substrate (1f) led to a complex mixture of intractable materials. The
unsubstituted oxiranyl MIDA boronate 1g was also tested in the
reaction, and no rearrangement took place, even at an elevated
temperature with a prolonged reaction time (40 °C, 24 h).
The regioselectivity of the reaction prompted us to investigate
its mechanism. 1-Deuterated oxiranyl MIDA boronates 3a and
3b, which were prepared from the corresponding deuterated
acetylenes, were thus subjected to the BF3-promoted rearrange-
ment (Scheme 3). We found that the deuterium label was
exclusively incorporated at the carbonyl carbon of the resulting
R-boryl aldehydes 4a and 4b. These results unambiguously prove
that a 1,2-boryl migration took place. The MIDA-boryl group
migrated not only in preference to the deuteride (hydride) but
also to the alkyl or aryl substituents. While a similar rearrange-
ment of silyl epoxides with migration of the silyl group is known,4
the boron version of this process has remained elusive because
As part of a program aimed at the discovery of new amphoteric
molecules, we became interested in preparing aziridinyl boronic
acid derivatives from oxiranyl MIDA boronates (Scheme 2).
The latter molecules can be made by epoxidation of vinyl MIDA
boronates,3f which were recently developed by Burke and co-
workers.3 During our initial attempts to make the azidoalcohol
precursor to the target aziridinyl boronates, the starting oxiranyl
MIDA boronate 1a was charged with excess TMSN3 in the
presence of BF3 etherate (Scheme 2). In addition to the expected
azidoalcohol product, a white solid was isolated in 10% yield.
1
This material exhibited an IR stretch at 1701 cmÀ1 and an H
NMR chemical shift of 9.73 ppm, consistent with rearranged
R-boryl aldehyde 2a. This unexpected result led us to explore
further the rearrangement of oxiranyl MIDA boronates into
R-boryl aldehydes. We found that exposure of oxiranyl MIDA
boronate 1a to BF3 etherate in anhydrous CH2Cl2 (À30 to 0 °C)
provided 2a in nearly quantitative yield within 30 min. No ketone
byproduct was detected during the reaction. The resulting
R-boryl aldehyde 2a proved to be stable during aqueous workup,
Received: June 24, 2011
Published: August 02, 2011
r
2011 American Chemical Society
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|

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Table 1. Preparation of r-Boryl Aldehydes 2 via Rearrange-
Scheme 4. Synthetic Transformations of r-Boryl
ment of Oxiranyl MIDA Boronates 1a
Aldehydesa,b
a Unless stated otherwise, the reactions were carried out using 1 equiv of
epoxide and 1 equiv of BF3 Et2O in anhydrous CH2Cl2 from À30 to
complex mixture of intractable products. d The reaction was carried out
3
a Reagents and conditions: (a) t-BuS(O)NH2, CuSO4, DCM, rt, 85%
(6:7 = 85:15). (b) Br2, dioxane/DCM, rt, 77%. (c) NaClO2, NaH2PO4,
cyclohexene, t-BuOH/H2O, rt, 73%. (d) Allylbromide, In(0), THF-
H2O (1:1), rt, 97% (syn:anti = 93:7). (e) Ph3PdC(Me)CO2Et, DCM,
60 °C, 72%. (f) CBr4, PPh3, DCM, rt; 12a (60%), 12b (65%). (g)
Bn2NH, 4 Å molecular sieves, CHCl3, rt, 85%. (h) Pyrrolidin-2-one,
TsOH, toluene, 120 °C; 14a (50%), 14b (43%). (i) PhNTf2, KHMDS,
THF, À78 °C, 41%. (j) Method A for TBDMS enol ether: TBDMSCl,
Et3N, MeCN, 80 °C, 16a (55%, E:Z = 10:90). Method B for TIPS enol
ether: TIPSOTf, DBU, THF, rt; 16b (81%, E:Z = 20:80), 16c (80%, E:Z
< 5:95). b [B] = N-methyliminodiacetyl boronate.
c
0 °C for 30 min. b Isolated yields after silica gel chromatography. A
at 40 °C for 24 h. N.R. = no reaction.
Scheme 3. Deuterium Labeling Experiments
under a variety of conditions. With 2a and 2d as model sub-
strates, various novel boronic acid derivatives were obtained
(Scheme 4). The stability of R-boryl aldehydes first encouraged
us to investigate the synthesis of nitrogen-containing R-borylated
species.7 Different classes of amines and amides, including
benzylamine, aniline, p-toluenesulfonamide, and tert-butanesul-
finamide, were reacted with R-boryl aldehyde 2a in the presence
of dehydrating agents and/or Lewis acids.8 The majority of
amines and amides generated N-boryl enamines and enamides.
tert-Butanesulfinamide led to the formation of R-boryl imine 6
along with a small amount of N-boryl enamide 7.
a Isolated yields after silica gel chromatography.
any attempt to enlist a tricoordinate boron-containing fragment
has been met with rapid C-to-O transfer.5 The unique feature of
the trivalent MIDA ligand transforms the electron-deficient sp2-
hybridized boron center to an electron-rich sp3-hybridized boron
center, enabling the 1,2-boryl migration.6
In addition to 2-substituted oxiranyl MIDA boronates, a
representative 1-substituted substrate 5 was also tested in the
BF3-promoted rearrangement. Interestingly, R-boryl aldehyde
2a was isolated here also (eq 1). This result indicates that 1,
2-hydride migration instead of 1,2-boryl migration took place.
Although it is likely that the rearrangement is concerted, the
involvement of transient boryl-substituted cationic species dur-
ing the rearrangement of 5 has not been ruled out.
R-Boryl aldehyde 2a was also subjected to different oxidative
conditions. The highly functionalized R-bromo-R-borylaldehyde
8 was obtained in good yield through R-bromination and
characterized by X-ray crystallography.9 On the other hand,
oxidation with sodium chlorite generated the novel R-boryl
carboxylic acid 9. We also subjected 2a to oxidation with sodium
t
chlorite in BuOH/D2O, and no deuterium incorporation was
observed at the R-position of the acid product (see the Support-
ing Information). This result attests to the configurational
stability of the starting material during the course of the reaction
as well as to the configurational stability of the acid product.
A highly diastereoselective preparation of boryl alcohol 10 from
2a took place upon reaction with an allylindium species under
mild conditions. Meanwhile, olefination of the aldehyde group
afforded the novel (E)-γ-boryl-R,β-unsaturated derivative 11
equipped with an ester substituent at the terminal allylic carbon.
gem-Dibromoallyl boronates 12a and 12b were also obtained.
These allylboron reagents have not been reported to date. Since
In order to demonstrate the potential of amphoteric R-boryl
aldehydes in synthesis, we evaluated their chemical reactivity
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Scheme 5. Chemoselective Transformations of Allylated
Boryl Alcohol 10
Scheme 6. Preparation of Unnatural Amino Acids from
Functionalized Boronatesa
a Isolated yield after silica gel chromatography. b Diastereoisomeric
ratios were determined by 1H NMR analysis of the crude reaction
mixtures.
the utility of gem-dihaloalkene intermediates in metal catalysis is
well-documented,10 one can anticipate interesting possibilities
for these novel dibromoallyl boronates in tandem reactions.
Finally, by taking further advantage of the enolizable aldehyde
functionality, we synthesized a series of β-functionalized vinyl
boronates, including enamine 13, enamides 14a and 14b, and
triflate enol ether 15 as well as silyl enol ethers 16aÀc with high
chemo- and stereoselectivities.
The densely functionalized building blocks accessible with this
chemistry provide ample opportunities for synthesis. For example,
allylated boryl alcohol 10 was directly converted to 1,2-diol 17 via
oxidation in basic H2O2. In contrast, treatment of 10 with 1 M
aqueous NaOH resulted in the stereospecific formation of 1,4-diene
(“skipped” diene) 18. This reaction likely proceeds via a Wittig-type
syn-elimination process11 promoted by NaOH (Scheme 5).
Ambident silyloxy and amido vinyl boronate molecules such as
14 and 16 obtained using our method aroused further curiosity.
We recognized the bis-nucleophilic character of these species at
the carbon equipped with the boryl group and opted to inves-
tigate the possibility of chemoselective transformations of nu-
cleophilic CÀB bonds. We chose the Petasis borono-Mannich
reaction12 as a testing ground. MIDA boronates 14a, 14b, 16b,
and 16c were thus first transformed into the corresponding
pinacolyl boronates 19a, 19b, 20a, and 20b. These pinacolyl
boronates were subjected to the Petasis protocol with glyoxylic
acid in the presence of different secondary or primary amines in
1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) as the solvent.13 To
our delight, several novel unnatural amino acids 21aÀk with
intact silyl enol ether or enamide functionalities were successfully
obtained (Scheme 6). Importantly, no regular Mannich-type
products were detected in the course of these reactions, even in
the cases of silyl enol ether boronates 20a and 20b.
Generally, the alkyl-substituted boronates 19b and 20b were
found to be more reactive than their aryl-substituted counter-
parts 19a and 20a. Electron-rich substrates 20a and 20b posses-
sing silyl enol ether groups showed much higher reactivity than
their corresponding enamide counterparts 19a and 19b. Thus,
silyloxy vinyl boronates not only reacted more rapidly with
secondary amines than did the amido ones but also worked very
well with primary amines, with which amido vinyl boronates 19a
and 19b showed no reactivity. In addition, we were able to glean
insight into the diastereoselectivity of the Petasis reaction of
these functionalized vinyl boronates. While (S)-N-methyl-1-
phenylethanamine14 gave inferior results (21d, d.r. = 75:25),
the reaction of 2-methylpyrrolidine,15 glyoxylic acid, and pina-
colyl boronate 19a or 20b afforded alkenylglycine 21c or 21j with
excellent diastereoselectivity (d.r. > 95:5).
a Each Petasis reaction was carried out using 1 equiv of functionalized
alkenyl pinacolyl boronic ester, 1.2 equiv of glyoxylic acid, and 1.2 equiv
of amine (1° or 2°) in HFIP at 23 or 50 °C for 5À72 h. b Isolated yields
after silica gel chromatography. c Diastereoisomeric ratios (d.r.) were
determined by 1H NMR analysis of the crude reaction mixtures. d E/Z
ratios were determined by 1H NMR analysis of purified product
mixtures. e Stable methyl esters were generated by one-pot esterfication
using TMSCHN2.
to prepare using established methods can be accessed from
R-boryl aldehydes. These molecules have enabled the facile
and highly selective construction of densely functionalized molec-
ules such as unnatural amino acids. In view of the facility with
which amphoteric aziridine aldehydes orchestrate efficient bond-
forming processes,16 we expect that R-boryl aldehydes and their
derivatives will find utility in synthesis and may open up inter-
esting new opportunities for designing processes with high bond-
forming efficiency.
’ ASSOCIATED CONTENT
S
Supporting Information. Complete experimental details,
b
characterization data for new compounds, and crystallographic data
(CIF) for 8. This material is available free of charge via the Internet
at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
ayudin@chem.utoronto.ca
In summary, we have reported the discovery of stable ampho-
teric R-boryl aldehydes arising from a 1,2-boryl migration. A
range of boryl imines, alkenes, alcohols, enol ethers, enamides,
and other poly functionalized boron derivatives that are difficult
’ ACKNOWLEDGMENT
We thank NSERC for financial support and Dr. A. Lough for
the X-ray structural analysis of 8.
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’ NOTE ADDED AFTER ASAP PUBLICATION
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13773
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