Dimethylamine adducts of allylic triorganoboranes as effective reagents for Petasis-type homoallylation of primary amines with formaldehyde

Article abstract of DOI:10.1039/c8ob02152j

Dimethylamine adducts of triallyl-, triprenyl- and trans-cinnamyl(dipropyl)borane are effective reagents for mild homoallylation of primary amines with aqueous formaldehyde in MeOH without an inert atmosphere. A new concept is proposed for the explanation of the high stability of allylborane-amine adducts in aqueous MeOH.

Full text of DOI:10.1039/c8ob02152j

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Dimethylamine adducts of allylic triorganoboranes as effective
reagents for Petasis-type homoallylation of primary amines with
formaldehyde
Received 00th January 20xx,
Accepted 00th January 20xx
Nikolai Yu. Kuznetsov,*a Rabdan M. Tikhov, a Tatiana V. Strelkovaa and Yuri N. Bubnov a,b
DOI: 10.1039/x0xx00000x
www.rsc.org/
Dimethylamine adducts of triallyl-, triprenyl- and trans- stability in alcohol solutions of ammonia and amines.10 At the
cinnamyl(dipropyl)borane are effective reagents for mild same time reactivity of TAB moiety in the Petasis-type11 three-
homoallylation of primary amines with aqueous formaldehyde in component aminoallylation of carbonyl compounds is
MeOH without an inert atmosphere. For explanaiton of high completely preserved. All three allyl groups of TAB are
stability of allylborane-amine adducts in aqueous MeOH a new involved in the reaction, while compatibility with the
concept is proposed.
functional groups of the substrates similar to having lower
activity allylboronic acid and its pinacol esters. Unfortunately,
primary amines with α-branched radicals failed to react in the
aminoallylation reaction, and the corresponding adducts of
TAB with isopropyl- and adamantylamines were readily
protolyzed in MeOH.10 We found that in case of formaldehyde
into aminoallylation can be involved both linear and α-
branched amines (Scheme 1).
Аllylboranes and allylboration reactions are very important
tool of organic chemistry.1 Among the multiple allylborane
mediated reactions a particular attention is paid to the
synthesis of homoallylamines,2 which are versatile and useful
synthetic intermediates in pharmaceutical chemistry, in
synthesis of natural products, heterocyclic compounds and
biologically active molecules.3 The selected examples of the
transformation of a homoallylamine fragment by using of Ni-
catalysis,4 Ru- and Mo-catalysed RCM reactions,5
[Raeppel, 2004]
Bn
NH
hydroarylation
cycloaddition
reactions,6
to
aminocyclopropanes,7a
Rh-catalysed
carbonylative
cyclohydro-
BF3K
Bn
N
(CH2O)n, BF3•OEt2 (25 mol %)
PhMe, 90 °C, 5 h
40%
carbonylation,7b-e intramolecular aminofluorination,8
Pd-
catalyzed cycloisomerization,9a cyclization to pyrrole,9b 1-
pyrrolines,9c piperidines3e demonstrate the great relevance of
this type of molecules in construction of various
azaheterocycles.
OR1
[Furstner, 2008]
R2
N
H
69%
R2
NH2
(CH2O)n, PhMe, 90 °C, 1 h
(CH2O)n, EtOH aq., rt, 14 h
R1O
BPin
The most reactive in the allylboration of imines mono- and
triallylic organoboranes are extremely sensitive to oxygen (low
molecular weight allylboranes are self-igniting in air), water
(alcohols), to important functional groups; in addition, allylic
organoboranes are not useable in catalysis. Such chemical
properties not only complicate handling with these important
compounds, but in general hamper the development of
modern chemistry on their basis. Recently we have shown that
adducts of triallylborane (TAB) with linear primary and
secondary amines are low-sensitive to air and possess high
OR1
58%
NH2
R3
NH
R3
[Brummond, 2001]
[This work]
NHMe2
BR2
R1 R2
R3 NH2, CH2O aq. 37%, MeOH
25 °C, 30 min, then 40 °C, 1 h
H
N
R1
R3
27-95%
R2
R1 = H, Me, Ph; R2 = H, Me
R3 = H, Alk, Ar, HetAr
Scheme 1. Available examples of homoallylation of amines with formaldehyde
The Petasis reaction (Petasis-borono Mannich) was discovered
in 1993 on example of synthesis of tertiary allylic amines from
vinylboronic acids, paraform and secondary amines.11a
However, further use of formaldehyde in Petasis reaction was
rather scarce. The main obstacle was the extensive reduction
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of the iminium intermediate to N-Me derivatives.12a Raeppel formaldehyde giving rise to 2a. Excess of NH3 shifts the
elaborated improved method through aryltrifluoroborate salts equilibrium of the reaction with formaldehyde towards partial
in the presence of BF3xOEt2 or TiF4 that completely eliminated formation of 2b. It was found that excessive imination of 2b
the reduction process and produce tertiary amines in up to can be reduced by addition of NH4Cl (4 equiv.) for in situ
98% yield, with exception of allyltrifluoroborate13 where yield protonation of 2b. In this way, a predominant formation of 2b
of the homoallylamine derivative was only 40%.12a Successful is occurred (2a
/2b, 1:4), which was isolated as HCl salt 2b
application of formaldehyde was described in the synthesis of (54%). Homoallylation of allylamine with 1a proceeds
tertiary amines from acetylene trifluoroborates in ionic smoothly, giving hydrochloride of 2c (78%). Interesting to note
liquid,12b from aryl boronic acids under copper ferrite that although preparation of 2c via 4-bromo-1-butene was
(CuFe2O4) catalysis,12c from aromatic amines with aryl boronic known yet from 1981,16a,b pure 2c was obtained by 4-stages
acids.12d However, the synthesis of secondary homoallylamines synthesis.16c
is limited by only two examples of reactions with pinacol esters
1. CH2O 37%, MeOH
2a-p
NH2+Cl-
of oxy-substituted allylboronates, reported by Furstner in the
synthesis of (-)-haouamine A12e and Brummond in the
synthesis of the core structure of the immunosuppressant
FR901483 (Scheme 1).12f
R
NH2
2. 1a-c, 25 °C, 30 min; 40 °C 1 h
NH2+Cl-
NH3+Cl-
2
2a, 70% (55 °C, 3 d)
2b, 54% (55 °C, 3 d)
2c, 78%
The main objects of the present study were to demonstrate
unique allylborating properties of amine adducts of allylic
triorganoboranes and elaborate method for chemoselective
homoallylation of primary amines in MeOH. As starting
material, Me2NH adducts of triallyl- (1a), triprenyl- (1b) and
NH2+Cl-
HO
2d, 79%
NH
NH
2g, 80%
2f, 82%
NH2+Cl-
cinnamyl-dipropylboranes
(1c) were chosen which were
O
obtained from the corresponding boranes and THF solution of
Me2NH (Scheme 2).
2e, 80%
NH2
HO
NH
NH
Me
NH
2 M Me2NH in THF
-20 °C to rt
B
NH
Me
B
R
R
2j, 60%
2h, 95%
2i, 89%
3
3
N
N
N
N
R
R
HN
R = H (1a); R = Me (1b)
Br
2 M Me2NH in THF
-20 °C to rt
BPr2
Me NH
Ph
NH
NH
BPr2
Ph
2l, 62%
1c
2k, 39%
2m, 28% (92%)a
Me
N
(DABCO)
B
NMe3
Me3N
B
DABCO
1d
B
N
NH2+Cl-
Ph
2o, 22% (-20 °C, 1 h)
a obtained by NBS-mediated bromination of 2l
Ph
n-C5H12
3
n-C5H12
3
3
1e
Ph
NH
0 °C to rt
0 °C to rt
NH
2n, 74%
2p, 54% (-20 °C, 1 h)
(dr 2.5:1)
Scheme 2. Synthesis of amine adducts of allylic boranes
As we established previously adduct 1a is subjected to an
immediate exchange reaction upon mixing with primary
amines or ammonia.10 Actually Me2NH serves just as a carrier
of TAB and does not participate in the allylation reaction.
Formaldehyde was used as 37% aqueous solution and
reactions were run in MeOH which is the optimal solvent for
Petasis-type aminoallylation.14 Initially, the reaction of 1a (δ
11B -3.7 ppm in MeOH) with formaldehyde and equivalent
Scheme 3. Homoallylation of primary amines with adducts of allylic boranes
Under similar conditions, reaction of allylamine, CH2O and
adduct of triprenylborane 1b proceeds equally well with the
allylic rearrangement, resulting in 2,2-dimethyl substituted
amine 2d (79%), that confirms the general reactivity of the
triallylic borane adducts. Branched amines give weak
stabilization of TAB,10 therefore homoallylation of branched
diallylmethanamine17 (1 equiv.) was carried out with 10%
excess of 1a, though the clean formation of amine 2e (80%)
was observed. However, as the size of the substituents in the
branched amine increases, the yield of the product begins to
decline. Homoallylation of (R)-1-PhEtNH2 was carried out at
1
amounts of methanolic 7M NH3 was studied. According to H
NMR, formaldehyde and NH3 are immediately condensed into
urotropine (δ 1H singlet at 4.7 ppm),15 which slowly reacts with
adduct of TAB-NH3 (δ 11B -8.7 ppm)10 for 3 days at 55
°C.
Surprisingly, the only product of this reaction was
bis(homoallyl)amine 2a (70%) (Scheme 3). When trying to
optimize the reaction rate, it was found that in excess of NH3
(15 equiv.) together with 2a up to 30% of homoallylamine 2b is
formed. Evidently, 2b as first-step product is more nucleophilic
than NH3 and easily combined with second molecule of
the same conditions as for 2e, but the reaction was
accompanied by the propylene evolution and the NMR yield of
2f was 67%. The reaction with 2 equiv. of the amine gives an
insignificant increase in the NMR yield of 2f (75%). An attempt
to stabilize borane by the addition of 1 equiv. of Me2NH (8M
2 | J. Name., 2012, 00, 1-3
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solution in MeOH) led to an opposite effect; the NMR yield of
2f was dropped to 50%. Probably excess of Me2NH competes
with (R)-1-PhEtNH2 for formaldehyde, thus removing it from
the target homoallylation. We proposed that for additional
stabilization of TAB would be suitable a basic tertiary amine
having less sterically crowded nitrogen atom fixed in the
pyramidal configuration. An ideal candidate for those demands
would be 1-aza-adamantane,18 however, taking into account
the cost and availability DABCO is more preferable. Indeed,
addition of 1 equiv. DABCO to the reaction with 1a affords the
best yield of 2f, 82% (90% by NMR). It is important, because 2f
found an application in stereoselective synthesis of a number
of natural products.19 The original Petasis reaction tends to
proceed exceptionally well when there is a free hydroxy group
on the aldehyde fragment.20 Since in our case only amine
fragment is subjected to modification, the homoallylation of
(R)-phenylglycinol was studied. The reaction of 1a with the
aminoalcohol proceeds cleanly, even without DABCO. Pure
amine 2g (80%) was isolated as solid without chromatographic
purification, it is worth to note that the melting point of 2g
Ph
BPr2
Ph
BPr2
Me NH
Me
MeOH
Me NH
1c'
CH2O
R-NH2
CH2O
R-NH2
1c
Me
Ph
NHR
NHR
Ph
R
RR
Scheme 4. The observed allylic rearrangements with adduct 1c
The proposed mechanism of homoallylation with adducts
1
consists (Scheme 5) of three repetitive steps. On the first step
an exchange reaction between imine and Me2NH occur.
R
R
CH2O
NH
N
R
NH2
NHMe2 NHMe2
B(OH)3 BAll3
-H2O
Me2NH
H2O;
Me2NH
HO
OH
N
All3B
R
AllB
R
was almost 10 °
C higher than the literature values,7b,21 where
N
2g was obtained by heating with butenyl bromide. Similarly
homoallylation of the unprotected (S)-phenylalanine is
occurred almost quantitatively to give pure aminoacid 2h
Me2NH
N
H2O; Me2NH
R
R
HN
(95%). Aromatic amine aniline readily reacts with CH2O and 1a
,
NHMe2
NHMe2
All2BOH
but for the best yield of 2i (89%) DABCO have to be used. 1,2-
Phenylendiamine produces a mixture of mono- (2j) and 1,2-
bis(homoallyl)diamines in ratio 4.2:1, whereas pure 2j (60%)
was easily separated by FC. Homoallylation of the hindered
amine 1-aminoadamantane under optimized conditions with
DABCO gives 39% of the 2k together with the rest of the
unreacted amantadine, which were separated by FC. 5-
Aminopyrazole with a bulky cyclohexyl substituent expectedly
worse reacts with 1a and a significant portion of the TAB
decomposes. The yield of homoallylated pyrazole 2l was only
35%. The addition of DABCO significantly increases the yield of
2l (62%). The introduction of bromine into the 4-position of 5-
aminopyrazole has a detrimental effect on homoallylation,
even with DABCO the yield of 2m was only 28% (58% of the
starting amine). It should be noted that this yield is not related
to the decomposition of TAB, because the same amount of 2m
28% (54% of the starting amine) was obtained by the
homoallylation of the bromopyrazol-derivative with
allyl(dimethoxy)borane. Nevertheless an excellent yield of 2m
(92%) was reached in other way by bromination with NBS of
AllB(OH)2
OMe(H)
R
R
All2B
R
N
NH
N
H2O;
Me2NH
Me2NH
Scheme 5. Mechanism of the homoallylation with triallylborane-dimethylamine adduct
The formed imine adduct immediately undergoes
allylboration, and generated aminoborane hydrolyzed by
water to homoallylamine. Diallylborinic acid is trapped by
Me2NH followed by the second cycle of exchange-allylboration
and so on till the boric acid formation. In addition to the
reaction mechanism, which is generally understandable, the
key question remains why the amine adducts of allylic
triorganoboranes are stabilized by alcohols and amines. The
first reason for that is the excess of amine in the solution shifts
the equilibrium out of the dissociation of the borane adduct to
the amine and free allylborane, preventing it from
protonolysis. Another reason is not so obvious. Apparently, in
adducts either strong polarization or dissociation of the NH-
bond occurs, which leads to an increase in the negative charge
on nitrogen and an increase of stability of B-N bond. To
support this hypothesis, 11B spectra of 1a in CDCl3 in the
presence of tertiary amines (2 equiv.) was studied. Chemical
shift 11B of 1a -3.11 ppm, the addition of amines yields upper
filed shifts: DIPEA -3.17 ppm, urotropine -3.22 ppm, DBU -3.35
ppm, DABCO -3.33 ppm. The observed changes are not related
to the exchange of Me2NH with an excess of tertiary amines,
since their 11B shift lies in much weaker fields 1d 0.74 ppm, 1e
0.98 ppm; the 11B shift of the mixture of 1e and DIPEA 1.19
amine 2l
. Homoallylation of allylamine with adduct of
cinnamyl(dipropyl)borane 1c results in unexpected trans-
amine 2n (74%). Adduct 1c exists as trans-isomer (Scheme 2
and 4), so we reasonably expected the product with
rearranged cinnamyl group R, whereas 2n is a result of double
allylic rearrangement RR (Scheme 3 and 4). Surprisingly 1c is
subjected to 1,3-sigmatropic shift of boron22 to 1c’ despite the
coordination with dimethylamine23 and presence of protolytic
medium. Homoallylation of (R)-1-PhEtNH2 with 1c at -20 ⁰C
gave rise to a mixture of amines – doubly rearranged 2o (22%)
and normal product of rearrangement 2p (54%, dr 2.5:1)
isolated by FC (Scheme 3).
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6
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In conclusion we have elaborated convenient method of
Petasis-type homoallylation of ammonia and amines with
aqueous formaldehyde and Me2NH adducts of allylic
triorganoboranes. The mechanism of stabilization of the allylic
borane adducts in alcohols involves dissociation of NH-bond.
The authors gratefully acknowledge financial support from
Russian Foundation for Basic Research (grant ofi-m 15-29-
05870).
8
9
Conflicts of interest
P.-N. Liu, ACS Catal., 2018, 8, 6407.
10 N. Yu. Kuznetsov, R. M. Tikhov, T. V. Strelkova and Yu. N.
Bubnov, Org. Lett., 2018, 20, 3549.
There are no conflicts to declare.
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