New highly efficient method for the synthesis of tert-alkyl nitroso compounds

Article abstract of DOI:10.1055/s-2006-926442

The syntheses of tert-alkyl nitroso compounds RCH₂CMe ₂N=O from commercially available tert-alkyl amines RCH ₂CMe₂NH₂ proceed cleanly via the intermediacy of the benzoyl derivatives RCH₂CMe₂NHOC(O)Ph and the corresponding hydroxylamines RCH₂CMe₂NHOH. Since the intermediates require no purification in the course of the transformations, the overall yields of the isolated crystalline nitroso dimers (75-80% for R = H, 75% for R = Me and 66% for R = Me₃C) are based on the corresponding amine precursors. In the latter case (R = Me₃C), significant steric demands and hydrophobicity of Me₃CCH₂CMe₂ group necessitate the application of more efficient reagents and conditions on the debenzoylation and oxidation steps. The syntheses are perfectly suitable for scale-up and were successfully performed on up to 500-mmol scale. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2006-926442

PAPER
1423
New Highly Efficient Method for the Synthesis of tert-Alkyl Nitroso
Compounds
New
S
ynthesis
e
of tert-Alk
r
yl
N
itroso Com
K
pounds iang Quek,a,b Ilya M. Lyapkalo,*a Han Vinh Huynhb
a
Institute of Chemical & Engineering Sciences Ltd., 1 Pesek Road, Jurong Island, 627833 Singapore
Department of Chemistry, National University of Singapore, 3 Science Drive 3, 117543 Singapore
Fax +6563166184; E-mail: Ilya_Lyapkalo@ices.a-star.edu.sg
b
Received 13 December 2005
spite of the fact that this compound was described as early
as in 1903,7 we could not find a suitable synthetic litera-
ture method which would provide a good isolated yield
starting from inexpensive and easily available precursors.
Abstract: The syntheses of tert-alkyl nitroso compounds
RCH2CMe2N=O from commercially available tert-alkyl amines
RCH2CMe2NH2 proceed cleanly via the intermediacy of the ben-
zoyl derivatives RCH2CMe2NHOC(O)Ph and the corresponding
hydroxylamines RCH2CMe2NHOH. Since the intermediates re- Despite the recent advance in the development of new ef-
quire no purification in the course of the transformations, the overall
yields of the isolated crystalline nitroso dimers (75–80% for R = H,
75% for R = Me and 66% for R = Me3C) are based on the corre-
amine as a substrate.8 The nitroso compound A has been
sponding amine precursors. In the latter case (R = Me3C), signifi-
cant steric demands and hydrophobicity of Me3CCH2CMe2 group
ficient protocols for the oxidation of amines to nitroso
compounds, they are lacking widely available tert-butyl-
prepared earlier in low-to-moderate yields by the oxida-
tion of tert-butylamine with peroxyacids,7,9 H2O2–
necessitate the application of more efficient reagents and conditions
on the debenzoylation and oxidation steps. The syntheses are per-
10
Na2WO4 and H2O2–phosphotungstic acid.11 A singular
fectly suitable for scale-up and were successfully performed on up
to 500-mmol scale.
report has also appeared that describes a high-yielding ox-
idation of the tert-alkyl amines to the corresponding ni-
troso compounds with peracetic acid in ethyl acetate.5
However, the amines were used in a large excess, and the
nitroso compounds obtained were not isolated from the
solutions. The difficulties in the synthesis of tert-alkyl ni-
troso compounds have been attributed to their extremely
high volatility,9a,12 thermal instability9a and the ease of the
over-oxidation10a of the nitroso monomers. It is worth not-
ing that these problems can be circumvented by oxidizing
Me3CNHOH (3a) with aqueous NaOBr such that the
product A is precipitated from the aqueous mixture as a
white crystalline dimer which can be isolated by simple
filtration in a good yield.12 Therefore, it is the accessibility
of the hydroxylamine 3a that constitutes a main challenge
in the straightforward preparation of the product A.13 As a
convenient precursor for the hydroxylamine 3a we envis-
aged the benzoyl derivative Ph(O)ONHCMe3 (2a) which
can easily be obtained from the reaction of tert-buty-
lamine (1a) with inexpensive benzoyl peroxide although
variable yields and conditions have been reported.14,15
When trying to optimize the synthesis of the benzoyl de-
rivative 2a using an excess of concentrated aqueous solu-
tion of potassium carbonate as an auxiliary base, we
observed that the reaction proceeded very cleanly albeit
with appreciably incomplete conversion of the benzoyl
peroxide in spite of the excess of the tert-butylamine (1a)
used.
Key words: tert-alkyl amines, benzoyl peroxide, steric hindrance,
selective oxidation, tert-alkyl nitroso compounds
Nitroso compounds have been known for more than a cen-
tury and still continue to play a significant role in the de-
velopment of the chemistry of nitrogen compounds.1 2-
Methyl-2-nitrosopropane (A), the simplest representative
of tert-alkyl nitroso compounds (Figure 1), was success-
fully employed as a trap for free radicals,2 N-electrophile
in the preparation of various stable nitroxyl radicals,3 con-
venient building blocks for the syntheses of nitrones4
azoxy- and azo-compounds1a as well as sterically hin-
dered amines.5
R
Me
O–
N+
Me
Me
R
crystalline state
solution
+
N
Me
2
N
Me
O
–O
Me
R
A–C
Figure 1 Tertiary nitrosoalkanes synthesized: structure and dyna-
mic equilibrium; A (R = H), B (R = Me) and C (R = Me3C)
In search for suitable N-electrophilic equivalents for the
introduction of R3CNH group to heterocyclic moieties, we
considered tert-alkyl nitroso compounds to be the re-
agents of choice. However, the only commercially avail-
able representative, 2-methyl-2-nitrosopropane (A) is
only offered in small quantities and at a very high cost.6 In
We established that the incomplete conversion observed
was due to the formation of the salt S, which apparently
co-exists with potassium carbonate in the reaction mixture
and develops in the voluminous precipitate towards the
end of the reaction16 (Scheme 1 and Table 1).
SYNTHESIS 2006, No. 9, pp 1423–1426
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x
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x
x
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Advanced online publication: 11.04.2006
DOI: 10.1055/s-2006-926442; Art ID: P19405SS
© Georg Thieme Verlag Stuttgart · New York

1424
S. K. Quek et al.
PAPER
Table 1 Optimization of Conversion 1a to 2a: The Effect of the Inorganic Base Used
Conversion (%)
Aqueous base (pK17)
K2CO3 (10.33)a
Initial 1a:(PhCO2)2 ratio
Final 2a:S ratio
1a → 2a
(PhCO2)2
1.2:1
1.4:1
n.d.b
3:1c
69
69
83
97
K3PO4 (12.32)
1:1.05
100:0
100
ca. 99
a pKa[Me3CNH3 ]aq = 10.86.18
+
b Not determined.
c Traces of PhC(O)NHCMe3 (up to 3% yield) were formed with K2CO3 as a base.19
CMe3
the intermediates 3a,b afforded deep blue-green nitroso
monomers A, B, which gradually turned to the corre-
sponding white crystalline dimers. The latter were isolat-
ed from the reaction mixtures by simple filtration in good
overall yields.
(PhCO2)2
0 °C to r.t.
HN
Me3C NH3+ PhCO2
–
Me3C NH2
O
MeOCMe3
aq base
O
1a
S
Ph
2a
However, the above conditions were not quite efficient for
synthesis of the nitroso compound C. Thus, sterically hin-
dered O-benzoyl-N-(1,1,3,3-tetramethyl)butylhydroxyl-
amine (2a) failed to react with aqueous potassium
hydroxide at ambient temperature, whereas the hydrolysis
upon mild heating (40–50 °C) was accompanied by the
formation of appreciable amount of side products. The
subsequent oxidation with aqueous NaOBr did not pro-
ceed cleanly either and led to a mixture of products from
which the desired nitrosoalkane C could be isolated only
by distillation and in a low yield. These problems were
circumvented by using more efficient hydrazine hydrate
for the benzoyl group cleavage followed by oxidation of
the intermediate 3c by MeOBr22 under homogenous con-
ditions in methanol as a solvent. This modified procedure
afforded the 2,2,4-trimethyl-4-nitrosopentane (C) as a
pure white crystalline dimer23,24 in a good overall yield.
Scheme 1
The problem was circumvented by replacing potassium
carbonate with more basic K3PO4,20 which prevented the
formation of the salt S and thus provided the complete
conversion of the amine 1a to the benzoyl derivative 2a in
a virtually quantitative yield.21
With the efficient method for the synthesis of the O-ben-
zoyl-N-tert-alkylhydroxyl amines 2 in hand, we finally es-
tablished a protocol for the overall conversion of the
commercially available tert-alkyl amines 1a–c to the de-
sired nitroso compounds A–C (Scheme 2).
O
Ph
NH2
Me
O
i
ii
HN
Me
Me
Me
In conclusion, we have developed a general and conve-
nient method for synthesis of tert-alkyl nitroso com-
pounds. The procedures are simple and reliable and can be
efficiently scaled up to 500 mmol using normal laboratory
glassware. Our method provides an easy access to pure
isolable tertiary nitrosoalkanes, which are efficient traps
for free radicals to investigate the reaction mechanisms,25
and useful building blocks in synthesis of stable high-spin
organic molecules26 and sterically hindered nitrogen
bases.5,27
R
R
1a–c
2a–c
O
N
NHOH
Me
A: R = H (75–80%)
B: R = Me (75%)
C: R = CMe3 (66%)
iii
Me
Me
Me
R
R
A–C
3a–c
(as crystalline dimers)
Scheme 2 Reagents and conditions: 1a,b to A, B: i: (PhCO2)2,
Me3COMe–aq K3PO4, 0 °C to r.t.; ii: aq KOH, r.t.; iii: aq NaOBr
(from Br2 + aq NaOH), –15 °C to r.t. 1c to C: i: (PhCO2)2, Me3COMe–
aq K2CO3, r.t. to 45–50 °C, 5 h; ii: N2H4·H2O, MeOH, r.t.; iii: MeOBr
(from Br2 + MeONa), MeOH, –78 °C to –15 °C.
NMR spectra were recorded on a Bruker 400 UltraShield spectrom-
eter in CDCl3 unless stated otherwise. 1H and 13C chemical shifts are
expressed in ppm downfield from TMS (d = 0 ppm) that was used
as an internal standard.
Conversion of the starting amines 1a–c to the correspond-
Starting tert-alkyl amines: tert-butylamine (1a; Fluka), 1,1-dimeth-
ing benzoyloxy derivatives 2a–c was carried out under ylpropylamine (1b; Aldrich), 1,1,3,3-tetramethylbutylamine (1c;
Fluka) were stored over potassium hydroxide pellets and were used
optimized conditions in the two-phase mixture of
without further purification. Benzoyl peroxide (75% assay) was
used as purchased from Aldrich.
Me3COMe and appropriate aqueous inorganic base (see
above). The following saponification of 2a,b furnished
homogenous aqueous solutions of the hydroxylamines
3a,b which were added dropwise directly to the freshly
generated NaOBr in water. The instantaneous oxidation of
Synthesis of 2-Methyl-2-nitrosopropane A
K3PO4·H2O (138.2 g, 600 mmol) and Me3CNH2 (37.07 g, 500
mmol) were added to a two-phase mixture of Me3COMe (250 mL)
Synthesis 2006, No. 9, 1423–1426 © Thieme Stuttgart · New York

PAPER
New Synthesis of tert-Alkyl Nitroso Compounds
1425
and water (470 mL). The mixture was cooled to 0 °C before
(PhCO2)2 (169.6 g, 525 mmol) was added. The temperature was
gradually allowed to rise to 20–25 °C, and the reaction mixture was
vigorously stirred overnight (17 h). It was then diluted with hexane
(400 mL) and water (100 mL); the aqueous layer was extracted with
hexane (2 × 100 mL), and the combined hexane fractions were
washed with water (2 × 50 mL), evaporated under vacuum and dried
(r.t., 0.05 mbar, 2 h) to furnish a pure compound 2a. A solution of
KOH (70.1 g, 1250 mmol) in water (625 mL) was added to the re-
sidual 2a. The resulting emulsion was vigorously stirred at ambient
temperature overnight. At the end the mixture turned essentially ho-
mogenous indicating complete saponification. It was then filtered
through porosity 4 sinter funnel, and the filtrate was added dropwise
at –15 °C to the vigorously stirred NaOBr solution freshly prepared
from Br2 (96 g, 600 mmol) and aq NaOH (50.0 g, 1250 mmol NaOH
pellets dissolved in 300 mL of water) at –10 °C to –5 °C. The in-
tense deep-blue color of the nitroso monomer A developed instan-
taneously. The temperature was allowed to rise to 20–25 °C with
continuous stirring while the blue monomer gradually turned to the
white crystalline dimer. The reaction mixture was kept for several
hours in the refrigerator and then filtered. The residue was washed
with water to give a pure white crystalline 2-methyl-2-nitrosopro-
pane dimer A (33.54 g, 77% yield).
vigorously stirred at ambient temperature for 75 h before diluting
with hexane (500 mL) and water (200 mL). The hexane fraction was
washed with water (2 × 50 mL), evaporated in vacuum and dried
(r.t., 0.05 mbar, 17 h) furnishing a pure Me3CCH2CMe2NHOH (3c)
as a crystalline solid.28 Compound 3c was then dissolved in MeOH
(45 mL) and added dropwise at –78 °C to the vigorously stirred
MeOBr solution freshly prepared from Br2 (30.6 g, 192 mmol) and
methanolic MeONa (418 mmol in 400 mL of MeOH) at –78 °C to
–70 °C. The essentially homogenous deep-blue mixture was al-
lowed to gradually warm to –20 °C and left overnight in a deep
freezer at –18 °C. It was then transferred to the separating funnel
containing pentane (400 mL) and ice-water (500 mL) to extract the
nitroso monomer C with pentane. The aqueous layer was extracted
with pentane (2 × 100 mL); the combined pentane phase was
washed with water (60 mL) and dried (Na2SO4). The solvent was
slowly removed at the rotatory evaporator (up to 200 mbar, bath
temperature <20 °C), and the residue was slowly crystallized within
7 d at –18 °C (see ref. 24). It was then treated with very cold (<–50
°C) MeOH, filtered off and dried, furnishing the product C (18.88
g, 66% yield) as a slightly bluish crystalline solid.
C
Dimer–monomer mixture.
A
Monomer
Dimer–monomer mixture.
1H (400.23 MHz, CDCl3): d = 0.81 (s, 9 H, CMe3), 1.08 (s, 6 H,
CMe2), 2.46 (s, 2 H, CH2).
Monomer
13C NMR (100.65 MHz, CDCl3): d = 23.2 (CMe2), 31.3 (CMe3),
31.61 (Me3C), 51.4 (CH2), 100.8 (CN).
1H (400.23 MHz, CDCl3): d = 1.26 (s, 9 H, CMe3).
13C NMR (100.65 MHz, CDCl3): d = 23.1 (CMe3), 96.1 (CN).
Dimer
Dimer
1H (400.23 MHz, CDCl3): d = 0.98 (s, 9 H, CMe3), 1.61 (s, 6 H,
1H (400.23 MHz, CDCl3): d = 1.54 (s, 9 H, CMe3).
CMe2), 2.13 (s, 2 H, CH2).
13C NMR (100.65 MHz, CDCl3): d = 25.3 (CMe3), 76.5 (CN).
13C NMR (100.65 MHz, CDCl3): d = 26.3 (CMe2), 31.1 (CMe3),
31.57 (Me3C), 47.4 (CH2), 81.8 (CN). The dimer dissociated almost
completely (>95% monomer) in the NMR sample after 80 min at r.t.
Synthesis of 2-Methyl-2-nitrosobutane B
The nitroso compound B was prepared according to the above pro-
cedure from 1,1-dimethylpropylamine (1b; 1.743 g, 20.0 mmol)
and (PhCO2)2 (6.782 g, 21.0 mmol). After the last oxidation step,
the reaction mixture was allowed to stay at +4 °C overnight to en-
sure complete crystallization resulting in the product B (1.508 g,
75% yield) as a white crystalline dimer.
Acknowledgment
Generous support by the Agency for Science Technology and Re-
search (A*STAR Singapore) is most gratefully acknowledged.
B
References
Dimer–monomer mixture.
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Monomer
1H (400.23 MHz, CDCl3): d = 0.75 (t, 3J = 7.6 Hz, 3 H, CH2CH3),
1.12 (s, 6 H, CMe2), 2.03 (q, 3J = 7.6 Hz, 2 H, CH2CH3).
13C NMR (100.65 MHz, CDCl3): d = 8.0 (CH2CH3), 20.4 (CMe2),
29.77 (CH2CH3), 99.5 (CN).
Dimer
1H (400.23 MHz, CDCl3): d = 0.87 (t, 3J = 7.5 Hz, 3 H, CH2CH3),
1.54 (s, 6 H, CMe2), 2.11 (q, 3J = 7.5 Hz, 2 H, CH2CH3).
13C NMR (100.65 MHz, CDCl3): d = 8.6 (CH2CH3), 23.9 (CMe2),
29.80 (CH2CH3), 80.4 (CN).
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Synthesis of 2,2,4-Trimethyl-4-nitrosopentane C
K2CO3 (33.2 g, 240 mmol), Me3CCH2CMe2NH2 (25.85 g, 200
mmol) and (PhCO2)2 (71.1 g, 220 mmol) were consecutively added
to a two-phase mixture of Me3COMe (150 mL) and water (40 mL).
The mixture was vigorously stirred at 50 °C for 5 h and then diluted
with hexane and water, extracted and concentrated as described
above. Added to the residual 2c were MeOH (20 mL), water (20
mL) and N2H4·H2O (41.2 g, 823 mmol). The resulting mixture was
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Synthesis 2006, No. 9, 1423–1426 © Thieme Stuttgart · New York

1426
S. K. Quek et al.
PAPER
–
2–
(6) USD 285.80 for 5 g (ALDRICH 2005–2006).
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(20) The use of yet more basic KOH led to the complete
hydrolysis of (PhCO2)2 to PhCO2K with the starting tert-
butylamine (1a) remaining intact.
(21) Interestingly, K2CO3 was efficient enough to provide a
complete conversion of the more hydrophobic amine 1c to
the benzoyl derivative 2c. Apparently, a fine balance
between hydrophilicity, basicity and aqueous solubility
precluded the formation of the respective tert-alkyl-
ammonium benzoate under the reaction conditions.
(22) Dicks, P. F.; Glover, S. A.; Goosen, A.; McCleland, C. W.
Tetrahedron 1987, 43, 923.
(23) Dimerization time and hence the crystallization time
dramatically increases with the increase of the steric
demands of the tert-alkyl substituent: ca. 20–30 min for A,
several hours in the refrigerator (+4 °C) for B, and at least a
week in a deep freezer (–18 °C) for C. For the NMR study
on the monomer–dimer equilibrium, see ref. 10a.
(24) NMR monitoring of the blue liquid nitroso monomer C prior
to the crystallization showed that it was sufficiently pure to
be used as a reactant in subsequent transformations.
(25) Usuki, T.; Mita, T.; Lear, M. J.; Das, P.; Yoshimura, F.;
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(16) Salt S is insoluble in CHCl3 but soluble in water and DMSO.
1H NMR (400.23 MHz, DMSO-d6): d = 1.20 (s, 9 H, CMe3),
+
4.36 (br s, NH3 , exchange peak with water), 7.31–7.41 (m,
3 H, Ph, 2 × CHm + CHp), 7.88–7.91 (m, 2 H, Ph, 2 × CHo).
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(27) The research on new type of metal-free nitrogen bases is
currently in progress in our laboratory.
13C NMR (100.65 MHz, DMSO-d6): d = 28.9 (CMe3), 48.6
+
(CNH3 ), 127.0, 128.5 (CHm, CHo), 129.5 (CHp), 168.2
–
(CO2 ).
(28) Stowell, J. C.; Lau, C. M. J. Org. Chem. 1986, 51, 3355.
Synthesis 2006, No. 9, 1423–1426 © Thieme Stuttgart · New York