Chemoselective Schwartz Reagent Mediated Reduction of Isocyanates to Formamides

Article abstract of DOI:10.1021/acs.orglett.6b01226

Addition of the in situ generated Schwartz reagent to widely available isocyanates constitutes a chemoselective, high-yielding, and versatile approach to the synthesis of variously functionalized formamides. Steric and electronic factors or the presence of sensitive functionalities (esters, nitro groups, nitriles, alkenes) do not compromise the potential of the method. Full preservation of the stereochemical information contained in the starting materials is observed. The use of formamides in the nucleophilic addition of organometallic reagents (Chida-Sato allylation, Charette-Huang addition to imidoyl triflate activated amides, Matteson homologation of boronic esters) is briefly investigated.

Full text of DOI:10.1021/acs.orglett.6b01226

Letter
pubs.acs.org/OrgLett
Chemoselective Schwartz Reagent Mediated Reduction of
Isocyanates to Formamides
Vittorio Pace,*,† Karen de la Vega-Hernandez,†,‡ Ernst Urban,† and Thierry Langer†
́
†Department of Pharmaceutical Chemistry, University of Vienna, Althanstrasse, 14, A-1090 Vienna, Austria
‡Institute of Pharmacy and Food Sciences, University of Havana, 23rd Street, 21425-13600 Havana, Cuba
S
* Supporting Information
ABSTRACT: Addition of the in situ generated Schwartz reagent to widely available isocyanates constitutes a chemoselective,
high-yielding, and versatile approach to the synthesis of variously functionalized formamides. Steric and electronic factors or the
presence of sensitive functionalities (esters, nitro groups, nitriles, alkenes) do not compromise the potential of the method. Full
preservation of the stereochemical information contained in the starting materials is observed. The use of formamides in the
nucleophilic addition of organometallic reagents (Chida−Sato allylation, Charette−Huang addition to imidoyl triflate activated
amides, Matteson homologation of boronic esters) is briefly investigated.
socyanates represent a versatile class of organic molecules1
the synthesis of amides (e.g., the reaction of amines with
Iwhich, due to the excellent electrophilicity of the hetero- carboxylic acid derivatives),8 the key factor accounting for the
success of the reactions is the easy attack of the organometallic
nucleophile to the isocyanate carbon, whose reactivity is
practically uninfluenced by the steric and electronic properties
of the substituent on nitrogen. If the procedure could be
extended to the addition of a hydride nucleophile, a
straightforward and direct route to inaccessible formamides9
could be envisaged. The reduction of isocyanates with LiAlH4 has
been known since the 1950s when Wessely and Swoboda10
reported their first full reduction to N-methylamines, confirmed
shortly after by Finholt.11 The same outcome was also observed
with other powerful reducing agents such as Ph2SiH2/
B(C6F5)312 or NaBH4/TFA.13
As depicted in Scheme 1b, the ideal reducing agent for the
desired transformation should present two main features: (1) it
should chemoselectively halt the reduction at the formamide
level (i.e., partial reduction), and (2) it should not react with
additional functionalities under the reductive conditions
employed. In this context, such a likelihood has been considered
in the literature: Lorenz and Becker reported two examples of N-
arylformamides obtained via the reduction of isocyanates with
Ph3SnH with poor yields (40−50%),14 while Noltes observed
that trialkyltin hydrides reacted better with arylisocyanates than
alkyl counterparts.15a Moreover, scattered examples of this
chemistry come from work by Ojima (Pd-catalyzed hydro-
silylation)15b and Howell (amine-catalyzed hydrogenation).16 As
noticed by Baldwin, the reduction of isocyanates can be further
complicated by their reduction to isocyanides in the presence of
Ph2(t-Bu)SiLi or Cl3SiH/amine.17 Taking into account these
cumulene carbon, enables the smooth addition of a plethora of
nucleophiles including alcohols (synthesis of carbamates),2
amines (synthesis of ureas),3 and carbanions (synthesis of
amides, vide infra). The reaction of Grignard reagents and
isocyanatesa long known reaction since the seminal work by
Gilman,4 although nonfrequently applied to synthetic pro-
cesses5constitutes an excellent method to access hindered
amides, as elegantly showcased by Bode in 2012.6 Subsequently,
our group documented the addition of lithium halocarbenoids
(LiCH2Z and LiCHYZ) to isocyanates to reach halomethyl- and
dihalomethylamides (Scheme 1a).7 Both Bode’s and our own
methods were general in scope, high yielding (in many
circumstances, no chromatography was required), and adaptable
to the synthesis of optically active amide derivatives starting from
chiral isocyanates. Compared to conceptually distinct tactics for
Scheme 1. General Context of Presented Work
Received: April 26, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b01226
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
examples and considering the lack of studies on the subject in the
last >30 years, we looked at redesigning the transformation,
considering that crucial for its success was identifying a reliable
reducing agent possessing the chemoselective features defined
above.
We began our investigations with model isocyanate 1, which
presents an ester as an additional electrophilic moiety susceptible
to reduction. As summarized in Table 1, treatment with various
added at rt, chemoselectivity was affected as a consequence of the
reducing capability of the same LiAl(O-t-Bu)3H (entry 9).
However, performing such an addition at 0 °C and removing the
cooling bath enabled us to access 2 in 87% isolated yield with full
chemocontrol (entry 10). Further improvement could be
observed by running the reaction in 2-methyltetrahydrofuran
(MTHF, entry 11), nowadays considered a versatile alternative
to THF in terms of eco-compatibility for organometallic
reactions.26
With these optimized conditions in hand, we explored the
scope of the reaction (Scheme 2). Both aromatic and aliphatic
isocyanates react smoothly, providing the corresponding
formamides 5−22 and 23−32, respectively, in high yields.
Substitution on the aromatic ring does not adversely influence
the reaction outcome: both electron-releasing (alkyl, alkoxy, e.g.,
6−12) and electron-withdrawing [halogens (13 and 14), nitrile
(18), nitro (19), esters (20,21)] functionalities are tolerated.
Interestingly, sterically hindered isocyanates provide the
expected products in high yield not only in the case of aromatic
substituents (7 and 8) but also for aliphatic analogues (26 and
28), thus overcoming the well-known issues concerning the low
reactivity of the corresponding amines toward acylating agents.8b
The following additional points on the chemoselectivity of the
protocol are worthy of mention: (a) Hydride delivery takes place
exclusively on the isocyanate without affecting sensitive
functionalities such as nitrile 18 or nitro 19. (b) Additionally,
the presence of esters decorating aromatic nuclei at different
positions does not alter the chemoselectivity (20 and 21); the
concomitant reduction of this functional group was not observed
at all. (c) Olefinic fragments (known to be susceptible to the
Schwartz reagent under different conditions)19c,e,22b,27 were not
affected under the reaction conditions (28 and 29). (d) Use of
optically active isocyanates allowed us to prepare the
corresponding formamides in excellent enantiopurity without
minimal erosion of the stereochemical information (30−32). (e)
Heteroaromatic rings in the cases of both aromatic and aliphatic
isocyanates (17 and 27) or a boronic acid pinacol ester (15)
susceptible of further derivatization28 did not compromise the
efficiency of the methodology. (f) Multiple reduction of a bis-
isocyanate was possible as indicated in the case of diformamide
16. In line with previous studies by Georg,20a a highly reactive
aromatic ketone was concomitantly reduced to the secondary
alcohol under the reaction conditions (22). Of particular
significance is the one-step, straightforward access to formamide
10 known to manifest mutagenic properties.29
Because of our interest toward the addition of organometallic
reagents to amide type substrates,30 and being cognisant of the
fact that formamides have not been studied recently in reactions
with analogous species, we tested their behavior in such
nucleophilic additions (Scheme 3). In particular, the activation
of the inert amide bond could be realized through two
conceptually distinct approaches, namely, (a) formation of an
iminium ion with the same Schwartz reagent according to the
protocol of Chida−Sato31 and (b) formation of an imidoyl
triflate species reported by the groups of Charette32 and
Huang.33 Thus, the former strategy31 enabled the smooth
addition of an allylic fragment via the reaction of formamide 5
with allylSn(n-Bu)3, providing homoallylic aniline 33 in high
yield. When the formamide link of 5 was preactivated with an
electrophilic reagent (Tf2O), subsequent addition of a Grignard
reagent afforded, upon acidic hydrolysis, aldehyde 34 (Charette
conditions).32a By employing Huang’s chemoselective NaBH4-
mediated reduction of a preactivated amide,34 the complete
Table 1. Model Reaction: Optimization
reducing agent
(equiv)
temp (°C)/
time (h)
2/3/4
yield of
a
entry
solvent
ratio
2 (%)
1
2
DIBAL-H (1.0)
LiAlH4 (0.3)
NaBH4 (0.3)
NaBH4 (1.0)
NaBH4 (1.0)
Et3SiH (1.5)
THF
THF
THF
THF
THF
THF
0/0.5
0/0.5
0/0.5
0/0.5
40/2
0/1
2:1:1
0:0:1
12
b
3
1:0:0
30
34
37
4
1:0:0
5
1:0:0
c
6
1:1:1
6
7
Hantzsch ester (1.2) THF
0/1
1.5:1.5:1
1:0:0
36
78
65
87
d
8
Cp2ZrHCl (1.2)
THF
rt/1
e
9
Cp2ZrHCl (1.2)
THF
rt/1
1:0.2:0
1:0:0
ef
,
10
11
Cp2ZrHCl (1.2)
THF
rt/1
ef
,
Cp2ZrHCl (1.2)
MTHF
rt/1
1:0:0
92
c
a
b
Isolated yield. Compound 4 was obtained in 46% isolated yield. An
d
overall conversion of 22% was observed by 1H NMR. Cp2ZrHCl was
e
supplied from a commercial source. Cp2ZrHCl was prepared
according Snieckus.22b LiAl(O-t-Bu)3H (1 M in THF) was added
f
to Cp2ZrCl2 at 0 °C.
reducing agents could, in principle, provide a mixture of three
different products, namely, the targeted 2-ethoxycarbonyl
formamide 2, 2-methylaminobenzoate 3 (in which the starting
isocyanate was fully reduced at the corresponding amine), and
the completely reduced 2-methylaminobenzylic alcohol 4. The
use of DIBAL-H afforded a mixture of the three possible
products with a limited conversion (entry 1). When the reducing
power of the reagent (LiAlH4) was increased, fully reduced
compound 4 was formed as the unique product (entry 2), in
agreement with the seminal studies discussed above. A milder
reagent such as NaBH4 improved the selectivity dramatically.
However, the yield was far from optimal (entry 3), even in the
presence of increased loading or after higher temperatures/
longer reaction times (entries 4 and 5). Other common reagents
such as triethylsilane (entry 6) or the Hantzsch ester18 (entry 7)
provided mixtures of the three possible products in limited yields.
A satisfactory 78% yield of desired compound 2, under full
chemocontrol, was achieved by employing the commercially
available Schwartz reagent19 (Cp2ZrClH, entry 8). Indeed, this
reagent features excellent chemoselectivities, as showcased in
works by Georg,20 Ganem,21 Snieckus,22 and Furman23 among
others,24 during independent investigations on amide-type
reduction chemistry. Because of the well-known issues associated
with the use of commercially available reagents (e.g., moisture,
air, light sensitivity, limited solubility in common organic
solvents),25 we found it highly beneficial to generate it according
to the recently described Snieckus protocol from Cp2ZrCl2 and
LiAl(O-t-Bu)3H.22b Interestingly, when the latter reagent was
B
DOI: 10.1021/acs.orglett.6b01226
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Letter
Scheme 2. Scope of the Reaction
Scheme 3. Reactivity of Formamides toward Organometallic
Reagents
presence of a N-formylamide derivative 15. The attack of the
carbenoidic iodomethyllithium took place exclusively at the
boronic moiety (no modification occurred at the formamide
moiety), affording compound 36 in very high yield.
In conclusion, we have developed an expeditious route to N-
formamides through the highly chemoselective nucleophilic
addition of the in situ generated Schwartz reagent to widely
available isocyanates. Key characteristics of the transformation
are (1) uniformly high yields and full retention of the steric
information contained in the starting materials and (2) excellent
chemoselectivity in the presence of functionalities sensitive to the
Schwartz reagent such as nitro, cyano, ester, and alkene groups.
The synthetic potential of the formamide moiety in reactions
with organometallic reagents such as stannane, hydride,
Grignard, and carbenoid reagents has been documented.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.6b01226.
Experimental procedure, NMR spectra, HPLC traces, and
analytical data for all compounds (PDF)
AUTHOR INFORMATION
Corresponding Author
■
a
b
With 3.0 equiv of Schwartz reagent. With 2.0 equiv of Schwartz
reagent.
*E-mail: vittorio.pace@univie.ac.at.
Notes
The authors declare no competing financial interest.
reduction of formamide 5 was observed, thus yielding N-
methylaniline 35. Finally, with the aim of gaining further insights
into the portfolio of lithium carbenoid mediated homologations
currently underway in our group,35 we evaluated the chemo-
selectivity of the Matteson boronic ester homologation28,36 in the
ACKNOWLEDGMENTS
■
The authors thank the University of Vienna for financial support.
K.d.V.H. acknowledges the OEAD for a postgraduate Mach
grant.
C
DOI: 10.1021/acs.orglett.6b01226
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