Reaction of stable trialkylsilyl esters with Ph3P(SCN)2: A novel method for the preparation of acyl and aroyl isothiocyanates under neutral condition

Article abstract of DOI:10.1081/SCC-120014983

An efficient, mild and novel method is described for one-pot conversion of stable triisopropylsilyl- and tert-butyldimethylsilyl carboxylates to their corresponding acyl- or aroyl isothiocyanates using in-situ generation of Ph₃P(SCN)₂ at room temperature under neutral condition. This method has also been applied for the high yield preparation of 2-thioxo-3,4-dihydro-2H-1,3-benzoxazine-4-one, which has fungicidal and bactericidal activities.

Full text of DOI:10.1081/SCC-120014983

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REACTION OF STABLE TRIALKYLSILYL ESTERS WITH
Ph P(SCN) : A NOVEL METHOD FOR THE PREPARATION
3
2
OF ACYL AND AROYL ISOTHIOCYANATES UNDER
NEUTRAL CONDITION
a
b
a
N. Iranpoor , H. Firouzabadi & H. R. Shaterian
a
Chemistry Department , College of Sciences , Shiraz University , Shiraz, 71454, Iran
Chemistry Department , College of Sciences , Shiraz University , Shiraz, 71454, Iran
b
Published online: 16 Aug 2006.
To cite this article: N. Iranpoor , H. Firouzabadi & H. R. Shaterian (2002) REACTION OF STABLE TRIALKYLSILYL ESTERS WITH
Ph P(SCN) : A NOVEL METHOD FOR THE PREPARATION OF ACYL AND AROYL ISOTHIOCYANATES UNDER NEUTRAL CONDITION,
3
2
Synthetic Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry, 32:23,
653-3657, DOI: 10.1081/SCC-120014983
3
To link to this article: http://dx.doi.org/10.1081/SCC-120014983
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SYNTHETIC COMMUNICATIONS
Vol. 32, No. 23, pp. 3653–3657, 2002
REACTION OF STABLE TRIALKYLSILYL
ESTERS WITH Ph P(SCN) : A NOVEL
3
2
METHOD FOR THE PREPARATION OF
ACYL AND AROYL ISOTHIOCYANATES
UNDER NEUTRAL CONDITION
N. Iranpoor,* H. Firouzabadi,*
and H. R. Shaterian
Chemistry Department, College of Sciences,
Shiraz University, Shiraz 71454, Iran
ABSTRACT
An efficient, mildandnovel methodis de scribedfor one-pot
conversion of stable triisopropylsilyl- and tert-butyldimethyl-
silyl carboxylates to their corresponding acyl- or aroyl isothio-
cyanates using in-situ generation of Ph P(SCN) at room
3
2
temperature under neutral condition. This method has also
been appliedfor the high yieldpreparation of 2-thioxo-3,4-
dihydro-2H-1,3-benzoxazine-4-one, which has fungicidal and
bactericidal activities.
*
Corresponding authors. E-mail: firouzabadi@chem.susc.ac.ir
3
653
DOI: 10.1081/SCC-120014983
Copyright & 2002 by Marcel Dekker, Inc.
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3
654
IRANPOOR, FIROUZABADI, AND SHATERIAN
Aroyl andacyl isothiocyanates have foundinterest in synthesis
The methods of preparation of isothiocyanates are
[1,2]
of heterocycles.
[1]
limited. The reaction of carbonimidoyl dichloride with metal thiocyanate
is reportedto furnish corresponi dn g isothiocyanate, but the starting
[
3]
material is not easily available andthe yieldof the reaction is too low.
Other synthetic methods which are developed for the preparation of
isothiocyanates are mainly basedon the reaction of acyl hali de s with thio-
[
1,4,5]
[6]
cyanate ion.
The reaction of carboxylic acids with Ph P(SCN)
3
or
2
[7]
PO(NCS)3 has also been performed. In the case of former reaction, the
reagent is preparedusing leadthiocyanate andbromine to pro du ce poison-
ous thiocyanogen. The produced thiocyanogen solution was then decanted
ꢁ
[6]
and added to triphenylphosphine solution in CH Cl at ꢀ40 C to give
2
2
Ph P(SCN) . In the case of later reaction, the phosphoryl isothiocyanate,
3
2
PO(NCS) , preparedfrom the reaction of POCl with alkali metal isothio-
3
3
cyanate in liquidSO or in the presence of 18-crown-6-ether andit shouldbe
2
[7]
freshly distilled before use. Preparation of the reagents in these methods
[
6,7]
suffers from difficult procedures and the reactions take long time.
Recently, we have reporteda simple mo di fiedproce du re for in-situ
generation of Ph P(SCN) , andits application for efficient thiocyanation
2
3
or isothiocyanation of alcohols, silyl ethers andtetrahy dr opyranyl ethers
[
un de r mildandneutral reaction con di tions.
8,9]
In continuation of this work, now we wish to report a direct, facile and
efficient methodfor imme di ate conversion of stable tri isopropylsilyl- and
tert-butyldimethylsilyl carboxylates to their corresponding aroyl and acyl
isothiocyanates (Sch. 1).
In this method, due to the pronounced tendency of acyl and aroyl
[
1]
thiocyanates to isomerization, isothiocyanates are obtainedas the sole
product of the reaction.
Using this method, triisopropylsilyl- and tert-butyldimethylsilyl
[10]
carboxylates
obtainedfrom aliphatic andaromatic carboxylic acisd
were convertedimme di ately to their correspon di ng acyl andaroyl isothi-
cyanates at room temperature under neutral condition in high to excellent
Scheme 1.

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ACYL AND AROYL ISOTHIOCYANATES
3655
Table 1. Preparation of Acyl Isothiocyanate with Ph P(SCN) from TIPS and
3
2
TBDMS Carboxylates in Acetonitrile at Room Temperature
a
Yield(%)
X ¼ TIPS, TBDMS
Entry
Substrate
Product
1
2
3
4
5
6
7
8
9
PhCO
4-MeOC
4-MeC
2
X
PhCONCS
4-MeOC
4-MeC
3-BrC H CONCS
92, 96
90, 92
91, 93
90, 91
90, 91
91, 92
90, 91
90, 89
90, 90
6
H
4
CO
2
X
6
H
4
CONCS
6 4
H CONCS
6
H
4
CO
2
X
3-BrC H CO X
2
6
4
6
4
3-ClC
PhCH
6
H
4
CO
X
2
X
3-ClC
PhCH
6
H
4
CONCS
2
CO
2
2
CONCS
PhCH¼CHCO X
PhCH¼CHCONCS
2
CH
CH
2
¼CH(CH
(CH
2
)
8
CO
X
2
X
CH
CH
2
¼CH(CH
2 8
) CONCS
3
2
)
16CO
2
3
(CH
2
)
16CONCS
CO X
2
O
1
0
90, 91
NH
OH
O
S
CO X
2
CONCS
1
1
87, 89
OH
OH
a
Reaction was completedspontaneously at room temperature andthe yieldrefers to
isolatedpure pro du ct.
yields (86–95%). The reactions of trialkylsilyl carboxylates carrying electron-
donating or electron-withdrawing groups on the aromatic ring were also
performedefficiently at room temperature (Table 1, Entries 1–10).
This novel direct isothiocyanation method can be applied efficiently
for the one-pot synthesis of 2-thioxo-3,4-dihydro-2H-1,3-benzoxazine-4-one
from triisopropylsilyl- and tert-butyldimethylsilyl salicylates. In the reported
methods for this synthesis, salicylic acid or their corresponding derivatives
such as salicyloyl chloride, salicylamide, alkyl and aryl salicylates is used as
[
11–14]
starting materials.
condition and low product yield. For example, the reaction of carboxylic
All of these reactions suffer from harsh reaction
ꢁ
acids has been performed at ꢀ40 C for several hours to furnish the synthesis
[
6]
of the desired compound in 70% yield. When we appliedour methodto
TBDMS or TIPS salicylates, 2-thioxo-3,4-dihydro-2H-1,3-benzoxazine-4-
one, a compound with fungicidal and bactericidal activities, was obtained
in 91% yield(Table 1, Entry 11).
The present methodis a new, inexpensive, andconvenient proce du re
for one-pot conversion of stable TBDMS andTIPS carboxylates to their

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3
656
IRANPOOR, FIROUZABADI, AND SHATERIAN
corresponding isothiocyanates under neutral condition. The excellent yields,
short reaction times, andmildreaction con di tions make this methoda
useful synthetic procedure for one-pot preparation of acyl and aroyl isothio-
cyanates and also for the preparation of 2-thioxo-3,4-dihydro-2H-1,3-
benzoxazine-4-one as a fungicide and a bactericide compound from their
TIPS andTBDMS carboxylates.
EXPERIMENTAL
All yields refer to isolated products. The products were purified by
column chromatography and the purity determination of the products were
accomplished by GLC on a Shimadzu model GC-8A instrument or by TLC
on Silica-gel polygram SIL G/UV254 plates, mass spectra were run on a
Shimadzu GC MS-QP 1000EX at 20 eV. IR spectra were record on a
Perkin-Elmer 781 spectrophotometer. The NMR spectra were recorded on
a Bruker Avance DPX 250 MHz instrument.
General Procedure for the Synthesis of
Acyl- or Aroyl Isothiocyanates
A three-necked flask equipped with a dropping funnel, stirrer, drying
tube chargedwith CaCl andN gas inlet was chargedwith Ph P (2.2 mmol)
anddry CH CN (5 mL). To this solution Br (2.2 mmol) was added dropwise
2
2
3
3
2
at room temperature under N2 atmosphere. When the addition was
completed, a solution of NH SCN (4.4 mmol) in CH CN (5 mL) was added
4
3
dropwise to the mixture. On addition of TBDMS- or TIPS esters (2 mmol)
to the resulting mixture, the reaction was completedimme di ately (Table 1).
Silica gel (1 g) was added to the mixture and the solvent was evaporated on a
rotary evaporator under reduced pressure. The resulting solid mass was
appliedon a silica gel column andwashedwith petroleum benzene
ꢁ
60–80 C/ethyl acetate (9/1) to give the desired isothiocyanates. All the
products are known compounds and were identified by comparison of their
1 13
IR, H, and C NMR data with those obtained for known samples.
[6,7]
ACKNOWLEDGMENT
We acknowledge the partial support of this work by Shiraz University
Research Council.

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ACYL AND AROYL ISOTHIOCYANATES
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Receivedin the UK August 6, 2001

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