The chemistry of cyclic carbaphosphazenes: The first observation of (R 2PN)(ClCN)2 (R=Cl, Ph) as a reagent for the conversion of alcohols to aldehydes, ketones, and alkyl chlorides

Article abstract of DOI:10.1080/10426500600733947

The oxidation of nine primary and secondary alcohols to the corresponding aldehydes and ketones has been carried out under mild conditions and in good yields using the cyclocarbaphosphazenes (R₂PN)(ClCN)₂ [R₂P = Cl₂ P(1), Ph₂P(2)] along with dimethylsulfoxide. While both the P-Cl and C-Cl bonds of the carbaphosphazene can in principle bring about the reaction, we observed an increased preference for the C-Cl bonds over the P-Cl bonds in the oxidation of alcohol. Blocking the reactive P site on the heterocyclic ring with the phenyl groups was found to reduce the yields of the oxidized products, while blocking the C- sites with diethylamino groups resulted in no reaction. In addition, along with DMF, the same cyclocarbaphosphazene has been found to be useful for the conversion of alcohols to alkyl chlorides. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/10426500600733947

Phosphorus, Sulfur, and Silicon, 181:2445–2452, 2006
Copyright © Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500600733947
The Chemistry of Cyclic Carbaphosphazenes: The First
Observation of (R2PN)(ClCN)2 (R Cl, Ph) as a Reagent
for the Conversion of Alcohols to Aldehydes, Ketones,
and Alkyl Chlorides
Nabakrushna Behera
Pradyumna Kumar Mishra
Anil J. Elias
Department of Chemistry, Indian Institute of Technology, Delhi,
Hauz Khas, New Delhi, India
The oxidation of nine primary and secondary alcohols to the corresponding aldehy-
des and ketones has been carried out under mild conditions and in good yields using
the cyclocarbaphosphazenes (R PN)(ClCN) [R P = Cl P(1), Ph P(2)] along with
2
2
2
2
2
dimethylsulfoxide. While both the P Cl and C Cl bonds of the carbaphosphazene
can in principle bring about the reaction, we observed an increased preference for
the C Cl bonds over the P Cl bonds in the oxidation of alcohol. Blocking the re-
active P site on the heterocyclic ring with the phenyl groups was found to reduce
the yields of the oxidized products, while blocking the C- sites with diethylamino
groups resulted in no reaction. In addition, along with DMF, the same cyclocar-
baphosphazene has been found to be useful for the conversion of alcohols to alkyl
chlorides.
Keywords Aldehydes; alkyl chlorides; cyclocarbaphosphazene; dimethylformamide;
dimethyl sulfoxide; oxidation
INTRODUCTION
The impact of inorganic and hybrid inorganic-organic heterocycles, such
as halogenated cyclophosphazenes and cyclocarbaphosphazenes, has
not only been in use as precursors for inorganic polymers but also in
finding the unique heterocyclic framework useful for novel reactions
and applications. For example, cyclophosphazenes are found to be useful
as stable cores for dendrimer synthesis,1 as scaffolds for multicenter lig-
and synthesis,2 in the design of unique supramolecular networks,3 and
Received February 3, 2006; accepted March 25, 2006.
We thank Department of Science and Technology, India, for financial assistance in the
form of a research grant to Anil J. Elias.
Address correspondence to Anil J. Elias, Indian Institute of Technology, Department of
Chemistry, Delhi, Hauz Khas, New Delhi 11001, India. E-mail: elias@chemistry.iitd.ac.in
2445

2446
N. Behera et al.
as supramolecular hosts,4 all of which result from their unique struc-
tural framework, reactivity of the P-halogen bonds, and orientation of
substituents around the heterocycle. N3P3Cl6 has also been used as a
reagent for the conversion of carboxylic acids to acid anhydrides5 and
aldoximes as well as acid halides to nitriles.6,7 Related cyclic carbaphos-
phazenes having both PCl2 and C Cl moieties in their ring framework
has revealed a chemistry that differs in many ways when compared to
cyclophosphazenes (Scheme 1).
SCHEME 1
Reactions such as dealkylation8 and ring opening of tertiary amines9
and those showing regioselectivity in nucleophilic substitution10 have
been unique for carbaphosphazenes. The difference in the reactivi-
ties of C Cl and P Cl bonds of carbaphosphazenes accounts for these
unique reactions. Quite recently, Chandrasekhar and coworkers have
prepared the first examples of pendant carbaphosphazene contain-
ing monomers and polymers making use of the unique reactivity of
carbaphosphazenes.11 New examples of carbaphosphazenes and bet-
ter methods for their synthesis have also been reported recently.12 In
this article we report the first observation of the use of chlorinated car-
baphosphazenes as a reagent along with dimethylsulfoxide for the mild
oxidation of alcohols to aldehydes and ketones. We also report herein
the conversion of alcohols to alkyl chlorides when DMF is used instead
of DMSO along with the carbaphosphazene.
RESULTS AND DISCUSSION
The oxidation of alcohols using activated DMSO has been first observed
by Mancuso and Swern, who used oxalyl chloride along with DMSO.13
Although routinely used, the Swern oxidation still has some disadvan-
tages as the reaction is violent, oxalyl chloride is sensitive to hydrol-
ysis, and its vapors toxic. The use of reactive halides has since been
reported for the same reaction with DMSO, and some of them, such as
hypervalent phosphorus14 as well as iodine compounds15 and cyanuric
chloride,16 are highly reactive compounds that are relatively difficult

The Chemistry of Cyclic Carbaphosphazenes
2447
to handle. Our interest in using (Cl2PN)(ClCN)2 for this reaction stems
from the fact that this carbaphosphazene, which is easily prepared by
a one-step method, is easier to handle and is more stable than some of
the highly reactive DMSO activators. In addition, since P Cl, C Cl, or
both bonds can in principle bring about such an activation, the unique
structure of the hybrid heterocycle also provides an opportunity to de-
termine the relative reactivity of the P Cl and C Cl bonds in such
mild oxidation reactions. We have chosen three different carbaphosp-
hazenes in the present study with one having both reactive P Cl and
C Cl bonds(1), one with only reactive C Cl bonds(2), and one with only
reactive P Cl bonds(3) (Scheme 2).
SCHEME 2
Reactions of cyclocarbaphosphazene 1 were carried out with a vari-
ety of alcohols. It was observed that (Cl2PN)(ClCN)2/DMSO efficiently
brings about the oxidation of the alcohols in the presence of triethy-
lamine (Scheme 3). Nine different alcohols were tried that were found
to undergo the reaction resulting in oxidized products with yields vary-
ing from 31 to 87%. The results are given in Table I.
SCHEME 3
The oxidation of the alcohols were conveniently carried out using
CH2Cl2 or THF as a solvent. The relatively better procedure with higher
yields involved the treatment of cyclocarbaphosphazene with five equiv-
alents of DMSO in CH2Cl2 at −40◦C for 1 h followed by the addition
of alcohol. After an additional 30 min, triethylamine was added, and
the reaction was allowed to run for 2.5 h. Bringing the reaction to r.t.

2448
N. Behera et al.
TABLE I The Conversion of Alcohols Into the
Corresponding Carbonyl Compounds
ROH
RꢀCHO/R1R2CO
Percentage yield
87
53
70
72
87
85
75
57
31
and working up yields the carbonyl compound. The reactions were car-
ried out at −40◦C to prevent the eventual formation of undesirable
byproducts, such as chloro derivatives and thiomethyl ethers. The de-
sired product along with unreacted alcohol, if any, was obtained, from
which yields of the products were determined by NMR analysis. Pure
aldehydes and ketones were isolated in all cases, and their identity was
confirmed by standard spectroscopic methods and comparison with au-
thentic samples.
Taking benzhydrol as a standard, the ideal conditions were worked
out so as to maximize yields. It was observed that after the addition of
a base, the more the time given for reaction the more was the yield. In
order to see the effect of the temperature on yields, reactions were also
carried out at −20◦C and 0◦C. In both cases, the yields were relatively
poor, i.e., 32% and 21%, respectively. But when attempted at −60◦C, the
reaction was complete, and the yield was almost quantitative (>96%).
By changing the solvent (THF instead of CH2Cl2), the reaction can be
carried out, but with a decrease in yields of the products. The oxidation
of 2-phenylthioethanol was slow, resulting in a lesser yield, which was

The Chemistry of Cyclic Carbaphosphazenes
2449
also been observed in a previously reported oxidation reaction of this
alcohol.16
The substitution of Cl by OH groups on chlorinated phosphazenes
and cyanuric chloride has been shown to result in the partial ring sat-
uration and formation of an exocylic P O or C O bond, respectively,
with the adjacent ring nitrogen getting protonated.8,17 The formation
of P O and C O bonds of relatively higher stability in comparison to
P N and C N bonds is the driving force for this reaction in such ring
systems. The carbaphosphazene used in the present study has both
Cl2P N and ClC N moieties in its framework, and it can in princi-
ple get converted to (A) or (B) during the oxidation and chlorination of
the alcohols (Scheme 4). To verify this aspect, compounds 2 and 3 hav-
ing partially blocked phosphorus and carbon sites were prepared and
reacted with an alcohol.
SCHEME 4
The reaction of compound 3 having only reactive P Cl bonds was car-
ried out with benzyl alcohol along with DMSO under conditions that
yielded maximum oxidized products in the reaction of alcohols with
1. However, there was no desired product, and the unreacted alcohol
was isolated from the reaction mixture. On the other hand, in a reac-
tion of compound 2 having only reactive C Cl bonds, the same alco-
hol was found to react, and benzaldehyde was obtained in 17% yield.
This study clearly indicates that C Cl bonds of the carbaphosphazene
are more reactive in comparision to the P Cl bonds in the previously
discussed transformation. A decrease of yield in the oxidized product
may be attributed to steric and electronic effects induced by the phenyl
groups present on the phosphorus site on the reactivity of the C Cl
bonds.
Reactions of the carbaphosphazene (Cl2PN)(ClCN)2 with the core-
agent DMF brings about an efficient conversion of alcohols to the cor-
responding alkyl chlorides in CH2Cl2 at r.t. (Scheme 5). Among the sec-
ondary alcohols used, only 1-phenylethanol gives a better yield than
benzhydrol. It is noteworthy that benzyl alcohol, cinnamyl alcohol, and
4-methoxybenzyl alcohol give yields that are almost quantitative. The

2450
N. Behera et al.
SCHEME 5
reaction of n-hexanol resulted in a mixture of 1- and 2-chlorohexane,
and NMR analysis of the mixture revealed that the latter is the major
product in the chlorination reaction.
Details of the alcohols used and chlorinated products obtained are
given in Table II.
As shown in Schemes 3 and 5, oxidation and chlorination reactions
proceed possibly through the formation of alkoxysulfonium and imi-
dinium salts of carbaphosphazene as intermediates. It is important to
note that while DMSO gets converted to Me2S in an oxidation reaction,
the role of DMF is more or less catalytic in nature in the conversion of
alcohols to alkyl chlorides (Scheme 5).
In conclusion, we report herein the first use of a chlorinated car-
baphosphazene as a reagent for the mild oxidation of alcohols as well
TABLE II The Conversion of Alcohols to Alkyl
Chlorides
ROH
RCl
Percentage yield
94
68
72
50
95
81
71
66
Mixture of isomers

The Chemistry of Cyclic Carbaphosphazenes
2451
as the conversion of alcohols to alkyl chlorides in good yields in the
presence of DMSO and DMF as coreagents.
EXPERIMENTAL
All solvents and alcohols used were obtained from commercial sources
and were distilled and dried prior to use by standard procedures.
Cyclocarbaphosphazene, (Cl2PN)(ClCN)2,18 (Ph2PN)(ClCN)2,19 and
8
(Cl2PN)(Et2NCN)2 were prepared according to literature procedures.
All products obtained from the reactions of alcohols were separated, pu-
rified, and characterized by 1H and 13C NMR and IR spectroscopy, and
wherever possible, their spectra were compared with authentic samples
of aldehydes, ketones, and alkyl chlorides from the literature.
The General Procedure for the Oxidation of Alcohols
DMSO (1.25 mL, 17.6 mmol) was added to a solution of (Cl2PN)(ClCN)2
(0.86 g, 3.6 mmol) in CH2Cl2 (20.0 mL) and was stirred and maintained
at −40◦C under a nitrogen atmosphere.20 After 1 h, alcohol (3.0 mmol)
in CH2Cl2 (10.0 mL) was added slowly at −40◦C with stirring, followed
by triethyl amine (2.0 mL, 14.3 mmol) after an additional 30 min. After
2.5 h, the reaction mixture was brought to r.t., and the solvent evap-
orated under vacuum. Diethyl ether (50.0 mL) was then added to the
viscous solid formed. The mixture was then quenched with 1N HCl.
It was observed that the organic phase selectively dissolved the alde-
hyde/ketone along with any unreacted alcohol present, and the oxidized
carbaphosphazene remained in the aqueous layer. The organic phase
was washed with 15 mL of a saturated solution of NaHCO3 followed
by brine. After that, it was dried with anhydrous Na2SO4, and the sol-
vent evaporated off to get the product. Impure products were purified
by column chromatography over silicagel using hexane/ethyl acetate.
All products were characterized by spectral data and compared with
authentic samples.
The General Procedure for the Conversion of Alcohols
to Alkyl Chlorides
(Cl2PN)(ClCN)2 (1.2 g, 5.0 mmol) was slowly added to DMF (1.0 mL) un-
der a nitrogen atmosphere and maintained at 25◦C.21 A reddish brown
colored mass formed, which was stirred for 30 min, and then CH2Cl2
(25.0 mL) was added, followed by alcohol (4.5 mmol). After the addi-
tion, the mixture was stirred at r.t. for 2 h. Water was added, and the
organic phase was then washed with 15 mL of a saturated solution of

2452
N. Behera et al.
Na2CO3, followed by 1N HCl and brine. The organic layers were dried
with anhydrous Na2SO4, and the solvent evaporated off to yield the
chlorides. Compounds were purified by column chromatography over
silicagel using hexane/ethyl acetate as an eluant.
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