Nucleophilic oxygen transfer from a perepoxide to phosphites

TetrahedronLetters, Vol32, No 7, pp 863-866, 1991 Printedm Great Bntain

NUCLEOPHILIC

0040-4039/91 $3 00 + Pergamon Press plc

OXYGEN TRANSFER FROM A PEREPOXIDE

Manolis Stratakis, Michael Orfanopoulos, Department

of Chemistry

University

and Christopher

TO PHOSPHITES

S Foote*

and Bmchenustty

of California, Los Angeles

Los Angeles, CA 90024- 1569, USA

Summary:

Phosphites

trap the perepoxide

The phosphite reacts as an electrophtle In reactions

of ‘0,with

various substrates,

trapping agent

ion mtermediates

(Ad=Ad)s,

intermediate

sulfoxides

nucleophilic

carbonyl

in the reaction

to sulfoxides.

oxides6

are more effective sulfide (k&

of lo,

formed

than either sulfides

= 87, Scheme below)ts

or sulfoxides prompted

sulfide.XV

from

and their

of ‘0,

with sulfides2.3,4,

was used to trap peroxonium

and oxadisiliranes.7.8

with diethyl

and the perepoxide

A recent observation

sulfoxlde

in attack on sulfoxides,

Adam has used thtanthrene-5-oxide

many of these reactions.lcJ1.12

the epoxlde.

have been found to be the most

in the reaction

Diphenyl

of strained disiliranes

in sulfide oxidation acted as a nucleophile

was electrophilic4;

forming

have been proposedi,

Sulfoxides

trap intermediates

and diazoalkanes.6

in the singlet oxygenation

also been used to trap intermediates

several oxenoid intermediates

or by trapping experiments.

For example,

adamantyhdeneadamantane

efficiently,

m this process.

existence has been verified kinetically effective

from adamantylideneadamantane

Diphenyl

sulfide has

Sawaki found that the

although m attack on sulfides it

Ad=Ads

were

also found

as a nucleophilicity/electrophilicity

that phosphites,

which are relatively

in trapping intermediates

us to mvestigate

probe for

inert towards

m the lo2 reaction

their efficiency

to be

m trapping

102,

of diethyl

other proposed

oxenoid mtermediates

‘02

EtzS -[xl

EtzB

2 EtzSO

ks

EtzSO + (MeO)aPO

The hindered intermediate epoxide.5

Ad=Ad

reacts

in this reaction,

with lo,

thought

to be the perepoxide,

We carried out the photooxidation

was produced,

accompanied

with the concentration

by an equimolar

of phosphite

= 1.7 x 1O-3M (see scheme trappmg the mtermediate

to give a stable

l,Z-dioxetane.t4.t* was readily

of Ad=Ad in benzene

amount of the phosphate;

(Table 1). A plot of the reciprocal

below and Fig. 1). Triphenylphosphite

perepoxide

than mmethylphosphne

X63

trapped

in the presence

Schaap

showed

by sulfoxides

that an

to give the

of phosphitesis.

the epoxide/dioxetane

Epoxide

ratio increased

of thus ratio vs. phosphitel-1

gave k r&r,

was roughly three tunes more efficient

and roughly equal to diphenylsulfoxide

J

in

864

Table 1. Product ratio as a function of [Triphenylphosphite]’

EpoxideiDioxetane

K$0)~P1,mM 0

7f93

1

42J58

2

56/44

3

70130

5

82/18

8 10

9713 99/l

25

100x)

*[Ad=Ad] = 3.5 mM, C,H,. 0

o-o

OC

Ad=Ad

d--‘Ad -

-

‘02 b 0

ALL

bl

I

WbP

AdAd +

(RObP=O

16

6 B

0.8

4

0.6

5Ko

0.4

9

0

0.2

0.2

04

0.6 [Phosphtte]

Fig. 1. Dioxetandepoxide

-‘,

0.8

1.13

mM-’

rano (Table 1, corrected for epoxide formed with no phosphite) vs [Phosphitel-I.

One possible route to epoxide and phosphate would be reduction of dloxetane by the phosphite. process is known to proceed with tialkylphosphines.

Such a

15 However, a g-fold excess of triphenylphosphite did not

result in significant (cl%) formanon of epoxide from dioxetane under the experimental conditions.

865

The relative reactivities (kx&.J of aryl substituted phosphites toward oxygen transfer from perepoxide in competition experiments in benzene 17 are: m-Cl = 4.4, p-Cl = 2.9, p-F = 2.0, m-Me = 0.7, p-Me = 0.6, pMe0 = 0.35. A Hmmett plot (Frg. 2) gave p = +1.76 (r = 0.957). This result shows that oxygen acts as a nucleophlle in attack on phosphorus.

Analogous experiments in acetone gave p = +1.85 (r = 0.962). The

scheme below shows a possible mechanism, although no intermediate is required by the data. Hammett

Plot for Triaryl Phosphites

0.4 -

3 B

02-

0.0 -

-02-

-04:

A’

-0 3

I”“I”“I”“I’“‘I”“1”” -0.2 -0 1

00

0.1

02

03

04

a

Fig. 2. Hammett plot for substituted triarylphosphites in benzene. Log k, for p-OMe (l), p-Me (2), m-Me (3), p-F (4); p-Cl (5), and m-Cl (6), relative to H.

? cO*-0-P(OArb

When the reacnon was carried out in benzene/CHsOH

-

(l/l), a substantial decrease in relative reactivity

was observed, and p was only +0.98 (r = 0.951). This lower electrophihcity can be attributed to reductron of the nucleophilicrty of the the neganve oxygen of the perepoxide by hydrogen bonding to methanol.

Similar

hydrogen bonding was suggested to increase the electrophilicity of persulfoxides in alcoholic solvents.2 These results demonstrate that phosphites, especially those bearing electron-withdrawing

substituents, are

effecuve trapping agents for oxenoid species and that (somewhat surprisingly) they react as electrophiles, as do sulfoxides. However, the p values are larger than with sulfoxides: Schaap reported a p of only +0.52 for aryl methyl sulfoxides in trapping adamantylideneadamantane for triarylphosphites

is more than threefold larger.

perepoxide in benzene;5 by comparison,, the value This probably reflects the fact that there are three

substituents in the phosphites and only one in the sulfoxides. are in progress.

Further trappmg experiments using phosphites

866

After this work was submitted,

a paper appeared suggesting

that the production

of adamantyhdeneadamantane

is an artifact, caused by hydrogen

our hands, hydrogen

does not cause epoxidation

peroxide

peroxide

of epoxide in the reaction

production

as a side reactlon.ls

In

of adamantylideneadamantane

under the conditions

and NATO grant no. 880120.

M. S. and M 0 are

of the photooxidations. Acknowledgment

This work was supported by NSF grant no. CHE89-11916 at the Department

of Chemism,

University

of Crete, P. 0. Box 1470 Iraklio, Crete Greece.

References

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Press, New York, N. Y., 1979; pp 244-286. (16) The solution was irradiated with a Cermax 300W Xenon lamp with a NaNO, filter solution for 20-30 minutes usmg TPP (2.5 x lOA M) as sensitizer,

3 5 mM [Ad=Ad].

(17) A 4 mM olefm solution in benzene (TPP as sensitizer) total 15 xnM equimolar amount of triphenylphosphite

was u-radiated for 20 mtnutes in the presence and the aryl substituted

mixture was analyzed by G.C. The relative rcactivities

(18) Jefford, C. W.; Estrada, M. J.; Barchietto, 1653-1657. (19) S. Silverman (Received m USA

and C. S Foote, unpublished. 31 October 1990)

hi=

lc+W)

kH

log++$$

were determmed

G.; Berclaz, T.; Geoffroy,

phosphate.

of a

The reaction

using the expression:

M. Helv. Chim. Acta 1990,73,