Photocyclization of ortho-benzoyl N-alkylanilinium ions

Tetrahedron Letters, V-0132. No 32. pp 3915.3918.1991

0040.4039/91 $3.00 Pergamnn Press plc

Prmted m Great Bram

Photocyclization

of Ortho-Benzoyl

Peter

J

Wagner’

N-Alkylanilinium

and Qunjran

l

Ions

Cao

Chemistry Department, Michigan State University, East Lansing, MI 48824

Abstract: Ortho-benzoyltrimethylanilrnium tetrafluoroborate effloently photocyclfzes by triplet state S-hydrogen abstract/on to the 1, I-d/methyl-3-hydroxy-3phenyl-2,3-drhydromdollum

salt

both

in acetonitrlle

solutron

and

Ortho-

as a sol/d

benzoyl-N,N-dibenzylaniline hydrochloride photocyclizes IR acetonltr!le to N-benryl2,3-diphenylindole. The rate constant for S-hydrogen abstraction is high thanks to a favorable 0 02.

geometry

Both

but

rapid

Ortho-alkoxyphenyl hydrogen to

to

prepare

ketones

have

not

electron

transfer

and

quenching undergo wrth

been

and

by

promotes

to

normal

or

examples

of

abstraction

from

high

yield

both

and felt

alkylation

of with

crystals

of

1

resulted

Near In

Its

3-phenyl-2,3_dihydroindolium recrystallrzatron

from

tying

lone and

involving

excess

irradiation9

quantitative

of

Its

6

Rapid

competes

Padwa’s pair

on

now

CT

bad

luck

nitrogen

report

intramolecular 1

methyl erther

transformation

tetrafluoroborate methanol.

result

the

by first

hydrogen

salts.

with uv

phenyl

o-amidobenzophenones

up the

cyclrzahon

cyclrze

suppresses

Despite

tetrafluoroborate

o-amrnobenzophenone AgBF4.

that

the

mtramolecular,

S-

y~etds 3

o-alkylamino

being

and

dealkylation.8

promote

Ortho-benzoyltrimethylanilrnlum exchange

of

However,

7

undergo

usefut

of a-ammoketones

photoreactrvrty

anllrnrum

to only

that

synthetlcalty

usually inter-

Acylation

photocyclization

o-benzoyl

yield

I,5brradrcals

In

photocyclrzations mixtures

could

quantum

ketones4

generate

former

messy

we

protonation

to

the

carbon@,

deacylatron

a-amrnoacetophenones,7a

quaternizatron

states ,5

abstraction.

photolytic

the

o-m-butylphenyl

comparable

successful, nitrogen

hydrogen

prrmanly

triplet

lowers

quantitatively.

and

derrvabves

indoles

from

impedes

their

indanot

Attempts

with

from

or

quenching

essentially

ketones1-3

abstraction

benzofuran

internal

proceed

react,ons

NMR

3915

2.10 spectrum

Iodide

dilute to The closely

was

prepared

followed

acetonitrlle

by

by

solutions

eon

or

1 ,l -dimethyl-3-hydroxyproduct

was

resembles

Isolated that

of

by the

3916

lndanol

formed

differences

from

wlthrn

experimental

stable.

Aoyama

isoelectronrc

shifts

error of

efficiency

o-m-butylbenzophenone

due to the posrtive

of

reported

photocyclizatron quantum

the

in chemrcal

100%

very

The

The

product

appears

solid

low chemical

yields

and

The

reactron

were

IS readily

performed

quenched

by measuring

that the reaction

lQOO-fold and

with

finding

with

The positive which

that

for the

1 yrelds 2 with a

typical

triplet

Quantrtative

quenchers.

ethyl

sorbate

on the loss of

With a value for in the monitoring region. we calculate a triplet lifetime of 16 + 4 ns

by g-hydrogen

as occurs

between

an electron-deficient

carbonyl

earlier

efficiencies

abstraction

In OTBBP.4

and then

The triplet

cyclization

lrfetime of OTBBP

The positive charge on nitrogen has the expected effecti of abstraction, but the 30-fold effect is much smaller than the

difference

valerophenone.14

towards the

reactivity

proceeds

Intermediate,

is only 0.5 ns in hexane.4 slowing down &hydrogen

indefrnltely

2

the effect of added

since a minor byproduct absorbs strongly kq In acetonitrile of 1 x 1010 M-l s-1 ,ll.Jz of the 1,5-blradrcal

quantum

be

for was

A linear Sternin carbonyl IR absorption at 1680 cm-1 of 160 f 40 M-1. UV analysis proved to be Inaccurate,

1 as Indicated by the change Volmer plot gave a kqz value

We assume

to

balance

rn acetonitrlle

solld

studies

except

material

In contrast,

o-dimethylamrnobenzophenone.”

of 0.56

OTBBP,s

nitrogen.

charge

reagent: it

protonated

in 1 deactrvates

but it also

of

pyridyl

the adjoining

enhances

This

conjugated.

IS

protonation

y-drmethylamlnobutyrophenone the

conclusion

ketones

intrinsrc

C-H

IS rn accord

increases

their

bond

reactivity

tnplet

with

of our

reactivity

by a factor of ten.15 o-Benzoyl-N,N-dibenzylanlline in acetonitrile x-ray

solution

crystallography.

low: 0.02

hydrochloride

to yield Quantum

In dry acetonrtrile

3 also

yields

and 0.001

were with

readily 6% water

which

measurable present.

for all lnmal reaction but itself is gradually converted secondary product. Quenching studies with ethyl sorbate M-1 and thus a triplet

lifetime

undergoes

N-benzyl-2,3-drphenylindole,

of only 0.25 ns

These

photocycllzation was

identified

by HPLC The

lndole

by

and were accounts

to an as yet umdentlfied provided a kg7 value of 2 4

figures

indicate

a rate constant

3917

for

hydrogen

yields

abstraction

rndrcate

a raprd

of 8 x

quenching

competrng

to that

107 s-1, comparable

for

Two

process.

Ph. .H

the

strongly

proton spectra its

acidic

transfer indicate

weak

enhanced

triplet

to the that

conlugate

anrlinrum

carbonyl in solution base,

by added

which

can

ground

IS

a

Phh

decay

probably

the

which

water,

ion

oxygen

by

occurs, state

the

IS

4,2

despite

the

The

lowest

energy

predicted

by

on

and

exist

primarily

abstraction, the as

with

N-benzyl triflic.

charge,

low

In

the

case: The

o-benzoylanrlrnes

to

it does

not

combination

population

group of

of

that

twisted

photocyclize lowered

reactive

the

in acetonitrrle intrinsic

conformatrons,

in

with

reactivity rapid

an

because

group

2

ketones

internal

contarnrng

3,

unreactive

o-alkoxy

carbonyl.2

close

Unlike

benzyl

carbonyl

of

benzylic

away, 17

the

the

IS

N-H

one

A

rapidly

primarily

IS

distance

and

undergo

from

The

2.8

react

for

calculations

left.

only

mimic

cannot

away

nitrogen

bondrng

ant1 to the

undoubtedly

conformatrons

N-alkyl

MMX

oxygen

conformation

monoalkyl

3

rapidly

conformation

the

is

exists

triplet

more

positive

hydrogen

carbonyl

held

that

o-benzyloxybenzophenone

hydrogen 4

IS

yields

a b-hydrogen

enough

Uv wrth

drssocratron

noteworthy

pictured

rapid 1s

equilibrium

This

does

the

and ketones

in

than

within

quantum likely*

CH,Ph

IS

filter.

quantum It

Simple

!-

Ion

abstracts

3

low are

deprotonatatron;

internal

lowered

The

as in o-hydroxy

anrlmrum

powerful

explains

1

possrbrlrtres

hydrogen We looked

acids of

radratronless

as

the

at

strong positive

decay

by

3918

deprotonatlon,

and

cyclization and

probable

In these

trialkylanihnium

salts

Acknowledgement. was

purchased

Internal

cases

NSF grant

by

unprotonated

Nonetheless,

suggests

This work

with

frlterlng

unlikely.

several

was

the

avenues

for

further

by NSF grant

supported

amlne

successful

make

efficient

cyclizatlon

of

dl-

exploration.

CHE

88-15052.

The

NMR

CHE 88-00770.

No

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