Steric course, kinetics and mechanism of the 2+2 cycloadditions of alkylphenylketenes to ethyl cis - and trans -propenyl ether

Steric course, kinetics and mechanism of the 2+2 cycloadditions of alkylphenylketenes to ethyl cis - and trans -propenyl ether

TotrahodronLetteraNo. 34, pp 2y6q - 2972, 1975. STERlC COURSE, KINETICS AND ALKYLFHENYLKETENES MECHANISM TO ETHYL klf lnstitut Rtr Orgrmis&e...

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TotrahodronLetteraNo. 34, pp 2y6q - 2972, 1975.

STERlC

COURSE,

KINETICS

AND

ALKYLFHENYLKETENES

MECHANISM

TO

ETHYL

klf lnstitut

Rtr Orgrmis&e

Chemie

(Bcceiredin UK 23 June The reaction 40’

quantitatively

tional

reaction

(Table

by nmr I).

The double

In the preceding the transition

is placed

sure while

it is phenyl

between

3

ond hydrogen.

bulk of R the formation less stable

Karlstr.23,

ethyl

propenyl

of ketenophile

Germany 1975)

ethers in beruonitrile structure.

at

The configum-

1 - $_of the Jr2a

complexes

+n2;l

6’ bonds.

that the orientation complexes

complex

is a good model for

of type -3 and 4- in which

In 1 and 2 the ketene

equal

via 1 and 2 to a nearly __ for the sterically

and other cis disubstituted

On the other hand, duct via the more favored -

5

olkyl

is in the zone

the ketene

of steric

pres-

an increase

: 4’

This is attributed

ethylphenyl-

is analogous

ether

is favored

to the increase

and pmpylphenylketene

andi-butylphenylketene

to the tmns enol ether (2 for methyl

of the enol ether

of the ratio _-3’

cis-pmpenyl -

of van use the

react only via 2. --

to the behavior

over

of cyclopentadiene

The 4

mixed ketenes.

of --3 : 4 rests on the assumption

If methyl and hydrogen

from ethyl

preferred.

isopropylphenyl-

towards

complex

2’

Whereas

cyclobutanone

the cycloaddition orientation

is always

to t-butyl.

extent,

more crowded

ethylenes

of cyclobutanone

product

pathways

armngement,

ETHER

in 2 and 4. -

increasing

ful interpretation

2,

of the orientation

we have demonstmted

R from methyl

dmgen.

isomeric

the incipient

der Waals stmin in 1 on varying

predilection

8 Monchen

the structure

OF

Mayr

with retention -1’ - 4’ -

to deduce

ethoxy

CYCLOADDITIONS

TRANS-PROPENYL

with the cis-trans

communication

2+2

accepted for publication10 July

1975;

arrows indicate

the thermodynamically

x,2.,

G-AND

state and that trans enol ethers choose orientation

substituent

With

allows

THE

Huisgen * and Herbert

produces the cyclobutanones

elucidation

OF

der Universitlft,

of alkylphenylketenes

2

PergoaonPrcoo. Printed in Grvot Britain.

to pmpyl,

that the ketene were to interact

should accompany 2969

furnishes predominantly ,4 for,isopropyI

substituent

is located

with the ketens the bmnching

lf

the more stable

and t_-butyl). between

substituent R in contrast

pro-

A meoning-

ethoxy

and hy-

in the orthogonal to observation.

1970

Table

We.

I.

Orientation

Activation

Complexes,

Free Energies

in Bennronitrile

Pmduct Distribution,

for the C)&additions

Rsrtial

Rate Constants

of Alkylphenylketenes

to Ethyl

I*

(18k2, e-

mol-‘set-‘)

34

and

and

at 40”

P c

czv,

M-5

Y

,Aj= p\



‘gH5

CzY\

,H

,C

c=lj=c c/ H’

v’s R

CiS

R :f

3



II

0

CH3

0

trons

2

G 1

1

CrH50

CgH5

0

c6”5,8,h

RW

N

o

2

CH3 2’

7. Yldd

I?

kl

T

4

t

01. yuztd

k,

AC+

‘I. yw Ld

0

-

-

16

-

10

0.40

27.54

22

164

67

2.4

26.42

13

0.35

27 62

27

176

16

0.022 29.34

65

0.13

26.24

200

0.028

29.19

183

23 94

100

46

65

24.37

90

CH&HzCHJ

51

65

49

62

24.40

-

-

100

26

24 94 25.95

0

CHKIi3)Z CKH,),

-

-

The combination photometrical composition

addition (Table

-

of kinetic

data with

rate constants

I),

i.e 2

kcis

=

Pmbably, energy

:

two electronic the electron

bmnching

k, + k2,

in the steric

factors coopemte

release by R (Taft’s

results

=

kg+

100

allows

interaction

as well

a quantitative

ether are partitioned

evalution.

according

The

to product

k4.

bulk of R cannot have steric

reasons because R is pointing

of ketene and ketenophile

in reducing d’)

-

pmpenyl

and and ktmns

growing

-

the cycloadditions as the out-of-plane

substituents

rate by lifting twisting

out-

in 2 and 4. -

the ketene LUMO

of phenyl

increase with

of R.

On varying 4 kcal/mol

the stereochemical

for the e-

The decrease of k2 and k4 with wards and is hence not involved

5.1

100

i;-

24.41

130

24.26 24.37

k,

AG:

26.19

12

77

-26.60

k4

3.5

22.66

64

88

.I. ylald

4

61

985

CHzCHJ

CH3

k3

and

nal 3 kcal/mol

the alkylphenylketene

AG2* in

&B,*

from methyl

only by 1 .O kcal,&rol. reflect

With

the greater steric

to isopropyl

one observes thot

the same electronic stmin

in the tmnsition

deactivation

dG,*

increase

by

in both series the additio-

state on replacing

methyl

by isopropyl,

2971

No. 34 Analogously,

.AGrincreases

mom steeply than

In the complexes land

AG,?

3 the ketone olkyl is oriented toward the interior,

mpulsion. The increase of k/k3

the mgion of van der Wools

from 16 for R = methyl to 27 for propyl mirrors the additional

tion in overcoming the interfemnce

energy consump-

of R with ethoxy and hydrogen in 2 compared with thot of R between two

hydrogens in 1.

On the other hand, the cis-tmns rote ratio k2+/k4 stays in the mnge of 180 ; i.e., md to conquer the additional two hydtogens in i.

3.2 kcal/mol

om mqui-

steric mpulsion of phony1 between ethoxy and hydrogen in _4 vs. that of phony1 and

The mtio kfi4

does no longer depend an the size of R which juts outwards and, themfom,

plays no active role in the steric intemction of the substituents in the tmnsition state. position to ethoxy in 1’ and at the tmns location in c,

the constancy of k2/k4

As Rends up at the cis

is only mconciloble

with on

early transition state.

Ethyl e-

ond E-pmpenyl

ether having nearly equal free energies,’

the mte ratios k,/k3

and k/k4

mveol unique steric effects in the tmnsition states. Though the singular pmfemnce for cis 1,2-disubstituted ethylenes or ketenophiles is a significant mechanistic criterion, ‘lT2a +

82s

3

it is not an unequivocal proof for the

process. The findings om also consistent with a concerted pathway [Gs

+n2s +rr2s] 7 for

which a diagonal attack of the ketenophile fl systemon one orbital each of the CC and the CO double bond of the ketone is postulated. While the orientation complexes am still diffemnt, tually indistinguishable as the mhybridizotion

Appamntly,

the two schemes am becoming vir-

proceeds.

the cis pmfemnce does not occur in 2+2 cycloadditions via zwitterionic -

the cyclobutane formation from tetmcyanoethylene to mtios of TCNE with e-

and tmns-l-alkenyl

intermediates ;

and enol ethers may be mgorded as a prototype.

olkyl ethers om close to unity.

9

8

The m-

No. 34

2972

REFERENCES

1)

The new cyclobutanones gave satisfactory CH onalyser and were chamcterired

2)

H. Mayrond

3)

R. Huisgen and H. Mayr,

4)

P.R. Brook, J.M.

Harrison, and A.J.

A. Dieffenbacher,

and A. S. Dreiding,

R. Huisgen, Tetmhedron Lett.,

J.Amer.Chem.Soc., 5)

T. Do-Minh

Tetmhedron Lett.,

6)

T. Okuyama,

7)

R.B. Woodward,

8)

Brady, F.H.

Duke,

s,

J.C.S.

Chem.Comm.,

Helv.Chim.Acta,

2,

Bmdy and E.T. Sot.,

Parry, ond J.D.

g,

417 (1970) ; W.T. Hoff,

J.Org.Chem.,

Rey, S. Roberts,

Brady and R. Roe, g,

3733 (1970).

Brady and R. b,

Stockton, J.Org.Chem.,

5,

36, 1486 (1971) ; R.C. =

2908 (1972).

private communication to R.H.,

1968;

5409 (1969). see R. Huisgen, L. A. Feiler,

and G. Binsch,

102, 3460 (1969).

95, 5056 (1973);

J.Amer.Chem.Soc.,

95, 5054, 5055 (1973) ; G. Steiner and R. Huisgen, =

R. Huisgen, R. Schug, and G. Steiner,

81 (1974). 9)

589 (1970) ; M.

1766 (1970) ; W.T.

T. Fueno, and J. Furukowo, Tetmhedron,g,

R. Huisgen and G. Steiner, ibid., -=

preceding communication.

and 0. P. Stmusz, J.Amer.Chem.

De Selms and F. Delay, ibid.,

Chem.Ber.,

1349 (1975).

92, 4618 (1970) ; W.T.

93, 1662 (1971) ; W.T. -

by their spectra.

R. Huisgen and G. Steiner, Tetmhedron Lett.,

3763 (1973).

Angew.Chem.,

Int.Ed.Engl.,

13, 80, 4