NMR emission and enhanced absorption resulting from the radical reactions of grignard reagents with alkyl halides.

Tetrahedron Letters No.7, pp. 521-524, 1970.

NMR

EMISSION AND REACTIONS

(Received

The

in USA

degrees

reagents

have

reactions

in

of

believed

with

alkyl

important

only

in

the

purpose

the

reactions

iodides,

of

this of

taken

on

in

tetrahydrofuran

(the

**

has

slowly

presence

of

many

(3)

certain

reaction

course

slow

reaction

(t

the

evidence

reagents

the

metal

in

02912

and

alkyl

enhanced

Grignard these

radical

the

bond

inter-

case

of

Grignard

homolysis

salts

benzyl

for

with

for

For

that

of

Although

(1).

(2).

as

emission

reactions

evidence

transition

report

Grignard of

years

concluded

stable, to

observation

the

accumulated

particularly

the

in

ionic,

Kharasch

alkyl

bromide

methylbutane

*

Island

or

was

when

the

systems.

radical

It

is

character

bromides

(4)

absorption

in and

in

nmr

(5). -10

40°)

minutes,

of

a

four-fold

exce.ss

l/2

t-butyl

course

for

predominantly

simple

relatively

Theresa A. Marzilli Providence, Rhode

character

concern

communication

durinq

of

the

be

halides,

were

RESULTING FROM THE RADICAL WITH ALKYL HALIDES.

G. Lawler, University,

radical

of

cases

the

formed

based

The

to

selected

so

and

a matter

reactions

radicals

ionic

Ronald Brown

Printed in Great Britain.

1969; received in UC for publication 10 January 1970)

2 December

been

are

mediates

spectra

ENHANCED ABSORPTION OF GRIGNARD REAGENTS

Harold R. Ward, Research Laboratories,

Metcalf

Pergamon Press.

with under

(6%)**. of

multiplet

the

the

The

argon

gave

nmr

spectra

reaction,

effect)

Grignard

(5a)

reagent*

(2M)

isobutylene

(6%),

displayed

and

clearly

for

the

show

vinyl

prepared

in

protons

isobutane

Figure

enhanced of

from

1 were

t-butyl (39%) taken

chloride

and

tetra-

during

absorption

and

emission

isobutylene

and

the

methyl

refer to t-butylmagnesium The terms "t-butyl Grignard" and "n-butyl Grignard" chloride and n-butylmagnesium bromide respectively. Analyses are from integrated nmr spectra taken at the end of the reaction. Reported yields based on initial Grignard concentration have been corrected isobutylene and tetramethylbutane formed during for the amount of isobutane, The excess of isobutane over isobutylene the preparation of the Grignard. may result in part from hydrogen atom abstraction by t-butyl radicals from and in part by reaction of the isobutylene with t-butyl tetrahydrofuran, Diisobutylenes can be detected in the product by gas chromatography radicals. and mass spectrometry.

521

522

No.7

protons

of

is clear

evidence

t-butyl

one-electron to

has

also

of

for

radicals,

lit

1-butene

intermediacy

the

halide.

formed

Grignard

formed

from

observed

that

polariz,ation

presumably

alkyl

been

strating

the

transfer

the

n-butyl

Such

isobutane.*

in

by

the

vinyl

protons

reaction

z-butyl

iodide,

less

be

formed

a

organometal-

the

the

of

Polarization**

from and

*

stable

of demon-

primary

I’,\

radi-

/1 cals

also For

alkyl it

can

the

copper

Grignard

has

been

in

this

catalyzed

reagents

proposed

reaction

and

that

i i

way.

alkyd

of

bromides,

olefins

were 4.26

formed arose

by

a non-free

only

from

the

radical

process

and

Grignard

reagent

(6).

1.0 P,,6

1. Nmr spectrum taken at 60 IHz approximately one minute after :he addition of t-butyl bromide to ,-butyl Grignard reagent in tetraThe vinyl protons of iydrofuran. .sobutylene (64.6-4.7) were scanned it a sensitivity 2.5 x that of the Iethyl protons of isobutane (6 0.8 Absorption for t-butyl ..0). ;rignard (6 0.931, and tetramethyllutane (b 0.89) can be seen between :he peaks of the isobutane doublet.

'igure

The

experiments

complicated exchange

between

and

salts

of

HOWeVer,

both

**

of

free

either or

that

claim

formed both

from

transthis

r-butyl

show from

the

alkyl

added

exchange,

were

halide.

bromide

reactions

isobutylene

indicating

*

alkyl

and

without are

iodide

evidence

the

Grignard

Mixtures

;-butyl

this

cooper-catalyzed

conducted

metal

culty.

support

Ly the

Reactions ition

to

and

Grignard fi-butyl

within strong the

halide

diffi-

the

Grignard

limits

spectral

the

_t-butyl

of detectibility enhancements

disproportionation and

and

Grignard

of give

the rise

and

bromide by

nmr

show

absorption.

dehancements

t-butyl

no

for

radical,

to olefins

by

a

Polarization of the kind observed in isobutylene and isobutane sums to zero when the emission and absorption for a set of magnetically equivalent protons is combined. Consequently, no polarization is expected or observed in the single line of tetramethylbutane. The absorptions for the methyl protons of isobutylene and the methyl proton of isobutane are obscured by solvent. Nuclear polarization here only in terms nmr lines.

of

is an appropriate the positive and

description of the effect described negative enhancements of individual

No.7

523

radical

reaction*.

Using

the

destroyed

only

radical at or

approximate by diffusion

concentration below

rates

the

sensitivity

of

the

over

direct

esr

have

failed

to

10

detection. detect

the

there

evidence

that

nmr

(8) and

methods

radical

are

reporteLI

intermediacy

Since

it is

even

the

in

absence by

reagents.

Addition

nmr

the

Ni,

enhancements

*

**

***

the

magnesium

spectra.

(0.001%

here

suffer be

to

of use

However, for are

triply

little if

example),

by

demonstrates

the

or

no

the

at

ambiguity, in the strongly of

salts.

magnesium**

effect***

reaction

rate,

the

level

on

either

of

added

and

systems

while

esr

This and

of

the

by

reaction

metal

consequently

ion

is the

The of

reaction. transition

impurities,

possibility

carefully

(7)

ketyl

evidence

by

of possible the

give

the

has

ketones

reaction.

catalyzed

catalysis

aryl

to

since

products

greater

these

with

reaction

such

about

of

forming

is

intermediates

Further,

product

metal

the

studies

main

This

the

radical

a side

question

concentration

of

are

a maximum

calculated.

in reactions

are

the

be

radicals.

directly

sublimed

no

esr

usually

reactions

salts

and

inference

free

transition

of metal

had

can

esr

thatradicals

reactions,

mole/liter

represent

from

assuming

radical-radical

in the

consider

added of

of

detected

halide

and

preliminary

radicals

involved

can

reactions

systems,

not

important

diminished

in

such

Grignard-alkyl

metals,

for

presence

in Grignard

-7

limit

Indeed,

radicals

radicals

-10

techniques

detected is

-6

detection

nmr

the

controlled

of

usual

of

was

purified impurities** rate

or

the

increased signal

incued.

While it is not yet possible to differentiate conclusively between free and it is worth noting that the complexed radicals by nmr observations, polarization exemplified here is similar to that observed in cases such as peroxide decomposition where the intermediate radicals are formed in the absence of likely complexing agents. Impurities Dow Chemical Company. a) Kindly supplied by Dr. J. F. Pashak, Ni<0.0005%) are established by the (Cu< O.OOl%, Fe <0.0005%, Mn
No07

524

The

sensitivity

transition

metals

(reagents

prepared

clearly

magnesium conclude

by or

that

react that

reaction

requires

organonagnesiums show

of

that

the

of

orqanomagnesium

with

reagents

alkyl

halides

This

Petroleum

Fund,

Research

the

presence

research

of minute

be made

organohalides In the

carefully

prepared

compounds was

Administered

by

the

in part American

Griynard

case

from

reagents

best

same

and

results available

is premature

the by

of

metal)

our

the

It

show

amounts

magnesium

first

route,

would

supported

between

with

halides.

by a radical

arganomagnesium

Acknowledgment:..

fo

a distinction

reaction

Grignard

pure

rates

to

reaction

a grant

from

Chemical

Society.

38,

(1905).

mode.

the

REFERENCES 1.

2.

E.

SpQth,

Ii.

G. Gough

H.S.

J!h.chem.

Mosher,

and J.

J,

&q,Xd*s A.

(19131,

Dixon,

OrganometaZ.

R. Abegq,

Ber.,

4112

Org. Chems,z, 2148 (1968), C. BLomberg and therein. Chem., 13, 519 (1968), and references

J.

3.

M, S. Kharasch "Grignard Reactions of Non-metallic and 0. Reinmuth, Substances", Prentice-Hall, Inc., New York, N.Y., 1954, pp. 1048-1053.

4.

The reduction of a gem-dibromocyclopropane by methyl Grignard in tetrahydrofuran has been shown to be a radical reaction. D. seyferth and B. Prokai, J. Org. Chem., z, 1702 f1966).

5.

a) see 8. Fischer and 3. Barqon, Ace. Chem. Res. 2, 110 (1969) for a listing For of other radical reactions which lead to spectral abnormalities. b) examples of nmr emission and enhanced absorption resulting from the reactions of alkyl halides with organolithium compounds see: H.R. Ward and R. G. Lawler, J. Amer. Chem. Sot., 89, 5518 (1967), H. R. Ward, R.G. Lawler and H. Y. Loken, ibid., zt 7359 (1368), H. R. ward, R.G. Lawler and R.A. Cooper, ibid., 91, 746 (19691, A. R. Lepley and R.L. Landau, ibid., z, 748 (1969), and A. R. Lepley, ibid., 91, 749 (1969).

6.

v.

7.

K. Maruyama,

8.

J.

D.

Parker

Billet

and

and

C.R.

&?I. S. G.

Noller,

J.

Amer. Chem. Sot., 5,

Chem. Sac., Japan, Smith.

3.

Amer.

34,

897

1112

(1964).

(1969).

Chem. Sot., 90, 4108

(1968).