A Guidebook to Mechanism in Organic Chemistry

^ Steric Effects

The 2,6-dimethyl derivative (XXXII) does not couple under these

conditions, however, despite the fact that the methyl groups that have

been introduced are too far away for their not very considerable

bulk to interfere directly with attack at the />-position. The failure to

couple at this position is, in fact, due to the two methyl groups in the

o-positions to the NMe 2 interfering sterically with the two methyl

groups attached to nitrogen and so preventing these lying in the same

plane as the benzene nucleus. This means that the p orbitals of nitro­

gen and the ring carbon atom to which it is attached are prevented

from becoming parallel to each other and their overlapping is thus

inhibited.: Electronic interaction with the nucleus is thus largely pre­

vented and transfer of charge to the /^-position, with consequent acti­

vation to attack by PhN 2 e as in (XXXI), does not now take place:

QCXXW)

The most common steric effect, however, is tfce classical 'steric
hindrance' in,which it is apparently the sheer bulk of groups thjj is
influencing the reactivity of a^site in a compound directly and not by
.promoting or inhibiting ^ectrbn-availability. This has been investi-
:-Wted closely in connection with the stability of the complexes formed
%y trimethylboron with a wide variety of amines. Thus the complex
(XXXIII) formed with triethylamine dissociates extremely readily
whereas the complex (XXXIV) with quinuclidine, which can be
looked upon as having^iree ethyl groups on nitrogen that are 'held
back' from interfering sterically with attack on the nitrogen atom,
is very stable:

Me

/

CH 2 Me

\© o/

Me—CH 2 —N : B—Me

/ \

CH 2 Me

\

Me

(XXXIII)

CH,—CH 2 Me

/ \e 0/

CH ,N : B—Me

\CH 2 -CH 2 ^ \

CH2"~CHf2 Me

(XXXIV)