Chemistry Reference
In-Depth Information
5.5
The Molecular Motor
It is not common to use a gaseous poison like phosgene to fuel a motor, but it is a
reality. Devised by Kelly at Boston in 1999, it led to the world's first molecular motor,
319, sometimes called the Boston motor [306-309]. It consists of just 78 atoms (320)
and has a spindle of triptycene, which rotates about a carbon-carbon single bond
as an axle between the triptycene and a helicene as a base plate. The triptycene can
only rotate in a clockwise direction because the chiral helicene acts like a friction or
back-pedaling brake shaped in an asymmetric skew. Chirality is the reason for the
unidirectional rotation.
The thermodynamics is outlined in the reaction sequence 320a
320b. Mole-
cule 320a is one of three low-energy rotational isomers (rotamers) and has a rather
high rotational energy barrier of about 25 kcal mol
1
(caused by the friction brake
of the helicene; the barrier to rotation about a typical C
!
C single bond is only 3-5
kcal mol
1
). Rotamer 320a is activated by reaction with phosgene (4 atoms) to give
the isocyanate 321 via a carbamoyl chloride. Isocyanate 321 is chemically prepared
to react with the alcohol function of the hydroxypropyl ether chain attached to the
helicene, but in the rotational ground state 321 the isocyanate and the alcohol
group are too far apart to interact. However, in the event of a clockwise rotation of
the triptycene to 322 (as opposed to an anti-clockwise rotation, which would take
the reacting groups further apart and prevent the essential carbamate formation),
the isocyanate and alcohol groups are brought su
ciently close to react, such that
carbamate 323 can be formed. In this way, the triptycene is irreversibly trapped in a
relatively high-energy conformation (E
act
) about the triptycene-helicene axle. Am-
bient thermal energy then drives the exoergic unidirectional rotation from 323 to
324 releasing 25 kcal mol
1
(E
act
). Finally, the carbamate function in 324 is cleaved
with water to give 320b, thereby completing the chemically driven rotation of 320a
to 320b, which is the second low-energy rotamer. Hydrolysis of the carbamate
group in 324 is accelerated by a relative rate enhancement of 2
a
10
4
using metal-
bonded water or hydroxide as the nucleophile [310].
spindle
axle
base plate
319
Molecular Motor
