Highlights
DOI: 10.1002/anie.201002914
Trityloxy Radical
The Rearrangement of the Trityloxy Radical: Sherlock
Holmesꢀ Most Recent Case
Gꢀtz Bucher*
analytical methods · radicals · rearrangements ·
time-resolved spectroscopy
N
owadays when the police attempt to solve a criminal case,
was re-investigated many years after 1911, when new
experimental methods like laser flash photolysis (LFP) and
modern quantum chemistry could give new insights into the
mechanism, does not come as a surprise.[2–5] In the context of
our analogy, laser flash photolysis can be compared to a
closed-circuit video recording of the crime scene. Ideally, one
obtains a direct spectroscopic (UV/Vis, IR, or ESR) portrait
of the suspect. A disadvantage of LFP lies in the fact that
reactive intermediates are generated photochemically in an
LFP experiment. While the photochemical fragmentation of
labile compounds frequently involves the same intermediates
as the thermal reaction, this cannot be assumed. A study
aimed at elucidating the photochemistry of 3 by LFP with
picosecond resolution was published in 1990.[6] In contrast to
Wieland’s report, the authors found practically no triphenyl-
methanol in the product mixture, and the formation of 2 was
observed to occur during the laser pulse. Based on these
findings, they determined the lower limit for the rate constant
of the rearrangement of 1 to be k1 > 5 ꢁ 1010 sÀ1.
they make use of modern analytical and physical techniques
such as DNA analysis and the calculation of a projectileꢀs
trajectory. But until only a few decades ago, the methods of
investigation police had at their disposal were limited to
comparing fingerprints, questioning suspects, and applying
detectiveꢀs intuition. This is reflected in detective novels—
Sherlock Holmes had a working style very different from that
of modern-day police inspector on television.
The “criminal case” to be considered here has been on
record for a very long time, to be accurate, since 1911. Back
then, Heinrich Wieland described the rearrangement of
triphenylmethoxyl (or trityloxy, 1) into phenoxydiphenyl-
methyl (2).[1] Thermolysis of ditritylperoxide (3) in xylene
yielded diphenoxytetraphenylethane (4) as the main product
(65–75% yield). As side products, benzophenone, phenol,
and an unspecified amount of triphenylmethanol (5) were
formed (Scheme 1).
The discrepancy between the results of the LFP study[6]
and Wielandꢀs preparative work is obvious. Along the lines of
our metaphor, we have a video recording of the crime scene
and we have witness testimony, but they contradict each
other. Did the video camera and the witnesses observe the
same crime? A group of researchers from Canada, Poland,
and the Netherlands has now searched for a different
precursor for 1; they have studied the thermal decomposition
of hyponitrite 6 in great detail by means of classical product
analysis.[7] In other words: away from video recordings and
back to the painstaking interrogation of witnesses and
deduction, back to the method of Sherlock Holmes. However,
the new study does utilize some more recent techniques like
HPLC which had not been available to Heinrich Wieland in
1911.
Hyponitrite 6 is a thermally highly labile compound that
quantitatively decomposes in CH2Cl2 within a couple of hours,
even at ambient temperature. In the presence of air, ether 4,
phenol, benzophenone, a little phenyl benzoate, and ditrityl-
peroxide 3 (10% yield) are formed. In the absence of air, but
with the addition of 80% 1,4-cyclohexadiene as a hydrogen
donor, 5 is formed as main product, while ether 7 is only a side
product along with peroxide 3 in 2.6% yield (Scheme 2). The
rate constant for the abstraction of a hydrogen atom from 1,4-
cyclohexadiene by 1 can be estimated quite accurately as k2 =
4 ꢁ 107 mÀ1 sÀ1, as there are published values for a range of
Scheme 1. Heinrich Wieland’s experiment.
Wielandꢀs experiment is easily interpreted in terms of a
rapid rearrangement of 1 into the more stable carbon-
centered radical 2, which then dimerizes. Hydrogen abstrac-
tion from the solvent, yielding the stable alcohol 5, competes
with the rearrangement of 1. To return to our detective
analogy: Wieland proved the rearrangement of the suspect 1
by questioning the witnesses 4 and 5. However, has the course
of events been sufficiently elucidated? The motive is obvious
(thermodynamics), and the weapon used is well known (an
oxygen-centered radical). But many details are not so clear.
How fast was the rearrangement? Had there been accom-
plices (oxygen…), intermediates? The fact that this reaction
[*] Dr. G. Bucher
University of Glasgow, Joseph-Black-Building
University Avenue, Glasgow G12 8QQ (UK)
E-mail: goebu@chem.gla.ac.uk
6934
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2010, 49, 6934 – 6935