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Cyril A. Grob (1917Ц2003) Fragmentation and Inductivity

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Obituary
Cyril A. Grob (1917–2003):
Fragmentation and
Inductivity
Cyril A. Grob died in his home in Basel
(Switzerland) on December 15, 2003 at
the age of 86. He was born in London
(UK) to Swiss parents, studied chemistry at the Eidgen-ssische Technische Hochschule
Z0rich
(ETH
Z0rich), and completed his
PhD in 1943 under the guidance of Leopold Ruzicka
(Nobel Prize in Chemistry
1939) on artificial steroidal
antigens. He then moved to
Basel to work with Taddeus
Reichstein first at the pharCyril A. Grob
maceutical institute and
from 1947 at the organic
chemistry institute of the university,
where he moved up the academic
career ladder to become the director of
the institute and holder of the chair
there as Reichstein9s successor in 1960.
He retained this function until he was
given emeritus status in 1987.
Initially Grob9s research focused
mainly on natural products, with synthetic studies on biotin, lysergic acid,
and the steroid framework. He also
completed an elegant synthesis of sphingosin. A research stay with Saul Winstein at the University of California,
Los Angeles (1951/52) gave him the critical impetus: After his return to Basel he
dedicated himself more and more to the
elucidation of reaction mechanisms and
structure–reactivity relationships.
An investigation of the reductive
elimination of bromine from 1,4-dibromides in the presence of zinc led in
1955 to the recognition of heterolytic
fragmentation as a general reaction
principle.[1] The structural and stereochemical prerequisites for a fragmentation to occur were investigated with
model compounds, and the fundamental
mechanisms of this reaction type were
elucidated.[2] The heterolytic fragmenta-
4392
tion has now entered textbooks under
the name of its discoverer. These studies
also led to the elucidation of the mechanism of the Beckmann rearrangement.
Experimental evidence for vinyl cations
as discrete reactive intermediates was
also first provided by Grob.[3]
The inductive substituent effect
occupied Grob for many years. Substituted quinuclidines were chosen as
model compounds, as they bear the
polar functional groups at a clearly
defined distance from basic or protonated nitrogen atoms as a result of
their rigid molecular framework.
Through the exact measurement of protonation equilibria a large number of
substituent inductivity constants were
determined.[4]
In solvolytic reactions intermediate
carbon-centered cations often undergo
rearrangement of the carbon framework
with the transfer of charge to neighboring centers. These reactions do proceed
via a nonclassic intermediate or transition state, in which a carbon atom is formally bonded to five other atoms.
Whether a charge transfer occurs prior
to the complete displacement of the
leaving group or in a subsequent elementary step was highly controversial
for a long time. Grob saw an answer in
the determination of the so-called
inductivity; that is, the sensitivity of the
rate of the ionization process on neighboring substituents. These substituents
should influence the incipient charge
transfer in the transition state through
their inductive effect. A kinetic study
showed that the nature of the transition
depends on the individual reaction.
Only in exceptional cases, for example,
for the notorious norbornyl cation,
does the nonclassic structure seem to
be more stable than one with a localized
charge.[5] This result was compared in
1983 in a “symposium in print” with
the reports by H. C. Brown and G. A.
Olah, who had approached this question
from different angles with a similar
result. As C. Walling, the “arbitrator”,
observed, this publication settled peacefully a long and intense controversy.[6]
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/anie.200461144
Except in kinetic studies, Grob was a
forerunner in the field of physical
organic chemistry in continental
Europe. Students and colleagues at the
university and in his working environment profited considerably from this
role of his. As a lecturer he had the ability to present complex concepts in a
simple, clear, and precise way, and to
convince his audience through well
thought out arguments. He allowed his
numerous co-workers much freedom,
but always gave them good advice at
the right time.
Cyril Grob never acted impulsively,
but always calmly and deliberately. He
never sought attention in public, but fulfilled his social duties efficiently, reliably, and without a fuss. In this way as
dean he directed the affairs of the faculty for a year and presided over the
Swiss Chemical Society during a twoyear term. With his scientific success
came frequent invitations to almost all
countries in Europe, as well as the
USA and other parts of the world. He
always prized these trips very highly as
recognition of his scientific achievements.
In public Cyril Grob was reserved
and he did not particularly enjoy socializing. Only in intimate circles did he
shine as a relaxed, witty, cultured, and
kind person. However, his scientific
interests always came first, not only in
his professional but also in his private
life.
Peter Schiess
Universitt Basel
[1] C. A. Grob, W. Baumann, Helv. Chim.
Acta 1955, 38, 94.
[2] C. A. Grob, Angew. Chem. 1969, 81, 543;
Angew. Chem. Int. Ed. Engl. 1969, 8, 535.
[3] C. A. Grob, Chimia 1971, 25, 87.
[4] a) C. A. Grob, Angew. Chem. 1976, 88,
621; Angew. Chem. Int. Ed. Engl. 1976,
15, 569; b) C. A. Grob, Helv. Chim. Acta
1985, 68, 882.
[5] C. A. Grob, Angew. Chem. 1982, 94, 87;
Angew. Chem. Int. Ed. Engl. 1982, 21, 87.
[6] C. Walling, Acc. Chem. Res. 1983, 16, 448.
Angew. Chem. Int. Ed. 2004, 43, 4392
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