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Experiments Models Paper Tools. Cultures of Organic Chemistry in the Nineteenth Century

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Angewandte
Books
Chemie
Metal-Catalyzed Cross-Coupling
Reactions
Two completely
revised and
enlarged edition.
Edited by Armin
de Meijere and
Franois Diederich.
Wiley-VCH, Weinheim 2004.
916 pp., hardcover
E 329.00.—ISBN
3–527–30518–1
Just as Technicolor changed the movies,
so has cross-coupling chemistry enriched
organic synthesis. Cross-coupling reactions, which are transition-metal-catalyzed coupling reactions of organometallic compounds with organic halides or
related electrophiles, are arguably some
of the most important reactions of
organic chemistry. Is the exploratory
phase over, and have we reached the
optimization phase? Probably not! In
the last few years, fundamentally new
classes of ligands and reactions of great
synthetic value have been developed.
Sterically demanding phosphine and Nheterocyclic carbene ligands, the Hartwig–Buchwald amination, iron-catalyzed
cross-couplings, and the coupling of sp3
carbon atoms, have only recently
moved into synthetic laboratories.
The first edition of this work, published in 1998 with a healthy 540 pages,
has become one of the most popular
books on cross-coupling reactions. The
appearance of a completely revised
and enlarged second edition, consisting
of two volumes, is an indication of the
rapidly growing interest in cross-coupling chemistry.
Angew. Chem. Int. Ed. 2005, 44, 1157 – 1159
There is no best actor, but only a
good cast for a certain play, and the
same is true for cross-coupling reactions.
Each method has its advantages and disadvantages, and the best choice for a
particular transformation depends on
many criteria. Desirable preconditions
are commercial availability of the starting materials, reagents, and catalysts, a
high yield under mild reaction conditions, high atom economy with minimal
formation of side-products, wide applicability and functional group tolerance,
stability of products under the reaction
conditions, and low toxicity of all compounds involved. The book deals with
all of these aspects, and provides the
reader with a good grasp of the field.
In 15 well-written chapters, renowned
experts such as Bckvall, Brse, Buchwald, Denmark, Echavarren, Haley,
Kazmaier, Knochel, Marek, de Meijere,
Mitchell, Miyaura, Negishi, Snieckus,
and Tsuji discuss the most relevant
developments in cross-coupling chemistry and illustrate them with examples.
Each chapter provides the reader with
background information, discusses the
latest developments, and ends with representative experimental procedures
for the reactions that were described.
However, the user-friendliness could
be further improved by indicating in
the main text whether the description
of a reaction is followed at the end by
an experimental procedure.
In the first chapter, important mechanistic aspects of cross-coupling reactions
are discussed, a good warm-up for the
remainder of the book. Five chapters
deal with key players in cross-coupling
chemistry: the various organoboron, -tin,
-silicon, -zinc, and -magnesium reagents.
In addition, other groundbreaking
classics such as the Heck reaction,
cross-coupling reactions to sp carbon
atoms (e.g., the Sonogashira reaction),
carbometalations, reactions of p-allyl
intermediates (e.g., the allylic alkylation), and also reactions to form bonds
between nitrogen and aromatic carbon
atoms (the Hartwig–Buchwald amination) are refreshingly presented. Three
more chapters deal with 1,4-addition to
conjugated dienes, the palladium-catalyzed coupling of propargyl compounds,
and directed ortho-metalation.
A final highlight is the chapter on
the Negishi coupling, which gives an
www.angewandte.org
insight into the historic relationship of
the different cross-coupling reactions.
This chapter also discusses the advantages and disadvantages of the different
types of reactions.
This book is not meant to be a comprehensive encyclopedia. In fact, for the
sake of time-pressed readers, the editors
skillfully focus on the most important
aspects of modern cross-coupling
chemistry. All the chapters are of high
didactic value, are easily readable, and
present state-of-the-art science. Therefore, students as well as experienced
chemists in academia and industry will
benefit from this book, and it can be recommended unreservedly. This new edition represents much more than an
update. It is a reader-friendly definitive
book of cross-coupling chemistry!
Frank Glorius
Fachbereich Chemie
Philipps-Universitt Marburg (Germany)
Experiments, Models, Paper Tools
Cultures of Organic
Chemistry in the
Nineteenth Century. By Ursula
Klein. Stanford University Press, Stanford 2003. 305 pp.,
hardcover
$ 65.00.—ISBN
0–8047–4359–2
Traditionally, historians, and philosophers of science have analyzed the
dynamics of discovery by taking into
account the experiments and theories
of a period, and setting these into the
context of particular social and economic conditions prevailing at the
time. Nevertheless, a few historians and
philosophers of science now maintain
that these time-honored approaches,
exemplified for the history of chemistry
by such canonical works as Partingtons
A History of Chemistry and Ihdes The
Development of Modern Chemistry, are
insufficient for a complete analysis.
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1157
Books
These scholars contend that an important and previously overlooked component of scientific research in chemistry,
physics, genetics, and other disciplines
is the development and use of interpretive models derived from so-called
“paper tools”. In this book, Ursula
Klein, director of a research group at
the Max Planck Institute for the History
of Science in Berlin, aims to show how
the manipulation of one such set of
“paper tools” strongly influenced
events in early organic chemistry.
Klein describes “paper tools” as “a
category that serves to focus the historical analysis and reconstruction on the
performative, cultural, and material
aspects of representations, without disregarding the goals and other intellectual preconditions of human actors”.
A central point of the book is that by
manipulation of “representations”, on
paper or mentally, scientists organize
experimental results into patterns of
ontological significance. The general
idea is not new, and Klein refers to studies by others on this theme. She implies,
however, that what she offers here has
not been done before. She seeks to support the argument that “… chemists
began applying chemical formulas not
primarily to represent and to illustrate
preexisting knowledge, but rather as
productive tools on paper or paper
tools for creating order in the jungle
of organic chemistry”.
In the context of chemical history,
the key word in her formulation is “representations”, which refers to any of the
symbols and signs that identify, in written form, the objects and actions of laboratory and theoretical work. Thus,
Kleins self-assigned task draws not
only upon history of science but also
explicitly upon semiotics.
Kleins analysis leads to conclusions
about early 19th century chemistry that
may startle some chemists and historians. She maintains that, contrary to conventional views, the period before the
rise of the structural theory was not
only extraordinarily fruitful in producing many new compounds, but also
that the introduction and employment
of Berzeliuss atomic formulas and
molecular “formula models” derived
from them during that time constituted
a cognitive advance that was not
merely a “preparation” for the struc-
1158
tural theory, as other scholars have proposed, but a “decisive precondition” for
it. We must now ask whether the specific
facts and interpretations that Klein puts
forward in justification are persuasive.
Klein points out that until the first
quarter of the 19th century, most of the
known “organic” compounds were isolated by extraction from living natural
sources, and their classification into categories followed from the observations
that the members of a category had similar properties as judged by sensory perception. Thus, acids turned litmus red,
sugars tasted sweet, “ethers” (a category
that also included what we now call
esters) were volatile and had pleasant
odors, and so forth. Also, similarities
could be seen in substances obtained
by corresponding extraction methods
from similar sources, such as, for example, essential oils obtained by distillation
of various plants. Classification was thus
tied to nature.
Soon after, however, under the
important influence of the rising discipline of quantitative elemental analysis,
a classification based upon constitution
began to emerge, and was eventually to
dominate. Chemical experiments transformed many of the naturally occurring
compounds into non-natural derivatives,
and it was soon evident that what all
these substances had in common was
not their natural origin but rather the
fact that they all contained carbon.
Thus, Klein writes “… plant and animal
chemistry before the late 1820s and
carbon chemistry after 1840 were so
structurally different that they should
be considered two scientific cultures. I
apply the term scientific culture to
describe the relatively coherent relations
between different social, material, symbolic, and epistemic elements and practices on the communal level of a science”.
The major focus of organic chemistry thus shifted from elucidating its significance for biology to understanding
chemical properties in terms of molecular constitution. This background picture is not one that I would seriously dispute. However, there is much more to
Kleins argument. I will examine only
two of the major themes here. The first
is the attempt to show that the manipulation of Berzelian empirical formulas,
that is the employment of “paper
tools”, led to important advances in
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
understanding the constitution of
organic compounds and their reactions.
The second is the claim that these developments were a “decisive precondition”
for structural theory.
A simple example of the use of
“paper tools” is given by the early
attempts to understand the nature of
ethanol. The Berzelian empirical formula for ethanol is C2H6O (or an integral multiple thereof). Early 19th century chemists, such as Berzelius and
Dumas, tried to rationalize this in
terms of a “binary constitution” of this
substance. They proposed that ethanol,
and in fact organic compounds generally, were made up by a combination of
two smaller “immediate constituents”.
These components were believed to be
substances actually or potentially isolable, that is, independently existing species in their own right. According to
Klein, Berzelius favored the “formula
model” C2H6 + O, C = 12, Liebig
favored C4H10 + H2O, C = 12, and
Dumas favored C8H8 + H4O2, C = 6).
The Berzelius “formula model”, as
Klein calls it, corresponds to an empirical formula C2H6O, and so does the
Dumas formula model when converted
into a C = 12 basis and divided by 2:
(C4H8 + H4O2)/2 ! C2H4 + H2O, C =
12.The Liebig formula model implies
an incorrect empirical formula C4H12O.
Such attempts to force chemistry
into a binary format soon foundered
when no binary schemes could be
devised for the constitution of other
compounds. This ultimately led to elaborate sequential ternary schemes, in
which a binary combination gave a
hypothetical intermediate, which then
combined with another component to
give a product. Such hypotheses thus
implied not only constitutional but also
mechanistic concepts.
Klein maintains that the manipulation of formula models provided a way
to balance the masses of reactants and
products in the reactions of organic
compounds. This avoided the practical
experimental problem of accounting
quantitatively for all of the products.
However, in this process, chemists
found themselves calling upon hypothetical unknown “immediate constituents” to achieve the balance. In some
cases, these supposed reactants not
only were unknown but had empirical
Angew. Chem. Int. Ed. 2005, 44, 1157 – 1159
Angewandte
Chemie
formulas that by 1860 would be recognized as unacceptable under the rules
of valence, and hence were completely
chimerical.
A case in point was the compound
known as “sweet wine oil” and assigned
the formula C4H3. In Chapter 6, Klein
states the importance of this material,
which played a key role in the theory
of Dumas and Boullay regarding the
long-known formation of ether from
ethanol and sulfuric acid. She credits
those authors with providing the first
analysis of two products of the reaction,
“sweet wine oil” and also of “sulfovinic
acid” (later called ethyl hydrogen sulfate), and goes on to say approvingly:
“These analyses fulfilled a minimum
requirement for the experimental
embedding of their work on paper with
formulas and the construction of formula models”.
Similarly, Klein writes that the
Dumas–Boullay experimental analyses
and formula model manipulations
brought their investigation to the point
where they “… could eliminate the possibility that additional, still-unknown
reaction products existed”. It is only in
a footnote that Klein reveals that “the
substance sweet wine oil was identified
in 1879 by P. Claesson as a mixture
made up of polymerized ethylene and
ethyl sulfate”. It seems odd that Klein
does not point out that this finding
directly contradicts her immediately
preceding statement.
Klein continues her discussion of the
ethanol-to-ether reaction in a lengthy
and confusing discourse. We can focus
on the question that Klein places at the
center of the story. Other than water,
the two major products of this reaction
are ether and “sulfovinic acid”, that is,
ethyl hydrogen sulfate. Are these
formed in two independent parallel
reactions or in a sequential reaction?
Klein accepts with approval the claim
of Dumas and Boullay that their work
proves that the two reactions are independent. In Kleins words, “they could
exclude the possibility that the formation reaction of sulfovinic acid interfered in any way with the simultaneous
production of ether”. To a modern
chemist, this is a questionable assertion.
It seems to imply that Dumas and Boullay, by invoking the nonexistent compound “sweet wine oil”, solved this
Angew. Chem. Int. Ed. 2005, 44, 1157 – 1159
extremely complex mechanistic problem entirely by stoichiometry, and without recourse to any of the tools of physical organic chemistry. If Dumas and
Boullay even carried out a control
experiment to test whether there can
be crossover between the two pathways,
that is, whether ether can be formed
from ethanol and sulfovinic acid, Klein
does not mention it.
These studies and others of a similar
kind, which are described at some
length in the book, led to dubious or
simply false conclusions. In a number of
instances, the errors turn out to result
not from experimental mistakes but
from the manipulation of formula
models to fit the spurious binary constitution theory. Curiously, Klein never
makes this point. These procedures, as
we have seen, are the ones she lauds as
intellectual forerunners of the structural
theory, but readers of her book will
have difficulty in making the connection.
On the positive side, Klein does a
better job in describing the significance
of the later work on substitution reactions by Dumas and by Laurent (Chapter 7). In contrast to the binary constitution studies, these substitution experiments can be said to have contributed
to the later ideas of organic chemistry.
Also, the introduction to some of the
semiotic concepts in Chapter 1 is informative, although most chemists will
find the terminology spiky and difficult
to penetrate. Even elsewhere, her writing can be obscure, and a number of sentences are rendered unintelligible by
omissions of words or punctuation. A
more-demanding editor might have
clarified some of the text. Moreover,
Klein is not at home in the graphical display of concepts, so that her intended
point sometimes becomes obscured
rather than illuminated by reaction diagrams or diagrammatic plots. For example, Figure 8–1 shows the time-dependence of a parameter Klein calls “cultural
extension”, without further definition.
No justification is given for her evident
assumption that “cultural extension”
can be characterized in some quantitative way. Her protracted presentation
of Liebigs many attempts to determine
the nature of “wood spirit” (methyl
alcohol) ends anticlimactically when
she reveals that Liebigs samples were
later shown to be mixtures. We are not
www.angewandte.org
told what point she was trying to make
with this story.
It is true that organic chemistry grew
rapidly in the early 19th century. As is
often the case, theory stimulated experiment, and certainly the imaginative
hypotheses proposed in the name of the
binary theory and its modifications provoked new laboratory observations. However, much of the effort put into manipulating the formula models did not guide
chemists towards the ideas of structural
theory and valence, but rather into intellectual blind alleys. Thus, a central claim
of the book strikes me as dubious.
Of course, in history the issue of
what is a “necessary precondition” is
often a matter of dispute. However, the
interpretive difficulties and inconclusive
aspects of organic chemistry in the
period that Klein studies here seem to
cry out for the structural theory. In
most of her examples, Klein does not
show how the work of the formula modelers helped this transition. In fact, many
historians have characterized the process as a fundamental change.
The things that really were the necessary preconditions for the structural
theory included, first, correct molecular
weights combined with quantitative elemental analysis, truly a seminal innovation of that period. These provided the
identity and the numbers (or at least
the ratios) of the atoms of the molecule.
Second was the concept of valence, and
third was the visualization of atom-toatom connectivity using these valences.
As Rocke has pointed out, the third of
these was reflexively related to the first
two, in that only one set of atomic
weights would fit self-consistently with
assignments of structure.
The author may well be correct in
stating that the present book deals with
a period of organic chemistry that has
received relatively little attention. I
think chemists should be interested in
some of the still unfamiliar ideas of the
time. However, in my view, what is
most evident from Kleins account is
just how sharp was the intellectual discontinuity between that era and the
structural age of organic chemistry.
Jerome A. Berson
Department of Chemistry
Yale University, New Haven (USA)
DOI: 10.1002/anie.200385227
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1159
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