2. The Nobel Prize in Chemistry 1979 was
awarded jointly to
Professor Herbert Charles Brown,
Professor Georg Wittig,
"for their development of the use
of boron- and phosphorus-
containing compounds,
respectively, into important
reagents in organic synthesis"
Summary
3. Herbert C. brown (1912 – 2004)
• Born: 22 May 1912, London, United Kingdom
• Died: 19 December 2004, USA
• Known for: Organoboranes
• Subjects of Study :Organoboron compound
Chemical reaction
Carbonium ion
Borane
• Field: organic chemistry
H. C. Brown was only the second American organic chemist to win a Nobel Prize
4. Georg wittig (1897 –1987)
• Born: 16 June 1897, Berlin, Germany
• Died: 26 August 1987, Heidelberg, West
German
• Known us: Wittig reaction
• Subjects of Study : Organic Phosphorus
Compound
Wittig reaction
• Field: organic chemistry
6. In 1950, Herbert C. Brown, has developed new reagents containing
boron.
His several most significant contributions in the area of boron
chemistry include (1) codiscovery of sodium borohyride.
His studies on applications of the borohydrides and diborane
to organic synthesis had a revolutionary impact on synthetic
organic chemistry.
His seninal work on customized reducing agents and
organoborane compounds in synthetic organic chemistry .
Herbert C. Brown has systematically studied various boron
compounds and their chemical reactions. He has shown how
various specific reductions can be carried out using
borohydrides.
Work over view
7. One of his reagents is sodium boro-hydride, which has become
the reagent of choice for reduction of carbonyl compounds (Fig 1).
modified the boro-hydrides into reagents for highly selective.
Brown created more than 50 combinations of boron and
hydrogen.
In addition Brown has introduced an entirely new class of
compounds, (Fig 2).
The organoboranes have become the most versatile reagents.
. (Boron-hydrogen compounds and their derivatives are known as borane.)
8. Research Activities
Herbert Brown studied the reactions of diborane, B2H6.
Hermann Schlesinger's laboratory at the University of Chicago
was one of two laboratories that prepared diborane.
His dissertation, under the direction of Hermann Schlesinger,
involved the reaction of diborane with aldehydes and ketones.
He discovered that diborane reacts with aldehydes and ketones
to produce dialkoxyboranes, which are hydrolyzed by water to
produce alcohols.
9. Until this point, organic chemists did not have an acceptable
method of reducing carbonyls under mild conditions.
Diborane was too rare to be useful as a synthetic reagent.
In 1940, they began to research volatile, low molecular weight
uranium compounds for the National Defense Research Committee.
Brown and Schlesinger successfully synthesized volatile uranium(IV)
borohydride.
We were requested to supply relatively large amounts of the
material for testing, but diborane was in short supply.
U(BH4)4.
10. They discovered that the reaction lithium hydride with boron
trifluoride in ethyl ether, solution provieded such a route.
Unfortunately Lithium hydride was also in short supply, so Brown
and Schlesinger needed to use sodium hydride instead.
World War II soon came to dominate the research carried out by
Schlesinger’s group desire for a convenient method for the field
generation of hydrogen.
Of more lasting importance, though, were the discovery and large
scale preparation of sodium borohydride
and lithium aluminum hydride.
11. Soon they were informed that sodium borohydride could be useful
in generating hydrogen.
They began to look for a cheaper synthesis and discovered that
....
In this way it was discovered that sodium borohydride is a
valuable reagent for the hydrogenation of organic molecules.
Sodium borohydride is a mild reducing agent that works
well in reducing aldehydes, ketones, and acid chlorides.
12. As a consequence of his work, organic chemists obtained an
unparalled array of reducing agents carefully tailored to specific
synthetic requirements.
It was this work that was alluded to in the first part of the citation
for his Nobel Prize.
Further, Brown and his coworkers showed that organoboranes,
produced by the hydroboration reaction, were in fact capable of
an extraordinary range of synthetically important reactions.
This work was addressed in the second part of his Nobel Prize
citation.
13. His several most significant contributions in the area of boron
chemistry include...
(1) Codiscovery of sodium borohyride
(2) Systematic exploration and methodological development of the
reduction of a wide variety of organic compounds with sodium
borohydride as well as other related borohydrides and
aluminohydrides
(3) Discovery of hydroboration and subsequent developments of
hydroboration-based organic synthetic methods and
(4) Development of asymmetric allylboration, crotylboration, and
related reactions as a group of widely used methods for asymmetric
carbon-carbon bond formation.
Major work
14. SODIUM BOROHYDRIDE
Sodium tetrahydridoborate is versatile reducing agent
that finds wide application in chemistry.
Large amounts are used for bleaching wood pulp.
The compound is insoluble in ether, and soluble in
methanol and water, but reacts with the latter two in the
absence of base.
(Sodium tetrahydridoborate)
15. REACTIVITY
NaBH4 will reduce many organic carbonyls, depending on the
precise conditions.
Reduce ketones and aldehydes to alcohols, acid chlorides and
imines also.
No effect on C=C, C=C, N=N, esters, nitriles and nitro groups.
However, unlike the powerful reducing agent lithium aluminium
hydride, NaBH4 typically will not reduce esters, amides, or
carboxylic acids.
16. APPLICATIONS
Production of pharmaceuticals
used for reductions, rearrangements and additions
This reaction is used in the production of various antibiotics
including chloramphenicol, dihydrostreptomycin, and
thiophenicol.
Various steroids and vitamin A are prepared using sodium
borohydride in at least one step.
20. • Georg Wittig has developed new synthetic methods of
considerable importance and has studied reaction mechanisms.
• In investigating reactions involving carbanions, negatively charged
organic species.
• In 1953 Georg Wittig discovered a class of organic phosphorus
compounds called ylides that became known as the Wittig
reaction.
• In this, phosphorus ylides, allowed to react with carbonyl
compounds. An exchange of groups takes place and the
result is a compound in which two carbon atoms have been
joined by a double bond.
• This reaction proved of great value in the synthesis of complex
organic compounds such as vitamins A and D2 and steroids.
Work over view
21. • Wittig's research covered a wide selection of areas of organic
chemistry over the span of his career.
• Both a rearrangement and a reaction that is widely used for organic
syntheses now carry the name Wittig.
• There were available a relatively small number of chemical
reactions for forming such bonds as late as the 1940s.
• Forming new carbon to carbon linkages is inarguably an
indispensable task for organic syntheses.
• Main research into organic reactions of alkali metals and
elaboration of carbon-based chemistry.
Research Activities
22. • Georg Wittig discovery of the halogen-metal exchange reaction
(simultaneously with H. Gilman). Ex. phenyl lithium
• Wittig published a series of papers in the late 1940s on the
reactions that result from reaction of quaternary salts with very
strong bases;
• The element phosphorus is directly below nitrogen in the periodic
table of the elements and correspondingly displays very similar
chemical properties.
• In the early 1950s Wittig thus expanded his investigation of
quaternary bases to their phosphorus counterparts, called
phosphonium salts. were found to link very efficiently to aldehydes
and ketones.
23. • The transform, called a Wittig reaction, can be visualized by
replacing oxygen in the aldehyde or ketone by the carbon that had
been attached to phosphorus.
• This new reaction found almost immediate use in organic
syntheses.
• Many extensions and modifications of the Wittig reaction have
been developed by other investigators over the years.
• George Wittig received numerous awards in recognition of his
contributions to organic chemistry in addition to a share of this
Nobel Prize.
24. The Wittig reaction
In 1953 Georg Wittig discovered a reaction between a
phosphorous carbon compound and another carbon compound that
resulted in formation of a carbon compound with a least one double
bond between carbon atoms.
In the Wittig reaction (Fig 3) an organic phosphorus compound
with a formal double bond between phosphorus and carbon is
reacted with a carbonyl compound.
25. The oxygen of the carbonyl compound is exchanged for carbon,
the product being an olefin.
This method of making olefins has opened up new possibilities,
not the least of which are for the synthesis of biologically active
substances containing carbon-to-carbon double bonds.
Example,vitamin A is synthesized industrially using the Wittig
reaction.
• Industrial preparation of Vitamin A
26. Major Works
It is widely used in organic synthesis for the preparation of
alkenes.
Georg Wittig discovered the 1,2-Wittig rearrangement which is a
categorization of chemical reactions in organic chemistry, and
consists of a 1,2-rearrangement of an ether with an alkyllithium
compound.
He is also credited with the discovery of the [2,3]-Wittig
rearrangement which refers to the transformation of an allylic ether
into a homoallylic alcohol via a concerted, pericyclic process.
27. Forms of Wittig reagent
Described in the Phospheorane form or the ylide form :
30. Advantages
Generation of new carbon-carbon double bond in the product at
a fixed position.
Used to prepare symmetrical alkenes.
E- and Z- stereoselectivty are controlled through careful selection
of the phosphorus reagent and reactions conditions.
For the synthesis of sensitive alkenes e.g. Highly unsaturated
compound like the cartinoid.
31. The preparation of Olefins
Witting reagent provides a method for the synthesis of olefins.(Wittig reaction)
Synthesis of natural
product
1. Squalene
