Page 12 - Advanced Organic Chemistry Part B - Reactions & Synthesis

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Introduction
O O O
+ – – – + – + –
R' 3 P C HR (R'O) PC HR RC HSR' R' S C HR R' 2 S C HR
2
2
O
phosphonium phosphonate sulfone sulfonium sulfoxonium
ylide carbanion anion ylide ylide

Among the olefination reactions, those of phosphonium ylides, phosphonate anions,
silylmethyl anions, and sulfone anions are discussed. This chapter also includes a
section on conjugate addition of carbon nucleophiles to -unsaturated carbonyl
compounds. The reactions in this chapter are among the most important and general
of the carbon-carbon bond-forming reactions.
Chapters 3 to 5 deal mainly with introduction and interconversion of functional
groups. In Chapter 3, the conversion of alcohols to halides and sulfonates and their
subsequent reactions with nucleophiles are considered. Such reactions can be used to
introduce functional groups, invert configuration, or cleave ethers. The main methods
of interconversion of carboxylic acid derivatives, including acyl halides, anhydrides,
esters, and amides, are reviewed. Chapter 4 discusses electrophilic additions to alkenes,
including reactions with protic acids, oxymercuration, halogenation, sulfenylation,
and selenylation. In addition to introducing functional groups, these reagents can
be used to effect cyclization reactions, such as iodolactonization. The chapter also
includes the fundamental hydroboration reactions and their use in the synthesis of
alcohols, aldehydes, ketones, carboxylic acids, amines, and halides. Chapter 5 discusses
reduction reactions at carbon-carbon multiple bonds, carbonyl groups, and certain other
functional groups. The introduction of hydrogen by hydrogenation frequently estab-
lishes important stereochemical relationships. Both heterogeneous and homogeneous
catalysts are discussed, including examples of enantioselective catalysts. The reduction
of carbonyl groups also often has important stereochemical consequences because
a new stereocenter is generated. The fundamental hydride transfer reagents NaBH 4
and LiAlH and their derivatives are considered. Examples of both enantioselective
4
reagents and catalysts are discussed, as well as synthetic applications of several other
kinds of reducing agents, including hydrogen atom donors and metals.
In Chapter 6 the focus returns to carbon-carbon bond formation through cycload-
ditions and sigmatropic rearrangements. The Diels-Alder reaction and 1,3-dipolar
cycloaddition are the most important of the former group. The predictable regiochem-
istry and stereochemistry of these reactions make them very valuable for ring formation.
Intramolecular versions of these cycloadditions can create at least two new rings, often
with excellent stereochemical control. Although not as broad in scope, 2+2 cycload-
ditions, such as the reactions of ketenes and photocycloaddition reactions of enones,
also have important synthetic applications. The [3,3]- and [2,3]-sigmatropic rearrange-
ments also proceed through cyclic transition structures and usually provide predictable
stereochemical control. Examples of [3,3]-sigmatropic rearrangements include the
Cope rearrangement of 1,5-dienes, the Claisen rearrangement of allyl vinyl ethers, and
the corresponding reactions of ester enolate equivalents.