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  • 1

    CHAPTER 1

    Synthesis of amides using Lewis acid catalyst: Iodine

    catalyzed Ritter reaction

    Part-1

    Chemistry of Lewis acids and amides

  • 2

    1.1.1. General overview of Lewis acids

    Acid and base reactions are of tremendous importance in organic chemistry,

    as they are also in inorganic and biochemistry. In 1884, Svante A. Arrhenius defined

    acids as substances dissociating into protons (H+) and the corresponding anions (A)

    in an aqueous medium.1

    Correspondingly, bases were characterized as substances

    dissociating into hydroxyanions (OH) and its respective cations (C+). Clearly this

    concept is restricted to acids and bases like HBr or KOH in water. In 1923 Johannes

    N. Brnsted and Thomas M. Lowry independently elaborated the extension of the

    acidbase theory by defining acids as proton(H+)donors and bases as

    protonacceptors, no longer restricting the concept to specific substance classes or

  • 3

    solvents.2

    Following these ideas, a number of reactions could be interpreted

    accordingly.

    Since many reactions do not involve direct proton transfers, the

    generalization of the acid base definition by Gilbert N. Lewis in 1938, was an

    important step for the understanding of chemical compounds and their

    transformations.3

    A Lewis acid is a compound with electron demand, (i.e.) having an

    unoccupied orbital which is able to accept an electron pair. On the other hand a

    Lewis base donates an electronpair from an occupied orbital. The reaction products

    of Lewis acids and bases are Lewis adducts, bound through a coordinative (or

    dative) bond. By joining the two concepts it becomes clear, that a Brnsted acid is a

    compound consisting of a Lewis base and a proton. This leaves the proton as the

    smallest of all Lewis acids, bearing free space in its 1sorbital.

    Further refined by Ralph G. Pearson in 1963, Lewis acids and bases are

    categorized into hard and soft acids and bases (HSABconcept).4 This approach

  • 4

    describes the quality of a bond in a Lewis adduct by consideration of atomic or ionic

    radii, charge distribution and polarizability.

    Accordingly, moieties of small size, high charge density and high polarizing

    potential are considered hard, whereas large radii, low charge densities and low

    polarizing potential characterize soft acids or bases. The association of hard acids

    with hard bases, and soft acids with soft bases is preferred. This preference is based

    on the bond character present in these combinations. A bond between a hard acid

    and a hard base, as in NaCl, has an ionic character, whereas the bond between a soft

    acid and a soft base results in a combination exhibiting a covalent bond character.

    With these concepts a manifold of chemical reactions and compounds can be

    qualitatively described. Especially for the understanding of structure and reactivity

    of transition metal complexes, the HSABconcept is of very high value (Scheme

    1.1).

    L A + LB LA LB L A LB

    Lewis ad du ct bind ing through a coo rdinative (d ative ) bond

    LA LB

    Examples:

    H + OH H2O

    F3B + OEt2 BF3 OEt2

    Cl4Si + OC(CH3)2 Cl4SiH OC(CH3)2

    Hard acids: BF3+, H+, Na+, K+, Fe3+, Mg2+; Hard bases: OH-, F- , Cl-, NH3, NR3

    Sof t acids: BH3, pd

    2+, Au+, Cu+, SiCl4; Soft bases: RSH, CN-, R2S, R3P,CO

    LA=Lewis acid, LB=Lewis base

  • 5

    Scheme 1.1: Interaction of Lewis acids and Lewis bases and the HSABcategorization.

    A great potential of Lewis acids is their application in the activation of

    molecules bearing Lewis basic sites. Thus Lewis acids play a key role for the

    activation of and systems. The formation of complexes is present in the

    activation of substrates such as carbonyl compounds or imines.5

    Due to the

    interaction of the lowest unoccupied molecular orbital (LUMO) of the Lewis acid

    with the highest occupied molecular orbital (HOMO) of the Lewis base, adjacent

    bonds get polarized and a nucleophilic attack from external or internal nucleophiles

    is facilitated. This mode of action can be summarized in a commonly accepted

    catalytic cycle which accounts for this activation mode, Lewis acid catalysis

    (Scheme 1.2).

    L A

    P

    libe ration

    S

    activation

    LA P Lewis adduct (intermediate II)

    LA S Lewis addu ct (intermediate I)

    transformation

    Scheme 1.2: General reactive principle of Lewis acid catalysis.

    Together with Lewis base activation, Brnsted acid activation and Brnsted

    base activation it represents the pillars of asymmetric catalysis.6

    However,

    Lewis acid activation is not restricted to electron donors. Some Lewis acids

  • 6

    are very powerful catalysts for the activation of electronsystems as well. By

    these interactions, nucleophilic attacks on allenes, alkenes and alkynes can be

    efficiently triggered. Usually these catalysts are complexes bearing a soft metal

    center.7

    Lewis acids

    Some archetypical Lewis acids for donor activations are group 3 and

    4element halides, such as BF3, AlCl3, SnCl4 or SiCl4. Many other Lewis acids are

    based on transition metals, such as TiCl4 and FeCl3. Based on the periodic table, a

    vaguely defined borderline between (transition)metal and (transition)metalfree

    Lewis acids can be drawn. As a common example from the main group elements,

    AlCl3 finds its application for various purposes.8

    One important example is the

    FriedelCrafts alkylation (see scheme 1.3). This reaction, first reported in 1877 by

    French chemist Charles Friedel and American chemist James Crafts, is an

    electrophilic aromatic substitution, occurring through an additionelimination

    mechanism.9

    The role of the Lewis acid is the interaction of its empty orbital with a

    lone pair of electrons on chlorine. This interaction polarizes the halogencarbon

    bond in RCl and results in a higher electrophilicity on carbon. Subsequently a

    nucleophilic attack by the aromatic ring takes place. Upon rearomatization via

    elimination of a proton, the product and the catalyst are liberated.

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    HCl +

    CH3

    AlCl3 Cl

    R

    Me H

    Cl AlCl3

    R Cl

    Cl

    Al Cl

    Cl

    Scheme 1.3: Mechanism of the FriedelCrafts alkylation of benzene

    Lewis acids

    As already mentioned, the activation of multiple carboncarbon bonds is

    another domain of Lewis acid catalysis. Here the systems of, for example, allenes,

    alkenes and alkynes are activated. An extremely important reaction using this

    slightly different mode of action is the ZieglerNatta polymerization of ethylene or

    propylene.10

    Z iegler-Aufbau:

    R Al

    R

    n+1 R Al

    R

    R Al

    R

    ( ) n

  • 8

    Scheme 1.4: The ZieglerAufbau reaction.

    In early studies Ziegler developed the socalled Aufbaureaction (Aufbau

    (Ger.) = assembly), the multiple insertion of ethylene into the carbonaluminium

    bond of trialkylaluminium compounds (Scheme 1.4).

    1.1.2. Application of Lewis acid catalysts in organic synthesis

    The use of Lewis acids in organic synthesis, especially in catalysis is one of

    the most rapidly developing fields in synthetic organic chemistry. In addition, Lewis

    acid catalysis is one of the key technologies for asymmetric synthesis, and

    combinatorial chemistry as well as for large-scale production. Lewis-acid (LA)-

    catalyzed reactions are of great interest because of their unique reactivities

    and selectivities and mild reaction conditions used. A wide variety of reactions

    using Lewis acids have been developed, and they have been applied to the

    synthesis of natural and unnatural compounds. Some of the examples are given

    below.

  • 9

    Alkylation reactions

    Friedel-Crafts alkylations are of great interest due to their importance and

    common use in synthetic and industrial chemistry. In 1887 Charles Friedel and

    James Mason Crafts11

    isolated amylbenzene after the treatment of amyl chloride

    with AlCl3 in benzene (Scheme 1.5). This was not only one of the first descriptions

    of a Lewis acid used in organic synthesis but also the first example of what as later

    to be called Friedel-Crafts alkylation after its inventors.

    + Cl AlCl3

    -HCl

    Scheme 1.5: Friedel-Crafts alkylation of amyl chloride with benzene

    Over the intervening years many other Lewis acids including BF3, BeCl2,

    TiCl4, SbCl5 and SnCl4 have been described as catalysts for the FC alkylation.12

    Acylation reactions

    FriedelCrafts acylation is the acylation of aromatic rings with an acyl

    chloride or acid anhydride using FeCl3 or AlCl3 as a strong Lewis acid catalyst.

    Reaction conditions are similar to the FriedelCrafts alkylation as mentioned above.

    O

    RCOCl or (RCO)2O R

    FeCl3

    Scheme 1.6: FriedelCrafts acylation of benzene with acylchlor

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