Chapter 14

Organometallic Compounds

Chapter 14 suggested problems: 13, 17, 18, 19, 20, 21, 22

web sites: Organic Synthesis and Carbon-Carbon Bond Forming Reactions, Multi-step Organic Synthesis

  1. General

    1. Organometallic compounds: carbon is bonded to a metal, such as Li, Na, K, Zn, Hg, Pb, Tl, etc.

      1. Carbon-metal compounds of virtually every sort have been synthesized

      2. The properties vary depending on a number of factors, including the various metal atoms involved

    2. Organometallic nomenclature

      1. Organometallic compounds are named as substituted metal compounds

      2. The metal is the base name

      3. Alkyl groups are named as prefixes

        1. Propyllithium

        2. Dipropylmagnesium

      4. If the metal also is bonded to an atom other than carbon, that atom is treated as an anion w.r.t. nomenclature unless it is also a metal atom

        1. Butylmagnesium bromide

        2. Phenylmagnesium iodide

        3. Lithium diethyl copper (Et2CuLi)

    3. Carbon-metal bonds in organometallic compounds

      1. When carbon bonds with other nonmetals, the bond is often polar and carbon bears a partial positive charge

      2. When carbon bonds to metals, the bond is generally polar but since carbon is the more electronegative of the two bonding atoms it bears a partial negative charge

      3. Carbanions: an anion that contains a negatively charged carbon atom

      4. Organometallic compounds are not true carbanions but have carbanion character

      5. The greater the difference in EN between carbon and the metal to which it bonds, the greater the partial negative charge on the carbon atom and the greater the carbanionic character of the compound

      6. Side bar: are the carbon-metal bonds ionic or covalent? (percent ionic character graphic)
        perecnt ionic character based on electronegativity differences

      7. Why lithium and magnesium?

        1. "The metals in these two groups are the most electropositive of the elements. The polarity of the bond is such as to place high electron density on the carbon." (C&S B:249)

        2. They seem to be the best at balancing stability with reactivity

    4. Preparation of Group 1 organometallic compounds

      1. RX + 2M -> RM + M+X-

      2. R can be 1°, 2°, 3°, cycloalkyl, alkenyl, or aryl

      3. The carbon can be either sp3 or sp2 hybridized, although sp2 hybridized carbons react more slowly

      4. The halogen can be any of the four, with reactivity I > Br > Cl > F (mostly unreactive)

      5. The synthesis of Group 1 organometallic compounds must take place in anhydrous solvents, typically alkanes such as pentane

        1. Group 1 metals are extremely reactive with water and alcohols

        2. Organolithium compounds are powerful Brönsted bases and react with even weak Brönsted acids (see below)

      6. Mechanism: reaction takes place at the metal surface

        1. RX + Li· -> Li+ + RX·- ( formation of radical anion)

        2. RX·- -> R· + X- (fragmentation of radical anion)

        3. R· + Li· -> R-Li

    5. Preparation of organomagnesium compounds: Grignard reagents

      1. RX + Mg -> RMgX

      2. R can be 1°, 2°, 3°, cycloalkyl, alkenyl, or aryl

      3. The carbon can be either sp3 or sp2 hybridized, although sp2 hybridized carbons react more slowly (this is implied by the need for "more vigorous conditions" such as THF)
         
        BP (°C)
        dielectric constant
        dipole moment (D)
        pentane
        36
        1.84
        0
        diethyl ether
        35
        4.335
        1.15
        THF
        66
        7.58
        1.75
        water
        100
        84.2
        2.2

      4. The halogen can be any of the four, with reactivity I > Br > Cl > F (mostly unreactive)

      5. The synthesis of Group 1 organometallic compounds must take place in anhydrous solvents, typically diethyl ether or TFH

      6. Mechanism: similar to that of lithium except that each Mg atom can participate in two x 1-step electron transfers

        1. RX +Mg:-> Mg·+ + RX·- ( formation of radical anion)

        2. RX·- -> R· + X- (fragmentation of radical anion)

        3. R· Mg·+ -> R-Mg-X

    6. Organolithium and organomagnesuim compounds as Brönsted bases

      1. The stronger the acid, the weaker its conjugate base; the weaker the acid, the stronger its conjugate base

      2. compound
        formula
        Ka
        pKa
        conjugate base
        2-methylpropane
        (CH3)3C-H
        10-71
        71
        (CH3)3C-
        ethane
        CH3CH2-H
        10-62
        62
        CH3CH2-
        methane
        CH3-H
        10-60
        60
        CH3-
        ethylene
        CH2=CH-H
        10-45
        45
        CH2=CH-
        benzene
        C6H5-H
        10-43
        43
        C6H5-
        ammonia
        H2N-H
        10-36
        36
        H2N-
        acetylene
        H-C=_C-H
        10-26
        26
        H-C=_C-
        ethanol
        CH3CH2O-H
        10-16
        16
        CH3CH2O-
        water
        H-OH
        1.8 x 10-16
        15.7
        -OH

      3. Think of the implication of these numbers as equilibria constants, given that pKa + pKb = pKw

      4. Any weak Brönsted acid (containing O-H, S-H, N-H) will react with a carbanion to form the hydrocarbon through proton transfer

      5. Can take advantage of this to reduce any alkyl halide to the corresponding hydrocarbon by first converting it to an organomagnesium compound and then adding water or alcohol

  2. Synthesis of alcohols: Grignard and organolithium reagents are most commonly used in the reduction of carbonyl compounds to form alcohols

    1. Using Grignard reagents: see Table 14.3, p. 555
      Grignard addition to a carbonyl to form an alcohol

      1. Grignards react with formaldehyde to give primary alcohols

      2. Grignards react with all other aldehydes to give secondary alcohols

      3. Grignards react with ketones give tertiary alcohols

    2. Using organolithium reagents: somewhat more reactive than Grignard reagents

    3. Preparation of tertiary alcohols from esters and Grignard reagents

      1. By using an ester as the carbonyl compound can form a tertiary alcohol by adding two alkyl groups to the carbonyl carbon

  3. Other syntheses using organometallic compounds

    1. Wurtz Reaction (Wurtz coupling): organosodium compounds can be formed as described in (I.D) above, but the resulting compounds are so reactive that combine as they are being formed and result in a symmetric alkane as the product

      1. R-X + Na -> R-R + 2 NaX

        1. RX + Na· -> Na+ + RX·- ( formation of radical anion)

        2. RX·- -> R· + X- (fragmentation of radical anion)

        3. R· + R· -> R-R

      2. n-C4H9Cl + Na -> n-C8H18 + NaCl

      3. t-C4H9Cl + Na -> C8H18 + NaCl (product: 2,2,3,3-tetramethylbutane)

    2. Alkane synthesis using organocopper compounds: a variety of organocopper compounds have been prepared, but the most useful are the lithium dialkylcuprate compounds (R2CuLi)

      1. General reaction: R2CuLi + R'-X -> R-R' + RCu + LiX

      2. Dialkylcuprate compounds also add to a,b-unsaturated aldehydes and ketones (discussed later)

      3. Preparation of lithium dialkylcuprates and lithium diarylcuprates

        1. 2 RLi + CuX -> R2CuLi + LiX

        2. Mixed in ether at low temperatures

      4. Reactivities

        1. Primary alkyl halides (especially iodides) are best (if not the only); 2° & 3° alkyl halides may have problems with elimination

        2. Note: according to March (p. 401) primary alkyl, allylic, benzylic, aryl, vinylic, and allenic

        3. Primary organocuprates are best; 2° & 3° often decompose before reacting with the alkyl halide

      5. Examples



    3. An organozinc reagent for cyclopropane synthesis

      1. Carbenes and carbenoids

        1. Carbenes: highly reactive molecules with lifetimes far quicker than 1 second

        2. Highly unstable but have been studied at low temperature

          1. Neutral divalent carbon atom

          2. Only six valence electrons, four involved in bonding and two that may be paired or not

          3. Only forms two single bonds, no multiple bonds

        3. Parent species CH2 is called methylene or carbene

        4. Most common carbenes are methylene and dichloromethylene (dichlorocarbene)

        5. Dihalocarbenes are formed by the reaction of a strong base (potassium t-butoxide) with trihalomethane compounds

          1. The trihalomethane loses its hydrogen atom without its electron pair

          2. A halogen atom leaves with an electron pair

        6. Carbenoids: if a compound appears to form a carbene during the course of a reaction but there is no evidence that free carbene is formed or when there is doubt

      2. Iodomethylzinc iodide (ICH2ZnI) reacts with alkenes to form cyclopropane and its derivatives seemingly via a carbene intermediate

      3. Simmons-Smith reaction: RCH=CHR' + ICH2ZnI -> cyclopropyl derivative + ZnI2

      4. Stereospecific: cis substituents remain cis, and trans remain trans

      5. Example: the reaction of iodomethylzinc iodide with 2-pentene

    4. Transition metal organometallic compounds

      1. Many transition metal complexes obey the "18 electron rule:" the sum of bonding and nonbonding electrons around a central transition metal atom should equal 18

      2. 2s + 10 d + 6 p = 18 e-

      3. Examples

        1. Nickel carbonyl Ni(CO)4

        2. Ferrocene

  4. Retrosynthetic analysis: the process of working backwards from the desired product to available reactants (start complex and work to simple)

    1. Steps (on paper)

      1. Examine the target molecule

      2. Can it be made in one step?

      3. What functional groups are in the molecule and how can they be prepared?

      4. What is the carbon backbone like? Will carbons, rings, etc. need to be added?

      5. Work backwards

    2. Examples