6 Organometallic Chemistry

Home , Electron counting

9/26/10

W 4:00- 6:50 PM (08/23/10 – 12/13/10) 3615 Bayou Building

Zerong Wang, 3531-6 Bayou, Tel. (281)-283-3795, Fax (281)-283-3709

Office Hours: TTH 1:00-2:30 PM

Middle Term Test & Final Exam

Oct. 13

Nov. 10

Dec. 8

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Scores

Item Points Total points Attendance 200 points 200 points Middle Term 200 points 400 points Final Exam 400 points 400 points

Total Points 1000 points

Grade Percentage

A 92.0-100% B- 78.0-79.9% D+ 63.0-64.9%

A- 89.0-91.9% C+ 75.0-77.9% D 58.0-62.9%

B+ 86.0-88.9% C 68.0-74.9% D- 55.0-57.9%

B 80.0-85.9% C- 65.0-67.9% F <55.0%

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Attendance & Performance

Each class for 20 points of attendance for the following dates

Sept. 22 Ch. 5,6 Sept. 29 Ch. 7,8 Oct. 6 Ch. 9,10 Oct. 20 Ch. 11 Oct. 27 Ch. 12,13 Nov. 3 Handout Nov. 17 Handout

In addition, performance will be counted into your total scores, things considered for the performance include coming to class on time (e.g., not late for more than 15 minutes), and not leaving the class before the lecture ends.

Organometallic Chemistry

• Definition of organometallic chemistry: transformations of organic compounds using metals.

• Organometallic chemistry is at the interface between inorganic and organic chemistry. – Inorganic: subset of coordination chemistry – Organic: subset of synthetic methods

• Other interdisciplinary areas – Bioorganometallic chemistry – Surface organometallic chemistry – Fullerene-metal complexes

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Traditional Organometallic Compounds in Organic Chemistry

Organometallic Compounds - Reagents with carbon- metal bonds

H H C C - + H3CH2CH2CH2C-Li (H3C)2Cu Li H MgBr Butyllithium vinylmagnesium bromide Dimethylcopper lithium

Carbon-Metal Bonds in Organometallic Compounds

C X C MgX

- + δ+ δ- δ δ _ C X C MgX C

Alkyl halides: Carbanions: nucleophile electrophiles react with electrophile

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Polarity of Bonds

Metal Alkyls

• General formula R-M (R = alkyl, M = metal) • The C-M bond is a covalent bond! • However, C--M+ or R--M+ bond tend to be polarized. • This is especially true for organometallic compounds conataining the more electropositive metals, i.e. alkali and alkaline earth metals. • Generally, the alkyl fragment of the organometallic compound is very reactive; however this depends on the metal, changing the metal alters the polarization of the R-M bond. • Thus different organometallic compounds are used in many different types of organic reactions.

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Alkyl halides will react with some metals (M0) in ether or THF to form organometallic reagents Preparation of Organolithium Compounds

2 Li(0) R-X R-Li + LiX diethyl ether

- + δ δ _ very strong bases C Li C very strong nucleophiles

organolithium reagents are most commonly used as very strong bases and in reactions with carbonyl compounds

M(0) H O R-X R-M 2 R-H + M-OH

Preparation of Organomagnesium Compounds: Grignard Reagents Mg(0) R-X R-MgX (Grignard reagent) THF R-X can be an alkyl, vinyl, or aryl halide (chloride, bromide, or iodide)

Solvent: diethyl ether (Et2O) or tetrahydrofuran (THF)

H CH C CH CH 3 2 O 2 3 O

diethyl ether (Et2O) tetrahydrofuran (THF) Alcoholic solvents and water are incompatible with Grignard reagents and organolithium reagents. Reactivity of the alkyl halide: -I > -Br > -Cl >> -F alkyl halides > vinyl or aryl halides

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Most halides can be used directly to make an organometallic lithium or magnesium compound except alkynyl halides. These can be made, but special methods (later) are required.

R-X + 2 Li R-Li + LiX R-X + Mg R-Mg-X

alkyl sp3 -C-X -C-Li or -C-MgX halide vinyl sp2 =C-X =C-Li or =C-MgX halide sp2 aryl X Li or MgX halide alkynyl sp C-X doesn’t work directly * halide X

R-CO-CH=CH-R HO-CH2CH2-R R1-CO-CH=CH2 epoxide R-COOH

CO R1-CO-R

CuI 2 R R1-X R1-COOR2 R1-R R-Li R1-C-OH R1-CO-R2 R1-C=CH R2

R-CO-Cl O 2

R1-CHO R-OH R-CO-R R1-C=C-Li R-CO-R1

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Reactions of Grignard reagents

The solvent or alkyl halides can not contain functional groups that are electrophilic or acidic. These are incompatible with the formation of the organomagnesium or organolithium reagent.

Grignard reagents will deprotonate alcohols

0 _ Mg + H3O HO Br HO MgBr O H HO H _ BrMg Other incompatible groups:

-CO2H, -OH, -SH, NH2, CONHR (amides)

Reactive functional groups: aldehydes, ketones, esters, amides, halides,

-NO2, -SO2R, nitriles

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( CARBON-METAL ) carbanion - + δ δ - Carbon-metal bonds : + are polar bonds that can be represented by a resonance hybrid of covalent and ionic covalent ionic structures.

Carbon-metal bonds most reactive bond percent ionic strongest base are frequemtly classified as to how C-K 51 much “ionic character” they have, by using C-Na 47 Li and Mg an index called the C-Li 43 are the percent ionic character. C-Mg 35 metals used This index states the most often importance of the C-Zn 18 ionic resonance con- C-Cd 15 tributor relative to the covalent structure. C-Cu 9 best nucleophile least reactive

Organocopper Compounds

- + • Lithium Dialkylcopper (organocuprate ) [(R)2Cu] Li • Cuprates are less reactive than organolithium • R acts as a Nucleophile • Oxidation state of copper is Cu(I). • Nucleophile “R” will attack various organic electrophiles. • Organocuprates are used in cross-coupling reactions to form higher alkanes. • Cross-Coupling Reaction: coupling of two different alkyls R and R’ to yield a new alkane (R-R’). This type of reaction is used to make new C-C between alkyl groups.

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Molecules in Organometallic Chemistry

 Involves at least one TRANSITION METAL that form more than two bonds with other groups (coordinates), known as organometallic complex.  Have more than one coordinates that do not involve in the reactions but stablize the complex.  Coordinates bind to the transition metal via s, p bonds, involving dsp or spd orbitals, and possibly with antibonding orbital participated.  Coordinates can be as simple as hydrogen atom

(hydride), H2, NO, N2, CO, CN, or complicated molecules such as alkene, carbene, phosphine, etc.

Types of Metal-Ligand Interactions • Sigma (σ) donor ligands Pi (π) donor ligands

Examples of donor and acceptor ligands

• Pi (π) acceptor ligands Sigma donor Pi donor* Pi acceptor*

- NH3 HO CO - - - H2O F , Cl CN - - H RCOO PR3 These ligands also act as σ donors.

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Why 18 electrons?

antibonding

Electron Counting

understanding structure and reactivity Cr = 6 Rh = 9 2*Bz = 12 + 3* P→ = 6  e-count 18 Cl = 1 +  e-count 16 tot. charge 0 tot. charge  0 2*Bz - -   ox state 0 3*P→ - Cl -1 -  ox state +1

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Why Count Electrons ?

• Basic tool for understanding structure and reactivity. • Simple extension of Lewis structure rules. • Counting should be “automatic”. • Not always unambiguous ⇒ don’t just follow the rules, understand them!

Predicting Reactivity

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Predicting Reactivity

Most likely associative:

II 16-e Pd 16-e PdII

18-e PdII

Predicting Reactivity

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Predicting Reactivity

Almost certainly dissociative:

16-e Cr(0)

18-e Cr(0) 18-e Cr(0)

The Basis of Electron Counting • Every element has a certain number of valence orbitals: 1 { 1s } for H 4 { ns, 3´np } for main group elements 9 { ns, 3´np, 5´(n-1)d } for transition metals

s px py pz

dxy dxz dyz dx2-y2 dz2

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The Basis of Electron Counting

• Every orbital wants to be “used", i.e. contribute to binding an electron pair. • Therefore, every element wants to be surrounded by 1/4/9 electron pairs, or 2/8/18 electrons. – For main-group metals (8-e), this leads to the standard Lewis structure rules. – For transition metals, we get the 18-electron rule. • Structures which have this preferred count are called electron-precise.

Compounds Are Not Always Electron-Precise ! The strength of the preference for electron-precise structures depends on the position of the element in the periodic table. • For very electropositive main-group elements, electron count often determined by steric factors. – How many ligands "fit" around the metal? – "Orbitals don't matter" for ionic compounds • Main-group elements of intermediate electronegativity (C, B) have a strong preference for 8-e structures. • For the heavier, electronegative main-group elements, there is 2- the usual ambiguity in writing Lewis structures (SO4 : 8-e or 12-e?). Stable, truly hypervalent molecules (for which every

Lewis structure has > 8-e) are not that common (SF6, PF5). Structures with < 8-e are very rare.

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Compounds Are Not Always Electron-Precise !  The strength of the preference for electron-precise structures depends on the position of the element in the periodic table.  For early transition metals, 18-e is often unattainable for steric reasons  The required number of ligands would not fit.  For later transition metals, 16-e is often quite stable  In particular for square-planar d8 complexes.  For open-shell complexes, every valence orbital wants to be used for at least one electron  More diverse possibilities, harder to predict.

Prediction of Stable Complexes

Cp Fe, ferrocene: 18-e 2 Cp Co, cobaltocene: 19-e Cp Ni, nickelocene: 20-e Very stable. 2 2 Chemically reactive, Behaves as an aromatic Strong reductant, reacts with air. easily loses a Cp ring, organic compound in e.g. Cation (Cp Co+) is very stable. reacts with air. Friedel-Crafts acylation. 2

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If there are not enough electrons...

 Structures with a lower than ideal electron count are called electron-deficient or coordinatively unsaturated.

 They have unused (empty) valence orbitals.

 This makes them electrophilic, i.e. susceptible to attack by nucleophiles.

 Some unsaturated compounds are so reactive they will attack hydrocarbons, or bind noble gases.

Reactivity of Electron-Deficient Compounds

18-e Fe(0) 16-e Fe(0) 18-e Fe(0) unreactive very reactive

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If there are too many electrons...  "Too many electrons" means there are fewer net covalent bonds than one would think.  Since not enough valence orbitals available for these electrons.  An ionic model is required to explain part of the bonding.  The "extra" bonds are relatively weak.  Excess-electron compounds are relatively rare, especially for transition metals.  Often generated by reduction (= addition of electrons).

Where Are the Electrons ?

 Electrons around a metal can be in metal-ligand bonding orbitals or in metal-centered lone pairs.

 Metal-centered orbitals are fairly high in energy.

 A metal atom with a lone pair is a σ- donor (nucleophile).

 susceptible to electrophilic attack.

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Counting Electrons

Method A Method B

Determine formal oxidation state of metal Ignore formal oxidation state of metal Deduce number of d electrons Count number of d electrons for M(0)

Add d electrons + ligand electrons (A) Add d electrons + ligand electrons (B)

The end result will be the same

A simple classification of the most important ligands

L2X

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Ligands in Organometallic Chemistry

 Neutral 2e donors: PR3 (phosphines), CO (carbonyl), R2C=CR2 (alkenes), RC≡CR (alkynes, can also donate 4e), N≡CR (nitriles)

- - - -  Anionic 2e donors: X (halide), CH3 (methyl), CR3 (alkyl), Ph (phenyl), H- (hydride) The following can also donate 4e if needed, but initially count them as 2e donors (unless they are acting as bridging ligands): OR- - - - (alkoxide), SR (thiolate), NR2 (inorganic amide), PR2 (phosphide) - 2- 2- 2-  Anionic 4e donors: C3H5 (allyl), O (oxide), S (sulfide), NR 2- (imide), CR2 (alkylidene) - - - and from the previous list: OR (alkoxide), SR (thiolate), NR2 - (inorganic amide), PR2 (phosphide)

 Anionic 6e donors: Cp- (cyclopentadienyl), O2- (oxide)

 Z ligands: do not bring e to the metal: BR3, AlR3

How Do You Count ? "Covalent" count:  Number of valence electrons of central atom from periodic table  Correct for charge, if any.  but only if the charge belongs to that atom!  Count 1 e for every covalent bond to another atom.  Count 2 e for every dative bond from another atom.  no electrons for dative bonds to another atom!  Delocalized carbon fragments: usually 1 e per C  Three- and four-center bonds need special treatment.  Add everything.  There are alternative counting methods (e.g. "ionic count"). Apart from three- and four-center bonding cases, they should always arrive at the same count. We will use the "covalent" count in this course.

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Starting Simple...

N = 5 H = 1 3* H = 3 H = 1  +  +  e-count 8 e-count 2 N has a lone pair. Nucleophilic!

C = 4 4* H = 4 C = 4 +  2* H = 2 e-count 8 2* C = 2 +  e-count 8 A double bond counts as two covalent bonds.

Predicting Reactivity

C = 4 C = 4 + chg = -1 2* H = 2 3* H = 3 +  +  e-count 6 e-count 6 "Singlet carbene". Unstable. Highly reactive, Sensitive to nucleophiles electrophilic. (empty orbital) and electrophiles (lone pair). C = 4 - chg = +1 "Triplet carbene". Extremely 3* H = 3 reactive as radical, not +  especially for nucleophiles e-count 8 or electrophiles. Saturated, but nucleophilic.

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When is a line not a line ?

is C = 4 3* H = 3 C = 1 or +  e-count 8 B = 3 N = 5 B = 3 N = 5 - chg = +1 + chg = -1 3* H = 3 3* H = 3 3* H = 3 3* H = 3 N→ = 2 +  N = 1 B = 1 +  e-count 8 + + e-count 8   e-count 8 e-count 8

Covalent or Dative ?

How do you know a fragment forms a covalent or a dative bond? • Chemists are "sloppy" in writing structures. A "line" can mean a covalent bond, a dative bond, or even a part of a three-center two-electron bond.

• Use analogies ("PPh3 is similar to NH3"). • Rewrite the structure properly before you start counting.

Pd = 10 dative 1 e 2 e covalent Cl = 1 bond  bond P→ = 2 allyl = 3 "bond" to the 3 e +  allyl fragment e-count 16

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What Kind of Bridge Bond Do I Have ? A 3c-2e bond will only form when the central (bridging) atom does not have any lone pairs. When lone pairs are available, they are preferred as donors.

A methyl group can form one more single bond. After After chlorine forms a single bond, that, it has no lone pairs, so the it still has three lone pairs left. best it can do is share the Al-C One is used to donate bonding electrons with a to the second Al: Al = 3 second Al: Al = 3 3* Cl = 3 3* Me =  Cl→ = 2 3 +  MeAl→ = 2 e-count 8 +  e-count 8

3c-2e vs Normal Bridge Bonds

• The orbitals of a 3c-2e bond are bonding between all three of the atoms involved.

– Therefore, Al2Me6 has a net Al-Al bonding interaction.

• The orbitals involved in "normal" bridges are regular terminal-bridge bonding orbitals.

– Thus, Al2Cl6 has strong Al-Cl bonds but no net Al-Al bonding.

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Handling Charges

"Correct for charge, if any, but only if it belongs to that atom!" How do you know where the charge belongs? Eliminate all obvious places where a charge could belong, mostly hetero-atoms having unusual numbers of bonds. What is left should belong to the metal...

Any alkyl-SO3 group would normally be anionic Rh = 9 (c.f. CH SO -, the anion 3 3 CH2 = 1 of CH3SO3H). 3* CO→ = 6 So the negative charge +  does not belong to the metal! e-count 16

Handling Charges Even overall neutral molecules could have "hidden" charges!

A boron atom with 4 bonds would be -1 (c.f. BH -). 4 Co = 9 No other obvious centers of + chg = -1 charge, so the Co must be +1. 3* P→ = 6 2* CO→ = 4 +  e-count 18

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A few excess-electron examples

PCl P would have 10 e, but only has 4 valence 5 2- 5- orbitals, so it cannot form more than 4 “net” P-Cl SiF6 , SF6, IO6 and bonds. You can describe the bonding using ionic noble-gas halides can structures ("negative hyperconjugation"). Easy be described in a dissociation in PCl3 & Cl2. "PBr5" actually is PBr4 similar manner. +Br- !

P = 5 P = 5 + chg = -1 5* Cl = 5  4* Cl = 4 +  +  e-count 10 e-count 8

Does It Look Reasonable ?

Remember when counting:

• Odd electron counts are rare.

• In reactions you nearly always go from even to even (or odd to odd), and from n to n-2, n or n+2.

• Electrons don’t just “appear” or “disappear”.

• The optimal count is 2/8/18 e. 16-e also occurs frequently, other counts are much more rare.

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Electron-counting Exercises

Me2Mg Pd(PMe3)4 MeReO3

ZnCl4 Pd(PMe3)3 OsO3(NPh) 2- ZrCl4 ZnMe4 OsO4(pyridine) - - Co(CO)4 Mn(CO)5 Cr(CO)6 - 4+ V(CO)6 V(CO)6 Zr(CO)6

PdCl(PMe3)3 RhCl2(PMe3)2 Ni(PMe3)2Cl2

Ni(PMe3)Cl4 Ni(PMe3)Cl3

How Do You Calculate Oxidation States ? 1. Start with the formal charge on the metal

2. Ignore dative bonds

3. Ignore bonds between atoms of the same element

This one is a bit silly and produces counterintuitive results

4. Assign every covalent electron pair to the most electronegative element in the bond: this produces + and – charges

Usually + at the metal

Multiple bonds: multiple + and - charges

5. Add

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Examples - Transition Metals

2- chg = -2 4* Cl-Pd+ = +4 +  ox st +2 no chg = 0 1* O-Mn+ = +1 3* O2-=Mn2+ = +6 +  ox- chg st = +7-1 4* O2-=Mn2+ = +8 +  ox st +7

Normal Oxidation States

For group n or n+10: – never >+n or <-n (except group 11: frequently +2 or +3) – usually even for n even, odd for n odd – usually ≥ 0 for metals – usually +n for very electropositive metals – usually 0-3 for 1st-row transition metals of groups 6-11, often higher for 2nd and 3rd row – electronegative ligands (F,O) stabilize higher oxidation states, π-acceptor ligands (CO) stabilize lower oxidation states – oxidation states usually change from m to m-2, m or m+2 in reactions

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Coordination Number and Geometry 2 linear 3 triangle or T-shape 4 tetrahedral or square 5 trigonal bipyramid or square pyramid 6 octahedral 7 capped octahedral or pentagonal bipyramid

IV. Complexes of π-bound Ligands

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Outline

1. Alkene complexes 2. Butadiene complexes 3. Alkyne complexes 4. Enyl Complexes 5. Complexes the cyclic p-perimeters CnHn 6. Metal π-complexes of Unsaturated ligands with heteroatoms 7. Characterization of organometallic compounds

1. Alkene complexes A. Structure and bonding of mono-olefin complexes

Examples:

Bonding:

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Q1. Which interaction is dominant? High valent metal (e- poor) systems: Low valent metal (e- rich) systems: Q2. What are the effects of complexation?

• d(C-C) • bond angle at C • ν(C=C) • e- density on C

Q3. Do you expect that the complexed olefins be more reactive or less reactive towards nucleophiles?

(a) (b) (c) 59

Q4. Consider the fragment Pt(PPh3)2. Which of the following olefins would you expect to form the most stable olefin adduct A?

(a) CH2=CH2 (b) CH3-CH=CH2 c) CH2=CHCN

Q5. Which of the following olefin complexes is more stable?

* Usual stability of olefin complexes:

Q6. Which of the following olefin complexes is more stable?

Release of ring strain

Q7. Which of the following olefin complexes is more stable?

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B. Preparation of olefin complexes 1. From Substitution reactions: - 3. From alkyl and related species: LnM-L' + olefin ----> L' + LnM (olefin) Examples:

2. Metal salt + olefin + reducing agents. e.g.

C. Reactivity of olefin complexes 1. Insertion reactions

Oxidative additon of C-H

Example: Example:

Example:

Examples:

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3). Nucleophilic attack on coordinated olefins.

Examples:

Summary: Reactions of olefin complexes.

2. Butadiene complexes A. Most common structures of butadiene complexes B. Synthesis of butadiene complexes 1. by substitution reactions. e.g.

2. Reaction of coordinated ligands. e.g.

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C. Reactions of butadiene complexes

Examples:

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3. Alkyne complexes

A. Structure and bonding of alkyne complexes R-C≡C-R can be simple η2-ligand or bridging ligand. As simple η2-ligand, they can be 2e or 4e donors. e.g.

Q1. How can they be 2e or 4e donors?

In terms of bonding:

As bridging ligands:

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Q2. For each pair of the following complexes, which one is more stable?

B. Preparation of alkyne complexes • 1. By substitution reactions: e.g.

2. By other methods: e.g.

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C. Reactivity of alkyne complexes

Similar to olefins complexes, e.g.

Q1. Give the product for the following reaction.

4. Enyl Complexes

Enyl: Organic ligands with odd number of carbon chains. Examples:

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A. Allyl complexes

1. Structure of allyl complexes

2. Preparation of allyl complexes a. Metal salt + main group organometallic reagents.

Examples:

b). Low valent metal complexes + allylhalide.

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c) Metal salts + olefin + base. A previously mentioned example:

Question. What is the product for the following reaction?

Examples

Reactivity of allyl complexes

a). With Nucleophiles:

b). With electrophile (for η1-allyls):

c). Insertion reactions.

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B Examples of other enyl complexes and their preparation

They can be prepared similar to allyl complexes.

Examples: a). Metal salt + main group organometallic reagents.

b). Transfer of H+ or H-

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5. Complexes of the cyclic π- perimeters CnHn

Common structural types a). Sandwich complexes, e.g.

b) Half-sandwich complexes c) Multidecker sandwich

d). Complexes with tilted sandwich structures. 79

A. C3R3 as ligands

Examples and preparation.

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B. C4R4 as ligands

Q1. What are the common starting materials for preparing M(h4-C4R4) complexes?

Example and preparation.

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Important chemical properties:

a. Show aromatic properties. e.g.

C. C5R5 as ligands

• Common C5R5

Binding mode of C5R5

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A).How to prepare CpM complexes? • Common starting materials:

Examples.

1). From a source of Cp- : most common route

- Cp reagents: NaCp, CpMgBr, TlCp, CpSnMe3

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2). From C5H6 or related ligands. e.g.

2). From C5H6 or related ligands. e.g. c). From a source of Cp+. e.g.

B. What are the typical chemical properties of η5-Cp ligand?

--- η5-Cp ligand is usually unreactive. 5 --- The following reactions may occur to η -Cp.

* Electrophilic substitution-like benzene

* Metallation reactions

* Nucleophilic attack on Cp

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D. Arene complexes

• 1). Common structures

As simple ŋn ligands.

As bridging ligands

2). Common synthetic routes

a) From benzene

By substitution reactions

Benzene + metal salts + reducing agents

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b) From reactions of coordinated ligands

3). Common Chemical reactions

Effect of complexation

Expected reactivity:

Less reactive toward E+, more reactive toward Nu-.

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E. C7H7 and C8H8 as ligands

+ Tropylium cation = C7H7 Aromatic 6 e-. - - C7H7 : antiaromatic 8 e : for Electron counting and oxidation state assignments

1 3 5 7 * C 7H7 can be a η , η , η and η ligand. e.g.

2 4 6 8 * C8H8 can be a η , η , η and η ligand. e.g.

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* COT complexes can be made from:

7 8 • The chemical properties of (η -C7H7)M or (η -C8H8)M are similar to n other (η -CnHn)M complexes.

Metal Ⅱ-complexes of unsaturated ligands with heteroatoms.

Many unsaturated ligands with heteroatoms can form complexes.

Examples:

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Synthesis: similar to organic ones. e.g

Chiral Olefins as Steering Ligands in Asymmetric Catalysis

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Scheme 1. Selection of commercially available transition-metal ole!n complexes. coe: cyclooctene, dcp:endo-dicyclopentadiene, cod: 1,5- cyclooctadiene, nbd: norbornadiene, dba: dibenzylideneacetone, dvds: divinyltetramethyldisiloxane.

Scheme 2. Synthesis of chiral (E)-cyclooctene (( )-(P)-11) according to Cope et al.

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-1 Inversion barriers ΔGinv of planar chiral cyclooctatetraene derivatives in kcal mol

Scheme 5. Natural common chiral dienes used for the formation of RhI and IrI complexes.

Scheme 6. RhI and Fe0 complexes containing non-natural chiral dienes as ligands.

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Scheme 7. Dewar–Chatt–Duncanson (DCD) model for metal–ole!n binding.

Table 1: CDA analyses for some HAu-L complexes (DFT, B3LYP, basis set II [36b])

L d b d/b

C2H 4 0.36 0.13 2.9

C2H 2 0.16 0.12 1.3 CO 0.27 0.22 1.2

PMe3 0.53 0.16 3.3 imidazol-2-ylidene 0.36 0.12 3.0

NMe 3 0.20 0.01 32.7

Structural parameters of various chiral metal–diene complexes

Christian Defieber, Hansjrg Grtzmacher, and Erick M. Carreira, Angew. Chem. Int. Ed. 2008, 47, 4482 – 4502

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Chiral Ferrocenes , 2006 . , and Juan Adrio . Chem. Int. Ed Angew , Arrays, Javier

Carretero Gmez

, 7674 – 7715 Ramn Carlos 45

Figure 1. Representative families of chiral ferrocene ligands with outstanding applications in asymmetric catalysis.

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Scheme 4. Rh-catalyzed asymmetric hydrogenation of dehydro-a-amino acids and itaconic acid derivatives; 10 psig= 0.689 bar; cod= cyclooctadiene; nbd= norbornadiene.