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5 α, β, γ, And δ Elimination

5 α, β, γ, And δ Elimination

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presumably because a coplanar M–C–C–H arrangement is much harder

to achieve (Eq. 7.28).



In a series of analogous nickel complexes in the presence or absence

of excess phosphine, three different decomposition pathways are found,

one for each of the different intermediates, 14e, 16e, and 18e, that can

be formed (Eq. 7.29).



An alkyl and its alkene hydride elimination product can occasionally

be seen in equilibrium together (Eq. 7.30).19


Alkoxide complexes β eliminate readily to give ketones or aldehydes,

accounting for the ability of basic isopropanol to reduce many metal

halides to hydrides with formation of acetone by the pathway of Eq.

3.27. β Elimination of amides and amines to imines also occurs but tends

to be slow.20

α Elimination

Common for alkyls that lack β hydrogens, this is the reverse of 1,1

insertion (e.g., Eq. 7.14). β elimination being impossible, LnM–Me can

only undergo an α elimination to give LnM(=CH2)H. While any β

process gives an alkene, a stable species that can dissociate from the

metal, an alkylidene ligand from an α elimination is unstable in the free

state and cannot dissociate. Methylene hydride complexes are therefore rarely seen because they are thermodynamically unstable with


Insertion and Elimination

respect to the corresponding methyl complex, but α elimination can

still occur reversibly in a reaction sequence. For this reason, the α

process is less well characterized than β elimination. Isotope exchange

studies on both Mo and Ta alkyls suggest that α elimination can be up

to 106 times faster than β elimination even in cases in which both α- and

β-H substituents are present.21 A coordinatively unsaturated methyl

complex can be in equilibrium with a methylene hydride,22 that can be

trapped either by nucleophilic attack at the carbene carbon (Eq. 7.31)

or by removing the hydride by reductive elimination with a second

alkyl (Eq. 7.32):



Other Eliminations

A great variety of other ligands may lack β-Hs but possess γ- or δ-H’s

and can thus undergo γ or δ elimination to give cyclic products

(Eq. 7.33).


• 1,1-Insertion occurs for ηl ligands such as CO and 1,2 insertion

occurs for η2 ligands such as C2H4. In each case an X ligand

migrates from M to L (Eq. 7.1 and Eq. 7.2).

• Insertions are kinetically favored for X = H over X = R, but for

CO, insertion into M–H is thermodynamically disfavored (Eq. 7.1

and Eq. 7.2).




  1.  A. J. Pontiggia, A. B. Chaplin, and A. S. Weller, J. Organometal. Chem., 696,

2870, 2011.

  2.  A. Derecskei-Kovacs and D. S. Marynick, J. Am. Chem. Soc., 122, 2078,


  3. K. Fukumoto and H. Nakazawa, J. Organometal. Chem., 693, 1968, 2008;

M. Rubina, M. Conley, and V. Gevorgyan, J. Am. Chem. Soc., 128, 5818,


  4.  Z. X. Cao, S. Q. Niu, and M. B. Hall, J. Phys. Chem., A 104, 7324, 2000.

  5.  C. P. Lilly, P. D. Boyle, and E. A. Ison, Organometallics, 31, 4295, 2012.

  6.  S. A. Macgregor and G. W. Neave, Organometallics, 23, 891, 2004.

  7. Y. H. Zhang, R. J. Keaton, and L. R. Sita, J. Am. Chem. Soc., 125, 8746, 2003.

  8.  C. Lu, Q. Xiao, and P. E. Floreancig, Org. Lett., 12, 5112, 2010.

  9.  J. Vela, S. Vaddadi, T. R. Cundari, J. M. Smith, E. A. Gregory, R. J. Lachicotte,

C. J. Flaschenriem, and P. L. Holland, Organometallics, 23, 5226, 2004.

10.  M. E. Evans, T. Li, A. J. Vetter, R. D. Rieth, and W. D. Jones, J. Org. Chem.,

74, 6907, 2009; G. Choi, J. Morris, W. W. Brennessel, and W. D. Jones, J. Am.

Chem. Soc., 134, 9276, 2012 and personal communication, 2012.

11.  E. A. Standley and T. F. Jamison, J. Am. Chem. Soc., 135, 1585, 2013.

12.  R. H. Crabtree, New J. Chem., 27, 771, 2003.

13. L. Luan, P. S. White, M. Brookhart, and J. L. Templeton, J. Am. Chem. Soc.,

112, 8190, 1990.

14. L. Boisvert, M. C. Denney, S. Kloek Hanson, and K. I. Goldberg, J. Am.

Chem. Soc., 131, 15802, 2009.

15.  C. Lu and J. C. Peters, J. Am. Chem. Soc., 124, 5272, 2002.

16.  A. Macchioni, Chem. Rev., 105, 2039, 2005.

17. M. M. Konnick, N. Decharin, B. V. Popp, and S. S. Stahl, Chem. Sci., 2, 326, 2011.

18.  J. Zhao, H. Hesslink, and J. F. Hartwig, J. Am. Chem. Soc., 123, 7220, 2001.

19. K. Umezawa-Vizzini and T. R. Lee, Organometallics, 23, 1448, 2004.

20.  J. Louie, F. Paul, and J. F. Hartwig, Organometallics, 15, 2794, 1996.

21.  R. R. Schrock, S. W. Seidel, N. C. Mosch-Zanetti, K. Y. Shih, M. B.

O’Donoghue, W. M. Davis, and W. M. Reiff, J. Am. Chem. Soc., 119, 11876, 1997.

22.  H. Hamilton and J. R. Shapley, Organometallics, 19, 761, 2000.


7.1. Predict the structures of the products (if any would be expected)

from the following: (a) CpRu(CO)2Me  +  PPh3, (b) Cp2Zr­

HCl  +  butadiene, (c) CpFe(CO)2Me  +  SO2, and (d)

Mn(CO)5CF3 + CO.


Insertion and Elimination

7.2. Me2NCH2Ph reacts with PdCl2 to give A; then A reacts with

2,2-dimethylcyclopropene and pyridine to give a mixture of C and

D. Identify A and explain what is happening. Why is it that

Me2NPh does not give a product of type A, and that A does not

insert ethylene?

7.3. In the pyrolysis of TiMe4, both ethylene and methane are observed;


7.4. Suggest mechanisms for the following:

7.5. The reaction of trans-PdAr2L2 (A, Ar = m-tolyl, L = PEt2Ph) with

MeI gives 75% of m-xylene and 25% of 3,3′-bitolyl. Explain how

these products might be formed and list the possible Pd-containing

products of the reactions. When the reaction of A was carried out

with CD3I in the presence of d0-PdMeIL2 (B), both d0- and d3xylene were formed. A also reacts with B give m-xylene and

3,3′-bitolyl. How does this second result modify your view of the


7.6. [Cp*Co{P(OMe)3}Et]+ has an agostic interaction involving the

β-H of the ethyl group. Draw the structure. It reacts with ethylene

to form polyethylene. How might this reaction proceed? RhCl3/

EtOH and other late metal systems usually only dimerize ethylene to a mixture of butenes. Given that a Rh(I) hydride is the

active catalyst in the dimerization, what mechanism would you

propose? Try to identify and explain the key difference(s) between

the two systems.

7.7.Design an alkyl ligand that will be resistant to β elimination (but

not the ones mentioned in the text; try to be as original as pos-



sible). Design a second ligand, which may be an alkyl or an arylsubstituted alkyl, that you would expect to be resistant to β

elimination but have a high tendency to undergo β–C–C bond

cleavage. What products are expected?

7.8. Given the existence of the equilibrium shown:

how would you change L, M, and the solvent to favor (a) the

right-hand side and (b) the left-hand side of the equation?

7.9. trans-PtCl(CH2CMe3){P(C5H9)3}2 gives 1,1-dimethylcyclopropane

on heating. What mechanism is most likely, and what Pt-containing

product would you expect to be formed? If the neopentyl group

is replaced by –CH2Nb (Nb = 1-norbornyl), then CH3Nb is formed

instead. What metal complex would you expect to find as the

other product?

7.10.  In mononuclear metal complexes, β elimination of ethyl groups

is almost always observed, rather than α elimination to the

ethylidene hydride LnM(=CHCH3)H. In cluster compounds,

such as HOs3(CO)10(Et), on the other hand, α elimination to give

the bridging ethylidene H2Os3(CO)10(η1,μ2-CHCH3) is observed

in preference to β elimination. Suggest reasons for this difference.

7.11. Consider the three potential rate-accelerating effects on CO

insertion mentioned in Section 7.2: steric, Lewis acid, and oxidation. For each effect, discuss whether an acceleration of the overall

reaction rate is to be expected if the reaction in question is (a)

first order, (b) second order, (c) an intermediate case, and (d) an

apparent insertion of the type shown in Eq. 7.18.



In reductive elimination or migratory insertion, ligand transformations

occur within the coordination sphere of the metal. In contrast, we now look

at outer sphere processes in which direct attack of an external reagent can

take place on a ligand without prior binding of the reagent to the metal.


The attacking reagent can be a nucleophile or an electrophile, but for

reasons discussed here, the nucleophilic version is much more controllable and generally applicable. Nucleophilic attack on L′ is favored

when the metal fragment LnM–L′ is a poor π base but a good σ acid,

for example, if the complex bears a net positive charge or has electronwithdrawing ligands. In such a case, L′ is depleted of electron density

to such an extent that the nucleophile, Nu− (e.g., LiMe or OH−), can

attack. Electrophilic attack is favored when the metal is a weak σ acid

but a strong π base, for example, if the complex has a net anionic charge,

a low oxidation state, and good donor ligands, L. The electron density

on L′ is so much enhanced by back donation that it now becomes susceptible to attack by electrophiles, E+ (H+, MeI, etc.).

The Organometallic Chemistry of the Transition Metals, Sixth Edition.

Robert H. Crabtree.

© 2014 John Wiley & Sons, Inc. Published 2014 by John Wiley & Sons, Inc.




Both nucleophiles and electrophiles can give either addition or abstraction. In addition, the reagent becomes covalently attached to L′, and the

newly modified ligand stays on the metal. In abstraction, the reagent

detaches a part or even the whole of ligand L′ and leaves the coordination sphere of the metal. A nucleophile abstracts a cationic fragment,

such as H+ or Me+, while an electrophile abstracts an anionic fragment,

such as H− or Cl−. Often, reaction with an electrophile generates a positive charge on the complex and prepares it for subsequent attack by a

nucleophile. We will see an example of alternating Nu−/E+ reactivity

steps in Eq. 8.10; Eq. 8.17 shows the reverse sequence of reagents.

Equation 8.1–Equation 8.9 show some examples of these reactions

and reagents. In Eq. 8.1 and Eq. 8.2, the nucleophiles reduce the hapticity of the ligands because they displace the metal from the carbon to

which they add. In Eq. 8.2, we convert an η5-L2X into an η4-L2 ligand

and subtract one unit from the net ionic charge, for a zero net change

in the metal valence electron count. In general, an LnX ligand is converted to an Ln ligand, and an Ln ligand is converted to an Ln−1X ligand.

Electrophilic reagents, in contrast, tend to increase the hapticity of the

ligand to which they add (Eq. 8.6 and Eq. 8.7). Electrophilic attack on

a ligand depletes the electron density on that ligand, often compensated

by the attack of a metal lone pair on the ligand. For instance, in Eq. 8.7,

an η4-L2 diene ligand becomes an η5-L2X pentadienyl. At the same time,

a net positive charge is added to the complex, which leaves the overall

electron count unchanged. In general, an LnX ligand is converted to an

Ln+1 ligand and an Ln ligand is converted to an LnX ligand. Equations

8.3 and 8.4 show that nucleophilic abstraction of H+ is simply ligand

deprotonation. Nucleophilic abstraction of a methyl cation from

Pt(IV) by iodide was the key step in the reductive elimination mechanism of Fig. 6.2:

1. Nucleophilic addition:1



2. Nucleophilic abstraction:2



Addition and Abstraction



3. Electrophilic addition:3




4. Electrophilic abstraction:4



Attack often occurs at the metal rather than at the ligand. For a nucleophile, this is simply associative substitution (Section 4.5) and can lead to

the displacement of an existing ligand. If the original metal complex is 16e,

nucleophilic attack may take place directly on the metal; if 18e, a ligand

must usually dissociate first. In an 18e complex, a nucleophile is therefore

more likely to attack a ligand, rather than the metal. The pyridine in Eq.

8.1 is a potential 2e ligand, but it does not attack the metal because the

resulting 18e configuration is unfavorable for Pt(II). By attacking the

ligand, the nucleophile does not raise the metal electron count.

For an electrophile, the situation is different. As a 0e reagent, an

electrophile does not increase the electron count of the metal whether

it attacks the metal or the ligand. Attack at the metal is thus always a

possible alternative pathway even for an 18e complex except for d0

complexes, which have no lone pairs on the metal. Of course, large

electrophiles, such as Ph3C+, may still have steric problems that prevent

attack at the metal. This lack of selectivity has made electrophilic attack

less useful.

Nucleophilic Addition to CO


Organic free radicals are a third class of reagent that can give addition and abstraction reactions, but these reactions are less well understood and have not been widely employed. Radicals are typically

reactive transients, so addition and abstraction steps tend to occur only

as part of a multistep reaction scheme (e.g., Section 16.2).


When bound to weakly π basic metals, CO becomes very sensitive to

nucleophilic attack at carbon;5 L-to-M σ donation not being compensated

by M-to-L back donation, the CO carbon becomes positively charged. RLi

can now convert a number of metal carbonyls to the corresponding anionic

acyls. The resulting net negative charge now promotes electrophilic addition to the acyl oxygen to give the Fischer (heteroatom-stabilized) carbene

complex, 8.1. Equation 8.10 also illustrates a common pattern in synthetic

pathways—alternation of nucleophilic and electrophilic attack. Addition

of one prepares the system for attack by the other.


The cationic charge makes [Mn(CO)6]+ much more sensitive to

nucleophilic attack than [Mo(CO)6]. Hydroxide, or even water, can now

attack coordinated CO to give an unstable metalacarboxylic acid intermediate that decomposes to CO2 and the metal hydride by β elimination

(Eq. 8.11). This can be a useful way of removing one CO from the metal.


Nucleophilic attack of methanol instead of water can give a metalaester, LnM(COOR), stable from having no β-H.

The polar solvent favors loss of Cl− over loss of PPh3 (=L) in the first

step of Eq. 8.12. The resulting 1+ ionic charge sets the stage for a subsequent nucleophilic attack by MeOH on the activated CO. Acid can reverse

the addition reaction by protonating the methoxy group, leading to loss of

MeOH. This methoxide abstraction reaction is a case of a nucleophilic

addition being reversed by a subsequent electrophilic abstraction and

shows how the Nu/E alternation strategy can fail, perhaps from unsuitable

workup conditions. For example, the product of a nucleophilic addition


Addition and Abstraction

may revert to starting materials if excess acid is added to the reaction

mixture with the object of neutralizing the excess nucleophile.


We saw in Chapter 4 that Et3NO can remove coordinated CO from

18e metal complexes.6 Its nucleophilic oxygen (Et3N+–O−) can attack

the CO carbon with subsequent breakdown to Et3N, CO2, and a 16e

metal fragment (Eq. 8.13). The cis-disubstituted product is obtained

selectively because a CO trans to another CO is activated toward a

nucleophilic attack by receiving less back donation. Two problems arise:

the amine also formed can sometimes coordinate if no better ligand is

available and successive carbonyls become harder to remove as the

back bonding to the remaining CO groups increases, and so only one

CO is usually removable in this way.


A complexed isonitrile is more easily attacked by nucleophiles than

is CO (Eq. 8.14); isonitriles are not only intrinsically less π-acidic but

also tend to bind to higher oxidation state metals where back donation

is reduced.7




Free polyenes, such as benzene and butadiene, normally undergo

electrophilic, not nucleophilic attack. In a complete reversal of their

Nucleophilic Addition to Polyenes and Polyenyls


chemical character, called umpolung, complexation enhances nucleophilic, but suppresses electrophilic attack. The metal can therefore be

considered either as an activating group for nucleophilic attack or a

protecting group against electrophilic attack.

The nucleophile normally adds to the face of the polyene opposite to

the metal. Since the metal is likely to have originally bound to the least

hindered face of the free polyene, we expect to see selective attack at what

was the more hindered face of the free polyene, a useful selectivity pattern.

Davies–Green–Mingos Rules

In a complex with several polyene or polyenyls, we often see selective

attack at one site only. Davies, Green, and Mingos8 systematized these

reaction outcomes in terms of rules that usually correctly predict the

site of addition:

Rule 1: Polyenes (even or Ln ligands) react before polyenyls (odd or

LnX ligands).

Rule 2: Open ligands with interrupted conjugation react before

closed ligands with cyclic conjugation. Rule 1 takes precedence

over rule 2 if they conflict.

Rule 3: Open polyenes give terminal addition. Open polyenyls

usually give terminal attack, but nonterminal if LnM is particularly

electron donating.

Rule 4: A cation [LnM]c+ with an c+ net ionic charge is often subject

to attack c times, but the selectivity for later steps has to be considered in light of the structure produced by the preceding addition.

Polyenes or even ligands have an even-electron count on the covalent model and include η2-C2H4 and η6-C6H6; odd ligands with an oddelectron count include η3-C3H5 and η5-C5H5. Closed ligands include Cp

or η6-C6H6, while open ligands include allyl or cyclohexadienyl. Some

ligands and their classification are illustrated in 8.2–8.5.

Diagrams 8.6, 8.7 8.8, 8.9, 8.10, 8.11, 8.12, and 8.13 show the rules in

action with the point of attack indicated by the arrow(s) in the diagram.

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