Green–Davies–Mingos rules
In organometallic chemistry, the Green–Davies–Mingos rules predict the regiochemistry for nucleophilic addition to 18-electron metal complexes containing multiple unsaturated ligands.[1] The rules were published in 1978 by organometallic chemists Stephen G. Davies, Malcolm Green, and Michael Mingos. They describe how and where unsaturated hydrocarbon generally become more susceptibile to nucleophilic attack upon complexation.[1]
Rule 1[]
Nucleophilic attack is preferred on even-numbered polyenes (even hapticity).[1]
![GreenDaviesMingRule1.png](http://upload.wikimedia.org/wikipedia/commons/thumb/2/2a/GreenDaviesMingRule1.png/400px-GreenDaviesMingRule1.png)
Rule 2[]
Nucleophiles preferentially add to acyclic polyenes rather than cyclic polyenes.[1]
![Green-Davies-Mingos2.png](http://upload.wikimedia.org/wikipedia/commons/thumb/8/8c/Green-Davies-Mingos2.png/350px-Green-Davies-Mingos2.png)
Rule 3[]
Nucleophiles preferentially add to even-hapticity polyene ligands at a terminus.[1] Nucleophiles add to odd-hapticity acyclic polyene ligands at a terminal position if the metal is highly electrophilic, otherwise they add at an internal site.
Simplified: even before odd and open before closed
The following is a diagram showing the reactivity trends of even/odd hapticity and open/closed π-ligands.
The metal center is electron withdrawing. This effect is enhanced if the metal is also attached to a carbonyl. Electron poor metals do not back bond well to the carbonyl. The more electron withdrawing the metal is, the more triple bond character the CO ligand has. This gives the ligand a higher force constant. The resultant force constant found for a ligated carbonyl represents the same force constant for π ligands if they replaced the CO ligand in the same complex.
Nucleophilic addition does not occur if kCO* (the effective force constant for the CO ligand) is below a threshold value [2]
The following figure shows a ligated metal attached to a carbonyl group. This group has a partial positive charge and therefore is susceptible to nucleophilic attack. If the ligand represented by Ln were a π-ligand, it would be activated toward nucleophilic attack as well.
Incoming nucleophilic attack happens at one of the termini of the π-system in the figure below:
In this example the ring system can be thought of as analogous to 1,3-butadiene. Following the Green–Davies–Mingos rules, since butadiene is an open π-ligand of even hapticity, nucleophilic attack will occur at one of the terminal positions of the π-system. This occurs because the LUMO of butadiene has larger lobes on the ends rather than the internal positions.
Examples of complexes[]
The following is an example of a complex containing three types of π-ligands, demonstrating the preferential attack of a nucleophile at one of the π-systems.
![Green–Davies–Mingos example double ligand reaction.png](http://upload.wikimedia.org/wikipedia/commons/thumb/2/24/Green%E2%80%93Davies%E2%80%93Mingos_example_double_ligand_reaction.png/400px-Green%E2%80%93Davies%E2%80%93Mingos_example_double_ligand_reaction.png)
The above complex contains three types of π-ligands. The cyclooctane ring contains a butadiene fragment on the left and an allyl fragment on the right. A cyclopentadiene ligand on the cobalt center gives the third type.
Attack of the cyanide nucleophile occurs preferentially at a terminus of the butadiene fragment. (The drawing above shows the incorrect product)
Following the rules given above, the butadiene fragment is an open ligand of even hapticity, which has greater reactivity than the allyl fragment, an open ligand of odd hapticity, or the cyclopentadiene, a closed ligand of odd hapticity.
Attack occurs at the terminus which will result in the conjugated product shown.
Internal attack[]
Here the ligand that is already attached to the metal acts as the nucleophile and attacks the metal center internally.[3][4]
![Addition to pi ligands Fig 5.png](http://upload.wikimedia.org/wikipedia/commons/thumb/7/73/Addition_to_pi_ligands_Fig_5.png/350px-Addition_to_pi_ligands_Fig_5.png)
Effects of types of ligands on regiochemistry of attack[]
Nucleophilic attack at terminal position of allyl ligands when π accepting ligand is present.[5]
![Addition to pi ligands Fig 6.png](http://upload.wikimedia.org/wikipedia/commons/thumb/1/14/Addition_to_pi_ligands_Fig_6.png/350px-Addition_to_pi_ligands_Fig_6.png)
If sigma donating ligands are present they pump electrons into the ligand and attack occurs at the internal position.
![Addition to pi ligands Fig 7.png](http://upload.wikimedia.org/wikipedia/commons/thumb/a/a2/Addition_to_pi_ligands_Fig_7.png/350px-Addition_to_pi_ligands_Fig_7.png)
Effects of asymmetrical ligands[]
When asymmetrical allyl ligands are present attack occurs at the more substituted position.[6]
![Addition to pi ligands Fig 8.png](http://upload.wikimedia.org/wikipedia/commons/d/de/Addition_to_pi_ligands_Fig_8.png)
In this case the attack will occur on the carbon with both R groups attached to it since that is the more substituted position.
Effects of large π ligands[]
When large π ligands [7] are present they can undergo different kinds of nucleophilic attacks. In the following figure the nucleophilic attack can occur from either the top or bottom and reduce the double bond and add the nucleophile.
![Wiki last image.png](http://upload.wikimedia.org/wikipedia/commons/thumb/6/60/Wiki_last_image.png/250px-Wiki_last_image.png)
This nucleophilic attack can occur at either the top or bottom also and add the nucleophile.
![Addition to pi ligands Fig 9.png](http://upload.wikimedia.org/wikipedia/commons/thumb/d/d4/Addition_to_pi_ligands_Fig_9.png/250px-Addition_to_pi_ligands_Fig_9.png)
Uses in synthesis[]
Nucleophilic addition to π ligands can be used in synthesis. One example of this is to make cyclic metal compounds.[8] Nucleophiles add to the center of the π ligand and produces a metallobutane.
![Addition to pi ligands Fig 10.png](http://upload.wikimedia.org/wikipedia/commons/thumb/9/98/Addition_to_pi_ligands_Fig_10.png/450px-Addition_to_pi_ligands_Fig_10.png)
![Addition to pi ligands Fig 10.2.png](http://upload.wikimedia.org/wikipedia/commons/thumb/2/2a/Addition_to_pi_ligands_Fig_10.2.png/350px-Addition_to_pi_ligands_Fig_10.2.png)
References[]
- ^ a b c d e Davies, Stepehn G.; Green, Malcolm L. H.; Mingos, D. Michael P. (1978). "Nucleophilic Addition to Organotransition Metal Cations Containing Unsaturated Hydrocarbon Ligands: A Survey and Interpretation". Tetrahedron. 34 (20): 3047–3077. doi:10.1016/0040-4020(78)87001-X.
- ^ Bush Russell C.; Angelici Robert J. (1986). "Metal carbonyl νCO force constants as predictors of π-ethylene and π-benzene complex reactivity with nucleophiles". Journal of the American Chemical Society. 108 (10): 2735–2742. doi:10.1021/ja00270a037.
- ^ Periana Roy A.; Bergman Robert G. (1984). "Rapid intramolecular rearrangement of a hydrido(cyclopropyl)rhodium complex to a rhodacyclobutane. Independent synthesis of the metallacycle by addition of hydride to the central carbon atom of a cationic rhodium π-allyl complex". Journal of the American Chemical Society. 106 (23): 7272–7273. doi:10.1021/ja00335a084.
- ^ Suzuki, Tomohiro; Okada, Goro; Hioki, Yasunori; Fujimoto, Hiroshi (2003). "Theoretical Study of the Reactivity of (π-Allyl)molybdenum Complexes". Organometallics. 22 (18): 3649–3658. doi:10.1021/om0207459.
- ^ Aranyos, Attila; Szabó, Kálmán J.; Castaño, Ana M.; Bäckvall, Jan-E. (1997). "Central versus Terminal Attack in Nucleophilic Addition to (π-Allyl)palladium Complexes. Ligand Effects and Mechanism". Organometallics. 16 (5): 1058–1064. doi:10.1021/om960950m.
- ^ Delbecq, F.; Lapouge, C. (2000). "Regioselectivity of the Nucleophilic Addition to (η3-allyl) Palladium Complexes. A Theoretical Study". Organometallics. 19 (14): 2716–2723. doi:10.1021/om0003032.
- ^ Schörshusen, Sonja; Heck, Jürgen (2007). "Metal-Mediated Transformations of Cyclooctatetraene to Novel Methylene-Bridged, Bicyclic Compounds". Organometallics. 26 (22): 5386–5394. doi:10.1021/om700539e.
- ^ Periana, Roy A (1986). "Carbon-carbon activation of organic small ring compounds by arrangement of cycloalkylhydridorhodium complexes to rhodacycloalkanes. Synthesis of metallacyclobutanes, including one with a tertiary metal-carbon bond, by nucleophilic addition to π-allyl complexes". Journal of the American Chemical Society. 108 (23): 7346–7355. doi:10.1021/ja00283a033.
- Reaction mechanisms