In Heck coupling, Pd(OAc)2 is used as catalyst. In addition, if we also add PPh3, what will its effect on the reaction rate be? Will it accelerate or slow down the reaction, or have no effect?
PPh3 reduces Pd(II) to Pd(0). Triphenyl phosphine is a ligand and it has been shown that it increases the reactivity of less reactive alkyl halides..So I think it must speed up your reaction.
Please check the following references:
http://pubs.acs.org/doi/pdf/10.1021/ja00811a029 (JACS, 1974, 96, 1133
Dear Dhirendra,
As per I know Pd does exist in Pd(OAc)2 and Pd(PPh3)4. So Incorporation of PPh3 may help in formation of complex in TS. By stabilizing the TS it will help in enhancing the rate of the reaction.
Using PPh3 or any other ligands by playing on the electronic structure, I mean elecron rich-poor, and bulkiness has an effect on the rate of your reaction. For example electron rich ligands will enhance oxidative addition step while bulky ligands will enhance the reductive elemination step. So, PPh3 is in between and because of that it is one of the most used ligands in cross coupling reactions. However when you should use it when using Pd(OAc)2 ? Pd(OAc)2 is 16 e- through the coordination of acetate to Pd as XL ligand. But, upon heating at temperature higher than 80 degrees C, decoordination of acetate ligand will occur, thus PPh3 is used to stabilize the Pd catalyst and help in the reduction of Pd(II) to the active form Pd(0).
Triphenyl phosphine reacts with alkyl halide &generates phosphonium complex which undergo inrection
Thanks, Mr. Nidal Saleh for you explanation. But, are you sure that will not affect the regio-selectivity of the product? I mean, that would not interfere in the clean formation of trans-alkene.
There are examples of Heck reactions that works also without ligand (ex. J. Org. Chem. 1976, 2, 265 and also the racemates obtained in Science 2012, 338, 1455). The ligand on the metal changes the electronic density on the metal itself and this can drive to different outcomes of the reation (e.g. different regioselectivity). One role of the ligand can be also to reduce Pd(II) to Pd(0) but this is still not completely clear (the same reduction can be carried out by the base, solvent, substrates, etc...).
Dear Ivan Franzoni, thanks for your answer. There are several literature reports where Heck coupling was carried out without using any phosphine/ligand, but still the issue is related to clean (pure trans) and efficient method. Off course the priority is the formation of clean trans product and for that the PPh3 is responsible or not? Will the PPh3 cause the formation of other undesired isomers along with better yield?
I don't think, according to my poor knowledge, that PPh3 interferes in the regioselectivity more than the base, and additives you use in your reaction setup. But the substituents on the stilbene system has a direct impact of course. To better understand the role of the base, additives, and substrates you should look to recent papers of grubbs where he was able to solve the provlem of cis, trans regioselectivity in metathesis reactiob. Jacs 2009, 2010,2011, and 2012. I will send you the whole reference soon
Dear Kumar, unfortunately from what I've seen in literature each case is special. There are many parameters that play toghether.
To answer to Nidal, I agree with you, the base in many cases changes completely the regioselectivity. I was not clear in my answer maybe but I meant that given a base and all the other parameters (also the nature of the substrate) the ligand can control the regioselectivy.
A Heck reaction can be carried out with Pd(OAc)2 alone. The requirements of the catalyst depends on the substrates you are using. In my experience an electron deficient ArI or ArBr can react with ligand free Pd. The usual problem with ligand free conditions is the Pd(0) can aggregate and form Pd black. The use of homeopathic palladium (0.02mol%) has been demonstrated by DeVris as a solution to reduce the formation of Pd black under ligand free conditions. There is also the option of using Jeffrey conditions where a tetraalkylammonium salt is used to stabilise the Pd and prevent aggregation. There is also the potential to use cationic Pd to control the selectivity of the reaction.
Solvent, ligand, base, Ar-X and alkene all have a significant influence on the selectivity of the reaction. If you are looking to react an acrylate the selectivity is fixed. With some styrenes the selectivity can vary and with electron rich alkenes it becomes more difficult.
To properly answer your question I would need to know what both coupling partners are and the product you desire.
Pd(II) in the form of Pd(OAc)2 etc needs to be reduced to the active species P(0) by the phosphine ligand (Butchwald bulky phosphine ligands serve better as they affect the reactivity of unreactive aryl halide) in Heck reaction (including most other Pd-catalyzed reactions) before the oxidative addition step.
Triphenylphosphine acts as a bulky ligand for variety of aryl cross couplings involving C-C and C-heteroatom bond forming reactions. It will enhance oxidative addition and the reductive elemination steps during the coupling reactions. .
D. Kumar Sir,
Several existing reports reveal that the the electronic property of PPh3 helps in accelerating the rate of reaction.