DCM not best extraction solvent for wacker

[lab_bitch]

Contrary to popular belief on the hive, DCM is probably the worst solvent to use to extract the reaction contents of a wacker.  It dissolves nearly all of the PdCl2 along with the oils, creating a very messy workup.  This happens because the two chlorides on the DCM complex with the PdCl2 forming the soluble complex

                  H   Cl    Cl
                   \ /  \  /
                    C    Pd
                   / \  /  \
                  H   Cl    Cl

By using toluene or pet. ether to extract the mixture, all of the PdCl2 remains in the aqueous layer.  After the oil is removed, the remaining aqueous layer can be extracted with DCM to remove the PdCl2.  After stripping off the DCM, you should be left with reusable PdCl2.  I haven't stripped off the DCM yet, so I don't know what will be left, but it is definitely >90% of the PdCl2.  Whatever it is, this is probably the best way to go about recycling PdCl2.

[Shambhala]

Ok, let's say we get the PD out.  It's sitting on the shelf like happy PD just waiting to be resued.  Well, how many times could it be reused?  When should one consider using new PD?  Red Phos can be recharged/cleaned, right?  Would this be necessary or possible with PD?  Mabey I need to go re-read that old pd recovery thread whatcha think?
Alittle further now, "YOU CAN DO IT!"(WaterBoy?)

[lab_bitch]

I found out last night that toluene does dissolve all of the PdCl2 crap.  This is probably because of pi interactions b/t d-orbitals and the aromatic system.  It seems that pet. ether is the only way to go.  As to how long the catalyst can be used, all I have to say is use it until is stops working.  I've never actually reused it before, so I don't know.

[slappy]

Palladium, being a late transitional metal, does not like to form arene sandwich complexes. ⁶-C₆H₆ does not make a good ligand anyway, the Cyclopentadienyl anion is many orders of magnitude greater. There will not be any pi complexes between the Pd⁰ and Pd² present in the solution. On a somewhat unrelated note, the bond formed in Metallocene complexes shows pi, sigma, and delta character. The interaction is not exclusive to the d-orbitals, as the p-orbitals are also involved.

And regarding your PdCl₂(CH₂Cl₂) bridged species: While Pd² does have 5 coordination sites, you seem to be unaware that chlorine only forms 1 coordinate species. Besides the fact that attaching a chlorine to a metal is an oxidation, and Pd with 4 coordinated Cl's would be Pd⁴, another unknown species. CH₂Cl₂ is well known to be a non-coordinating solvent.

[dred]

You can get chlorine as a three electron ligand. This probably isnt the place for a general discourse but take the AlCl3 dimer for example. I have an organometallic complex upstairs. Off the top of my head it was Pt2Cl2Cp2 gives the correct electron count. Yeah, inorganic chemistry... liked that one, prefer organic, dunno all that much about biochemical. Oh well, back to the books for me.. catch up with you in a few years okay?

[lab_bitch]

PdCl2 in a solution containing Cl- ions forms the following dimer when present in certain concentrations:

                       Cl    Cl    Cl   -2
                         \  /  \  /
                          Pd    Pd
                         /  \  /  \
                       Cl    Cl    Cl

Why couldn't it form a similar complex with DCM?  Are the Cl-C bonds too strong?

You're probably right about the toluene, though.  I just made a wild guess.

[Chromic]

>Besides the fact that attaching a chlorine to a metal is an oxidation
>and Pd with 4 coordinated Cl's would be Pd4, another unknown species.

Palladium can be in the +4 oxidation state with all of the halides... although the +2 is far more common. (3,5,6 are also known but they are much less common)

Chloride anions can be a didentate or tridentate ligands as said before... PdCl2 is a perfect example of this behavior.. two forms exist for PdCl2, one is hexameric units (stable at high temp) and the other more common form is a polymeric form with square planar palladium (II) centers ...

[lab_bitch]

When PdCl2 is in aqueous or alcoholic solution, it has four chloride ligands, provided they are available.  This is how the wacker works.  I have a book on oxidations using palladium, and it describes the entire method of the oxidation.  The reactive complex is formed by displacement of one of the four chloride ligands with the alkene pi system.  Then, another chloride ligand is displaced by a solvent molecule, followed by deprotonation of the solvent molecule (alcohol or water).  This complex then rearranges to a sigma system by a cis transformation of the ligands and attack of the deprotonated solvent molecule on one of the two carbons.  The palladium, which is now bonded to the opposite to the attacked carbon, breaks off carrying an electron pair with it, leaving the ketone.


           Cl    Cl                     Cl    Cl
             \  /            C            \  /
              Pd       +     \\    =>   Pd         +  Cl-
             /  \              C          /  \ C
           Cl    Cl                     Cl    //
                                              C

           Cl    Cl                          ROH  Cl
             \  /                               \  /
              Pd       +     ROH    =>     Pd       +  Cl-
             /  \ C                             /  \ C
           Cl    //                           Cl    //
                 C                                 C


          ROH    Cl                         RO   Cl
             \  /                               \  /
              Pd       +     ROH    =>     Pd       +  ROH2+
             /  \ C                            /  \ C
           Cl    //                          Cl    //
                 C                                 C


           RO    Cl         Cl    Cl
             \  /                \  /
              Pd          =>   Pd     
             /  \  C            /  \ C
           Cl    //           RO    //
                 C                  C


           Cl    Cl                   Cl    Cl
             \  /                       \  /
              Pd      +   Cl-  =>    Pd       OR
             /  \  C                    /  \     /
           RO    //                   Cl    C-C
                 C                         
                                           

           Cl    Cl                             Cl    Cl
             \  /                                 \  / 
              Pd     OR   +   ROH    =>     Pd  
             /  \   /                             /  \ 
           Cl    C-C                           Cl    H


The Pd0 species is then reoxidized by CuCl2, which is in turn reoxidized by O2 or RONO.  I'm not sure if the last step is right.  I'll have to check in the book.

[slappy]

When PdCl2 is in aqueous or alcoholic solution, it has four chloride ligands, provided they are available.  This is how the wacker works.I have a book on oxidations using palladium, and it describes the entire method of the oxidation.  The reactive complex is formed by displacement of one of the four chloride ligands with the alkene pi system.  Then, another chloride ligand is displaced by a solvent molecule, followed by deprotonation.  This complex then rearranges to a sigma system and oxidatively decomposes.

No, no. This is not how the Wacker oxidation works. There are three steps in the catalysis cycle, Oxidative addition, Crossmetallation, and Reductive elimination. When you have your PdCl₂, it oxidativly adds the alkene, then a -OH to the metal center. Next, it couples the -OH to the alkene, adds a molecule of water, and gives off a mol of HCl. Then it reductivly eliminates the alcohol as the ketone and another mol of HCl. You are left with Pd⁰, which is re-oxidised by the CuCl₂, which in turn is re-oxidised by the HCl released, forming a mini cycle.

I didn't make myself clear earlier about chlorine. There is only two ways that Chlorine will be bidentate or polydentate. Chlorine will form very weak coordination complexes with very electropositive atoms. Alkali metal ions, Hydrogen, early transitional metals, lower main group elements such as Sn, etc. The other is in hypervalent complexes, like ClF₃ or ClO₄. In order to form hypervalent species, the ligands have to be more electronegative than Chlorine itself.

[lab_bitch]

Did you read this in literature?  If so, where?  I'll post my reference soon.

[Ritter]

From my experience, there is one simple way to recover the Pd from an alcoholic/O2 wacker.  After the reaction, allow mixture to sit for several hours.  During this time, much Pd(0) and PdCL2 will simply settle to the bottom of the reactor.  Use a big syringe to suck up the supernatant leaving the Pd precipitate undisturbed.  Rotovap off ALL alcohol then break up resulting crude ketone into several  20ml alliquots and centrifuge at 2-3000 rpm for .5h.  At the end of centrifigation, a nice brown ppt will be found caked at the bottom of the centrifuge tubes  concicting of the rest of your palladium and some copper compounds. This process works fine especially when dealing with more than 5g PdCl2 and SWIM has used it several times. 

We REALLY need an effective chelation process for recovering our precious catalyst.  It would be so nice if we could just dump in some EDTA and filter off the chelate.  This topic has been hashed over a million times.  Anyone ever put any of that info to practical usage?

[lab_bitch]

When PdCl2 is in aqueous or alcoholic solution, it has four chloride ligands, provided they are available.  This is how the wacker works.  I have a book on oxidations using palladium, and it describes the entire method of the oxidation.  The reactive complex is formed by displacement of one of the four chloride ligands with the alkene pi system.  Then, another chloride ligand is displaced by a solvent molecule, followed by deprotonation of the solvent molecule (alcohol or water).  This complex then rearranges to a sigma system by a cis transformation of the ligands and attack of the deprotonated solvent molecule on one of the two carbons.  The palladium, which is now bonded to the opposite to the attacked carbon, breaks off carrying an electron pair with it, leaving the ketone.


           Cl    Cl                     Cl    Cl
             \  /            C            \  /
              Pd       +     \\    =>   Pd         +  Cl-
             /  \              C          /  \ C
           Cl    Cl                     Cl    //
                                              C

           Cl    Cl                          ROH  Cl
             \  /                               \  /
              Pd       +     ROH    =>     Pd       +  Cl-
             /  \ C                             /  \ C
           Cl    //                           Cl    //
                 C                                 C


          ROH    Cl                         RO   Cl
             \  /                               \  /
              Pd       +     ROH    =>     Pd       +  ROH2+
             /  \ C                            /  \ C
           Cl    //                          Cl    //
                 C                                 C


           RO    Cl         Cl    Cl
             \  /                \  /
              Pd          =>   Pd     
             /  \  C            /  \ C
           Cl    //           RO    //
                 C                  C


           Cl    Cl                   Cl   HOR
             \  /                       \  /
              Pd      +   ROH  =>   Pd       OR
             /  \  C                    /  \     /
           RO    //                   Cl    C-C
                 C                         
                                           

           Cl     HOR                     Cl    H
             \  /                            \  / 
              Pd     OR         =>       Pd  
             /  \    /                        /  \   C-OR
           Cl    C-C                      Cl     //
                                                  C


           Cl     H                       Cl    H
             \  /                           \  / 
              Pd        +  2ROH =>     Pd      OR   +  ROH2+
             /  \ C-OR                   /  \    /
           Cl    //                      Cl    C-C
                 C                                  \
                                                       OR


           Cl     H                                       OR
             \  /                                         /
              Pd       OR  =>     PdCl2+2 +  C-C
             /  \     /                                   \
           Cl    C-C                                      OR
                      \                            
                       OR

[lab_bitch]

I just discovered that pet. ether isn't even miscible with the oils left from the rxn.  Toluene, on the other hand, dissolves all the black crap.  Therefore, I found that the best way to extract the rxn is to first extract it with toluene and then slowly add pet. ether to crash out all of the black crap.  This black crap (Pd containing substance) will coat all of your flasks and make a complete mess.  Don't worry, though.  A little DCM removes all of the black crap.  My guess is that if you evaporate the DCM, heat the residue to red heat, and then dissolve in aqua regia, you'll recover your PdCl2.

Once the black crap is precipitated, toluene won't touch it, even though it dissolved just fine when it was mixed with the oil.  This leads me to believe that the black oil at the end of a rxn is a Pd-safrole complex.  This makes sense, b/c after you add the safrole to the catalyst/oxidizer solution at the beginning of the rxn, it turns black and starts reacting.  Anyway, this complex is soluble in toluene, but not pet. ether.  When pet. ether is added the the toluene/oil, it causes the equilibrium of complex formation to shift way to the left, resulting in precipitated Pd and dissolved oil.  This is only based on my observations, not literature.  Also, if the rxn goes to completion, you might not even have this problem, since no Pd-safrole complex should be present.