Lecture 2a - chem.ucla.edu

Lecture 2a - chem.ucla.edu

Lecture 2b Triphenylphosphine Introduction I Phosphines are frequently used as ligands in metalorganic and organometallic compounds because they can be tuned in terms of their steric and their electronic properties (see later lecture) Asymmetric phosphines are chiral because of their high barrier of inversion (DG> 130 kJ/mol) compared to amines DIPAMP: Synthesis of L-DOPA BINAP: Rhodium or ruthenium complexes are used in asymmetric hydrogenation Grubbs Catalyst (RuCl2(=CHPh)(PCy3)2): Olefin metathesis Crabtree Catalyst ([(COD)Ir(py)(PCy3)]+PF6- ) homogeneous hydrogenation Examples containing PPh3 as ligand: Vaskas Complex: Ir(CO)Cl(PPh3)2 Wilkinsons Catalyst: RhCl(PPh3)3 Tetrakis(triphenylphosphine) palladium(0): Pd(PPh3)4 Strykers Reagent: [(PPh3)CuH]6 Wittig Reagents: Ph3P=CHR+X- OCH3 P P OCH3 (R,R)-DIPAMP PPh2 (R)

(R)-BINAP PPh2 Introduction II Triphenylphosphine can be synthesize by reaction of Phosphorus trichloride with chlorobenzene and sodium metal (industrial process but not feasible for this course) Phosphorus trichloride with organometallic compound like phenyl lithium or phenylmagnesium bromide (Grignard reagent) Br 3 + 3 Mg Et2O MgBr 3 PCl3 PPh3 + 3 MgBrCl Triphenylphosphine is a white solid (m.p.: 80 oC, b.p.: 377 oC) Triphenylphosphine is slowly oxidized in air at room temperature to form triphenylphosphine oxide (Ph3P=O), which can be removed by recrystallization from isopropanol or ethanol The molecule displays a trigonal pyramidal structure with a high (DG(inversion)= >130 kJ/mol) Experimental I Assemble the setup as shown on the right side and previously discussed Hints: If the flask still contains a white solid, it has to be treated with diluted sulfuric acid, water and acetone The addition funnel has to be checked for leaks at the stopcock before assembling the setup

The water-jacketed condenser should not be connected to the water outlet The apparatus should be clamped at the center neck using a clamp that is appropriate for the neck size of the flask Make sure that there is no dirt or Mg-turnings stuck inside the joints when charging the flask After adding the crushed Mg-turnings to the three-necked flask, the setup is then flame-dried twice before the addition funnel is charged with the ethereal bromobenzene solution Experimental II Place the dry diethyl ether in the addition funnel Add the bromobenzene and mix well Attach the water hoses to the Liebig condenser and turn the water flow on to cool the condenser Where does the dry diethyl ether come from? Add about 5 mL of the solution to the Mgturnings To reduce for the formation Why is so little added only? of biphenyl From the solvent still Where does the water enter the The water enters the condenser on condenser? the lower end of the Liebig condenser

What should be observed here? 1. By heating How the reaction initiated? 2. can Addition of a fewbeiodine crystals After the addition is completed, gently reflux the mixture out The mixture has to maintain a How fastboil should the solution be added? gentle in Experimental III Assay of the Grignard solution Remove a 2 mL aliquot of the solution with a pipette and add it to water What happens here? The Grignard is hydrolyzed and hydroxide ions are formed PhMgBr + H2O Add 2-3 drops of phenolphthalein Ph-H + Mg(OH)Br

What should the change studentfrom observe The color should pink (or red here? depending on how much indicator was added) Titrate the solution with dilute hydrochloric acid until a permanent color change is observed How dilute hydrochloric DiluteisHCl (1:99): ~0.125 M acid obtained? Assuming that the flask contains 60 mL of solution, calculate the amount of PCl3 needed for the reaction In order to have an excess of Grignard reagent, a ratio of PCl3:PhMgBr= 1:3.5 is used inratio the experiment to reduce thefor? Which is the student aiming formation of Ph3-xPClx Conc. HCl: ~12.5 M

Experimental IV Dissolve PCl3 in diethyl ether in the addition funnel Add the solution slowly to a chilled solution of the Grignard reagent while stirring rapidly After stirring for about 30 minutes, water and conc. hydrochloric acid are added (in this sequence) What is the protocol here? Add the diethyl ether before the PCl3 (r=1.574 g/cm3) Why is the slow addition necessary? The reaction is exothermic due to the formation of the Mg-salts that are insoluble in ether What does the addition of water and hydrochloric acid do? 1. Water quenches the excess of the Grignard reagent 2. Hydrochloric acid dissolves the magnesium salts Experimental V Separate the layers How is this accomplish here? Separatory funnel Extract the aqueous layer with diethyl ether Dry the combined organic layers over anhydrous sodium sulfate

What theaprotocol here? of drying Start is with small amount agent Remove the diethyl ether by distillation under nitrogen Remove other byproducts (i.e., biphenyl, chlorophosphines) by distillation under nitrogen (Do not use a column here!) Recrystallize the remaining oil/solid from hot ethanol Why is the distillation performed To suppress the oxidation of PPh3 under nitrogen? Tmax: ~280 oC to remove o PhPClis2 (b.p.: 240temperature C) and What the final here? biphenyl (b.p.: 256 oC) Diphenylchlorophosphine (b.p.: 326 oC) Triphenylphosphine oxide (b.p.: 360 oC) Which compound is removed here? Characterization I Melting point

Infrared spectrum nas(PC)=499, 513 cm-1 ns(PC)=422, 432 cm-1 Mass spectrum m/z=262 (C18H15P) m/z=183 (C12H8P) m/z=108 (C6H5P) P Characterization II 13 C-NMR spectrum (162 MHz) Need to consider the 31P-13C coupling: J1=12.1 Hz, J2=19.6 Hz, J3=5.6 Hz, J4= ~0 Hz 120 115 134.09 134.21 110 105 128.75 128.79 128.83 C2 4 5 3

100 95 90 C3 85 6 2 C4 1 P 80 19 75 70 14 7 18 65 60 17 55 15 9 8

10 13 11 12 16 50 45 40 35 C1 30 25 20 15 137.97 138.05 10 5 0 139 138 137 136 135 134

133 132 131 130 129 128 127 126 125 Characterization III 31 P-NMR (I=, Abundance= ~100 %) Characterization IV P-NMR (part II) 31 The reactant (PCl3) and the product (PPh3) display significantly different chemical shifts in the 31PNMR spectrum, and so do the possible intermediates The phosphorus atom is very deshielded in PCl3 because of the electronegativity of the chlorine atoms The ligand exchange (Cl to Ph) causes a shift by ~80 ppm upfield for each

Compound d(ppm) PCl3 220 PPhCl2 162 PPh2Cl 81 PPh3 ~ -5 POPh3 26 PPh4Br 23.3 85 % H3PO4 0

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