Phenylmercury(II) Chloride1

PhHgCl

[100-56-1]  · C6H5ClHg  · Phenylmercury(II) Chloride  · (MW 313.15)

(phenylating agent for allylic halides3 and alkenes;4-8 converts primary and secondary allylic alcohols into 3-phenyl alcohols and 3-phenyl aldehydes;9 reacts with carbon monoxide to give ketone10-13)

Physical Data: mp 250-252 °C. 1H NMR:16,17 d (DMSO, ppm) Ho 7.46 (3JHo-Hg = +203 Hz); Hm 7.37 (4JHm-Hg = +49 Hz); Hp 7.46. 13C NMR:18,19 d (DMF + (CD3)2CO (20%) at 340 K (ppm)) Ca 150.5, (1JHg-C = 2530 Hz), Cb 136.4 (2JHg-C = 117 Hz); Cg 128.0 (3JHg-C = 205 Hz); Cd 127.9 (4JHg-C = 35 Hz). 199Hg NMR:18 (CH2Cl2) -1192, ((CD3)2SO) -1182 ppm. MS:20 mass of ions observed: C6H5HgCl+&bdot;, C6H5Hg+, HgCl+. IR: n(Hg-Cl) 331-332 cm-1.15

Solubility: sparingly sol H2O; sol C6H6, ether, pyridine; slightly sol hot ethanol.2

Form Supplied in: white satiny leaflets; stable in air; widely available.

Preparative Methods: by mercuration of benzene21 with Mercury(II) Chloride (eq 1) or red Mercury(II) Oxide.

Copper or Copper(I) Chloride22 promotes facile displacement of nitrogen from double salts of Benzenediazonium Chloride and mercury(II) chloride in good yield (eq 2).

Phenylmercury(II) chloride can also be prepared by the Peters reaction23 involving mercury(II) chloride and sulfinic acid (eq 3), by the decomposition of the phenyl-tin bond in PhSn(alkyl)3 with mercury(II) chloride (eq 4),24 by mixing equimolar mercury(II) chloride and Diphenylmercury (eq 5),25 or by photolysis of diphenylmercury in CCl4 (eq 6).26

In PhHgCl the Ph-Hg bond is less polar than the Hg-Cl bond. The Ph-Hg bond is weak and undergoes homolytic cleavage.

Handling, Storage, and Precautions: highly toxic irritant; LD50 orally in rats 60 mg kg-1. Use in a fume hood.

Coupling Reactions.

PhHgCl undergoes dimerization27 to biphenyl in the presence of 10% Palladium(II) Chloride and an excess of copper in pyridine solution at 115 °C. The disadvantage of using copper, pyridine, and a high temperature is overcome by replacing the catalyst by 0.5% [ClRh(CO2)2]2 (eq 7).28

PhHgCl undergoes disproportionation on halogenation29 (eq 8) and when heated in sulfolane with sulfur gives diphenyl sulfide (eq 9).30

PhHgCl undergoes oxidation to phenol31 by first treating it with Borane-Tetrahydrofuran and then oxidizing the resulting phenylborane with alkaline Hydrogen Peroxide (eq 10). It also undergoes transmetalation with borane halide to give aryl borates (eq 11).32

Replacement of Counterion.

The chloro group is replaced easily on treatment with LiCCl2CH3 in THF or with acidic hydrocarbon under basic conditions (eqs 12-14).33 Replacement of the chloro group by acetate, nitrate, sulfate or perchlorate may be effected using silver salts34 or Group 14 metals35 (e.g. Ge or Sn) (eqs 15 and 16).

Nucleophilic Phenylation.

PhHgCl couples with allylic halides in the presence of a catalytic amount of PdCl2/Lithium Chloride to give allylbenzene (eq 17). The yields are better when the catalyst used is a mixture of 10-30% (PdCl2/LiCl) and 10-30% Copper(II) Chloride (yield 31-87%).3 The reaction proceeds at below rt with the transposition of the double bond of the allylic chloride.

Alkenes undergo a PdCl2-catalyzed addition-elimination reaction with PhHgCl in a wide variety of polar organic solvents and in the presence of O2 and H2O to give vinylic hydrogen-substituted products (eqs 18 and 19).4,5

The direction of addition of PhPdCl is determined more by steric effects than electronic effects, the PdCl adding to the carbon with least hydrogens.6,7 HPdCl elimination can occur in more than one direction, leading to mixtures of regioisomers as well as stereoisomers. The reaction can be catalytic in palladium if a reoxidant such as CuCl2 is added. When the catalyst PdCl2 is used along with 2M CuCl2, 2-phenylalkyl chlorides are produced instead.8 The reaction is valuable for the synthesis of 1-halocarbonyl compounds (eqs 20 and 21).

The reaction of primary and secondary allylic alcohols takes an entirely different course and produces 3-phenyl aldehydes and ketones, respectively (eq 22).9 The reaction of enol esters provides 2-phenyl aldehydes and ketones.36

PhHgCl with conjugated dienes in the presence of LiCl/PdCl2 gives a p-allyl palladium compound (1)37 by palladium hydride rearrangement. The p-allyl palladium compound on decomposition with a tertiary or secondary amine gives a phenylated diene or phenylated secondary amine,38 respectively.

Direct carbonylation of PhHgCl by CO requires high temperature and pressure.10-13 PdCl2 promotes carbonylation to give PhCOCl in MeCN (eq 23), esters in alcohol, and anhydride in the presence of PdOAc.

PhHgCl reacts with CO under pressure (50 psi) in the presence of PdCl2 to give Ph2CO.39 Rhodium(I) and rhodium(III) complexes give better yields at atmospheric pressure. Reaction with Tetracarbonylnickel40 gives considerably higher yields of Ph2CO.

PhHgCl undergoes acylation to give the corresponding ketones (eq 24).41 Ketenes also react with PhHgCl to give ketones (eq 25).42,43

PhHgCl forms weak complexes with bidentate ligands. The latter undergo complete or partial disproportionation in boiling benzene solution (eq 26).44,45

Related Reagents.

Phenylthallium Bis(trifluoroacetate).


1. (a) Makarova, L. G.; Nesmeyanov, A. N. In Methods of Elemento-organic Chemistry; Nesmeyanov, A. N.; Kocheshkov, K. A., Eds.; North-Holland: Amsterdam, 1967; Vol. 4. (b) Bloodworth, A. J. In The Chemistry of Mercury; McAuliffe, C. A., Ed.; Macmillan: London, 1977. (c) Larock, R. C. J. Organomet. Chem. Libr. 1976, 1, 257. (d) Wardell, J. L. In Comprehensive Organometallic Chemistry; Wilkinson, G., Ed.; Pergamon: Oxford, 1984; Vol. 2, p 863. (e) Staub, H.; Zeller, K. P.; Leditschke, H. MOC 1974, 13/2b.
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Rajinder N. Khanna & Prem K. Singh

Delhi University, India



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