XXXV *. On the mechanism of the W( CO) ,-mediated C-S
bond cleavage reaction
Chi-Tat Ng
Department of Chemistry, The Chinese Universily of Hong Kong. Shatin, New Territories (Hong Kong)
and Tien-Yau Luh *
National Taiwan University, Taipei 10764 (Taiwan) (Received August 30th, 1990)
Ahstraet
The reaction of 2-naphthybnethyl mercaptan (I) with W(CO), in refluxing chlorobenzene was monitored by ‘H NMR. The formation of 2-methylnaphthalene (II) occurs via the hydrogen abstraction from the mercaptan (I) by the intermediate 2-naphthyhnethyl radical (VI) while IV is not formed directly from I but from the intermediate compound bis(2-naphthyImethyl)sulfide(III).
Metal carbonyls are useful in the cleavage of C-S bonds [l-6]. We recently reported that group 6 metal carbonyls are selective reagents for the reduction of C-S bonds [2,3] and for the desulfurdimerization of thioethers and dithioacetals [2,4-6). A free radical mechanism has been suggested on the basis of deuterium labelling experiments, complete racemization in the products using optically active substrates, isolation of disproportionation products, as well as novel fragmentation process of dithiolanes [2,4-61. It is noted that the reaction of W(CO), with benzylic mercaptans in refluxing chlorobenzene afforded not only reduced products but a significant amount of dimers as byproducts (eq. 1) 141.
ArCH,SH W(Co)a) A&H, + ArCH,CH,Ar (1)
The former product may be formed by hydrogen abstraction from the radical, and the latter by dimerization of the radical. It is well documented that the rate of the hydrogen abstraction reaction of a radical species is in the order of lo6 to lo7 M-’ s-l [7] and that of the coupling process of radical moieties is nearly diffusion
* For Part XXXIV see ref. 12.
b)
4.0 3.0 2.0 1.0 6
Fig. 1. ‘H NMR spectra of the reaction of (c)r=16h,(d)r=36h.
..i
.+&.ud& I.,’
4.0 3.0 2.0 1.0 6
I with W(CO), in chlorobenzene at (a) f = 0, (b) t = 6 h,
controlled [8]. These rate data suggest that the two pathways are competitive and it
is therefore highly desirable to determine at which stage the product is formed in
this W(CO),-mediated desulfurization reaction. In this paper, we describe a compli-
mentary investigation using ‘H NMR spectroscopy to monitor such a reaction to
further support the radical mechanism and provide a useful evidence on the actual
mode of the metal carbonyl-promoted desulfurization.
A mixture of 2-naphthylmethanethiol (I) and an equimolar amount of tungsten
hexacarbonyl in chlorobenzene was immersed in an oil bath preheated to 170°C.
(III). At t = 16 h (Fig. lc), I was almost consumed, and II and III were the only
organic products at this stage. Further reaction led to the disappearance of the
signal at 6 3.77 and appearance of a new singlet at 6 3.20 (Fig. Id). This new
absorption is due to the methylene group of IV. The ratio of II and IV in the final
mixture was 5 to 1.
I Ii
III IV
In order to show that thioether III was indeed the intermediate in this transfor-
mation, the reaction was stopped after 6 h of reflux and III was isolated in 3% yield.
In addition, the reaction of III under the same conditions afforded IV and II in 54%
and 15% yields, respectively *.
On the basis of these results, a plausible mechanism is proposed (Scheme 1).
Mercaptan I will first associate with the tungsten moiety to give a thiolato complex
V [9,10] in which the carbon-sulfur bond is activated [ll]. Homolytic cleavage of
the carbon-sulfur bond of V will generate the radical VI, which will rapidly abstract
the hydrogen atom from I yielding II and VII. The sulfur radical VII will then react
with tungsten carbonyl to give again a thiolato species similar to V which furnishes
the chain propagation process. Coupling of VI and VII will give III.
Our results indicate that the rate of the hydrogen abstraction by VI under the
reaction conditions would be much faster than that of the dimerization of VI as long
as a hydrogen source, or SH moiety, is available. Sulfide III can also undergo C-S
bond cleavage under the reaction conditions [2-41 and, therefore, the growth of the intensity of the peak at 6 3.77 was found to be slow.
* The origin of the hydrogen source for the formation of II is not clear. It may arise from the hydrogen abstraction from the benzylic hydrogen of III or the like by VI.
R-SH + W(CO), -+ RS-[W] (V) RS-[W] -+ R. + S-[W] (VI) R- +RS-H-+R-H+ RS. (VII) RS . + W(CO), + RS-[W] R. +RS. +RSR 2R.-RR Scheme 1. (R = 2-naphthylmethyl),
In conclusion, we have provided further evidence to support that W(CO),- mediated C-S bond cleavage occurs via a free radical mechanism. In the desulfuri- zation of mercaptans, the dimeric product is not from mercaptans but from thioethers generated in situ during the course of the reaction.
Experimental section
NMR study of the desurfurization of naphthylmethanethiol (I). A mixture of I (290
mg, 1.7 mmol) and tungsten hexacarbonyl(560 mg, 1.7 rmnol) in chlorobenzene (20
mL, freshly distilled from P,O,) was frozen in liquid nitrogen bath, evacuated and flushed with nitrogen. After warming to room temperature, the flask was then immersed in an oil bath preheated to 170°C. The mixture was heated under reflux and the color of the solution turn into yellow then black. Aliquots were withdrawn at different time intervals and subjected to ‘H NMR analysis on a JEOL PMX-60 NMR spectrometer. The results are shown in Fig. 1.
Isolation of III from the reaction of I with W(CO),. The reaction was carried out
as described above. A mixture of I (290 mg, 1.7 mmol) and tungsten hexacarbonyl (560 mg, 1.7 mmol) in chlorobenzene (20 mL) was heated under reflux for 6 h. After cooling to room temperature, chlorobenzene was removed by vacuum distillation and the residue was washed with diethyl ether. The organic solution was filtered and the filtrate was carefully evaporated to give a residue which was chromatographed on silica gel using hexane as eluent to yield II (60 mg, 25%), III (8 mg, 3%), and recovered I (142 mg, 48%). No IV was detected.
Acknowledgment
We thank the Croucher Foundation of Hong Kong and the National Science Council of the Republic of China for support.
References
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