An Undergraduate Experiment in Homogeneous Catalysis Synthesis of Phenylethoxycarbene Tungsten Pentacarbonyl and Polymerization of Norbornene and Phenylacetylene Dider Villemin Unite Associee 403 C.N.R.S., Ecole Nationale Superieure de Chimie de Paris. 11 rue P. et M.%urie, 75231 Paris Cedex 05-France One of the most interesting developments in transition organometallic chemistly and catalysis during the last decade has been the recognition that metal-carbene complexes ( I ) play an important role in a large variety of different major catalytic processes such as olefin metathesis, cycloolefin ring opening polymerization, the Fischer-Tropsch process, alkyne polymerization to polyenes, alkane metal reactions, and alkane oxidations. Metal carbenes are highly reactive species. However, with a good choice of metal and ligands, and with heteroatoms bound to the carbene, they become less reactive. The first metal-earbene complex was isolated by E. 0. Fischer and A. Maashol(2) in 1964 with the preparation of phenylmethoxycarbene chromium pentacarbonvl. Manv other carhenes of chronium, molybdenum, munganese,and rungsten wrre thereafter prepared using the same methodology 111. The novelty and the relatively common occurrence of these species crrtainly justify a mention ~ in an undergraduate course. However, the designing I I lib oratory experiments to illustrate the chemistry of metal carbenes represents a challenging problem. We have chosen the nhenvlethoxvcarbene tunesten ~entacarbonvl.a Fischertype carhene. Tungsten ht:xucarhonyl was preferred to other metals for its price, toxicity, srahility, and catalytic activity.
metal complexes (e.g., Cr, Mo, Mn) and other organolithium compounds. From an organic point of view the synthesis can be considered as a nucleophilic addition of the phenylcarhahion to the complexed carhonyl carbon. The resulting acylate is very close to a pseudoenolate. I t can he alkylated wit. a hard electrophile (oxonium fluoroborate or diazomethane) at the hard nucleophile site (oxygen) (3).
Synthesis of the Phenylethoxycarbene Tungsten Pentacarbonyl Tungsten hexacarbonyl is transformed into acylate with phenyllithium ( I ) , and the acylate is then alkylated with triethyloxonium fluoroborate (2). This type of synthesis, discovered by Fischer f2), can he extended to other carhonyl
enolate:
. -
-
hard
soft
hard
soft
pseudoenolate
'I't~ngsten hexncarbonyl'.- phenyllithitun.' and triethyluxmitun f l u u r ~ h o r a t e : ~ ~commercinllv arr avuilnhle. hut the latter two are expensive. Phenyllithium ib prepared'in ether from bromobenzene and lithium under argon (3). Phenyllithium (1mL of solution) is titrated with a 0.1 N solution of benzhydrol(18.424 g/L) in toluene with 2,2'-biquinoleineaor 1,3-diphenylacetone p tosylhydrazonebs indicator (4). Triethyloxonium fluoroborate is prepared from epichlorhy-
'
0
.&-magnetic stlrrer Figure 1. Preparation of phenylethoxycarbene tungsten psntacarbanyl.
Caution: All these compounds are toxic and must be manipulated under a hood. Alkylating agents in general are extremely toxic. Et30 + BF,- is poorly volatile and less dangerous than classical alkylating agents. Gloves should be worn when handling Et30+BF,-. The latter is also extremely hygroscopic. Available from Merck. Fluka. Available from Aldrich. Volume 64
Number 2
February 1987
183
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