Polyhedron Vol. 12, No. 9, pp. IIOI-1102, 1993 Printed in Great Britain

0277-5387/93 $6X43+ $4 0 1993 Pergmon Press Ltd

COMMUNICATION

A RAPID COBALT CARBONYL MEDIATED CLEAVAGE OF BOTH PHOSPHORUS-CARBON BONDS IN

BIS(DIPHENYLPHOSPHINO)METHANE UNDER VERY MILD CONDITIONS: EXTRUSION OF METHYLENE AS

KETENE

SHAMSUDDIN AHMED, DAVID G. HOLAH, ALAN N. HUGHES* and ROBERT T. MARKEWICH

Department of Chemistry, Lakehead University, Thunder Bay, Ontario, Canada P7B 5E1

(Received 29 July 1992 ; accepted 5 February 1993)

Abstract-In cobalt(II)/{bis(diphenylphosphino)methane}/NaBH,/CO reactions, in which the NaBH, is added very rapidly, both P-C bonds of the phosphine are cleaved with extrusion of the methylene group as ketene.

It has been established’ that the ease of transition- metal mediated phosphorus-carbon bond cleavage in tertiary phosphines is in the order P-C, > P-C,2 > P-C,3 and, therefore, the great majority of such reactions in the literature involve phosphorus bound to sp or sp2 carbon.2 While P- C,, cleavage is the least common and normally requires more forcing conditions, several recent reports of this type of bond scission in bis(di- phenylphosphino)methane (dppm) in the presence of di-iron,3 platinum,4 cobalt’ or nickel’ centres under comparatively mild conditions have appeared. These reactions normally result in the formation of products containing both the Ph2P- and Ph2PCH2- fragments (or derivatives thereof) arising from a single P-C cleavage.3as4 However, products containing bridging Ph,P- groups as in 1 and 2, but with no trace of the Ph,PCH,- fragment, have also been reported ;’ there is one very recent report of a product containing both Ph*P- fragments arising from a double P-C clea- vage. 3b In this last reaction (described as being unprecedented) [Fe2(CO)6(@XIC02Et)@-dppm)] under reflux in toluene generates3b ethyl acrylate

*Author to whom correspondence should be addressed.

and [Fe2(CO)&PPh2)J, probably via the for- mation of an Fe,@-CH,) unit followed by the intra- molecular linking of the -CH2-- and -CHC02Et bridges in the manner observed6 for some other di- palkylidene species. We report here evidence for another such double scission which occurs very rap- idly under extraordinarily mild conditions and with the extrusion of methylene as ketene.

Examination of the filtrates obtained from reac- tions in which NaBH, is added very rapidly (< 1 min) to cobalt(I1) and nickel(I1) salts in ethanol/ benzene under CO in the presence of dppm at room temperature (or below), giving’ systems of types 1 and 2, showed no traces of Ph,PMe or

/C”2\

I I

(1) (2)

any other derivative of the Ph2PCH2- fragment. However, filtrates obtained from the cobalt(I1) sys- tem (the cleanest of these reactions) always showed

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(GLC, MS)* the presence of ethyl acetate. Similar reactions performed in the absence of dppm showed no evidence of ethyl acetate formation. The -CH,- bridge in dppm is therefore extruded, together with CO, as ketene which then reacts7 with the ethanol in the solvent mixture to give ethyl acetate with the entire reaction, starting from co- balt(II), being complete iri ca 1 min at room tem- perature.

Clearly, there are impor nt differences between the di-iron case3 noted a 6” dve and the cobalt(I1) reactions leading to 1 in that the final product in the di-iron case contains two bridging phosphido

* In a typical experiment, s solution of CoCl, - 6H20 (0.22 g) and dppm (0.70 g) in degassed EtOH/&H, (1: 1, 4 cm’) was saturated with COifor 5 min and solid NaBH, (0.10 g) was added in one portion. After 1 min the mixture was filtered to remove 1 and some [Co,(p-dppm)z(CO),] and the filtrate was distilled to low volume. The metal- and phosphine-free distillate has examined by GLC on both Poropak Q and Chromosorb 101 columns. In each case ethyl acetate (not present in either of the solvents used for the reaction and not formed if dppm is absent from the reaction mixture) was detected. The retention times observed for each stationary phase were found to be identical to those observed for a standard sample of 1: 1 EtOH/C,H, containing 0.1% ethyl acetate, which, given the quantities of reagents and solvents used and the measured5 yield of 1, is representative of the low concentration which should , and was, observed. Ethyl acetate formation was co & ed by low-voltage elec- tron-impact mass spectrome+ic studies on the distillate. At 14 eV a weak peak assigned to the low concentration of ethyl acetate was observed at m/z 88. This peak de- creased in intensity with incr

e asing voltage, while a new

peak at m/z 60 (M -CzH4, ~McLafferty elimination of ethylene) grew. This trend is cIearer for the solvent system THF/C6Hs, which, howevej, is not suitable for GLC studies since THF and ethyl acetate have similar retention times.

units, while for 1 the second bridging species is hydride. Also, in the di-iron case the reaction requires more rigorous conditions (reflux) than per- tain in the cobalt case (room temperature) and it is possible that the two reactions are mechanistically quite different. Mechanistic studies are in progress and the results of these studies will be the subject of a separate report.

Acknowledgement-We thank the Natural Sciences and Engineering Research Council of Canada for financial support.

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