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Thermal Decomposition of Cyclobutanone 6271 tetramethylene biradicals which have been assumed to be intermediates in the photolysis of cyclopentanone.5•6 The formation of butene and ethylene from the tetramethylene biradical has been observed by Bawn and Milsted30 in their study of the reaction of 1,4-dibromobutane and sodium at 300°.In spite of the data which are available the com-(30) C. E, H, Bawn and J. Milsted, Trans. Faraday Soc., 35, 889 (1939).plexity of the cyclopentanone decomposition precludes a thorough consideration of the detailed mechanism at the present time.Acknowledgment.-The authors wish to thank Mr. Carl Whiteman for making the infrared measurements and Dr. F. L. Mohler of the National Bureau of Standards for the mass spectrometric analyses.

Many organoboron compounds possess properties which suggest theThis is discussed, presence in these compounds of carbon-boron x-bonding. and additional spectroscopic evidence is presented.x-BONDING between carbon and boron has frequently been postulated to explain the properties of certain organoboron compounds. a@-Unsaturated boronic esters and acids, for example, possess chemical and spectral properties strongly suggesting such bonding, a hypothesis in this instance supported by simple molecular orbital ca1culations.l We now supply additional experimental evidence for carbon-boron x-bonding, and discuss some earlier postulates of this bonding.Evidence from Chemical Bekaviowr.-Carbon tetrachloride adds to dibutyl vinylboronate to form dibutyl 1,3,3,3-tetrachloropropylboronate; the low value for the first transfer constant (3.3 & 0.7 x is as expected if the intermediate radical is stabilized by electron delocalization involving the carbon-boron T;-bonding, viz.,The relative stability towards disproportionation of vinyl-and propenyl-alkylboron compounds provides further chemical evidence for carbon-boron x-bonding2 Unless steric effects are a determining factor, as in large and branched-chain alkyl groups, unsymmetrical triorganoboron compounds (e.g., BMe,Et) are believed to have only a transient existence at ambient temperatures, disproportionating into the symmetrical triorganoboron compounds. These disproportionations undoubtedly proceed via a fourcentre mechanism involving the p,-orbital of boron. Clearly, if this +,-orbital is involved in internal dative bonding, as is believed to occur in alkoxy-and amino-boranes and might occur in alkenylalkylboron compounds,2 the tendency of the boron compound to disproportionate will be greatly reduced. Ritter and his co-workers found that dimethylvinylborane decomposes only to the extent of about 33% after storage at room temperature for about 5 days. Similarly, methyldivinylborane can be heated to 100" for short periods without significant decomposition. The stability of these unsymmetrical boron compounds contrasts with the transient existence of such species as BMeEt,, supporting the suggestion that mesomerism in alkenylboron derivatives restricts the boron atoms to a planar configuration and therefore inhibits disproportionation.The stability of the perfluorovinylboron compounds is also intere~ting.~ These are the first covalent organoboron compounds to contain a fully fluorinated organic group bonded to boron. Organoboron compounds having halogen atoms in the organic groups tend to decompose to form boron trihalides. Thus fluoromethylboron difluoride readily yields boron trifluoride,* and although 2-chloroethylboron compounds have not yet been obtained pure there is clear evidence that they are ~n s t a b l e . ~ It has long been held that a boron compound in which a highly fluorinated organic group is attached to boron would tend to decompose with formation of substances having boron-fluorine bonds. Thus, reaction between bispentafluoroethylmercury and boron trichl...

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