1886
Gas-Phase Hydroboration of Allene, Other Olefins, and Acetylenes Hans Hasso Lindner and Thomas Onak Contribution f r o m the Department of Chemistry, California State College at Los Angeles, Los Angeles, California 90032. Received September 3, 1965 Abstract: Terminal addition of diborane to allene results in the formation of the cyclic 1,24rimethylenediborane. Polymerization of this compound occurs rapidly in liquid phase and is attributed, in part, to release of ring strain. Poly(p-trimethy1ene)diborane depolymerizes quantitatively at 60" in vacuo, thus establishing the reversibility of the reaction n-B2C3HL0 [(CH2)$-B2H4],which probably proceeds via m.sbile bridge hydrogens. The gas-phase hydroboration of simple monoolefins is shown to be a general method for the synthesis of mono- and 1,l-dialkyldiboranes in yields of 55-65 and 30-40%, respectively, when carried out at 60-65" using a 5 : 1 diborane-olefin molar ratio, Nonvolatile organoboron polymers, the structure of which is discussed in the case of 2-butvne. are
formed nearly quantitatively by gas-phase hydroboration of acetylenes with excess diborane; in addition, small fractions of alkyldiboranes are isolated.
0
lefin hydroboration has been studied extensively in the liquid phase and in solvent systems, 1-4 but little in the gas phase.5 I n previous studies, hyboration was allowed to proceed to the trialkylborane stage without deliberate attempts to isolate the intermediate alkyldiboranes in preparative yields. Because alkyldiboranes are customarily prepared by equilibration of trialkylboranes with d i b ~ r a n e , ~itJ was anticipated that these compounds could be prepared directly from monoolefins and diborane if a sufficiently high molar ratio of B2H6to olefin is used. Weiss, et al., have shown the gas-phase reaction of 1,3-butadiene with diborane in the presence of Hz to result in the formation of cyclic organoboranes.* By analogy, it is not unreasonable to expect 1,2-trimethylenediborane to form by 1',3' addition of diborane to allene. However, the work of Devaprabhakara and Gardner indicate that the dihydroboration of allenes in solvents results in the preferential addition of the boron atoms to the central carbon atom.Q Since solvent and gas-phase reactions frequently lead to different products, as is clearly demonstrated in the 1,3-butadiene dihydroboration reaction, *A lo- l 4 the gas-phase allene dihydroboration reaction was carried out with some expectation that 1 ',3 ' diaddition would occur. Though there are apparently n o reports about the isolation of organoboranes obtained from dihydroboration of acetylenes, these products are generally con(1) H. C. Brown, "Hydroboration," W. A. Benjamin, Inc., New York, N. Y.,1962. (2) G. Zweifel and H. C. Brown, J . Am. Chem. Soc., 85, 2066 (1963). (3) R. Koester and G. Griasnow, Angew. Chem., 73, 171 (1961). (4) R. Koester, G. Griasnow, W. Larbig, and P. Binger, Ann. Chem., 672, l(1964). ( 5 ) D. T. Hurd, J . Am. Chem. SOC.,70, 2053 (1948). (6) H. I. Schlesinger and A. 0. Walker, ibid., 57, 621 (1935). (7) H. 1. Schlesinger, L. Horwitz, and A. B. Burg, ibid., 58, 407 ( 1936). (8) H. G. Weiss, W. J. Lehmann, and I. Shapiro, ibid., 84, 3840 (1962); Technical Research Report No. 225, Contract A F 33 (600)33920, Olin Mathieson Chemical Corp., Feb 1960. (9) D. Devaprabhakara and P. D. Gardner, J . Am. Chem. Soc., 85, 1458 (1963). (10) R. Koester, Angew. Chem., 71, 520 (1959). (11) R. Koester, ibid., 72, 626 (1960). (12) G. Zweifel, I
