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Summary The CNDO calculations of XCO and XCN indicate that the currently observed CN force constant increase on CN- coordination is primarily a result of reduced C-N repulsion via charge withdrawal from carbon. For the comparison CO + XCO, however, the covalent binding energy change is primarily responsible for the change in CO force constant. For moderately backbonding substituents, the parent Q and P binding changes, as measured by bond orders and diatom binding energies, are of comparable importance. Generally speaking, within either series of “adducts,” variation in CN and CO binding energy seems to be primarily a result of changes in C-N and C-0 P binding, although
the CNDO method appears to overestimate the importance of P changes, and this prevents us from placing the importance of Q changes in a completely subjugated position relative to the P changes. That both CN and CO do tend to have increased Q bond orders as a result of coordination has been traced to polarization of the nitrogen and oxygen lone-pair electron Q. Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research and to Kansas State University for its support through the Bureau of General Research and the Faculty Fellowship programs.
Diboranes Derived from the Hydroboration of 1,3-Butadiene. Structures, Hydrogen-Bridge Cleavage Reactions, and Factors Which Affect the Course of Bridge Cleavage D. E. Young and S. G . Shore
Contribution from the Evans Laboratory of Chemistry, The Ohio State University, Columbus, Ohio 43210. Received December 17, 1968 Abstract: Convenient syntheses of 1,2-tetramethylenediborane(6)(I) and 1,2-bis(tetramethylene)diborane(6) (11) have been developed. It has been possible to distinguish between I1 and 1,l-tetramethylene-2,2-tetramethylenediborane(6) (111) and show that I1 is the product prepared in the synthetic procedures employed. Ammonia and methylamines react with I and I1 to produce either symmetrical or unsymmetrical cleavage of the hydrogen-bridge system, with the unsymmetrical cleavage products being zwitterions. The type of cleavage product formed (identified by boron-11 nmr) depends upon the reactants. Factors which affect the course of cleavage are considered and discussed.
I
n recent years studies of cleavage reactions of the hydrogen-bridge system of diborane(6) by Lewis bases have suggested that unsymmetrical cleavage occurs more frequently than was previously thought. l--5 These studies have also suggested that steric requirements of the base can influence the course of bridge cleavage reactions. With increasing methyl substitution in the methylamine series, the tendency for the base to produce symmetrical cleavage increases. In the present study, we have attempted to obtain additional information which could be related to the role of steric factors in determining the course of hydrogen-bridge cleavage. To this end, we have been interested in the type of cleavage product produced by a given base as substitution of the terminal positions of diborane(6) is increased. Since Moews and Parry4 have shown that tetramethyldiborane(6) is cleaved unsymmetrically by ammonia, it would have been of interest to have determined the type of cleavage produced by methyl-substituted amines on the series of
methyl-substituted diboranes. However, because of the tendency of methyldiboranes to rearrange,‘j it was decided to work with other substituted diboranes. The compounds 1,2-tetramethyIenediborane(6)(I) and 1,2-bis(tetramethylene)diborane(6) (11) were chosen.
*b5
I
(1) G . E. McAchran and S. G. Shore, Inorg. Chem., 4, 125 (1965). (2) S . G. Shore, C. W. Hickam, Jr., and D. Cowles, J . Am. Chem. SOC.,87, 2755 (1965). References to the earlier work of Parry, Schultz,
and Shore which established the first example of unsymmetrical cleavage, BHz(NHa)z+BHd-, are given here, (3) 0. T. Beachley, Inorg. Chem., 4, 1823 (1965). (4) P. C . Moews, Jr., and R . W. Parry, ibid., 5, 1552 (1966). ( 5 ) M. Inoue and G. Kodama, ibid., 7,430 (1968).
I1
They have been prepared from the hydroboration of 1,3-butadiene and show no tendency to rearrange. However, their structures have been the subject of some debate. While boron-1 1 nmr spectroscopy has clearly established structure I,’ there is no simple spectroscopic approach which will distinguish between structure I1 and an alternative structure 111, 1,1-tetramethylene-2,2tetramethylenediborane(6).
111 (6) H. I. Schlesinger and A. 0. Walker, J. Ani. Chem. SOC.,57, 621 (1935); H. I. Schlesinger, L. Horwitz, and A. B. Burg, ibid., 58, 407 (193 6). (7) H. G . Weiss, W. J. Lehmann, and I. Shapiro, ibid., 84, 3840 (1962); H. H. Lindner and T. Onak, ibid., 88, 18.86 (1966).
Young, Shore
Dibornnes f r o m Hydroboration of I,3-Butadiene
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Brown, Zwiefel, and Nagase8 favor structure 11, while Koster and Iwasakig favor structure 111. In the present investigation we have been able to distinguish between these two structures and have shown that structure I1 is the one obtained from the preparative procedures which were employed. Convenient syntheses of I and I1 were developed, and the reactions of I and I1 with amine bases to produce symmetrical and unsymmetrical cleavage products were studied. Results Syntheses. 1,2-Tetramethylenediborane(6). Synthetic procedures have been developed for preparing I on a relatively large scale (25 mmoles). Previous reported syntheses7 were based on gas-phase reactions of diborane(6) with 1,3-butadiene, yielding only 1-2mmole quantities of product. In the present study, the reaction of 1,3-butadiene with a slight excess of diborane-(6) was allowed to proceed in an ether solvent for a period of 3 to 7 days at temperatures ranging from 25 to 45”.
solvent = (C,H,),O, (i.C,H,),O, (n:C,H,),O
Good yields were obtained from reactions carried out in each of the solvents listed above, but dibutyl ether proved to be the best solvent with respect to separation of pure product from the reaction mixture by vacuum line fractionation. Yields of over 6 0 x were obtained in addition to a small amount of the isomer, 1,2-(1’methyltrimethylene)diborane(6) (IV) which was re-
I and minimizes the formation of polymer. Polymeric material was not formed in reaction 3 since unlike BzH6, the hydrogen-bridge system of I is not cleaved by THF. B,H6
+
(B( H >B) /
H
H
\
2(1,3-C,H6)
THF
Polymer
(1)
k20J
-t 1,3-C4H6 40“
H
All of the products obtained from the three reactions had vapor pressures of approximately 1 mm at 25”. Their mass spectra were identical: the parent mass was 136 in each case which supports the molecular formula B2H,(C4Hs),. All materials had identical boron-1 1 nmr and proton nmr spectra. The proton nmr spectra of samples of I1 showed no evidence for isomeric species such as might be derived from structure IV. The n m r spectra of the C-H protons of I1 showed two types of methylene hydrogen in a 1 : 1 area ratio with chemical shifts in good agreement with those assigned to a-CH? and P-CH, in structure I by Lindner and Onak.‘ Structures. 1,2-Tetramethylenediborane(6). The assignment of structure I is based upon earlier reports’ of boron-I 1 nmr spectra, which have been fully confirmed in this laboratory. Furthermore, the products obtained from reactions with amines also confirm this structure. Koster and Iwasaki5 assign structure V to a material of
v H
Iv
H
ported in the earlier gas-phase studies.’ The key to obtaining good yields of product from reactions in solution appears to rest in the choice of a solvent which does not cleave the bridge system of diborane(6). By contrast, when 1,3-butadiene and diborane(6) were allowed to react in tetrahydrofuran, a solvent which reacts with B2H6to form THFBH3, the yield of I in the volatile portion of the reaction mixture was small. The product was principally a polymer of low volatility which upon heating to 125” rearranged to produce I and I1 in comparable amounts. 1,2-Bis(tetramethylene)diborane(6). This compound was prepared by three different methods which are outlined in reactions 1-3. Reaction 1 has been described earlier.8 Reaction 2 is analogous to the synthesis of (8) H. C. Brown, “Hydroboration,” W. A. Benjamin, Inc., New York, N. Y., 1962, pp 209-212; G . Zweifel, K. Nagase, and H. C. Brown,J. A m . Chem. Soc., 84, 183 (1962). (9) R. I
