It has been shown that 1,2-cyclononadiene and 1,2-cyclotridecadiene undergo 78 and 100% hydroboration respectively, with disiamylborane. 1,2-Cyclononadiene gave products which represented 83 % attack of boron at the central carbon atoll1 and 17 % attack at the terminal carbon atom. I,?-Cyclotridecadiene gave 62 % central carbon attack and 38 % terminal carbon attack. The reactivity and selectivity may be explained mainly in terms of steric effects on a four-centered transition state.Canadian Journal of Chemistry, 46, 2632 (1968 It has been shown that the monohydroboration of 1,2-cyclononadiene (1) with diborane results in the attack of diborane, primarily at the central carbon atom of the allenic linkage (1). The object of the present study was to examine the possibility of stereoselective addition of boron to the allenic linkage with disiamylborane (2), which might have revealed some useful and interesting synthetic routes. In particular we were interested to study the effect of the ring size on the reactivity and selectivity in the direction of addition. Since the monohydroboration of allenes might proceed through monoalkyl-, dialkyl-, and trialkylborane stages, the percentage of electrophilic attacks of diborane on the central and terminal carbon atoms of the allenic linkage, will be governed by the cumulative steric effect due to ring size of the allene and the intermediate organoboranes. The percentage electrophilic attacks of disiamylborane are controlled only by the steric effect of ring size of the allene. Accordingly, we undertook the hydroboration of 1,2-cyclononadiene (1) and 1,2-cyclotridecadiene (2) with disiamylborane which is known to provide greater steric control over diborane (2).
Results and DiscussionDisiamylborane was prepared by hydroboration of 2-methyl-2-butene at -5 to -10 "C under a static pressure of nitrogen (3). A solution of the appropriate allene in diglyme was added within 3 min to the above solution. This reaction mixture was left for approximately 3 h to attain room temperature. After stirring for 6 h at room temperature, the mixture was then hydrolyzed until hydrogen was no longer evolved. The product was oxidized with alkaline hydrogen peroxide in the usual manner. The percentage conversion of the allene to products was determined by gas chromatography (4). The results are summarized in Scheme 1.The percentage electrophilic attack of boron on the central and terminal carbon atoms of the allenic linkage, as calculated from the percentage yield of the products and also the percentage conversion of the allenes to products, is shown in Table I.These results suggest that the observed difference in the reactivity and selectivity, in the direction of addition of disiamylborane with 1 ,2-cyclononadiene (1) and 1,2-cyclotridecadiene (2), could be dictated solely by difference in steric requirements of the allenes in the Brown's well known postulated transition state for hydroboration (2). The electronic effect seems to be of secondary importance here, since it has been shown that ...