Boron hydride, reaction with alkynes

Boron is the prime metal in the area of stoichiometric interactions between metals and unsaturated bonds. Especially, boron hydride additions have been investigated, in particular by H. C. Brown and his students. Nowadays, these addition reactions are well-established text book subjects. A number of reviews on hydroboration have appeared . The development of a clear mechanistic picture lagged far behind the applications in synthesis. It was also the group of Brown that contributed to mechanistic understanding by performing careful kinetic measurements using 9-borabicyclo[3.3.1]nonane, abbreviated as 9-BBN-H, as reagent. Reactive alkynes such as 1-hexyne and 3-methyl-1-butyne exhibited first-order kinetics in 9-BBN-H with a rate constant equal to that of reactive... 

Reaction of the alkyne adducts with aluminum and boron hydride reagents such as lithium aluminum hydride, sodium boro-hydride, or lithium triethylborohydride, however, leads to the formation of unexpected thiirane derivatives (eq 3). These can be desulfurized to give divinyl sulfides. 

Thus, far, all of the known carboranes have been derived either directly or indirectly from reactions of alkynes with various boron hydrides. In these reactions only the carbons participating in the triple bond of the alkyne become skeletal atoms in the carborane. No routes to the hypothetical four-carbon carboranes have been suggested. However, the dehydrogenation of the cyclic diborane derivatives prepared from butadiene 98, 100) should lead to C4BHS and C4B2H6. Unfortunately, such a conversion may require electric discharge conditions, in which case extremely low yields can be expected. 

Several reactions in organometaUic chemistry also appear to contravene the rule, but which can be explained in a somewhat similar way. Hydrometallation [5.45, see (Section 5.1.3.4) page 162], carbometallation, metallo-metallation, and olefin metathesis reactions are all stereospecifically suprafacial [2 + 2] additions to an alkene or alkyne, for which the all-suprafacial pathway is forbidden. Hydroboration, for example, begins with electrophilic attack by the boron atom, but it is not fully stepwise, because electron-donating substituents on the alkene do not speed up the reaction as much as they do when alkenes are attacked by electrophiles. Nevertheless, the reaction is stereospecifically syn—there must be some hydride delivery more or less concerted with the electrophilic attack. The empty p orbital on the boron is the electrophilic site and the s orbital of the hydrogen atom is the nucleophilic site. These orbitals are orthogonal, and so the addition 6.126 is not pericyclic. 

Interestingly, depending on the catalyst, the diboration of vinylboranes with B2cat2 leads via seqnential diboration and dehydroboration steps either to 1,1,1-triborylalkanes (20) or to 1,1,2-triborylalkanes (21). 1,1,1-Triborylalkanes, 1,1,2,2-, and 1,1,1,2-tetraborylethane species have been proposed as intermediates in the reaction of borylated alkynes with excess Et2BH ( hydride bath ) that eventnally leads to the formation of small carboranes. Indeed, isolated triboryhnethane species with alkyl or hahde snbstituents on boron have recently been shown to rearrange to small carboranes upon thermal treatment. ... 

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