Alkynes boron hydrides

Hydroboration—the addition of boron hydrides to alkenes and alkynes... 

In hydroboration, a boron hydride (R2BH) adds across an alkene (R CH=CH2) to give R CH2CH2BR2. The 16-electron, d° Zr(IV) complex Cp2Zr(H)Cl, popularly known as Schwartz reagent, undergoes a closely related reaction. The mechanism involves coordination of an alkene to the electrophilic Zr center followed by migratory insertion of the alkene into the Zr-H bond. The reaction proceeds for alkynes also, by exactly the same mechanism. 

The first carboranes were discovered " ° 2 among the products of reactions between alkynes and boron hydrides. Such reactions remain the best routes to many dicarba species (Scheme 3.1, 3, route Derivatives... 

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... 

Addition of boron hydrides to alkenes, allenes, and alkynes to form organoboranes, such that boron adds to the less substituted carbon. Attack usually takes place on the less hindered side in a cis fashion ... 

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. 

The addition of boron hydrides to alkenes and alkynes is known as hydrobor-ation. It provides a simple laboratory route to boron alkyls and thence to a wide range of organic compounds (p. 66). It takes place under mild conditions at room temperature in an ether solvent. 

Wilczynski, R. Sneddon, L. G. Transition-metal-promoted Reactions of Boron Hydrides. 3. (R2C2)Co2(CO)6-catalyzed Reactions of Alkynes and Small Carboranes Synthesis of Boron-substituted Alkenylcarboranes. Inorg. Chem. 1982,21, 506-514. 

Monocarbaborane cluster chemistry is sparsely investigated compared to its flanking fields of binary boron hydride cluster chemistry and dicarbaborane cluster chemistry. Dicarbaborane compounds can be synthesized relatively easily by addition of alkynes to boron hydrides such as decaborane, but there are no simple routes to monocarbaborane compounds. Traditionally, the principal methods involve removal of one carbon atom from dicarbaboranes, or addition of cyanide or organic nitriles to borane clusters to give C-aminated monocarbaboranes, which can subsequently be deaminated. ... 

Polyhedral boron hydride derivatives with terminal alkyne group were prepared by oxonium ring disclosure with alkynyl alcoholates or amines (Figure 24.14) [32,56,59]. 

Because of the security shroud placed over these studies it is not yet clear exactly when and where the first work on the carboranes was performed and to whom the credit belongs. However, several sources (9) provide a degree of historical insight into this problem. Hopefully, a more clear iccount, if not a more lively one, will be forthcoming. It is interesting to note that as early as 1923 Stock and Kuss 10) were conducting experiments with carborane precursors, alkynes, and boron hydrides however, they were not able to isolate and characterize the products. 

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. 

Nucleophiles other than hydride can be added to support-bound imines to yield amines. These include C,H-acidic compounds, alkynes, electron-rich heterocycles, organometallic compounds, boronic acids, and ketene acetals (Table 10.9). When basic reaction conditions are used, stoichiometric amounts of the imine must be prepared on the support (Entries 1-3, Table 10.9). Alternatively, if the carbon nucleophile is stable under acidic conditions, imines or iminium salts might be generated in situ, as, for instance, in the Mannich reaction. Few examples have been reported of Mannich reactions on insoluble supports, and most of these have been based on alkynes as C-nucleophiles. 

The stepwise formulation explains why boron becomes attached to the less-substituted carbon, but does not account for the fact that the reactions show no other characteristics of carbocation reactions. This could be because of an expected, extraordinarily fast rate of hydride-ion transfer to the carbocation. A more serious objection to the stepwise mechanism is that alkynes react more rapidly than alkenes, something which normally is not observed for stepwise electrophilic additions (cf. Section 10-5). 

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. ... 

Terminal and internal (Z)-l-alkenylboronates are prepared from (Z)-(l-halo-l-alkenyl)boronates [23]. which can be readily obtained by hydroboration of 1-halo-1-alkynes (Scheme 16.2). The internal Sv,2-like displacement of the halogen with hydrides [24] or organolithiums [25] takes place wath complete inversion of configuration at the sp carbon. On the other hand, the palladium-catalyzed alkylation of the C—X bond with organozinc reagents provides ( )-l-alkenylboronates [26] which are not available by conventional hydroboration of internal alkynes. 

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