Dicyclohexylborane, reaction with

The use of reductive alkylation conditions has been employed to access tricycles from the azide 353 <2002S242> (Equation 95). Hydroboration of the alkene double bond with dicyclohexylborane followed by reaction with the azide and subsequent elimination of nitrogen and cyclization gave the linear tricyclic diketopiperazine 354 and 355 as a mixture of diastereoisomers. 

The reagent is used in a new sequence for the preparation of alkylmercuric halides via hydroboration-mercuration.3 A terminal olefin is converted into an organoborane by reaction with dicyclohexylborane, and the organobqrane is treated with mercuric acetate. An alkyl mercuric acetate is formed, which is then converted into the more easily handled alkylmercuric halide by reaction with a sodium halide and water. 

The following substrate is subjected to reaction with an equimolar amount of dicyclohexylborane to generate a single boron containing entity in quantitative yield. Without isolation, it is mixed with an equimolar amount of aqueous caustic soda to isolate 1 -tmns-6-trans-1 -methyl-1,6-cyclodecadiene as the sole product. Please present the events in a schematic fashion with due care of the involved stereochemical features. 

A synthetically useful virtue of enol triflates is that they are amenable to palladium-catalyzed carbon-carbon bond-forming reactions under mild conditions. When a solution of enol triflate 21 and tetrakis(triphenylphosphine)palladium(o) in benzene is treated with a mixture of terminal alkyne 17, n-propylamine, and cuprous iodide,17 intermediate 22 is formed in 76-84% yield. Although a partial hydrogenation of the alkyne in 22 could conceivably secure the formation of the cis C1-C2 olefin, a chemoselective hydrobora-tion/protonation sequence was found to be a much more reliable and suitable alternative. Thus, sequential hydroboration of the alkyne 22 with dicyclohexylborane, protonolysis, oxidative workup, and hydrolysis of the oxabicyclo[2.2.2]octyl ester protecting group gives dienic carboxylic acid 15 in a yield of 86% from 22. 

The synthesis of the rare amino acid 3-hydroxy-4-methylproline (8)3 involves an aldol reaction of the oxazoiidinone 5 with methacrolein to provide the a-bromo-0-hydroxy adduct 6. Azide displacement and removal of the chiral auxiliary gives 7. On treatment with dicyclohexylborane, 7 undergoes hydroboration-cycloalkyl-ation to provide, after hydrolysis, the methyl ester hydrochloride (8) of (2S,3S,4S)-3-hydroxy-4-methylproline in >97% de. This cycloalkylation should be a useful route to cyclic amino acids as well as pyrrolidines. 

The reaction of dicyclohexylborane and trifluoromethanesulfonic acid is highly exothermic. On one occasion, the checkers cooled the reaction in an ice bath during the addition period, with no effect on product yield. The submitter reports that he once experienced a sudden vigorous reaction under cooling conditions, probably due to accumulation of unreacted triflic acid. It thus appears safer to add the acid at room temperature, slowly, so that it reacts immediately. 

An important aspect of the metal catalyzed hydroboration reaction is its ability to selectively reduce certain functionalities within a molecule. For instance, a key step in the synthesis of a tripeptide derivative containing the Phe-Arg hydroxyethy-lene dipeptide iosostere is the selective rhodium-catalyzed hydroboration of a lactone. The use of disiamylborane, 9-H-BBN, dicyclohexylborane, and (.9)-alpmeborane, however, gave only low to variable yields of the alcohol due to competitive reduction of the y-lactone to the hemiacetal (equation 8). In another example, hydroboration of the diene illustrated in equation (9) with HBcat and RhCl(PPh3)3 gave exclusive formation of the terminal alcohol derived from reaction of the less substituted alkene. Interestingly, uncatalyzed reactions failed to hydroborate this substrate selectively. ... 

Hydroboration of allenes. With few exceptions, the 9-BBN hydroboration of allenes affords B-allylic-9-BBN derivatives. In contrast, the reactions of allenes with disiamylborane or dicyclohexylborane afiord predominantly vinylic boranes. With unsymmetrical allenes, 9-BBN binds to the less substituted carbon atom. Allene itself affords a 1,3-dibora derivative. The B-allylic-9-BBN derivatives are useful reagents for the allylic boration of carbonyl compounds. ... 

Borabicyclo[3.3.1.]nonane (9-BBN) is available by hydroboration of 1,5-cyclooctadiene with one equivalent of BH3," or commercially either as a crystalline dimer or as a THF solution. The reagent is thermally more stable than dicyclohexylborane. It is frequently used as an anchor group in organoborane reactions, allowing an efficient utilization of valuable alkenes. 

With sterically hindered dialkylboranes, such as dicyclohexylborane (Chx2BH) and disiamylborane (Sia2BH), the stoichiometric hydroborations of both terminal and internal alkynes stops at the monoaddition stage. However, the stoichiometric reaction of 1-alkynes with 9-BBN affords mainly the 1,1-diborylalkanes. ... 

Alkanoic acids. Monohydroboration of l-alkynyl(trimethyl)silanes (2) with dicyclohexylborane leads to 1-boryl-l-silylalkenes (3) almost exclusively. The usual alkaline peroxide oxidation of 3 gives alkanoic acids (4). Related reactions can be used to prepare a, - and /S,y-unsaturated acids. ... 

Predict the stereochemistry (erythro or threo) of the products resulting from the reaction of ( )- and (Z)-l-hexene-l,2-d2 with dicyclohexylborane. 

After hydroboration, oxidation of radioiodide is performed with Chloramine-T, forming HO-I, which then adds to the C-B bond. The reactivity of the C-B bonds depends on the position of boron, which can either be formed from a terminal double bond or from an internal alkene. While the former case produces two radioiodinated compounds, the latter forms only one. Therefore, dicyclohexylborane is used for the hydroboration step. Both reactions, hydroboration of the alkene and radioiodination can be performed in one sequence without isolating the intermediately formed trialkylborane. 

Carbonyl compounds are an important class among organic molecules. Literature records several methods for their synthesis. However, there are very few methods to convert carbon-carbon unsaturation to carbonyl compounds. Hydroboration of acetylenes, followed by oxidation provides a novel method for carbonyl synthesis. It has been noted that regioselectivities achieved in the monohydroboration of internal acetylenes with thexylborane [1], disiamylbo-rane [1], dicyclohexylborane [1], and catecholborane [2] are similar to, but less pronounced than, that realized by 9-BBN [3]. The B-alkenyl-9-BBN derivatives undergo oxidation to the corresponding ketones or aldehydes under aprotic conditions with trimethylamine N-oxide [4, 5] or under protic conditions by inverse addition to buffered hydrogen peroxide [3]. The inverse addition, i.e., the slow addition of the B-alkenyl-9-BBN in THF to the buffered H O, suppresses the otherwise undesirable protonolysis reaction and favors the oxidation pathway to the desired aldehyde or ketone. 

Soderquist and coworkers [17] have reported the synthesis of stable c/s-vinyl-OBBDs. TheB-methoxy-9-BBN on selective oxidation with anhydrous trimeth-ylamine-iV-oxide (TMANO) (85%, CHClj, 0°C) affords the corresponding borinic ester. The borinic ester on alkynylation, followed by demethoxylation gives the stable alkynylborinate. The hydroboration of alkynylborination with dicyclohexylborane (Chx BH) affords cleanly the 1 1 gem-diboryl adduct This is selectively protiodeborylated with acetic acid at 0 °C and gives the corresponding cis-B-vinylborinate. The reaction sequence is outlined in Scheme 20.5 [17]. 

The hydroboration-oxidation studies of 1,4-epoxy-1,4-dihydronaphthalene (1) with various hydroborating agents such as borane-methyl sulfide (BMS), dicy-clohexylborane, disiamylborane, and 9-BBN have yielded interesting results [1]. The reaction of 1 with dicyclohexylborane or disiamylborane affords the exo alcohol 2 without epoxy ring opening (Eq. 32.1). On the other hand, 9-BBN hy-droboration of 1 (1 1 mole ratio) in THE at 25 °C, followed by oxidation affords a homoallylic alcohol 3 after opening of the epoxy ring (Eq. 32.2). BMS, however, affords a mixture of both the alcohols. 

---