2- Hexyne, hydroboration

Thus hydroboration of 1-hexyne and oxidation of the 1-hexenylborane, 4, with hydrogen peroxide gives hexanal by way of the enol ... 

Good yields of the title compounds have been obtained when the cyclic hydroborating agents are used. The versatility of this reaction is illustrated in Eq. 103, where the reaction of 3,5-dimethylborinane with 1-bromo-l-hexyne affords l-pentylidene-3,5-dimethylcyclohexane 161). 

This synthesis can be accomplished in two steps. Hydroboration of 1 -hexyne with catecholborane forms a vinylborane. Coupling of this vinylborane with (Z)-2-bromostyrene gives the desired 1,3-diene. The E configuration of the vinylborane and the Z configuration of the vinyl bromide are both retained in the product. 

Hydroboration-iodination of aUtynes. Olefinic side chains can be introduced stercosclcctivcly into cyclic systems by hydroboration-iodination of alkynes. Thus the reaction of l-hexyne with bis(/ru/is-2-methylcyclohcxyl)borane (1) gives the vinyl-borane (2). Treatment of (2) successively with 6 N sodium hydroxide and a solution of iodine in THI yields rrans-l-methyl-2-(cM-l-hexenyl)cyclohexane (3) in 85% yield. Migration of the 2-methylcyclohexyl group thus occurs with retention of configuration. 

Hydroboration of 1-hexyne with bis(tran5-2-methylcyclohexyl)borane followed by treatment with iodine and aqueous NaOH results in the transfer of one alkyl group from boron to the adjacent carbon. The hydroxide-mediated deboronoiodination gives isomerically pure cA-alkenes. ... 

Alkynes can be reduced by hydroboration and protonolysis. The internal acetylene 3-hexyne reacts with diborane to form an unsaturated boron derivative which is... 

Competitive hydroborations with diborane in diglyme established that the reaction is relatively insensitive to the structure of the olefin. The most reactive olefin studied, 2-methy 1-1-butene, is separated by a factor of only 20 or 30 from the least reactive ones, 2,4,4-trimethyl-2-pentene and 2,3-dimethyl-2-butene. In hydrobora-tion with BMB a factor of 10,000 separates reactive 1-octene from cyclohexene, one of the least reactive olefins studied the study could not be extended to still more inert structures such as 2,4,4-trimethyl-2-pentene. 1-Hexyne and 3-hexyne are more reactive than the most reactive olefins studied. 

Dibromoborane-dimethyl sulfide exhibits high reactivity toward internal alkynes . The relative reactivity of 1-octene, 1-hexyne, (Z)-3-hexene and 3-hexyne is 100, 290, 20 and 5900, respectively. This makes possible selective hydroboration of internal alkynes in the presence of terminal unsaturation. With 1-bromoalkynes Br2HB SMe2 provides [Z]-l-dibromoalkenylboranes which can be transformed into [E]- or [Z]-l-alkenylboronic esters . 

Yields of the catechol derivative of 3-hexyne and 1-hexyne are 84 and 44%, respectively, indicating substantial dihydroboration of the terminal triple bond. Quantitative yields of 2-alkenyl-1,3,2-benzodioxaboroles are obtained by hydroboration of alkynes with 1,3,2-benzodioxaborole . ... 

Allylethylzinc A reagent (2) of this type is prepared by hydroboration of 1-hexyne with dicyclohcxylborane (1 cquiv.) to form an (E)-l-alkcnylboranc 1, which undergoes transmctalation to form 2 on treatment with diethylzinc. This reagent adds to an aldehyde in the presence of the catalyst (-)-3-ejco-(dimethylamino)isoborneol (DAIB, 3) to form a secondary (E)-allylic alcohol in 70-95% yield with 79-98% cc. 

Hydroboration of terminal alkynes, e.g. 1 -hexyne, 1 -octyne or cyclohexylacetylene, with a dialkylborane, such as bis(l, 2-dimethylpropyl)borane, followed by copper(I)cyanide and copper(II) acetate in HMPA containing a trace of water, gives isomerically pure ( )-l-cyanoalk-l-enes (equation 29)133. Successive treatment of 1-bromo-l-alkynes with dialkylboranes and sodium methoxide results in the borinate esters 208, which are converted into ( )-alkenes of greater than 99% isomeric purity by protonolysis. The action of alkaline hydrogen peroxide on the borinates produces ketones (equation 30)134. 

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... 

In recent years, besides hydroboration also haloboration has been used for the introduction of boron atoms into organic molecules. A short review of this subject has been published Some examples follow. For instance, Paetzold and coworkers showed that 1-hexyne easily reacts with diphenylboron bromide but not with the chloride. An equilib-... 

Hydroboration of 1-hexyne, for example, gives the vinylborane with boron on the less highly substituted carbon. Oxidation of this intermediate gives an enol that quickly tautomerizes to hexanal. 

Although the hydroborations of 1-hexyne, 3-methyl-1-butyne, 3,3-dimethyl-1-butyne, and cyclohexylethyne all exhibit the same first-order rate of reaction, they have very different relative reactivities toward (9-BBN)2 (Table 4.10) [2]. 

The rate of hydroboration of diphenylethyne, with (9-BBN)2 is slower than that of 1-hexyne in CCl at 25 °C. The reaction exhibits three-halves-order kinetics, first order in diphenylethyne and one-half order in (9-BBN)2. The kinetic behavior intermediate between first order and three-halves order is observed with phenylethyne in CCl at 25 °C, which is between that of 1-hexyne and diphenylethyne. 

It is important to note that unlike the disiamylborane dimer [4] and the dibro-moborane-dimethylsulfide complex [5], (9-BBN)2 does not exhibit a large increase of relative reactivities toward alkynes. The relative reactivity of 1-hexyne is only one seventh that of 1 -hexene, and 3-hexyne shows about the same relative reactivity as does ds-3-hexene. Consequently, one can selectively hydroborate the terminal alkene in the presence of internal alkyne [6]. 

Unlike the disiamylborane dimer [4] and the dicyclohexylborane dimer [7], the hydroboration of a terminal alkyne, such as 1-hexyne with equimolar ratio of 9-BBN produces, also, a considerable percentage of gem dibora derivatives (Scheme 4.1, Eq. 4.10) [8]. 

The regioselectivity and stoichiometry of hydroboration of simple 1-silylacet-ylenes and 1-alkynes with 9-BBN are examined [14] by high-field C NMR. The hydroboration of 1-hexyne, 3,3-dimethyl-1-butyne, and 1-decyne in dilute THF solution with 100% excess of alkynes give 94, 96, and 94% monohydroboration, respectively. 

In order to establish the effect of TMS, 1-hexyne (lb), l-(trimethylsilyl)-l-hexyne (7), and 9-BBN in 1 1 1 ratio are reacted at 25 °C for 5 h (Eq. 5.25). It is found that lb undergoes hydroboration (>98%) to produce 5b and 6b in essentially equal amounts with the corresponding quantity of unreacted lb. The silylacetylene (7) remains essentially unchanged with only a traces amount of monohydroboration to vinylborane (8). 

The comparative data of C NMRfor 1-hexyne and the 1-halo-l-hexyne describe the importance of the -M effect, which in haloalkynes decreases in the order I > Br > Cl. In fluoroalkynes, no -M effect is possible as no d orbitals are available. Only a +M interaction is possible in the 1-fluoro-l-alkynes, and one expects their hydroboration at the C-1 position would (1) proceed at a slower rate than the corresponding chloro analogs, or (2) switch to C-2 position in case the electron density at C-1 decreases enough. Indeed, in 1-fluoro-l-alkynes the nucleophilic attack occurs at the C-1 position, whereas the same occurs at C-2 position in 1-chloro-1 alkynes [22a]. Their comparative hydroboration studies are not conducted [21], as fluoroalkynes are difficult to prepare and handle, and often very unstable [22]. 

PROBLEM 10.64 Contrast the results of hydroboration/oxida-tion and mercury (Hg )-catalyzed hydration for 2-pentyne and 3-hexyne. Would any of these procedures be a practical preparative method Explain. 

Hydroboration of 1-hexyne with dicyclohexylborane produced the alkenylborane 55, which on treatment with the lithium acetylide 56 furnished the organoborate complex 57 (Scheme 14) (24). Trimethyltin chloride then induced a stereoselective migration of the 1-hexenyl group to the adjacent acetylenic carbon to afford the diene-allene 58 and subsequently the o-isotoluenes 59 and 60. A variety of other o-isotoluenes were likewise synthesized. 

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