Boranes isomerization

Interesting similarities can be advanced between nickel catalyzed hydrocyana> tion and hydroboration [181 reactions (the isomerization step being related to borane isomerization througli elirrunation-addition reactions), the drift of nickel toward the less hindered terminal position being genuinely reminiscent of organoborane chemistry. 

Isomerization of organoboranes can involve unhindered alkyl groups. Trihexylbor-anes derived from 2-hexene and the stoichiometric amount of borane isomerizes at 150°C more slowly than when 20% xs boron hydride reagent is present ... 

This process is synthetically important because a sacrificial alkene such as 1-decene or 1-hexadecene can be added to bias the overall reaction toward a specific target alkene. Mechanistically, thermal isomerization apparently involves a series of reversible elimination reactions of borane followed by re-addition in an anti-Markovnikov manner.2 If the sacrificial alkene has a significantly higher boiling point, then the desired alkene can he distilled from the reaction medium.22a xhe sacrificial alkene must not boil < 160°C, however, which is the temperature required for borane isomerization. This process makes it possible to isomerize the double bond of an alkene to a less substituted isomer, a process that Brown termed contrathermodynamic isomerization.29,30 methylenecyclohexane (41) was prepared from 1-methylcyclohexene by... 

Borabicyclo [3.3.1] nonane [280-64-8], 9-BBN (13) is the most versatile hydroborating agent among dialkylboranes. It is commercially available or can be conveniendy prepared by the hydroboration of 1,5-cyclooctadiene with borane, followed by thermal isomerization of the mixture of isomeric bicychc boranes initially formed (57,109). 

When a boron atom of a borane is replaced by a heteroelement, the compounds are called carbaboranes, phosphaboranes, thiaboranes, a2aboranes, etc, by an adaptation of organic replacement nomenclature. The numbering of the skeleton in heteroboranes is such that the heteroelement is given the lowest possible number consistent with the conventions of the parent borane. Thus C2B2H is dicarba- /(9j (9-pentaborane(5) and could occur as the 1,2-, 2,3-, or 1,5-isomeric forms (l,2-dicarba- /(9j (9-pentaborane(5) [23777-70-0] 2,3-dicarba- /(9j (9-pentaborane(5) [30396-61-3] and... 

Arachno Clusters 2n + 6 Systems). In comparison to the number of known closo and nido boranes and heteroboranes, there are relatively fewer arachno species. Partly because of the lack of a large number of stmctures on which to base empirical rules, arachno stmctures appear to be less predictable than their closo and nido counterparts. For example, there are two isomeric forms of one with the arachno [19465-30-6] framework shown... 

A novel and far-reaching type of isomerism concerns the possibility of valence isomerism between nonclassical (electron-deficient) clusters and classical" organoboron structures. Thus, n-vertexed /do-boranes. have cluster structures... 

Allyllithium reagents have also been used in the synthesis of (Z)-y-alkoxyallylboronates 23 2 5. Stereoselectivity is excellent in these reactions since the (Z)-y-alkoxyallyl carbanions prepared by metalation of allyl ethers are stabilized by chelation. The (Z)-y-alkoxyallyl(diisopinocam-pheyl)boranes are prepared at low temperature by an analogous procedure and must be used at — 78 "C otherwise reaction diaslereoselectivity suffers owing to the facile isomerization to the -isomer26. 

Several studies of reactions of configurationally unstable a-substituted allylboranes have also been reported19,29. The reactions of dialkyl[( )-l-alkyl-2-butenyl]boranes and aldehydes at — 78 °C provide a mixture of syn- and an/i -diastereomers. reflecting reactions by both the Z-and /f-isomers. When generated and used at — I00°C, however, the ff/m-diastereomer is obtained with >95% diastercoselectivity and >90% selectivity for the /T-olefin isomer by way of a transition state analogous to 429. This result suggests that the allylboron isomerization is slow at —100 JC relative to carbonyl addition. 

No matter which of the electrophilic methods of double-bond shifting is employed, the thermodynamically most stable alkene is usually formed in the largest amount in most cases, though a few anomalies are known. However, there is another, indirect, method of double-bond isomerization, by means of which migration in the other direction can often be carried out. This involves conversion of the alkene to a borane (15-16), rearrangement of the borane (18-11), oxidation and hydrolysis of the newly formed borane to the alcohol (12-28), and dehydration of the alcohol (17-1) ... 

Since the migration reaction is always toward the end of a chain, terminal alkenes can be produced from internal ones, so the migration is often opposite to that with the other methods. Alternatively, the rearranged borane can be converted directly to the alkene by heating with an alkene of molecular weight higher than that of the product (17-14). Photochemical isomerization can also lead to the thermodynamically less stable isomer. ... 

Silyl(pinacol)borane (88) also adds to terminal alkenes in the presence of a coordinate unsaturated platinum complex (Scheme 1-31) [132]. The reaction selectively provides 1,2-adducts (97) for vinylarenes, but aliphatic alkenes are accompanied by some 1,1-adducts (98). The formation of two products can be rationalized by the mechanism proceeding through the insertion of alkene into the B-Pt bond giving 99 or 100. The reductive elimination of 97 occurs very smoothly, but a fast P-hydride elimination from the secondary alkyl-platinum species (100) leads to isomerization to the terminal carbon. 

Furthermore, such boranes should undergo more facile cis, trans-isomerization to yield 33 as the contribution of 31c becomes more important. To our knowledge, the ease of isomerizing (Z)-vinylboranes (31a) thermally to their -isomers (33) has not yet been evaluated (Eq. 10), possibly because structures 31a are not readily available. 

The discovery of polyhedral boranes and polyhedral heteroboranes, which contain at least one atom other than in the cage, initiated a new era in boron chemistry.1-4 Most commonly, of the three commercially available isomeric dicarba-closo-dodecaborane carboranes(l,2-, 1,7-, and 1,12-), the 1,2-isomer 1 has been used for functionalization and connection to organic molecules. The highly delocalized three-dimensional cage bonding that characterizes these carboranes provides extensive thermal and kinetic stabilization as well as photochemical stability in the ultraviolet and visible regions. The unusual icosahedral geometry of these species provides precise directional control of all exopolyhedral bonds. 

The thermal isomerization of tert. -butyl-diisobutylborane to triisobutylborane and the stepwise isomerization of triisopropyl and tri-sec.-butyl boranes to the corresponding straight-chain isomers have been studied over a range of temperature, both neat and using either the end product trialkylborane or diglyme as solvent117 . In all cases the reactions were first order and showed no solvent effect. 

Instead of having the olefin insertion reactions, the calculations indicate that M2b and M2c can only proceed uphill with the reductive elimination of HB(OH)2, leading to the formation of M3, an olefin complex which could be in principle obtained directly from the addition of olefin to the catalyst Rh (PH3)2C1. The olefin complex M3 then could undergo a-bond metathesis processes with HB(OH)2, giving two isomeric products M4 and M5 depending on the orientation of the HB(OH)2 borane. The a-bond metathesis processes are however found to be unfavorable because of the very high reaction barriers (Figure 4). 

One might expect that a large number of isomers of polyboranes should exist as the number of boron atoms increases, similar to hydrocarbon chemistry. This, however, is not the case in borane chemistry. There are indeed only a few examples of proven isomerism, for instance for the B2oHi82 anions (see Section 5.8). However, this point has been addressed in many theoretical papers, and energy differences between various isomers of a polyborane are, in general, larger than in hydrocarbons. 

Isomeric with non-classical triboretanes of type 3 (Schemes 3.2-3 and 3.2-5) are triboraficyclo] 1.1,0 butanes of type 13 (Scheme 3.2-8). The first derivative 13a was obtained when 4b was reacted with dichloro(trimethylsilylmethyl)borane [19]. 

In the late 1960s, methods were developed for the synthesis of alkylated ketones, esters, and amides via the reaction of trialkyl-boranes with a-diazocarbonyl compounds (50,51), halogen-substituted enolates (52), and sulfur ylids (53) (eqs. [33]-[35]). Only one study has addressed the stereochemical aspects of these reactions in detail. Masamune (54) reported that diazoketones 56 (Ri = CH3, CH2Ph, Ph), upon reaction with tributylborane, afford almost exclusively the ( )-enolate, in qualitative agreement with an earlier report by Pasto (55). It was also found that E) - (Z)-enolate isomerization could be accomplished with a catalytic amount of lithium phenoxide (CgHg, 16 hr, 22°C) (54). 

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