Boranes thermal isomerization

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). 

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. 

The hydroboration of simple, open-chain dienes can be somewhat complicated, leading to mixtures of regioisomers, cyclic and open-chain structures, and oligomeric structures of different sizes, all depending on reaction conditions and/or reactant proportions. - However, by control of stoichiometry, thermal isomerization of product mixtures, control of temperature during the hydroboration stage, subsequent treatment of a trialkylborane with further borane, or similar tricks , it is often possible to arrive at an unique product. Several important cyclic dialkylboranes can be made in this way, as shown in equations (6), (7) " and (8). Similar treatment of trienes can lead to bicyclic systems (e.g. equation 9). -24..to... 

Thexylborane is more reliable than borane for permitting clean formation of ring compounds from dienes. Under kinetic control there is a preference for formation of five- or seven-membered rather than six-membered rings (equations 14 and IS). - This regioselectivity can be altered by modification of the substitution pattern or by thermal isomerization after the initial hydroboration. 

Less hindered dialkylboranes cannot generally be prepared by direct stoichiometric hydroboration of 2 equiv. of an alkene with borane (Section 3.10.2.1), although 3,5-dimethylborinane and 3,6-dimethylbore-pane are exceptions. Borinane itself is obtained by a sequence involving hydroboration (dieneiBHs = 3 2), thermal isomerization, and then redistribution with further borane-THF, but there is as yet no similar direct route to the parent borepane. ° ... 

Borabicyclo[3.3.1]nonane (9-BBN) has been prepared by the thermal redistribution of 9-n-propyl-9-BBN, and the hydroboration of 1,5-cyclooctadlene with borane-tetrahydrofuran complex followed by thermal isomerization of the mixture of dialkylboranes at BS C. Solutions of 9-BBN have been prepared from the hydroboration of 1,5-cyclooctad ene with borane-methyl sulfide in solvents other than THF.6 The present procedure involves the cyclic hydroboration of 1,5-cyclooctadiene with borane-methyl sulfide in 1,2-dimethoxyethane.7 Distillative removal of the dimethyl sulfide in this special solvent system provides a medium that gives high purity, large needles of crystalline 9-BBN dimer in excellent yield. The material can be handled in air for brief periods without measurable decomposition. 

Convenient procedures are available for isomerically uncontaminated borinane which can be obtained by the hydroboration of 1,4-pentadiene in a 3 2 ratio, followed by thermal isomerization and exchange with borane ... 

Little is known about the thermal isomerization of organoboranes derived from trienes and higher polyenes. Hydroboration of 1,4,8-nonatriene and its 5-methyl derivative with HjB-THF gives insoluble polymeric products. When this mixture is heated to 210-220°C, bicyclic boranes XI and XIII are formed in good yields, along with other minor bicyclic products. Carbonylation-oxidation of the bicyclic boranes provides 9-decalol (XII) and lO-methyl-9-decalol (XIV) in 73 and 46 % yields, respectively, based on the initial trienes ... 

The products are crystalline and can be stored at 0 "C under nitrogen for prolonged periods of time without any appreciable loss of hydride activity or isomerization. (lS)-Di-4-isocaranyl-borane is sensitive to thermal isomerization and its use above room temperature should be avoided. The opposite enantiomeric boranes have not been prepared since (—)-2- and (—)-3-carene are not readily available. 

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... 

Bis(boracyclyl)alkanes 23, 27), formed by the hydroboration of diolefins with B2H6 23) at room temperature or from trialkylamine boranes at high temperatures 27), isomerize in the same way as B-alkylboracyclanes when heated. The equilibrium mixture of B2(C H2 )3 (XVIII) ( = 5, 6) was determined, before and after thermal isomerization, by oxidation and hydrolysis to diols 2J). 

Pyrolysis gives a mixture of various mixed trialkyl boranes, because of the alkyl exchange on boron 8). For example, as a result of thermal isomerization the propyl groups on boron will be both 1- and 2-propyl. No B—H-containing products are obtained, but they appear as reactive intermediates. 

Trialkylboranes (B-R-9-BBN) derived from hydroboration of alkenes with 9-BBN are remarkably stable to thermal isomerization [6]. The alkylborane, derived from (Z)-hex-3-ene by hydroboration with 9-BBN, requires the heating at 150 °C for 163 h to attain the equilibrium distribution of boron along the hexyl chain. At the same temperature the isomerization of the alkylborane derived from (Z)-hex-3-ene using borane-THF is complete in 1 h. 

E. Mincione. Thermal isomerization of steroid-boranes. IX. Synthesis of the steroidic B/C ring junction with the 8)3, 9)3 unnatural configuration. Ann. Chim. Rome), 1977, 67,119. 

A stereospecific synthesis of B-( )-l-alkenyl-9-BBN derivatives that are not available by hydroboration reactions has been developed. The 9-BBN dimer has featured in thermal isomerization and thermolysis studies and similar studies on dimesityl-3-hexyl-borane show that, in comparison to the diphenyl compound, it is quite resistant to thermal isomerization and an explanation is offered. ... 

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 relative reactivities of different alkenes towards 9-borabicyclo[3,3,l]nonane (9-BBN) and di-isoamylborane have been studied. Di-isoamylborane is the more selective for cycloalkenes, the relative rates of hydroboration of cyclop>entene, cyclo-heptene, and cis-cyclo-octene being 1.0 18.6 68.6, respectively. Hydroboration of substituted cyclo-octenes using diborane is complicated by rapid isomerization of the initially formed alkyl-borane, leading to mixtures of alcohols on oxidation. However, 9-alkyl-9-borabicyclo[3,3,l]nonanes are thermally stable, and hydroboronation of 1-alkylcyclo-octenes using 9-BBN leads sp>ecifically to trans-2-alkylcyclo-octanols after oxidation. ... 

Metalloboranes have also been prepared in which a metal atom has been incorporated into a borane structure. While the lower boranes such as and BjHg are extremely air sensitive as well as being thermally rather unstable, the metalloboranes are often robust. Cobalt(II) chloride with a mixture of B5H and Cp affords a 5% yield of the red air-stable crystalline 2-CpCoB4Hg, which isomerizes at 200 C to the yellow 1-isomer (Fig. 11.9). The isoelectronic complex l-(0C)3FeB4Hg is one product from the reaction between Fe(CO)5 and BgH. The borane ligand in these complexes, B H, is isoelectronic with and hence forms complexes of similar structure to C H CoCp and C4H4Fe(C0)3 (p. 269). 

Another remarkable property of 9-BBN where it distinguishes itself from other dialkylboranes is its thermal stability. 9-BBN can be distilled at 195 C (12 mm) or heated for 24 h at 200 °C under nitrogen, without loss of hydride activity [2, 5], in sharp contrast to other dialkylboranes. For instance, dicyclo-hexylborane decomposes at 180-200 °C, yielding cyclohexene and polymeric boranes, and disiamylborane undergoes isomerization [5,17] at 75 C. 

The formation of the DADB salt comes from the reaction of small, hard bases (like ammonia) with a borane species. Considering B2H6, the base induces an asymmetric cleavage of borane into a BH2 fragment, and a BHq fragment [53]. The borohydride is stable, and BH2 coordinates two bases. In the thermal decomposition of amine borane, researchers often see induction periods wherein reactions are slow to begin, and then accelerate later. These induction periods are potentially isomerization of BH3NII3 into the DADB salt, followed by further reaction [83]. 

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