Borane-alkene complexes

The mechanism involves the dissociation of the coordinated borane 15 to generate a monoborane intermediate 16. Coordination of the alkene would generate the alkene borane complex. A /3-borylalkylhydride with B-H stabilization is certainly an important resonance structure of 17. An intramolecular reaction would extrude the alkyl boronate ester product and coordination of HBcat would regenerate the monoborane intermediate. 

In the Cp2TiMe2-catalyzed hydroboration of alkenes, a titanocene bis(borane) complex is responsible for the catalysis. This bis(borane) complex initially dissociates to give a monoborane intermediate. Coordination of the alkene gives rise to the alkene-borane complex, which is likely to be a resonance hybrid between an alkene borane complex and a 3-boroalkyl hydride. An intramolecular reaction extrudes the trialkylborane product, and coordination of a new HBR2 regenerates the monoborane intermediate. 

Similar to the addition of secondary phosphine-borane complexes to alkynes described in Scheme 6.137, the same hydrophosphination agents can also be added to alkenes under broadly similar reaction conditions, leading to alkylarylphosphines (Scheme 6.138) [274], Again, the expected anti-Markovnikov addition products were obtained exclusively. In some cases, the additions also proceeded at room temperature, but required much longer reaction times (2 days). Treatment of the phosphine-borane complexes with a chiral alkene such as (-)-/ -pinene led to chiral cyclohexene derivatives through a radical-initiated ring-opening mechanism. In related work, Ackerman and coworkers described microwave-assisted Lewis acid-mediated inter-molecular hydroamination reactions of norbornene [275]. 

By chance, the existence of the borane complex 330 of 329 was discovered. The liberation of 330 occurred with the best efficiency with sodium bis(trimethylsilyl)-amide from the borane complex 327 of 326. When styrene or furan was used as the solvent, three diastereomeric [2 + 2]-cycloadducts 328 and [4 + 2]-cycloadducts 331, respectively, were obtained in 30and 20% yield (Scheme 6.70) [156]. With no lone pair on the nitrogen atom, 330 cannot be polarized towards a zwitterionic structure, which is why its allene subunit, apart from the inductive effect of the nitrogen atom, resembles that of 1,2-cydohexadiene (6) and hence undergoes cycloaddition with activated alkenes. It is noted that the carbacephalosporin derivative 323 (Scheme 6.69) also does not have a lone pair on the nitrogen atom next to the allene system because of the amide resonance. 

Reduction of alkenes to alcohols.3 The combination of TiCI4 and NaBH4 in DMF produces a low-valent titanium-borane complex that converts alkcncs to alcohols in which the hydroxy group is introduced by an anti-Markovnikoff addition. 

Hartwig, J. F., Muhoro, C. N. Mechanistic Studies of Titanocene-Catalyzed Alkene and Alkyne Hydroboration Borane Complexes as Catalytic Intermediates. Organometattics 2000,19, 30-38. 

DMAP has also proved useful in releasing an imine-bound borane complex formed after an alkene hydroboration reaction, thereby providing higher yields than the usual workup (56% vs. 35%). ... 

Reactions catalyzed by titanocene and lanthanocene complexes occur by different mechanisms (Schemes 16.13 and 16.14). The mechanism of the hydroboration of vinylarenes catalyzed by titanium complexes is shown in Scheme 16.13. In this mechanism, a titanocene bis-borane complex dissociates borane to generate a 16-electron complex that coordinates the alkene or alkyne. Coupling of one carbon of the resulting metallacycle with the boron of the coordinated borane forms the final product. The mechanism of the lanthanocene-catalyzed reactions, shown in Scheme 16.14, relates to the mechanism of... 

A quite new class of catalysts based on early main group metals (Ca, Sr, and K) was recently reported to promote general conversion of conjugated double bond (137). The catalytic reaction is initiated by the formation of a highly reactive metal hydride that adds either to an alkene or to a silane. The regiochemistry for the hydrosilylation of 1,1-diphenylethylene (DPE) catalyzed by calcium complex can be completely controlled by the polarity of the solvent. Amine borane and phosphine borane complexes were successfully used as effective catalysts for hydrosilylation of organic compounds with internal unsaturated bond (138) that cannot be selectively hydrosilylated in the presence of Pt catalysts. 

AAHeterocyclic carbene-borane complexes have been shown to be useful as hydrogen sources through radical mechanisms, as Chu et al [85] determined after their initial work with the reduction of xanthates mentioned earlier. Also, substituted imidazol-2-ylidene and triazol-3-ylidenes have been employed in the reduction of dodecyl iodide to dodecane without the addition of any radical initiator. Furthermore, even in the presence of usually borane-reactive species, such as ketones or alkenes, the reduction of the alkyl halide is the only reaction observed. Later, however, Ueng et al foimd that alkyl halides with nearby... 

An experimental study of the hydroboration of prochiral alkenes with chiral Lewis base-borane complexes has been undertaken. The study used N-isobornyl-N-methylaniline-borane... 

Hydroboration attracted several theoretical studies in 1978. Publications which have appeared include an ab initio study of the reaction of BHs with ethylene, a CNDO/2 study of the nature of the ethylene-borane complex, the transition state in the hydroboration reaction, a MNDO study of hydroboration of alkenes and alkynes, and a MNDO study of hydroboration and borohydride reduction with implications concerning cyclic conjugation and pericyclic reactions. Finally, HaBCHO was one of the molecules which has been studied with regard to banana bonds of the carbonyl group. ... 

V. T. Perchyonok and Kellie L. Tuck of Monash University found (Tetrahedron Lett. 2008, 49, 4777) that a concentrated solution of Bu NCl and HjPOj in water effected free radical reductions and cyclizations. Stephane G. Ouellet of Merck Frosst demonstrated (Tetrahedron Lett. 2008, 49, 6707) that an oxazoline such as 3 could be converted to the alcohol 4 by acylation followed by reduction. Elizabeth R. Burkhardt of BASF developed (Tetrahedron I tt. 2008, 49, 5152) a protocol for scalable reductive amination using an easily metered liquid pyridine-borane complex. Mohammad Movassaghi of MIT devised (Angew. Chem. Int Ed. 2008, 47, 8909) a strategy for conversion of an allyUc carbonate 8 by way of the ally lie diazene to the terminal alkene 9. 

Various chiral ligands were studied in the Pd-catalyzed reaction of methyl-phenylphosphine with 1-ethynylcyclohexene (Scheme 8.56) [139] (for racemic version of the reaction see Ref. [140]). P-Stereogenic vinyl phosphines were prepared with enantiomeric excess up to 30% e.e. The best results were achieved with (R,R)-Me-Duphos and (R,R)-i-Pr-Duphos in toluene at 35 °C. Increasing the temperature to 50 °C led to better conversion of the reagents and slightly better enantioselectivity. It should be pointed out that hydrophosphination of the double bond of alkenes with phosphine-borane complex did not require a catalyst and was carried out under regular thermal or microwave heated conditions [141]. 

A more convenient hydroboratmg agent is the borane-tetrahydrofuran complex (H3B THF) It IS very reactive adding to alkenes within minutes at 0°C and is used m tetrahydrofuran as the solvent... 

We can consider the hydroboration step as though it involved borane (BH3) It sim phfies our mechanistic analysis and is at variance with reality only m matters of detail Borane is electrophilic it has a vacant 2p orbital and can accept a pair of electrons into that orbital The source of this electron pair is the rr bond of an alkene It is believed as shown m Figure 6 10 for the example of the hydroboration of 1 methylcyclopentene that the first step produces an unstable intermediate called a tt complex In this rr com plex boron and the two carbon atoms of the double bond are joined by a three center two electron bond by which we mean that three atoms share two electrons Three center two electron bonds are frequently encountered m boron chemistry The tt complex is formed by a transfer of electron density from the tt orbital of the alkene to the 2p orbital... 

Step 1 A molecule of borane (BH3) attacks the alkene Electrons flow from the 7C orbital of the alkene to the 2p orbital of boron A 7C complex is formed... 

A number of less hindered monoalkylboranes is available by indirect methods, eg, by treatment of a thexylborane—amine complex with an olefin (69), the reduction of monohalogenoboranes or esters of boronic acids with metal hydrides (70—72), the redistribution of dialkylboranes with borane (64) or the displacement of an alkene from a dialkylborane by the addition of a tertiary amine (73). To avoid redistribution, monoalkylboranes are best used /V situ or freshly prepared. However, they can be stored as monoalkylborohydrides or complexes with tertiary amines. The free monoalkylboranes can be hberated from these derivatives when required (69,74—76). Methylborane, a remarkably unhindered monoalkylborane, exhibits extraordinary hydroboration characteristics. It hydroborates hindered and even unhindered olefins to give sequentially alkylmethyl- and dialkylmethylboranes (77—80). 

Primary dialkylboranes react readily with most alkenes at ambient temperatures and dihydroborate terminal acetylenes. However, these unhindered dialkylboranes exist in equiUbtium with mono- and ttialkylboranes and cannot be prepared in a state of high purity by the reaction of two equivalents of an alkene with borane (35—38). Nevertheless, such mixtures can be used for hydroboration if the products are acceptable for further transformations or can be separated (90). When pure primary dialkylboranes are required they are best prepared by the reduction of dialkylhalogenoboranes with metal hydrides (91—93). To avoid redistribution they must be used immediately or be stabilized as amine complexes or converted into dialkylborohydtides. 

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