Boranes mechanism

Similar fragmentations to produce S-cyclodecen-l-ones and 1,6-cyclodecadienes have employed l-tosyloxy-4a-decalols and 5-mesyloxy-l-decalyl boranes as educts. The ringfusing carbon-carbon bond was smoothly cleaved and new n-bonds were thereby formed in the macrocycle (P.S. Wharton, 1965 J.A. Marshall, 1966). The mechanism of these reactions is probably E2, and the positions of the leaving groups determine the stereochemistry of the olefinic product. 

The regioselectivity and syn stereochemistry of hydroboration-oxidation coupled with a knowledge of the chemical properties of alkenes and boranes contribute to our under standing of the reaction mechanism... 

Retardation of the reaction rate by the addition of dimethyl sulfide is in accord with this mechanism. Borane—amine complexes and the dibromoborane—dimethyl sulfide complex react similarly (43). Dimeric diaLkylboranes initially dissociate (at rate to the monomers subsequentiy reacting with an olefin at rate (44). For highly reactive olefins > k - (recombination) and the reaction is first-order in the dimer. For slowly reacting olefins k - > and the reaction shows 0.5 order in the dimer. 

Polyhedral Expansion. The term polyhedral expansion is used to describe a host of reactions in which the size of the polyhedron is increased by the addition of new vertex atoms whether boron, heteroelements, or metals. In the case of the boranes, the pyrolysis of B2H has been used to obtain B H and industrially. Although a subject of much study, the mechanism of such pyrolytic expansions is not well understood. 

Though the mechanism has not been investigated thoroughly, it has been shown to be intramolecular by the failure to find crossover products when mixtures of boranes are used, The following scheme, involving three boron-to-carbon migrations, has been suggested. 

Synthesis depends on gas-phase pyrolysis techniques , and interconversion is by electrochemical oxidation . Understanding of the mechanisms by which the boranes interconvert and build up into larger clusters is growing . 

The answers to all three of these questions are encapsulated in the mechanism as usual. In order to explore the accepted mechanism, we must first acquaint ourselves with the reagents, hi the first step, the reagents are BH3 and THE The former is called borane. The element boron uses its three valence electrons to comfortably form three bonds ... 

This solvent is called tetrahydrofuran, or THF for short. Even though it somewhat stabilizes the empty p orbital on the boron atom in BH3, nevertheless the boron atom is very eager to look for any other sources of electron density that it can find. It is an electrophile—it is scavenging for sites of high electron density to fill its empty orbital. A pi bond is a site of high electron density, and therefore, a pi bond can attack borane. In fact, this is the hrst step of our mechanism. A pi bond attacks the empty p orbital of boron, which triggers a simultaneous hydride shift ... 

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. 

Catecholborane and pinacolborane are especially useful in hydroborations catalyzed by transition metals.163 Wilkinson s catalyst Rh(PPh3)3Cl is among those used frequently.164 The general mechanism for catalysis is believed to be similar to that for homogeneous hydrogenation and involves oxidative addition of the borane to the metal, generating a metal hydride.165... 

Secondary amines are formed by reaction of trisubstituted boranes with alkyl or aryl azides. The most efficient borane intermediates are monoalkyldichloroboranes, which are generated by reaction of an alkene with BHCl2 Et20.190 The entire sequence of steps and the mechanism of the final stages are summarized by the equation below. 

Reduction of ketones to triphenylsilyl ethers is effected by the unique Lewis acid perfluorotriphenylborane. Mechanistic and kinetic studies have provided considerable insight into the mechanism of this reaction.186 The salient conclusion is that the hydride is delivered from a borohydride ion, not directly from the silane. Although the borane forms a Lewis acid-base complex with the ketone, its key function is in delivery of the hydride. 

Prior literature indicated that olefins substituted with chiral sulfoxides could indeed be reduced by hydride or hydrogen with modest stereoselectivity, as summarized in Scheme 5.10. Ogura et al. reported that borane reduction of the unsaturated sulfoxide 42 gave product 43 in 87 13 diastereomer ratio and D20 quench of the borane reduction mixture gave the product 43 deuterated at the a-position to the sulfoxide, consistent with the hydroboration mechanism [10a]. In another paper, Price et al. reported diastereoselective hydrogenation of gem-disubstituted olefin rac-44 to 45 with excellent diastereoselectivity using a rhodium catalyst [10b],... 

Initially, two plausible mechanisms were considered, as depicted in Scheme 5.16. The first was a hydroboration route (a), where the B-H bond was added across the olefin from the same face of S-0 and upon aqueous work-up, the resulting C-B bond was replaced with a C-H bond. The tis B-H addition to the olefin led to the cis-stereochemistry of the two adjacent aryl substituents. The reduction of the sulfoxide oxygen occurs in the next step. The alternative mechanism was the borane reduction route (h), which was similar to 1,4-addition of hydride,... 

We have developed the efficient synthesis of the SERM drug candidate 1 and successfully demonstrated the process on a multiple kilogram scale to support the drug development program. A novel sulfoxide-directed borane reduction of vinyl sulfoxides was discovered. The mechanistic details of this novel reaction were explored and a plausible mechanism proposed. The sequence of asymmetric oxidation of vinyl sulfoxides followed by stereospecific borane reduction to make chiral dihydro-1,4-benzoxathiins was applied to the asymmetric synthesis of a number of other dihydro-1,4-benzoxathiins including the sweetening agent 67. 

Figure 33 The catalytic mechanism for the production of borane-terminated isotactic polypropylene (z-PPs) via in situ chain-transfer reaction by a styrene/hydrogen consecutive chain-transfer reagent allowing the utilization of MAO cocatalyst (50). (Adapted from ref. 74.)...

The peroxide 179 dissociates in the presence of a monomer giving rise to alkoxyl (CO-) and borinate (BO-) radicals, but the latter are believed to be too stable to initiate polymerization. It should be mentioned that the molecular weight continuously increases throughout the process implying the pseudo-living mechanism for chain growth. After the completion of the process borane residue is completely oxidized into diol <2004MM6260>. Thus, the 8-boraindane molecule not only initiates the polymerization, but also is a precursor to two functionalities in the polymer chain. 

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]. 

---