Boranes, elimination with alkenes

Insertions and (3-eliminations are also the microscopic reverse of each other. In an insertion, an A=B 77 bond inserts into an M-X bond (M-X + A=B —> M-A-B-X). The M-X and A=B bonds are broken, and M-A and B-X bonds are formed. Insertion is usually preceded by coordination of the A=B 77 bond to the metal, so it is sometimes called migratory insertion. In an insertion, an M-X bond is replaced with an M-A bond, so there is no change in oxidation state, d electron count, or total electron count. However, a new a bond is formed at the expense of a 77 bond. The nature of the reaction requires that the new C-M and C-H bonds form to the same face of the A=B 77 bond, resulting in syn addition. The reaction of a borane (R2BH) with an alkene to give an alkylborane is a typical insertion reaction that you have probably seen before. 

In comparison with the hydroboration and diborafion reactions, thioboration reactions are relatively limited. In 1993, Suzuki and co-workers reported the Pd(0)-catalyzed addition of 9-(alkylthio)-9-BBN (BBN = borabicyclo [3.3.1] nonane) derivatives to terminal alkynes to produce (alkylthio)boranes, which are known as versatile reagents to introduce alkylthio groups into organic molecules [21], Experimental results indicate that the thioboration reactions, specific to terminal alkynes, are preferentially catalyzed by Pd(0) complexes, e.g. Pd(PPh3)4, producing (thioboryl)alkene products, in which the Z-isomers are dominant. A mechanism proposed by Suzuki and co-workers for the reactions involves an oxidative addition of the B-S bond to the Pd(0) complex, the insertion of an alkyne into the Pd-B or Pd-S bond, and the reductive elimination of the (thioboryl)alkene product. 

Fig. 2.8 A d irect route to vinylboranes in rhodium-catalyzed hydroborations with phos-phine-free catalysts (including oxidative degradation of a rhodium phosphine). The key intermediate is a rhodium hydride, capable of reversible insertion into the alkene (step A), followed by addition of borane in step B. This leads to reductive elimination of RH in step C followed by boryl migration in step D. A further...

An alternative method of hydroboration is to use diisopinocampheylborane (12) (Scheme 4). This reaction is particularly useful for sterically hindered alkenes. Diisopinocampheylborane (12) is prepared from borane-dimethyl sulfide and (+)-pinene.[23-24] Treatment of 4-meth-ylenecyclohexanone ethylene ketal with diisopinocampheylborane (12) gives the borane 13.[25] Further treatment with 2 equivalents of an aldehyde results in the elimination of pinene and the formation of a new dialkyl boronate, e.g. treatment of 13 with acetaldehyde gives the diethyl cyclohexylmethylboronate 14J261 The dialkyl boronates thus produced can be transesterified with pinanediol to give 15[26] or with other cyclic diols. 

Asymmetric hydroboration 2171 of prochiral alkenes with monoisopinocampheyl-borane in the molar ratio of 1 1, followed by a second hydroboration of non-prochiral alkenes with the intermediate dialkylboranes, provides the chiral mixed trialkylbo-ranes. Treatment of these trialkylboranes with acetaldehyde results in the selective, facile elimination of the 3-pinanyl group, providing the corresponding chiral borinic acid esters with high enantiomeric purities. The reaction of these intermediates with base and dichloromethyl methyl ether provides the chiral ketones (Eq. 130)2l8>. A simple synthesis of secondary homoallylic alcohols with excellent enantiomeric purities via B-allyldiisopinocampheylborane has been also reported 219),... 

Attack of diborane occurs at the /2-carbon atom with the exception of enamines of acetaldehyde for which a-borane product is formed. When isomerism is possible, as in the case of a 2-methylcyclohexanone enamine, the less substituted isomer is the more reactive form, for steric reasons. Protonolysis with propionic acid in diglyme afforded alkenes, as a consequence of a trans elimination of the base and the borane group166. 

The reacticxis of a-TMS-substituted boranes with aldehydes proceed with elimination of the TMS group to give alkenylboranes as in equations (45) and (46). Such products can be hydrolyzed to alkenes or oxidized to aldehydes. The intermediate in the case of the dimesitylborane reaction was directly oxidized to aldehyde in 95% overall yield, a good formyl homologation process. ... 

Examples of alkene and alkyne insertions followed by transmetallation with organotin compounds as well as boranates, and eventually reductive elimination are shown in Scheme 16.[ 0l.ns].[37]-[39] reactions do not only require that the transmetalla-... 

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