Organoboranes preparation

Trans-fused bicyclic organoboranes prepared by this method serve as intermediates in a convenient annelation reaction ... 

Organoboranes undergo transmetallation. 1-Hexenylboronic acid (438) reacts with methyl acrylate via the transmetallation with Pd(OAc)2, giving methyl 2,4-nonadienoate (439)[399], The ( )-alkenylboranes 440, prepared by the hydroboration of terminal alkynes, are converted into the alkylated ( )-alkenes 441 by treatment with an equivalent amount of Pd(OAc)2 and triethylamine[400]. The ( )-octenylborane 442 reacts with CO in MeOH in the... 

Alkyl ketones can be prepared by the carbonylation of alkyl iodides in the presence of organoboranes. The carbonylation of iodocyclohexane with 9-octyl-9-BBN at 1 atm gives cyclohexyl octyl ketone in 65% yield[386]. This reaction is treated in Section 1.1.3.3. Methyl o-methylacetoacetate (919) is obtained by the reaction of the 2-bromopropionate 918, which has a /9-hydrogen, with CO and Me4Sn. PhjAs as a ligand gives better results than Ph3P[771]. 

Organoboranes react with propargylic carbonates. Usually, addition of a base is essential for the Pd-catalyzed reactions of organoboranes, but the reaction with propargylic carbonates proceeds without addition of the base, because methoxide is generated in situ from carbonates. For example, the 1,2,4-triene 80 is prepared by the reaction of alkenylborane under neutral conditions[36]. 

Boron trifluoride is used for the preparation of boranes (see Boron compounds). Diborane is obtained from reaction with alkafl metal hydrides organoboranes are obtained with a suitable Grignard reagent. 

Usually, organoboranes are sensitive to oxygen. Simple trialkylboranes are spontaneously flammable in contact with air. Nevertheless, under carefully controlled conditions the reaction of organoboranes with oxygen can be used for the preparation of alcohols or alkyl hydroperoxides (228,229). Aldehydes are produced by oxidation of primary alkylboranes with pyridinium chi orochrom ate (188). Chromic acid at pH < 3 transforms secondary alkyl and cycloalkylboranes into ketones pyridinium chi orochrom ate can also be used (230,231). A convenient procedure for the direct conversion of terminal alkenes into carboxyUc acids employs hydroboration with dibromoborane—dimethyl sulfide and oxidation of the intermediate alkyldibromoborane with chromium trioxide in 90% aqueous acetic acid (232,233). 

Others. The stereoselective deuteration by deuterioboration or hydroboration—deuteriolysis is straightforward (207,459,460). Olefins monodeuterated in the aHyhc position are readily prepared by deuteriolysis of aHylic organoboranes (461). 

This reaction, now termed hydroboration, has opened up the quantitative preparation of organoboranes and these, in turn, have proved to be of outstanding synthetic utility. It was for his development of this field that H. C. Brown (Purdue) was awarded the 1979 Nobel Prize in Chemistry . Hydroboration is regiospecific, the boron showing preferential attachment to the least substituted C atom (anti-Markovnikov). This finds ready interpretation in terms of electronic factors and relative bond polarities (p. 144) steric factors also work in the same direction. The addition is stereospecific cis (syn). Recent extensions of the methodology have encompassed the significant development of generalized chiral syntheses. 

Direct transmetalation of organoboranes to organocopper reagents is not a general reaction. Because of dieir similar bond energies and electronegativities, diis trans-nietalation is linided to die preparation of alkenylcopper and unfiinctionalized... 

Organoboranes react with a mixture of aqueous NH3 and NaOCl to produce primary amines. It is likely that the actual reagent is chloramine NH2CI. Chloramine itself,hydroxylamine-O-sulfonic acid in diglyme, and trimethyl-silyl azide " also give the reaction. Since the boranes can be prepared by the hydroboration of alkenes (15-16), this is an indirect method for the addition of NH3 to a double bond with anti-Markovnikov orientation. Secondary amines can be prepared by the treatment of alkyl- or aryldichloroboranes or dialkylchlorobor-anes with alkyl or aryl azides. 

For a method of preparing organoboranes from RMgX and BF3, where the RMgX is present only in situ, see Brown, H.C. Racherla, U.S. Tetrahedron Lett., 1985, 26, 4311. For reviews as applied to Si, B, and P, see Wakefield, Ref. 291, p. 149 Kharasch, M.S. Reinmuth, O. Grignard Reactions of Nonmetallic Substances Prentice-Hall Englewood Cliffs, NJ, 1954, p. 1306. 

Several alkylboranes are available in enantiomerically enriched or pure form and can be used to prepare enantiomerically enriched alcohols and other compounds available via organoborane intermediates.196 One route to enantiopure boranes is by hydroboration of readily available terpenes that occur naturally in enantiomerically enriched or pure form. The most thoroughly investigated of these is bis-(isopinocampheyl)borane (Ipc)2BH), which can be prepared in 100% enantiomeric purity from the readily available terpene a-pinene.197 Both enantiomers are available. 

The most widely used route to organoboranes is hydroboration, introduced in Section 4.5.1, which provides access to both alkyl- and alkenylboranes. Aryl-, methyl-, allylic, and benzylboranes cannot be prepared by hydroboration, and the most general route to these organoboranes is by reaction of an organometallic compound with a halo- or alkoxyboron derivative.1... 

It is now clear that pure pheromones can be synthesized in quantity. The problem is how to prepare them simply and efficiently. New synthetic methodologies are always welcome to improve the existing syntheses. Organoborane reactions and organotransition metal chemistry contributed much to improve the efficiency of carbon-carbon bond formation, while asymmetric epoxidations and dihydroxylations as well as enzymatic reactions greatly improved the enantiomeric purity of synthetic pheromones. 

Three basic methods have been used for the preparation of organodihaloboranes, RBX2 (1) the direct interaction of hydrocarbons with haloboranes, (2) the reaction of organometallic derivatives with haloboranes, and (3) the halogenation of organoboranes. One of the most convenient laboratory procedures involves the reaction of tetraorganotin derivatives with trihaloboranes, BX3.1-8 This reaction works particularly well for the preparation of alkyldihaloboranes as outlined below. 

A Simple and Convenient Method for the Oxidation of Organoboranes Using Sodium Perborate Preparation of (+)-Isopinocampheol. 

Scheme 2.17. Preparation of organocopper reagents from organoboranes.

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