Conjugated systems hydroboration

The double bonds in a conjugated diene are hydroborated separately, that is, there is no 1,4 addition. However, it is not easy to hydroborate just one of a conjugated system, since conjugated double bonds are less reactive than isolated ones. Thexylborane °(48) is particularly useful for achieving the cyclic hydroboration of dienes, conjugated or nonconjugated, as in the formation of 53." ... 

M. Matsumi, K. Naka, and Y. Chujo, Extension of ir-conjugation length via the vacant p-orbital of the boron atom. Synthesis of novel electron deficient ir-conjugated systems by hydroboration polymerization and their blue light emitting, J. Am. Chem. Soc., 120 5112-5113, 1998. 

It has been observed that some conjugated polyenes can be selectively hydroborated with borane, 3) (Chart 3) probably due to the low reactivity of conjugated system in the intermediate organoborane. In some cases, the diene moiety within the polyene system has been protected by complexation 14-16). 

Hydroboration of conjugated systems has not been studied extensively. However, the apparent 1,4-reduction [3 ] of the 5,7 diene system (3) (see Fig. 25) occurs by the formation of the 5a-H, 6a-borane, A -olefin structure (4) which on subsequent treatment with acid loses its boron content with rearrangement to give the 6,7-olefin (5) [38]. 

Homolytic cleavage of the carbon-boron bond of a trialkylborane can be promoted by oxygen. The so-formed alkyl radical can be used in synthesis (for radical reactions, see Section 4.1). Indeed, triethylborane in air can be used to generate radicals from precursors such as alkyl iodides or selenides. Hydroboration followed by addition of an a,(3-unsaturated aldehyde or ketone leads to transfer of an alkyl group from the boron atom via an alkyl radical intermediate. The reaction takes place by addition of the alkyl radical to the conjugated system to form an enol borinate, hydrolysis of which gives the aldehyde or ketone product (5.37). 

Hydroboration of Alkenes Addition of Hydrogen Additions to Conjugated Systems Free-Radical Additions Polyethylene Oxidation of Alkenes A WORD ABOUT... Ethylene Raw Material and Plant Hormone... 

Selective Hydroboration of Dienes and Enynes. The opposite reactivity trends of DBBS and other hydroborating agents makes possible the selective hydroboration of dienes (eq 3) and enynes. In conjugated systems, however, bromoboration of the triple bond is observed. ... 

However, in contrast to compounds with either isolated or cumulative double bonds, electrophilic addition reactions with alkenes that contain conjugated n-systems routinely produce products resulting from the interaction of both double bonds. While hydroboration appears to be an exception, with each double bond reacting separately (and the second more rapidly than the first), conjugated dienes, such as 1,3-butadiene (CH2=CH-CH=CH2), suffer addition across each double bond ( 1,2-addition ) as well as across the entire conjugated system ( 1,4-addition ). Commonly, the products ( 1,2-addition and 1,4-addition ) are formed concurrently. 

Grignard chemistry was used as an alternative to hydroboration reaction by the Chujo team in its search for new n-type conjugated polymers for utilization in polymer energy storage systems. By this route, the syntheses of 7r-conjugated poly(p-phenylene-boranes) (15)35 (Fig. 12) and poly(ethynylene-phenylene-ethynylene-borane)s were reported.36... 

The a,p-acetylenic ketones can be synthesized in good yields by the selective mono-hydroboration-oxidation process of conjugated diynes. The monohydroboration of conjugated diynes with disiamylborane places boron preferentially at the internal triple position of the diyne system. The resultant organoboranes on treatment with sodium hydroxide and 30% H202 afforded the a,P-acetylenic ketones (Eq. 33) 79). 

Conjugated dienes are deactivated relative to isolated double bonds towards 9-BBN-H, so monohydroboration is difficult to achieve unless the second hydroboration stage is slowed by intrinsic lack of reactivity of the system or by steric crowding, when it is possible to produce allylic organoboranes e.g. equation 33). ... 

In a vinylic system, the chlorine not only retards the rate of reaction but direct the hydroboration to the 2 position. The inductive effect of chlorine here reduces the electron availability of the it bond, thus severely retarding the rate of the reaction, while some conjugation returns some electron density to the site (3 to the chlorine, which then becomes the preferred site of hydroboration. 

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