Organoboranes stability

Abbey, K. J. and Kendall, J. L., Internally coordinated organoboranes Stability and activation in polyolefin adhesives, Polym. Mater. Sci. Eng. 2004(2). 

Internally coordinated organoboranes, for example, l, l -dialkyl-3,4-dihydro-2//-pyrido[2,l-/][ l, 2]azaborine ylides, have superior long-term air stability over externally coordinated organoborane <2004MI175>. 

The seven-membered fully unsaturated boron heterocycle is named l//-borepin or, more frequently, borepin. It has six 7r-electrons and is therefore a potential Hiickel aromatic, isoelectronic with the tropylium ion. This similarity has induced several groups to search for a synthesis. Recent theoretical calculations have predicted the -stabilization to be less than that of the tropylium ion. On the other hand, borepins are organoboranes and therefore expected to be sensitive to oxygen. 

One chemical property quoted in support of aromatic character is kinetic stability towards hydrolytic break-down. Many boron heterocycles have been stated to be more stable than expected for organoboranes. On the other hand, several saturated cyclic boron compounds are stable as well, suggesting that the cyclic structure itself is favorable. 

Complexation is believed to be involved in the first step of the hydrobo-ration of alkenes and alkynes rearrangement reactions of organoboranes most likely involve intermediates of a rc-complex type. However, the stability of such complexes is generally too low to allow their isolation (112). However, evidence for 7t-complex formation has been obtained by the device of anchoring the alkene function to the metal atom in question by means of a... 

The high reactivity of organoboranes to oxidizing agents can be ascribed to the availability of a vacant p-orbital on boron, whidi provides the oppmtunity for a kinetically favorable attadc, and to the thermodynamic stability of the B—O bond (111-120 kcal mol 1 cal = 4.18 J) compared with the B—C bond (81-88 kcalmol- ). ... 

Boron-stabilized carbanions may also be produced by selective cleavage of a heteroatom group from an a-substituted organoborane such as a borylstannylmethane (equations... 

It appears that the first example of deprotonation to give a boron-stabilized carbanim was that shown in equation (9). In general, however, early attempts to deprotonate organoboranes (equation 10) foundered because borate formation (equation 11) was favored. 

In general, all types of boron-stabilized carbanions are readily alkylated by primary alkyl halides. The situation for the i ylation of dimesitylboryl compounds is sununarized in Scheme 2. These reactions represent highly efficient homologation processes. Oxidation of the tertiary all l organoboranes is slow, but use of 4-methoxy-2,6-dimethylphenyl groups instead of mesityl groups renders the products sensitive to solvolysis. ... 

Boron-stabilized carbanions. Ordinarily bases coordinate with the boron atom of organoboranes. However, Rathke and Kow report that a highly hindered lithium amide such as lithio-2,2,6,6-tetramethylpiperidine or lithio-t-butylneopentylamine2 can remove the a-proton from an organoborane to generate carbanions. Thus treatment of the boron compound B-methyl-9-borabicyclononane (1)J in benzene with the former base for 12 hr. at room temperature followed by quenching with deuterium oxide results in deuterium incorporation of 50% (equation I). 

Organoboranes can be efficiently bis-homologated to the corresponding acetic acid derivatives by reaction with the dianion derived from phenoxy-acetic acid. The stability of the phenoxlde leaving group provides the driving force for the transformation. ... 

Organoboranes possessing one B—C bond in the molecule can also be obtained by hydroborating alkenes and alkynes with heterosubstituted borane derivatives, especially dihalogenoboranes, which can be stabilized by complexation with amines, phosphines, ethers and sulfides. The last two are useful complexing agents for both the preparation and the reactions of dihalogenoboranes . The most convenient synthesis of dichloroborane etherate is the reaction of LiBH. with BCl, in Et,0 ... 

A limitation of SiajBH is the low thermal stability of both the reagent and the organoboranes derived from it. Dicyclohexylborane showing similar hydroboration characteristics often substitutes for SiajBH. In applications requiring thermal treatment... 

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