Initiators trialkylboranes

An ingenious application of Corey s ylide (1) was discovered by the Shea group in 199 7 51,52 ugjj g trialkylboranes as initiator/catalyst and 1 as the monomer, a living... 

Trialkylboranes, especially triethylborane, are used in conjunction with 02 to generate radicals.297 The alkyl radicals are generated by breakdown of a borane-oxygen adduct. An advantage this method has over many other radical initiation systems is that it proceeds at low temperature, e.g., -78°C. 

One of the first synthetic applications of organoboranes in radical chemistry is the conjugate addition to enones (Scheme 23, Eq. 23a) and enals reported by Brown [58-61]. Addition to -substituted enones and enals are not spontaneous and initiation with the oxygen [62], diacetyl peroxide [63], or under irradiation [63] is necessary (Eq. 23b). A serious drawback of this strategy is that only one of the three alkyl groups is efficiently transferred, so the method is restricted to trialkylboranes derived from the hydroboration of easily available and cheap alkenes. To overcome this limitation B-alkylboracyclanes have been used but this approach was not successful for the generation of tertiary alkyl radicals [64,65]. 

Allenylboranes can be prepared by treatment of propargylic acetates with butyl-lithium and a trialkylborane [19]. The reaction proceeds by initial formation of an alkynylboronate followed by migration of an alkyl group from boron to carbon (Eq. 9.17). 

Triethylborane in the presence of very small amounts of oxygen is an excellent initiator for radical chain reactions. For a long time it has been known that trialkylboranes R3B react spontaneously with molecular oxygen to give alkyl radicals (Reaction 4.7), but only recently has this approach successfully been applied as the initiation [22]. The reactions can be run at temperatures as low as — 78 °C, which allow for a better control of stereoselectivity (see below). 

In the interim period, results have accumulated steadily, in endeavors to address and extend the chemistry beyond the initial perceived limitations. These limitations include the following (a) the effective catalytic syntheses are confined to the reactions utilizing catecholborane (b) the scope of alkenes for which efficient rate, regio- and enantio-selectivity can be achieved is limited, and (c) the standard transformation mandates the oxidation of the initially formed (secondary) boronate ester to a secondary alcohol, albeit with complete retention of configuration [8]. Nonetheless, for noncatalytic hydroboration reactions that lead to the formation of a trialkylborane, a wide range of stereo-specific transformations may be carried out directly from the initial product, and thereby facilitate direct C-N and C-C bond formation [9]. 

Thus, the initiator can be a trialkylborane to which less than one molecule of oxygen per molecule of borane is added, or it can be an alkylperoxydialkylborane to which additional trialkylborane is added. In the latter case, it is convenient to use a standardized hexane solution of an alkylperoxydialkylborane, which is stable for about two weeks if stored at — 78° C. It is important to recognize that use of excess oxygen with the borane in the former case will result in no initiation or very poor initiation. 

The polymerization of dimethylsulfoxonium methylide (17) initiated by trialkylborane, which Shea et al. developed, is classified as the first category. Propagation involves insertion of 17 into the terminal C-B bond with elimination of DMSO. The polymerization was carried out in toluene at 70-80 °C, followed by oxidative workup to yield hydroxyl-terminated polymethylene (Scheme 71). The Mn values were very close to the calculated values from the feed ratio of 17 to trialkylborane, and the Mw/Mn ratio ranged from 1.04 to 1.17. These results are consistent with the character of living polymerization. 

Unhindered alkenes react rapidly with borane to give initially monoalkylboranes, then dialkylboranes, and finally trialkylboranes. The reaction of borane with ethene is illustrated in Equation B1.4. 

These transformations share common initial steps in which the carbon monoxide interacts with the trialkylborane to give an intermediate organoborate. This readily transfers one of its alkyl groups to the carbon atom derived from carbon monoxide to give intermediate X (Figure B3.1). 

The reaction of bis(phenylsulfanyl)alkyllithiums with trialkylboranes provides the adduct 144, after elimination of phenylsulfanyllithium. Final oxidation with hydrogen peroxide-dioxane in aqueous sodium acetate afforded aldehydes or ketones in good yields (Scheme 40)151,152. However, this process cannot be carried out with 2-lithio-l,3-dithiane because the initially formed borate did not undergo the spontaneous alkyl migration reaction152. 

In another early example, C radicals, which were generated from the reaction of trialkylboranes with dioxygen, O2, were added to a-epoxy alkynes 6 to give allenes 7 (Scheme 2.2). In this reaction, the initially formed vinyl radicals (or a-oxiranyl... 

Niedenzu initiated studies of B-substituted analogs of (47) and (48) and various X = chloro, fluoro, alkyl, aryl, alkoxy, and amido derivatives of (49). These compounds should be very useful for further synthetic transformations. Koster and coworkers have examined the reactions of sterically congested pyrazoles with (9H-9-BBN)2 and unique neutral, monomeric products with tricoordinate boron centers (50) are obtained. The compounds are stable toward dimerization. When bulky pyrazoles are allowed to react with activated trialkylboranes, for example, BEts at 170 °C,... 

Homologation of trialkylboranes. Trialkylboranes, in the presence of oxygen or other free-radical initiators, react with 1,3-butadiene monoxide to give 4-alkyl-2-butenc-l-ols resulting from four-carbon-atom homologation, for example ... 

The reaction of alkenes with borane, monoalkyl and dialkylboranes leads to a new organoborane (see 15-16). Treatment of organoboranes with alkaline H2O2 oxidizes trialkylboranes to esters of boric acid." This reaction does not affect double or triple bonds, aldehydes, ketones, halides, or nitriles that may be present elsewhere in the molecule. There is no rearrangement of the R group itself, and this reaction is a step in the hydroboration method of converting alkenes to alcohols (15-16). The mechanism has been formulated as involving initial formation of an ate complex when the hydroperoxide anion attacks the electrophilic boron... 

Carbalumination of 271b with tri-isopropylaluminium affords acyclic products, the formation of which can be attributed to rearrangement of the initially formed cyclopropyl carbocation (equation 84). Ring-opened products also result from reactions with trialkylboranes, but only cyclopropanes, from addition to the n bond, are seen in reactions with organomagnesium reagents . 

In the Cp2TiMe2-catalyzed hydroboration of alkenes, a titanocene bis(borane) complex is responsible for the catalysis. This bis(borane) complex initially dissociates to give a monoborane intermediate. Coordination of the alkene gives rise to the alkene-borane complex, which is likely to be a resonance hybrid between an alkene borane complex and a 3-boroalkyl hydride. An intramolecular reaction extrudes the trialkylborane product, and coordination of a new HBR2 regenerates the monoborane intermediate. 

The reactions of alkenes (and of alkynes) can also be carried out at room temperature or below, using a trialkylborane as catalyst.95 97 The initiation depends on the presence of an adventitious trace of oxygen which reacts with the borane to generate alkyl and alkylperoxyl radicals and thence organoperoxyboranes 98... 

The general principles of hydrostannation are described in Section 4.4. The most common tin hydride to be used is tributyltin hydride, which is commercially available. Al-kynes are more reactive than alkenes. With non-polar alkynes, the reaction follows a radical chain mechanism, and is typically carried out at 60-80 °C, often with AIBN as a radical initiator, but reactions can be carried out at room temperature or below with initiation with UV light or sonication,8 or with a trialkylborane in the presence of a trace of air.9... 

In the preceding chapters it was noted that trialkylboranes are useful radical initiators as well as an efficient source of alkyl radicals. Organoboranes can also be used as chain transfer reagents. This approach is used when the direct reaction between the radical precursor and the radical trap cannot proceed (Scheme 6.7). Alkyl radicals generated from the organoboranes are not involved in product formation, but they produce the radicals leading to products. For this purpose, an extra step such as an iodine atom transfer or a... 

It has been suggested that trialkylborane initiators form a complex with the radical end95. This has been shown by comparing the activation parameters for the tri-n-butylborane-initiated polymerization of vinyl trimethylacetate and of vinyl tri-fluoroacetate in various solvents with those for the AIBN-initiated polymerization (Table 2). The difference of stereoregulating activation parameters between the initiators may be due to the solvent effect on the formation of a complex of trialkylborane with the radical end. 

It is known that certain organometallic compounds act as radical initiators. Especially, trialkylborane analogs have been used very well as initiators in many stereo-... 

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