Boron-alkyl migration

Similarly, organoaluminum compounds in which the aluminum center is tetra-coordinated by way of a peroxo three-membered heterocycle subsequently undergo migration of alkyls, giving aluminum alkoxides (Scheme 6.142) [182]. The reaction is very similar to the boron-alkyl migration via a peroxoboron complex, which produces a variety of alcohols very efficiently. Likewise, mild oxidation of aluminum alkyls by controlled introduction of dry air has found substantial application in industrial production of the corresponding alcohols. 

Treatment of the borates with iodine leads to boron- C2 migration of an alkyl group[9]. This reaction has not been widely applied synthetically but it might be more applicable for introduction of branched alkyl groups than direct alkylation of an indol-2-yllithium intermediate. 

Although both boronates and alanates react with allylic bromides, aldehydes and C02 to afford allenic products in satisfactory yield, the alanates are more efficient in additions to ketones (Table 9.14). Boronate reagents do not require a B-C alkyl migration for their preparation. Thus the starting acetylene possesses the structural elements of the product. Additionally, the issue of dummy ligands is irrelevant. 

Thermal isomerization of a-cyclohexylidenecycluhexanone oxide (liq. 472) was likewise found to proceed by acyl migration, 27 rather than by alkyl migration aa formerly believed.1 1 Curiouely, boron trifluoride failed to give recognizable products. 

Aldehydes from alkylhoronic esters, RB(OR1)2. This reagent reacts with al-kylboronic esters (2) to provide an ate complex (a). Addition of HgCl2 induces alkyl migration from boron to carbon to give 3, which is oxidized to a homologated aldehyde 4.1... 

The reactivity profiles of the boronate complexes are also diverse.43 For example, the lithium methyl-trialkylboronates (75) are inert, but the more reactive copper(I) methyltrialkylboronates (76) afford conjugate adducts with acrylonitrile and ethyl acrylate (Scheme 16).44 In contrast, the lithium alkynylboronates (77) are alkylated by powerful acceptors, such as alkylideneacetoacetates, alkylidene-malonates and a-nitroethylene, to afford the intermediate vinylboranes (78) to (80), which on oxidation (peracids) or protonolysis yield the corresponding ketones or alkenes, respectively (Scheme 17).45a Similarly, titanium tetrachloride-catalyzed alkynylboronate (77) additions to methyl vinyl ketone afford 1,5-diketones (81).4Sb Mechanistically, the alkynylboronate additions proceed by initial 3-attack of the electrophile and simultaneous alkyl migration from boron to the a-carbon. 

The reaction is thought to proceed as shown in Figure B4.2. After hydroboration and addition of hydroxide anion to the boron atom, an iodonium species is formed which creates an electrophilic terminus for alkyl migration. (The migration occurs with retention of configuration of the alkyl group.) Finally an antiperiplanar elimination reaction gives the (Z)-alkene. 

Boron alkyls are expected to be inactive for coordinated anionic polymerization of olefins because the boron-carbon bond is not sufficiently ionic. The diazomethane polymerization with boron alkyl catalyst reported by Bawn, Ledwith and Matthies (275) is a special case of the growth reaction. A coordination mechanism seems most probable, but it has not been ascertained whether the polymer chain migrates as a car-banion or as a radical. If the complex between diazomethane and boron decomposes into a boron-carbene complex, then the polymer chain could migrate as a carbanion with the driving force provided by the electrophilic carbonium ion ... 

The photostadDility of boron-containing orgamic complexes has been discussed, amd a study maule of the luminescent properties of p-diketonates of disubstituted boron amd of bis-p-diketonatoboron. A novel photochemical alkyl migration from B to C in the dialkyIboryl acetylacetonate complexes (1) ( R-cyclohexyl, Bu, isopinocaunpheyl, R -H R-cyclohexyl, Bu, R -Me ) has appeared. Crossover experiments show that the alkyl migration is essentially intraunolecular Silicon... 

These alkyl boranes are not used as precursors for alkyl-lithiums but more usually oxidised to alcohols 59 with alkaline H202. This reaction involves nucleophilic attack by HO—O anion on boron 83 followed by alkyl migration from boron to oxygen 85. 

In its most general form, the boronic acid Mannich or Petasis reaction18 involves the reaction of boronic acid 22, a carbonyl compound 8, and an amine 23 to produce secondary amines 24. If one uses a-keto acid 25 for the carbonyl component then the corresponding product from the Petasis reaction was a-amino acid 26. The key mechanistic step was proposed to occur intramolecularly with alkyl migration from intermediate boronate ester 28 formed from aminol 27. 

This is not stable, and it can decompose by a mechanism you should look at closely. It is not one which is familiar to you, but it makes sense if you think about it. The 0—0 bond is weak and can break, losing HO. As it does so, one of the alkyl groups on boron can migrate from B to O, relieving the boron atom of its negative charge, to give the structure shown below. 

Although carbenes are electron-deficient species, they react, as donors, with boranes. Secondary alcohols are produced via two successive alkyl migrations from the boron to carbenium centers of the intermediates when chlorocarbene (24) and methoxycarbene (25) are employed. 

Another way that the benzylonium-tetraalkylboranate ion pair can stabilize itself is by alkyl migration from boron to carbon, yielding 1-pentyl-benzene and tri-l-butylborane (76). 

The pathway leading to indole 15d involves initial generation of allylpalla-dium complex 16a and a subsequent intramolecular nucleophilic attack of indole on the allylpalladium intermediate from the opposite side of the palladium [21], with simultaneous 1,2-alkyl migration from boron to carbon [22] (Scheme 32.9). 

Schemes) in contrast to earlier work with bis(phenylthio)alkyl-lithiums (2,112) and 2-lithio-l,3-benzodithioles (4,139), two of the boron-to-carbon alkyl migrations occur spontaneously and only the third needs to be induced by Hg". A related approach to secondary alcohols from sulphone carbanions and trialkylboranes is illustrated in Scheme 6. ... 

The stereochemistry of base-induced alkyl migration from boron to carbon in y-acetylvinylboranes (36) has been studied and migration shown to occur predominantly in an anti fashion with respect to the leaving group, as in (37), to produce a trans-sAlylic alcohol on oxidation (Scheme 13). The availability of optically active propargylic alcohols (see refs. 36, 37 2, 116 4, 142) as precursors for (36) means that the overall sequence accomplishes alkylation and enantioselective [l,3]-hydroxyl transposition enantiomer ratios up to 87 13 were achieved in the alcohol product. 

Preparation.—Variations continue to appear on the theme of alcohol production by hydroboration-oxidation of olefins. 5-Methoxydialkylboranes react with olefins in the presence of lithium aluminium hydride to afford a new route to trialkylboranes and thence, by carbonylation-oxidation, to trialkylcarbinols. Carbonylation with carbon monoxide is avoided in a new procedure in the presence of an excess of trifluoroacetic anhydride, trialkyl cyanoborates undergo a triple alkyl migration from boron to carbon to give, on oxidation, high yields of trialkylcarbinols (Scheme 126). Tri-... 

Enol stannanes of cyclohexanone and propiophenone have been indicated to take part in r/treo-selective aldol reactions with benzaldehyde at low temperatures e.g. —78 °C), but to be erythro-seAsciiwe at higher temperatures ca 45 °C). Two complementary methods have been described for stereoselection in aldol-type reactions. Whilst a-mercurio-ketones show eryr/wo-selection in their reactions with aldehydes in the presence of boron trifluoride diethyl etherate, pre-formed lithium enolates and aldehydes, in the presence of simple trialkyl-boranes, lead to mixtures that are rich in the more stable threo-d do product. Aldol-type products arise from 1,3-alkyl migrations of alk-l-enyl alkyl acetals and ketals, in a reaction that is catalysed by boron trifluoride diethyl etherate (Scheme 52). Diastereoselection is possible, since (.E)-alkenyl acetals give the... 

Hydrogen fluoride boron fluoride C-Alkyl migration... 

Cis-olefins or cis./rjns-dienes can be obtained from alkynes in similar reaction sequences. The alkyne is first hydroborated and then treated with alkaline iodine. If the other substituents on boron are alkyl groups, a cis-olefin is formed (G. Zweifel, 1967). If they are cir-alkenyls, a cis, trans-diene results. The reactions are thought to be iodine-assisted migrations of the cis-alkenyl group followed by (rans-deiodoboronation (G. Zweifel, 1968). Trans, trans-dienes are made from haloalkynes and alkynes. These compounds are added one after the other to thexylborane. The alkenyl(l-haloalkenyl)thexylboranes are converted with sodium methoxide into trans, trans-dienes (E. Negishi, 1973). The thexyl group does not migrate. 

Fig. 3. Migration of an alkyl group from boron to carbon.

Moderate yields of acids and ketones can be obtained by paHadium-cataly2ed carbonylation of boronic acids and by carbonylation cross-coupling reactions (272,320,321). In an alternative procedure for the carbonylation reaction, potassium trialkylborohydride ia the presence of a catalytic amount of the free borane is utilized (322). FiaaHy, various tertiary alcohols including hindered and polycycHc stmctures become readily available by oxidation of the organoborane iatermediate produced after migration of three alkyl groups (312,313,323). 

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