Olefins hydroboration-carbonylation

Yates, M. H. One-pot conversion of olefins to carbonyl compounds by hydroboration/NMO-TPAP oxidation. Tetrahedron Lett. 1997, 38, 2813-2816. 

A route to aldehydes from olefins via carbonylation has also been developed. The 9-BBN reagent formed on hydroboration of 1,5-cyclooctadiene is used to effect hydroboration of the olefin. Carbonylation of the resulting trialkylborane proceeds... 

Table 7.1 Aldehydes produced ing 9-BBN [7a] via the hydroboration-carbonylation of representative olefins us- ...

The examples of hydrometalations are hydroboration [64,72-77], hydrosilylation [78], etc. [78,79] of unsaturated compounds such as olefins, acetylenes, carbonyls and imines. For example, styrene reacts with catecholboran in the presence of a rhodium catalyst to afford stereoselectively the R isomer of 2-phenylethanol as shown in eq. (18.34) [72]. 

The sulphur and nitrogen heterocycles (80) and (81) have been prepared by hydroboration-carbonylation of bis-olefinic precursors, and the latter case is reported to be the first example of such a synthesis in the presence of a reducible functional group. Another hydroboration-carbonylation sequence has been used in the preparation of methylene cycloalkanes from cycloalkenes, and is shown in Scheme 11. ... 

Section B of the Scheme 9.1 shows several procedures for the synthesis of ketones. Entry 6 is the synthesis of a symmetrical ketone by carbonylation. Entry 7 illustrates the synthesis of an unsymmetrical ketone by the thexylborane method and also demonstrates the use of a functionalized olefin. Entries 8 to 10 illustrate synthesis of ketones by the cyanide-TFAA method. Entry 11 shows the synthesis of a bicyclic ketone involving intramolecular hydroboration of 1,5-cyclooctadiene. Entry 12 is another ring closure, generating a potential steroid precursor. 

In the hydroboration of terminal alkenes, carrying a ketone or aldehyde group, with a variety of borane reagents, dicyclohexylborane has been identified as the most efficient reagent. Analogous hydroboration of alkynyl ketones and alkynyl aldehydes with dicyclohexylborane yields the corresponding olefinic carbonyl compounds after protonation, or dicarbonyl compounds after oxidation. ... 

Liquid injection molding, for silicone rubbers, 3, 674—675 Liquid ligands, in metal vapor synthesis, 1, 229 Liquid-phase catalysis, supported, for green olefin hydroformylation, 12, 855 Lithiacarbaboranes, preparation, 3, 114 Lithiation, arene chromium tricarbonyls, 5, 236 Lithium aluminum amides, reactions, 3, 282 Lithium aluminum hydride, for alcohol reductions, 3, 279 Lithium borohydride, in hydroborations, 9, 158 Lithium gallium hydride, in reduction reactions, 9, 738 Lithium indium hydride, in carbonyl reductions, 9, 713—714... 

Thexylborane adds to hindered olefins only once, and the product hydroborates only unhindered olefins thus leading to mixed trialkylboranes. This selective stepwise hydroboration with thexylboranes gives a simple synthesis of unsymmetrical ketones after carbonylation (Eqs. 19 and 20)41-58,59). 

Use of selected alkylboranes for H—B addition to olefins has broadened the scope of hydroboration. Bis-3-methyl-2-butylborane (BMB), obtained as noted above as in (1) even in the presence of excess olefin, reacts with unsymmetrical olefins to give increased yields of the less hindered products and permits selective reactions not otherwise possible. Since, unlike diborune, it does not reduce carbonyl groups, the reagent can be used for hydroboration of an unprotected acid (2). r-Lactones react with only one mole of reagent, even when the latter is in excess (3). 

Carbonyl compounds (after ref. 5]. McMurry58 desired to selectively reduce an olefinic double bond with diborane in the presence of a keto group. This was successfully achieved by conversion to the dinitrophenylhydrazone, hydroboration and removal of the protective group by ozonolysis in ethyl acetate at — 78°. Oximes and oxime acetates are reduced by BH3. Attempted protection by ketalization in this case was unsuccessful because of simultaneous migration of the double bond. 

The potential utility of an asymmetric addition to a prochiral carbonyl can be seen by considering how one might prepare 4-octanol (to take a structurally simple example) by asymmetric synthesis. Figure 4.16 illustrates four possible retro-synthetic disconnections. Note that of these four, two present significant challenges asymmetric hydride reduction requires discrimination between the enantiotopic faces of a nearly symmetrical ketone a), and asymmetric hydroboration-oxidation requires a perplexing array of olefin stereochemistry and regiochemical issues h). In contrast, the addition of a metal alkyl to an aldehyde offers a much more realistic prospect (c) or (d). 

Carbonylation. Dialkylchloroboranes have also been used in the preparation of asymmetric ketones via reduction-hydroboration-carbonylaton protocol. In the presence of 1.0 equiv of trimethyl silane, IpcBCl2 is reduced to IpcBHCl, which selectively hydroborates terminal olefins faster than internal olehns, furnishing the dialkyl chloroborane. Upon further reduction of the R2BCI with LiAlH4, the internal olefin undergoes hydrobora-... 

The B-allyl-9-BBN derivatives formed by hydroboration of allenes are utilized for addition of the allyl group to the carbonyl moiety of aldehydes, ketones, and other carbonyl derivatives (allylboration). Since allenes are easily prepared from corresponding olefins [12, 13], the route the olefin-allene-allylborane-carbonyl addition adduct is a general allylation procedure, which provides an attractive alternative to Grignard-based sequences for the syntheses of complex structures. 

Table 7.2 Conversion of functionally substituted olefins into aldehydes via hydroboration with 9-BBN and carbonylation in the presence of lithium tri-t-butoxyaluminohydride[7b]...

It has been established [5] by competition experiments involving 9-BBN that the carbonyl of aldehyde or ketone is reduced rapidly and cleanly to the corresponding alcohol, much faster than the hydroboration of olefins ( cydohexanone/ cydopentene = 7). This is Complementary to the behavior of borane, which shows greater reactivity toward alkenes. This characteristic of 9-BBN has been utilized for the selective 1,2-reduction of a,p-unsaturated aldehydes and ketones to the corresponding allylic alcohols (Chart 25.7) [5]. 

Hydroboration is mainly currently used to prepare primary alcohols from terminal olefins, but other functional groups are also accessible. The hydroboration reaction can also chemoselectively lead to the reduction of the carbonyl group of aldehydes and a, (3-unsaturated ketones. 

The exceptional selectivity exhibited by disiamylborane in the hydroboration of terminal olefins enables the efficient conversion of such olefins into aldehydes, since disiamylboranes have been shown to be oxidized readily by pyridinium chlorochromate. Thexylalkylchloroboranes, reduced in the presence of an alkene, produce mixed thexyldialkylboranes, which can be transformed into ketones by carbonylation or cyanidation. This advance in the preparation of trialkylboranes bearing three different alkyl groups removes limitations previously imposed on ketones available by carbonylation of boranes [equation (11)]. 

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-... 

Only a minimum of mechanistic attention has been paid to the catalytic effect that ethers have on the hydroboration reaction. After determining that the hydroboration of olefins with l,2-bis(3-methyl-2-butyl)diborane is first order in olefin and also first order in the substituted diborane, it was suggested that the role of the solvent is to coordinate with the dialkylborane monomer, the leaving group in this particular reaction 101). However, the room temperature hydroboration of ethylene with borine carbonyl 102) intimates, by analogy, that a rather loosely held adduct may account for acceleration of B—H addition to unsaturated systems in ether solvents. The use of a molybdena-aluminum catalyst has been moderately effective in converting an ethylene-diborane mixture to a mixture of ethyldiboranes at room temperature 103). 

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