Hydroboration allyl halides

Alkyl- or 2-aryl-substituted allyl halides 1, when treated first with a borane and then with a base, give substituted cyclopropanes in moderate to excellent yield. For the hydroboration step tetraalkyldiboranes 2 (method A) are superior to diborane (B2H ) itself (methods B or C, respectively), and sodium tetraalkylborates 4a or trialkylborohydrides 4b are preferred bases. 

Table 5. Cyclopropanes from Allyl Halides by Hydroboration and Treatment with Base...

Borabicyclo[3.3.1]nonane (9-BBN) has found use in the selective hydro-boration of alkenes in the presence of other reducible functional groups and its reaction with alkynylstannates has been studied. o-Stannyl- and a-silyl-substituted crotyl-9-BBN show promise as reagents for the stereo-regulated synthesis of acyclic systems. A series of papers covers the question of olefin-alkyl exchange in. 5-alkyl-9-BBN s, " the kinetics of reduction of substituted benzaldehydes with 9-BBN, and the kinetics and mechanism of hydroboration of alkynes with 9-BBN dimers. Selective dehalogenation of tertiary alkyl, benzyl, and allyl halides in the presence of secondary or primary alkyl or aryl halides is possible with (165). The... 

Pinacolatoboration. Hydroboration of 1-alkynes to give 2-pinacolato-l-alkenes is effected with bis(pinacolato)diboron in the presence of CuCl and KOAc. Perhaps a borylcopper species is involved in the reaction. The same reagent is useful as a nucelophile toward allylic halides and Michael acceptors. 

The last subject to be discussed in this section is one found with Group IV elements, and probably also with many Group II elements (cf. mercury, Section IV, E)—neighboring group effects and participation. Hawthorne (192) has pointed out that y-halopropylboranes, produced by hydroboration of allyl halides, undergo basic hydrolysis to yield cyclopropanes. 

The electronic effects of the hydroboration of allyl halides have been studied and it is found that a Cl, Br, or I substituent at the y position has a rate-reducing effect of 25, 21.7, or 14.1, respectively [11]. 

The well-known synthesis of cyclopropanes from allylic halides by hydroboration, followed by base-catalysed cyclization of the intermediate y-chloroborane (1), has... 

Many of the common laboratory methods for the preparation of alcohols have been discussed in previous chapters or will be considered later thus to avoid undue repetition we shall not consider them in detail at this time. Included among these methods are hydration (Section 10-3E) and hydroboration (Section 11-6D), addition of hypohalous acids to alkenes (Section 10-4B), SN1 and Sn2 hydrolysis of alkyl halides (Sections 8-4 to 8-7) and of allylic and benzylic halides (Sections 14-3B and 14-3C), addition of Grignard reagents to carbonyl compounds (Section 14-12), and the reduction of carbonyl compounds (Sections 16-4E and 16-5). These methods are summarized in Table 15-2. 

The 5-keto acid 11 is supposed to be formed by hydroboration and oxidation of the terminal CC double bond in 12, feasible by allylation of the enone 13. Enone 13, once again, originates from an intramolecular Knoevenagel alkenylation of the a,5-diketone 14 prepared by addition of ethylmagnesium halide to the enol lactone 15 of the 5-keto acid 16. Preceding this keto acid, the allyl compound 17 is formed by nucleophilic opening of the cyclopropane ring with hydride and mefliylation in a position of the carbonyl in the cyclopropyl ketone 18. 

Brown et ah discovered a simple preparation of organoboranes inaccessible by hydroboration, e.g., aromatic, benzylic, allylic, by sonication of the halides, most of the time bromides, with magnesium and trifluoroborane etherate. The Grignard reagent formed in situ undergoes metal exchange in high yields (Eq. 35). 70... 

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