Carbon nucleophiles borane compounds

C-Alkylations have been performed with both support-bound carbon nucleophiles and support-bound carbon electrophiles. Benzyl, allyl, and aryl halides or triflates have generally been used as the carbon electrophiles. Suitable carbon nucleophiles are boranes, organozinc and organomagnesium compounds. C-Alkylations have also been accomplished by the addition of radicals to alkenes. Polystyrene can also be alkylated under harsh conditions, e.g. by Friedel-Crafts alkylation [11-16] in the presence of strong acids. This type of reaction is incompatible with most linkers and is generally only suitable for the preparation of functionalized supports. Few examples have been reported of the preparation of alkanes by C-C bond formation on solid phase, and general methodologies for such preparations are still scarce. 

Occasionally hydroxylation of the carbon nucleophile is observed during Pd-cata-lyzed C-C bond formation (Scheme8.15 third reaction in Scheme8.14 [121]). These reactions may in some instances proceed by a mechanism analogous to the Wacker reaction [130], or to the hydroxylation of organometallic compounds or boranes by peroxides or air (Section 3.5). 

Carbon-carbon bonds can also be made with alkyl boranes. The requirement for a carbon nucleophile that bears a suitable leaving group is met by a-halo carbonyl compounds. The halogen makes enolization of the carbonyl compound easier and then departs in the rearrangement step. The product is a boron enolate with the boron bound to carbon. Under the basic conditions of the reaction, hydrolysis to the corresponding carbonyl compound is rapid. 

Reactions of chiral allylic boranes with carbonyl compounds Reactions of chiral allyl boranes with imines Asymmetric Addition of Carbon Nucleophiles to Ketones Addition of alkyl lithiums to ketones Asymmetric epoxidation with chiral sulfur ylids Asymmetric Nucleophilic Attack by Chiral Alcohols Deracemisation of arylpropionic acids Deracemisation of a-halo acids Asymmetric Conjugate Addition of Nitrogen Nucleophiles An asymmetric synthesis of thienamycin Asymmetric Protonation... 

Intennolecular termination processes by carbon nucleophiles will be considered separately for reactions with soft nucleophiles (i.e., mostly stabilized enolates) and with organometal-lic compounds such as stannanes, boranes, and zincates. 

Compounds with a low HOMO and LUMO (Figure 5.5b) tend to be stable to selfreaction but are chemically reactive as Lewis acids and electrophiles. The lower the LUMO, the more reactive. Carbocations, with LUMO near a, are the most powerful acids and electrophiles, followed by boranes and some metal cations. Where the LUMO is the a of an H—X bond, the compound will be a Lowry-Bronsted acid (proton donor). A Lowry-Bronsted acid is a special case of a Lewis acid. Where the LUMO is the cr of a C—X bond, the compound will tend to be subject to nucleophilic substitution. Alkyl halides and other carbon compounds with good leaving groups are examples of this group. Where the LUMO is the n of a C=X bond, the compound will tend to be subject to nucleophilic addition. Carbonyls, imines, and nitriles exemplify this group. 

This commoner type of reaction involves the attack of carbon or heteroatom nucleophiles onto carbonyl compounds, by direct or conjugate addition, and onto imines. Because we have just dealt with boranes we shall start with the reaction of allylic boron compounds with such electrophiles. You might strictly not call this nucleophilic attack. 

The Pd-catalyzed cross-coupling reactions of metal nucleophiles with carbon electrophiles are of considerable value for the regio- and stereocontrolled synthesis of functionalized organometalhc compounds, in particular, silanes, stannanes, and boranes, which are important reagents for Pd-catalyzed carbon-carbon cross-coupling as shown in Sects. in.2.2-in.2.4. Symmetrical bimetallic compounds such as disilanes, distannanes, and diborons are usually used as metal nucleophiles. The present metallation is applicable to aryl, benzyl, vinyl, acyl, and aUyl (Sect. V.2.3.3) electrophiles. 

Even with an excess of tert-butylmethylphosphide, the desired compound 55 was not obtained and only 20 could be isolated, probably due to both steric and electronic reasons. Accordingly, the X-ray structure of 20 shows effective shielding of the ipso carbon of the remaining fluorine atom by the phosphine borane group. Deboronated 20 (Scheme 5.10) did not react with the phosphide either. Unexpectedly, when the reaction mixture containing 19 and the phosphide treated with acid at low temperature the main product turned out to be the p ra-substituted diphosphine borane 54. Compound 54 arises from nucleophilic attack at the meta position to the fluoro group therefore the... 

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