Boron nucleophiles, 1,2-addition

The alkynyl reagent 9 was recently introduced for the dia stereoselective synthesis of tertiary propargylic alcohols144. 9 can be prepared as a solid 1 1 complex with tetrahydrofuran by treatment of 9-methoxy-9-borabicyclo[3.3.1]nonane with (trimethylsilylethynyl)lithium, followed by addition of boron trifluoride-diethyl ether complex. The nucleophilic addition of reagent 9 to (R)-2-methoxy-2-methylhexanal (10) afforded a mixture of the diastereomers 11 with a considerable preference to the nonchelation-controlled (3S,4R)-isomer144. 

An interesting example from carbohydrate chemistry is the boron trifluoride-diethyl ether complex catalyzed nucleophilic addition of silyl enol ethers to chiral imines (from n-glyceralde-hyde or D-serinal)22. This reaction yields unsaturated y-butyrolactones with predominantly the D-arabino configuration (and almost complete Cram-type erythro selectivity). 

Nucleophilic Addition of Allylic Groups from Boron Compounds... 

In contrast to the highly reactive organoboranes, borabenzene metal complexes are surprisingly inert toward nucleophiles. However, cationic complexes may undergo nucleophilic addition reactions, and nucleophilic substitution has been observed with compounds having a hydrogen or an electronegative substituent at boron. 

More often, nucleophilic addition at boron can only be observed spectroscopically and is followed by more or less fast boranediyl extrusion (see Section VI,A) (69). A speculative mechanism has been proposed which links this boranediyl extrusion with the boranediyl insertion described earlier (Section V,A) (2,69). 

The presence of a stereogenic center on the aldehyde can strongly inlinence the diastereoselectivity in allylboration reactions, especially if this center is in the a-position. Predictive rules for nucleophilic addition on snch a-snbstitnted carbonyl substrates such as the Felkin model are not always snitable for closed transition structures.For a-substituted aldehydes devoid of a polar substituent, Roush has established that the minimization of ganche-ganche ( syn-pentane ) interactions can overrule the influence of stereoelectronic effects. This model is valid for any 3-monosubstituted allylic boron reagent. For example, althongh crotylboronate (E)-7 adds to aldehyde 39 to afford as the major prodnct the diastereomer predicted by the Felkin model (Scheme 2), " it is proposed that the dominant factor is rather the minimization of syn-pentane interactions between the Y-snbstitnents of the allyl unit and the a-carbon of the aldehyde. With this... 

Boron—nitrogen coordination polymers, with ferrocene, 6, 208 Boron nucleophiles, in conjugate additions asymmetric 1,4-additions, 10, 388 mechanisms, 10, 384 to nitroolehns, 10, 388 to a,/3-unsaturated amides, 10, 386 to a,/3-unsaturated esters, 10, 386 to a,/3-unsaturated ketones, 10, 384 Boron-silicon bonds, addition to alkenes, 10, 760 to alkynes, 10, 758 to allenes, 10, 760 to carbenoids, 10, 766 to 1,3-dienes, 10, 762 to isocyanides, 10, 765... 

Little is known with certainty in connection with the key step that involves the nucleophilic addition of the organic ligand from the boronate to the imine. One proposal is that the transfer is actually intramolecular, and takes place via the adduct pictured above ... 

In spite of its unusual structural outcome, the N-fusion reaction is a very general process. It was observed in a number of other N-confused macrocycles. For instance, an N-fused intermediate forms in the synthesis of trans doubly N-confused porphyrin 96 [238], N-fusion was also induced by the reaction with PhBCl2, in which case the reaction was promoted by the small radius of the coordinating boron(III) center [251], The resulting boron complexes were also aromatic, even though in some cases further chemical modification of the macrocycle took place (as in 110). The fusion process, which appears to be a nucleophilic addition-elimination, is reversible, and N-fused macrocycles can often be reopened when treated with nucleophiles such as alkoxides [248, 249],... 

Hydroboration of allenes 65 with pinacolborane in the presence of Pt(DBA)2 and a trialkylphosphine provides either the allyl boronate 66 or the vinyl boronate 67 regioselectively, depending on the stereoelectronic factors of the phosphine employed (Equation 2) <1999CL1069>. Allyl and vinyl boronates are synthetically important because of their ability to undergo nucleophilic addition to carbonyl compounds as well as transition metal-catalyzed cross-coupling. 

One mechanistic possibility for this thioacetalation reaction invokes furanoside ring-opening of 17, initiated by complexation with boron trifluoride etherate (Scheme 12.9). Oxonium ion 28 could then get intercepted by the ethanethiol to produce 29. After further complexation with BF3, in the manner shown, thionium ion formation can again occur to give 30, which can then engage in yet another nucleophilic addition with the ethanethiol to produce 31 after protonation. 

The kinetically controlled nucleophilic addition of preformed lithium enolates onto carbonyl compounds is reversible with a low activation barrier, and the thermal conditions are likely to have a major impact on the stereoisomeric ratio of the final aldols through the retroaldolization and the thermodynamic equilibration of lithium enolates76. The tendency of aldolates to undergo retroaldolization increases with the stability of enolates, and when going from lithium to potassium. On the other hand, boron enolates usually undergo completely irreversible aldol reaction511,512. 

Thermal and photochemical reactions Nucleophilic addition on boron Reduction Ring expansion... 

Diastereoselective allylation of optically pure sulfinyl dienal complexes using tributyl allyltin can be obtained (Scheme 138). 2,4-Hexadien-1,6-dial iron tricarbonyl complex (88) undergoes nucleophilic addition reactions with diaUcylzincs in the presence of a catalytic amount of an optically active amino alcohol (Scheme 139). Very high enantio-and diastereoselectivity is observed. Related reactions of (88) with chiral allyl boronic esters give allylated alcohols in very high enantiomeric excess. 

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