Cyclic boranes

Applications of oxazaphospholidine-borane complexes, cyclic phosphoramides, compounds with N—P=0 function, and other P-heterocycles in enantio-selective catalysis 99SL377. 

Under the phase-transfer catalysis conditions, 2-bromo-8-methylquino-line (67) was coupled with 2-pyridylboronic ester 68 to furnish 2-(2-pyridyl)-8-methylquinoline (69) in 56% yield (91JOC6787). At this point, it is opportune to mention that the simple 2-pyridylborane, in contrast to 3- and (4-pyridyl)boranes, is considered an unsuitable Suzuki coupling partner because it forms an unusually stable cyclic dimer resembling a dihydroanthracene. In this case, the obstacle was circumvented by using 2-pyridylboronic ester in place of 2-pyridylborane (Scheme 9). 

In the next step, one of the borane-hydrogens is transferred to a sp -carbon center of the alkene and a carbon-boron bond is formed, via a four-membered cyclic transition state 6. A mono-alkyIborane R-BH2 molecule thus formed can react the same way with two other alkene molecules, to yield a trialkylborane R3B. In case of tri- and tctra-substituted alkenes—e.g. 2-methylbut-2-ene 7 and 2,3-dimethylbut-2-ene 9—which lead to sterically demanding alkyl-substituents at the boron center, borane will react with only two or even only one equivalent of alkene, to yield a alkylborane or mono alky Iborane respectively ... 

Cyclic divinyl boranes (bora-2,5-cyclohexadienes) also act as good complex ligands and are available from the corresponding stannacyclohexadienes by treatment with PhBCl2. They react photochemically or thermically with transition-metal complexes, e.g. ... 

Comparable reactions between cyclic divinyl boranes and metal carbonyls lead to borinato (borabenzene) complexes... 

Three kinds of B containing species may be used to react with metal halides cyclic compounds, carboranes and borane anions. The carboranes especially form an enormous number of complexes. 

Reaction with 2,5-dihydro-lH-l,2-azaboroles 6.S.3.3 Reaction with borolenes 6.5.3.1 Reaction with cyclic divinyl boranes 6.5.3.1... 

As is true for most reagents, there is a preference for approach of the borane from the less hindered face of the alkene. Because diborane itself is a relatively small molecule, the stereoselectivity is not high for unhindered alkenes. Table 4.4 gives some data comparing the direction of approach for three cyclic alkenes. The products in all cases result from syn addition, but the mixtures result from both the low regioselectivity and from addition to both faces of the double bond. Even 7,7-dimethylnorbornene shows only modest preference for endo addition with diborane. The selectivity is enhanced with the bulkier reagent 9-BBN. 

Allylic derivatives are particularly important in the case of boranes, silanes, and stannanes. Allylic boranes effect nucleophilic addition to carbonyl groups via a cyclic TS that involves the Lewis acid character of the borane. 1,3-Allylic transposition occurs through the cyclic TS. 

Allylic boranes such as 9-allyl-9-BBN react with aldehydes and ketones to give allylic carbinols. The reaction begins by Lewis acid-base coordination at the carbonyl oxygen, which both increases the electrophilicity of the carbonyl group and weakens the C-B bond to the allyl group. The dipolar adduct then reacts through a cyclic TS. Bond formation takes place at the 7-carbon of the allyl group and the double bond shifts.36 After the reaction is complete, the carbinol product is liberated from the borinate ester by displacement with ethanolamine. Yields for a series of aldehydes and ketones were usually above 90% for 9-allyl-9-BBN. 

Although the allylation reaction is formally analogous to the addition of allylic boranes to carbonyl derivatives, it does not normally occur through a cyclic TS. This is because, in contrast to the boranes, the silicon in allylic silanes has little Lewis acid character and does not coordinate at the carbonyl oxygen. The stereochemistry of addition of allylic silanes to carbonyl compounds is consistent with an acyclic TS. The -stereoisomer of 2-butenyl(trimethyl)silane gives nearly exclusively the product in... 

TS, which is usually based on the chair (Zimmerman-Traxler) model. This pattern is particularly prevalent for the allylic borane reagents, where the Lewis acidity of boron promotes a tight cyclic TS, but at the same time limits the possibility of additional chelation. The dominant factors in these cases are the E- or Z-configuration of the allylic reagent and the conformational preferences of the reacting aldehyde (e.g., a Felkin-type preference.)... 

The detection of the structural effects of B—C rr-bonding in unsaturated open-chain organoboranes should prove more difficult than in boracyclo-polyenes for several reasons. First, as already mentioned, there is the conformational freedom of the open-chain boranes, which reduces B(2pz)—C(2pz) overlap (cf. 17 vs. 19). Second, there is the linear overlap of 34 versus the cyclic, Htickel aromatic overlap of 35. 

In this context, it is interesting to note that the first synthesis of 2, 3 -0,0-cyclic phosphorothioate 22a was reported by Eckstein in 1968 [25], He also isolated pure Rp diastereomer by fractional crystallization of the triethylammonium salts [26] and used it as reference to determine the absolute configurations of the other phosphorothioate analogues [27], 2, 3 -0,0-Cyclic H-phosphonate 20a was used as a key substrate for the synthesis of uridine 2, 3 -0,0-cyclic boranophosphate 27. Silylation of H-phosphate 20a gave the phosphite triester 25 (two diastereomers). Its boronation, with simultaneous removal of the trimethylsilyl group, was achieved by its reaction with borane-A.A-diisopropylethylamine complex (DIPEA-BH3). 

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