Boron isomeric structures

A very interesting tautomeric system is presented by the civ-fused tetrahydro[l,3,2]dioxaborino [5,4-cf]-l,3,2-dioxaborin (122). This is prepared from a threitol type precursor and is obtained as a mixture with isomer (123) in a ratio of 3 1 (Equation (2)). The particular stereochemistry of (122) permits the molecule to adopt a conformation in which the two boron atoms and two of the oxygen atoms assume a four-centered transition state (124), thus allowing establishment of a dynamic equilibrium with the isomeric structure (123) (Scheme 3). The trans-fused analogue of (122), prepared from an erythritol type precursor, is unable to enter into this tautomeric equilibrium (80LAH76). 

An isomeric species (106), which formally contains a borate anion and a boronium cation center was also reported. Similar isomeric structures were discovered in the case of imidazaboles (107) and (108).195.196 Diborylated bipyridine derivatives (109 R = NMe2, 2-thienyl) were also studied. While amino substituents stabilize a monomeric structure through n bonding to boron, the thienyl derivative (109 R=Th) is believed to adopt an oligomeric... 

A structural perspective gave rise to predictions about reasonable rearrangement pathways for polyhedral molecules. He also measured isomerizations that confirmed the predictions. The calculations that describe the boron hydride structures also gave basis for predicting which atoms would be most reactive. These predictions stimulated much synthetic chemistry. 

There appears to be no end to the structural ingenuity of boron and, whilst it is true that many regularities can now be discerned in its stereochemistry, much more work is still needed to unravel the reaction pathways by which the compounds are formed and to elucidate the mechanisms by which they isomerize and interconvert. 

Epoxides can be isomerized to carbonyl compounds by Lewis acids.104 105 Boron trifluoride is frequently used as the reagent. Carbocation intermediates appear to be involved, and the structure and stereochemistry of the product are determined by the factors which govern substituent migration in the carbocation. Clean, high-yield reactions can be expected only where structural or conformational factors promote a selective rearrangement. 

The hydrogen fluoride catalyzed fluorination of norbornene by xenon difluoride at room temperature leads to a mixture of at least seven components,39 but under milder conditions (— 78 to 26 C, 22 hours) the reaction affords a mixture of two main products 2-e,xo-5-cxo-difluoro-norbornane and 2-c-wfo-5- Yo-difluoronorbornane, ratio 2 1, in a total yield of 51-76%. If the same reaction is carried out in a limited temperature range between — 46 and — 39 C the yield of these products decreases, their ratio becomes equal, and the main product is 2-exo-l-ff //-difluoronorbornane (42 %).40 The structure dependence of the fluorination products of norbornene with xenon difluoride was studied. Solvent, temperature, reaction duration, catalyst (hydrogen fluoride, boron trifluoride, trifluoroacetic acid, pentafluorobenzenethiol) and the routes of product isomerization were analyzed.41-42... 

Not only the reaction conditions, but also the structure of the starting material has a strong influence if fluorohydrins are formed or rearrangement occurs. Tricyclic epoxides 6a, b react with boron trifluoride-diethyl ether complex to give fluorohydrins 7 via fluoride transfer, whereas 6c undergoes isomerization.32... 

However, studies on the scope of this sequence revealed that the substrate has to be an N-tosyl sulfonamide and that certain boronic acids are not trans-metallated but rather give rise to the formation of the pyrrole 21 or a pyridine derivative 22 (Scheme 7). The peculiar outcome as a carbopalladation-Suzuki sequence is rationalized by co or dinative stabilization of the insertion intermediate 18 by the sulfonyl oxygen atom, as represented in structure 19, now suppressing the usual /3-hydride elimination. If the transmetallation is rapid the Suzuki pathway is entered leading to product 17. However, if the transmetallation is slow, as for furyl or ferrocenyl boronic acid, either /i-hydride elimination or a subsequent cyclic carbopalladation occurs. The former leads to the formation of the diene 20 that is isomerized to the pyrrole 21. The latter furnishes the cyclopropylmethyl Pd species 23, which rearranges with concomitant ring expansion to furnish piperidyl-Pd intermediate 24 that suffers a -hydride elimination to give the methylene tetrahydro pyridine 22. 

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