Boranes zwitterionic

By chance, the existence of the borane complex 330 of 329 was discovered. The liberation of 330 occurred with the best efficiency with sodium bis(trimethylsilyl)-amide from the borane complex 327 of 326. When styrene or furan was used as the solvent, three diastereomeric [2 + 2]-cycloadducts 328 and [4 + 2]-cycloadducts 331, respectively, were obtained in 30and 20% yield (Scheme 6.70) [156]. With no lone pair on the nitrogen atom, 330 cannot be polarized towards a zwitterionic structure, which is why its allene subunit, apart from the inductive effect of the nitrogen atom, resembles that of 1,2-cydohexadiene (6) and hence undergoes cycloaddition with activated alkenes. It is noted that the carbacephalosporin derivative 323 (Scheme 6.69) also does not have a lone pair on the nitrogen atom next to the allene system because of the amide resonance. 

Organoboranes react with ethyl 4-nitrobenzenesulphonyloxycarbamate under basic two-phase conditions in the presence of benzyltriethylammonium chloride or Aliquat to yield ethyl (V-alkylcarbamates [38]. The reaction probably proceeds via the initial formation of the nitrene, which reacts with the borane to form a B -N+ zwitterion. Subsequent rearrangement and solvolysis leads to the product. Aliquat is the better catalyst for the higher-molecular-weight boranes. 

The reductive cleavage of phosphonio-substituents attached to other heterocyclic frameworks was first demonstrated in the reaction of the 3,5-bis-phosphonio-l,2,4-diazaphospholide 15 with NaBH4 or LiBEt3H to yield the zwitterionic borane-complexes 27. Liberation of the free zwitterion 28 was readily feasible by treatment of the complexes with an excess of NEt3 (Scheme 9) [34]. 

In addition to the electrophilic alkylation, the phosphinyl-benzophospho-lide 25 reacted also with sulphur, BH3-thf, and trimethylsilyl azide under selective conversion of the Ph2P-moiety to give the corresponding thioxophos-phorane, borane-adduct, and iminophosphorane, respectively [33, 37], and quaternisation with chloro-acetonitrile followed by deprotonation gave access to a zwitterionic phosphonio-benzophosphohde with an exocyclic ylide-substituent [37]. 

The construction of a borole ring that is part of a metal complex from zirconium-coordinated 1,3-dienes and tris(pentafluorophenyl)borane has been recently introduced. Tris(pentafluoro-phenyl)borane is widely used as an activator of metallocene-based polymerization catalysts because of its resistance to aryl-transfer reactions, it acts as the boron source. The reaction utilizes the zwitterionic 14-electron bis(allyl)zirconium complexes 72 and 73 which are converted to the corresponding complexes 74 and 75 via activation of one of the C-H bonds of the B-CH2 moiety... 

Upon reaction with basic transition metal complexes the general tendency of electronically unsaturated boranes to form Lewis acid-base adducts should result in the formation of borane complexes (I). [CpzWILfBFs)] (1), which is known from many textbooks represents the best known example for such a compound. Recently, however, the proposed constitution of 1 was disproved, and it was shown that reactions of [Cp2WH2] with boranes yield salt like compounds 2 and zwitterionic species 3, respectively. For the formation of 2 one equivalent of HF is required, and in this reaction the borane acts as a fluoride source, while the WH2 moiety supplies a proton (Figure 3). Until now no structurally authentic borane complex was reported. 

Figure 3 Reactions of [CP2WH2] with boranes Proposed (1) and authentic (2) constitution of the product obtained from [CP2WH2] and BF3 zwitterionic complex (3) obtainedfrom [CP2WH2] and tBuBCh...

Zwitterionic bicyclic compounds 5a-c are obtained when 64 is allowed to react with di-methoxyborane or di(isopropoxy)borane at temperatures below -10°C. The constitution of 5c is established by the crystal structure shown in Figure 1 (left side). The similarity of the NMR data of 5a and 5b to those of 5c (relevant chemical shifts are included in the formulas through-out this report 8I3C in bold and 8MB in plain text) proves the constitutions of 5a and 5b. 

Borane yields zwitterionic compounds with partial ylide structure (equation 97). ... 

Treatment of the bis(propynyl)zirconocene 759 with B(C6Fs)3 results in a linear G-G coupling of the alkynyl ligands to form the zwitterionic complex 76 0582,583 (Scheme 186). Complex 760 reacts with nitriles R CN to form initially the 1 1 adduct 761 that concurrently equilibrates with 760 and the metallacyclocumulene 762 (and the nitrile-borane adduct) subsequently, irreversible reaction in the presence of excess nitrile yields the methylene-cyclopropene derivative 763.584,585 Calculations have shown that the conversion 760 —> 763 is probably triggered by nitrile addition to the metal with formation of a planar-tetracoordinate carbon intermediate that features coordination of the three-membered carbocycle through one of its G-G cr-bonds. 

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