Boron compounds rearrangement

Heating triacetylboron at temperatures above its melting poiat, 123°C, causes a rearrangement to B20(0CCH2)4 (15). An explosive hazard is also generated by dissolving BF ia anhydride (see Boron compounds). 

The reader will no doubt have realized that the y-hetero rearrangement is well-known, being none other than the familiar neighboring group effect. The a-hetero rearrangement is known for all of the elements mentioned but boron, aluminum, and silicon. Perhaps a suitably constituted boron compound would rearrange to a zwitterionic product as follows ... 

Astrophylline, via ring-rearrangement reactions, 11, 261 Asymmetric addition reactions boron compounds, 9, 215 with zinc reagents, 9, 109... 

The inherent plausibility of metastable bridged carbonium ions as intermediates is supported by two independent types of observation. One is the extensive rearrangements which can occur in allylic systems under conditions in which ionic displacement reactions are possible. A second is the existence of stable bridged compounds, including, in the case of boron compounds, pentavalent atoms. Thus diborane and substituted diboranes have stable bridged structures. 

Nucleophilic groups such as OH, OR, or R attack the boron atom, often followed by decomposition reactions. A very informative intramolecular nucleophilic attack has been observed with l-/-butyl-2,3-dihydro-2-methyl-3-(4-pentenyl)-l//-l,2-azaborole (49) <88AG(E)952>. This compound rearranges slowly on cooling below 80 °C to form a tricyclic crystalline product (Scheme 2). 

Both enantiomers of binaphthol have found many uses as chiral reagents and catalysts. Thus, they modify reducing agents (e.g., lithium aluminum hydride) for the reduction of ketones to chiral secondary alcohols (Section D.2.3.3.2.) or react with aluminum, titanium or boron compounds to give chiral Lewis acids for asymmetric Diels-Alder reactions (Section D. 1.6.1.1.1.3.) and ene reactions (Section D.I.6.2.). They have also been used as chiral leaving groups in the rearrangement of allyl ethers (Section D.l.1.2.2.) and for the formation of chiral esters with a-oxo acids (Section D. 1.3.1.4.1, and many other purposes. 

The difficultly accessible trans-syn-trans arrangement of the a-b-c ring system present in steroidal antibiotics has now been synthesized. The known enedione (39) was converted into a 6 1 mixture of the desired compound (40) and its isomer (41) by ketalization of the saturated carbonyl, followed by lithium-ammonia reduction and enolate trapping with methyl iodide. After separation, (40) was converted into the tricyclic enedione (42) by standard procedures. The transfused AB-system was then obtained by ketalization, peracid treatment, and boron trifluoride rearrangement of the resulting epoxide to the keto-diketal (43). Removal of the 6-keto-group was performed under mild conditions by a new... 

Another route to carbon-carbon bond formation is via iodine-induced rearrangement of adducts of trisubstituted boron compounds and organolithium reagents. The adducts of trialkylboranes with lithium acetylides give an akylated acetylene. The mechanism involves electrophilic attack by iodine on the acetylenic... 

Thus, unlike a-oxyalkylphosphines and their derivatives, which undergo an oxidative rearrangement on heating, P,B-containing heterocycles are transformed into compounds with tricoordinated phosphorus and boron atoms. 

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