Azines with Lewis acids

Modification of ligand systems in azine complexes was studied for the example of reactions (3.223) of r 6-coordinated 2,4,6-collidinechromiumtricarbonyl 773 (L = CO) [294] and its monotriphenylphosphine derivative 773 (L = PPh3) [492] with Lewis acids (AlBr3, SnCl4 TiCl4), leading to binuclear structures 774 ... 

Adamantylideneadamantane has been prepared by (1) photolysis of 2-adamantylketene dimer,2 (2) reduction of 4< -chloroadaman-tylideneadamantane with sodium in liquid ammonia,3 (3) rearrangement of spiro[adamantane-2,4 -homoadamantan-5 -ol] with Lewis acids,4,5 (4) reduction of 2,2-dibromoadamantane with magnesium6 or zinc-copper couple,7 and (5) treatment of the azine of 2-ada-mantanone with hydrogen sulfide, followed by oxidation with lead tetraacetate and heating with triphenylphosphine.8... 

The analogous reaction with 1,1,3-trimethyl-2-(trimethylsilyloxy)cyclohexene employing titani-um(IV) as the Lewis acid also proceeded in the same stereochemical sense and gave 1,2-benzox-azin-2-ium 2-oxide 6 as the only diastereomeric product in 75% yield16. 

Treatment of resin-bound a-arylazobenzhydryl 488 with a Lewis acid at room temperature followed by acidic cleavage furnished indazole 489 in quantitative yield, the reaction being monitored by single bead IR microspectroscopy (Equation 98) <1996TL8325>. Unsymmetrical azines 490 thermally cyclized to fused pyrazoles 491 (Equation 99) <2002TL6431>. Indazoles 493 were obtained from thermal cyclizations of (2-alkynylphenyl)triazenes 492 in the presence of methyl iodide as a solvent other solvents were tested where either no reaction or complex mixture of products was obtained (Equation 100) <2002JOC6395>. 

The pyrimidine system shows side-chain reactivity typical of azines. CH3 groups at the 2-, 4- or 6-position undergo aldol condensations (with aldehydes in the presence of Lewis acids) or Claisen condensations (with esters in the presence of strong bases) with marked preference at C-4, e.g. ... 

It is tempting to think that allylic cations would behave similarly to other stabilized cations in their reactions with alkyl azides. " In practice what happens is invariably an initial formal [3+3] cycloaddition of azide to the allylic cation, which is followed by a migration event (typically of hydride) or trapping by a nucleophile. Pearson and coworkers found that when sulfonylindole 27 was treated with SnCk at -78 °C, followed by basic workup, triazoline adducts 28 were obtained as mixture of chloride epimers (Scheme 7.24). When A -alkyl indoles (e.g. 29) were subjected to Lewis acidic conditions, tri-azines such as 30 were obtained as the sole products. In the former case involving an iV-sulfonyl indole, a [3+2] cycloaddition pathway explains the product, whereas the iV-alkyl indoles examined underwent [3+3] cycloaddition. 

Silyl cyanides react enantioselectively with such electrophiles as aldehydes, ketones, imines, activated azines, or,/ unsaturated carbonyl compounds, epoxides, and aziridines in the presence of chiral Lewis acid catalysts to give functionalized nitriles, versatile synthetic intermediates for hydroxy carboxylic acids, amino acids, and amino alcohols (Tables 3-6, 3-7, 3-8, and 3-9, Figures 3-6, 3-7, and 3-8, and Scheme 3-154). ° Soft Lewis acid catalytst, the reaction of epoxides with trimethylsilyl cyanide often leads to isonitriles, which are derived from silylisonitrile spiecies (Schemes 3-155 and 3-156). Soft Lewis base such as phosphine oxide also catalyzes the reaction and cyanohydrin silyl ethers of high ee s are isolated. 

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