Oxygen-boron bonds reactions with

Even greater diastereoselectivity in die aldol reaction can be achieved using boron etiolates as the carbon nucleophile. Boron etiolates are easily prepared from aldehydes and ketones, and the syn and die anti isomers can be separated as pure compounds. They react with aldehydes and ketones to give aldol products by a similar transition state. The difference is fliat boron oxygen bonds are shorter than lidiium oxygen bonds, and thus steric interactions in the transition state are magnified and result in greater diastereoselectivity. 

Zirconium-aryne complexes have also found application in the regiose-lective synthesis of halophenols.62 Reaction of 101 with methyl diethylbori-nate or triethylborate leads to insertion of a boron-oxygen bond into a Zr-benzyne bond with formation of 102 and 104, respectively (Scheme 12). Further reactions of 102 and 104 with bromine or iodine followed by excess NaOH and 30% hydrogen peroxide gives halomethyl(methoxy)phe-nols 103 and 105 in good yields. In most cases only one regioisomer of the halophenol was observed. 

For the preparation of the imide 2, use Bu2BOTf and EtjN (CH2CI2,0 °C). The boron aldol reaction often benefits from a work-up with H2O2, to cleave the boron-oxygen bond and release the alcohol product. 

The first step, addition of peroxidate ion (H—O—0 ) to the borane, is just the reaction of a filled orbital on oxygen with the empty 2p orbital on boron. It s another nucleophile-electrophile reaction. Notice that the boron becomes oxidized in this process. A discussion of the later steps is deferred for now, but at least you can see that the boron-oxygen bonds in the B(OR)3 (boronic ester) should be especially strong ones and the reaction will be favored thermodynamically. Rnally, in excess hydroxide the boronic ester equilibrates with boric acid and the alcohol. 

Thioketals are readily formed by acid-catalyzed reaction with ethane-dithiol. Selective thioketal formation is achieved at C-3 in the presence of a 6-ketone by carrying out the boron trifluoride catalyzed reaction in diluted medium. Selective protection of the 3-carbonyl group as a thioketal has been effected in high yield with A" -3,17-diketones, A" -3,20-diketones and A" -3,l 1,17-triones in acetic acid at room temperature in the presence of p-toluenesulfonic acid. In the case of thioketals the double bond remains in the 4,5-position. This result is attributed to the greater nucleophilicity of sulfur as compared to oxygen, which promotes closure of intermediate (66) to the protonated cyclic mercaptal (67) rather than elimination to the 3,5-diene [cf. ketal (70) via intermediates (68) and (69)]." " ... 

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