Addition to C=O bonds

Addition to C O bond. Excellent regioselective addition of organozinc reagents to one of the C=0 group of a cyclic anhydride (see equation below) can be attributed to precoordination to the electronically more favorable double bond. ... 

Germanium and silicon hydrides should be even more selective toward alkoxyl radicals relative to carbon radicals. Indeed, while silicon hydrides react rapidly with alkoxyl radicals, reactions with carbon radicals are too slow to propagate chains. The rapid addition of silyl and germyl radicals to C—O bonds is a possible complicating reaction. 

Addition of AUylic Suifinyl Carbanions to C—O Bonds 233.6 Addition of AUylic Suifinyl Carbanions to C—C—C= Bonds 2.33.7 Addition qfa-Sulfinyl Carbonyl Compounds to C =0 Bonds 2.33B Addition cfa-Halo Suffoxides to Bonds... 

For the regiochemistry of multiple additions to C o a complex picture is found. Starting from an initial 1,2-adduct, there are nine discernible sites for a second addition to another (6,6)-bond as depicted in Figure 2.35. The resulting regio-isomers are termed e, e", trans-1, trans-2, trans-3, trans-4, ds-1, cis-2, and cis-3. 

Formed in superacid media species exhibiting electrophilic properties are able to attack alkanes primarily via electrophilic addition to C-H bond followed by other reactions. In particular, Olah et al. observed O atom insertion in hydrogen peroxide reaction with methane in Magic Acid above 0 °C to produce methanol with very high (>95%) selectivity [54a]. The particle (OH)"", which may be considered to be a protonated oxygen atom in the singlet state is apparently the active species in the reaction. Methyl alcohol formed is immediately protonated to methyloxonium ion, and this prevents further... 

The simplest example of the triple bond is in the nitrogen molecule illustrated in Fig. 23.15. Two 7C bonds are formed in addition to the o bond. The unshared pairs are 180° from the sigma bond on each end of the molecule. The formation of two n bonds results in the charge cloud having the shape of a cylindrical sheath around the axis of the molecule. (Compare to Fig. 22.5 for m = 1.) Species that are isoelectronic with nitrogen and therefore have the same electronic structure are C=0, "C=N, and C=C. ... 

The mechanism of this reaction has been studied to a certain extent. It has been proposed that zinc is actually converted into a complexing Lewis acid. In addition, the C-O bond, as a part of the monoacetate at C-20, of these steroids never breaks and is transformed into the carbonyl group in the final product while the hydrogen at C-20 migrates to the C-17 position, accompanying with the inversion of stereochemistry at C-17, as illustrated... 

In a further demonstration of the broad scope of nickel(0) oxidative additions to C-0 bonds, cyclic anhydrides were illustrated to be versatile substrates in crosscoupling processes. For example, enantioselective desymmetrizations with diethylzinc and cyclic anhydrides provide efficient access to functionalized 1,4-dicarbonyl products. The enantioselectivity-detemuning step in this process is the oxidative addition of nickel(O) to anhydride C-0 bond. Several other functionalization processes derived fi-om oxidative addition of nickel(O) to anhydrides were previously described. ... 

Unsurprisingly due to their potential as medicinal agents there have been many reported synthetic routes to isoflavanones including the reduction of corresponding isoflavones, [12] benzylic oxidation of isoflavans, [13] gold-catalysed annulations, [14] addition of C-O bonds to arynes [15] and palladium catalysed a-arylation [16]. Despite the multitude of syntheses reported over the past 70 years few routes have been shown to be truly modular with respect to substituents on the isoflavanone skeleton and only one route was asymmetric, relying on the use of chiral auxiliaries [17]. 

In contrast to C-O bond-forming reactions, C-S and C-Se coupling are rare in oxidative Pd catalysis. However, a recent report by Dong and coworkers demcmstrated the Pd-catalyzed ligand-directed sulfonylation of arylpyridine, aiylpyrazole, and aryloxime ether derivatives with ArS02Cl (Eq. 6) [30]. The authors speculated that a Pd mechanism (involving oxidative addition into the S-Cl braid and subsequent C-S bond-forming reductive elimination from Pd ) was potentially operative. 

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