Stannanes, allylic oxidation

Transformations involving chiral catalysts most efficiently lead to optically active products. The degree of enantioselectivity rather than the efficiency of the catalytic cycle has up to now been in the center of interest. Compared to hydrogenations, catalytic oxidations or C-C bond formations are much more complex processes and still under development. In the case of catalytic additions of dialkyl zinc compounds[l], allylstan-nanes [2], allyl silanes [3], and silyl enolethers [4] to aldehydes, the degree of asymmetric induction is less of a problem than the turnover number and substrate tolerance. Chiral Lewis acids for the enantioselective Mukaiyama reaction have been known for some time [4a - 4c], and recently the binaphthol-titanium complexes 1 [2c - 2e, 2jl and 2 [2b, 2i] have been found to catalyze the addition of allyl stannanes to aldehydes quite efficiently. It has been reported recently that a more active catalyst results upon addition of Me SiSfi-Pr) [2k] or Et2BS( -Pr) [21, 2m] to bi-naphthol-Ti(IV) preparations. 

This chemistry forms the basis of a general method for 1,3-hydroxy transposition in allylic alcohols (equation 7). The starting alcohol is converted by 3,3-sigmatropic rearrangement of the 0-allyl-S-methyldithiocarbonate followed by hydrostannolysis to the allylic stannane, which is oxidized by MCPBA in a completely regiospecific manner. A similar sequence has been reported for allylsilanes. "... 

MCPBA oxidation of an allylic stannane is a key stq> in the overall converaon of an a,p-unsaturated aldehyde to an ( )>p-bromo-a-enone, as shown in equation (8). ... 

Allylic oxidation of olefins is a reaction of considerable value in organic synthesis [18] and selenium dioxide itself or in combination with other cooxidants remains a highly predictable and reliable reagent to perform these reactions. Thus, selenium dioxide oxidation of (Z)-tributyltin 1-alkenylcarba-mates 46 constitutes the first successful example of such a conversion ever reported with an element other than hydrogen [19]. Namely, it was found that with the allylic stannanes 46a or 46b oxidation occurred smoothly within 15 min to deliver in good yields the expected corresponding allylic alcohols 47a and 47b, respectively (Eqs. 6 and 7). 

A sequence was later developed for the synthesis of enantioenriched a-oxygenated allylic stannanes that did not require resolution (Eq. 33) [53]. This sequence, like the former, starts with the addition of BusSnLi to an enal. The resulting lithio alkoxide is oxidized in situ to the corresponding acylstannane. Reduction of the acylstannane with (M)-BINAL-H affords the (5)-a-hydroxy allylic stannane in > 95% ee. The use of (F)-BINAL-H leads to the (R) enantiomer with comparable ee. These hydroxy... 

The radical anions of carbonyl groups can also be generated via PET from activated alkenes, e.g. allylic silanes or stannanes. Triplet excited aromatic ketones, a-dicarbonyls and Michael systems are suitable substrates for oxidizing allylic Group 14 organometallic compounds with subsequent formation of homoallylic alcohols or S-allylated ketones (Scheme 32) [120-122]. 

Three other groups reported alternative methods for the synthesis of 3-alkenyl-substituted P-lactams. Durst [71] prepared these compounds via a Peterson olefination reaction. Thus, for example, treatment of 3-trimethylsilyl-azetidinone 116 with lithium diisopropyl amide followed by addition of pro-pionaldehyde gave a mixture of 117 and 118 in 66% yield. Tanaka [72] converted allylic stannanes 119 to bromides 120 which were smoothly cyclized to P-lactams 121 in good yield. Ley [73] treated 7i-allyltricarbonyliron complexes with benzylamine in the presence of Lewis acids to afford the corresponding lactam complexes. Oxidation with cerric ammonium nitrate served to liberate the desired P-lactams (Scheme 14). 

Catalyst in Oxidation Reactions. DBTO has been used as a catalyst in Fe -mediated oxidation of thiols to disulfides, even though Tri-n-butyl(methoxy)stannane seems to be better suited for this purpose. Epoxidation of terminal alkenes in a two-phase system (chloroform-water) containing H202/ammonium molyb-date/DBTO has also been reported. A combination of DBTO and t-Butyl Hydroperoxide oxidizes allylic alcohols with moderate regio- and stereoselectivity. Tri- and tetrasubstituted double bonds are most easily oxidized and the selectivities are comparable to those of the corresponding Vanadyl Bis(acetylacetonate) mediated reactions. 

Lithiation at C2 can also be the starting point for 2-arylatioii or vinylation. The lithiated indoles can be converted to stannanes or zinc reagents which can undergo Pd-catalysed coupling with aryl, vinyl, benzyl and allyl halides or sulfonates. The mechanism of the coupling reaction involves formation of a disubstituted palladium intermediate by a combination of ligand exchange and oxidative addition. Phosphine catalysts and salts are often important reaction components. 

Allyl carbamates also can serve as amino-protecting groups. The allyloxy group is removed by Pd-catalyzed reduction or nucleophilic substitution. These reactions involve formation of the carbamic acid by oxidative addition to the palladium. The allyl-palladium species is reductively cleaved by stannanes,221 phenylsilane,222 formic acid,223 and NaBH4,224 which convert the allyl group to propene. Reagents... 

The Stille reaction of 2-chloro-3,6-diisopropylpyrazine (7) and 2-chloro-3,6-diisopropylpyrazine 4-oxide (9) with tetra(p-methoxyphenyl)stannane (readily prepared in situ from the corresponding Grignard reagent and SnCU) led to the corresponding arylation products 8 and 10, respectively [9]. Additional Stille coupling reactions of chloropyrazines and their N-oxides have been carried out with tetraphenyltin [10] and aryl-, heteroaryl-, allyl- and alkylstannanes [11]. 

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