Stannanes radical addition

Stannylation of lithiated allyl ethers gives (Z)-3-alkoxyallylstannanes (1)115,116, whereas mixtures of (Z)- and ( )-tributyl(3-methoxy-2-propenyl)stannanes (2) were obtained from free-radical addition of tributyltin hydride to l-methoxy-l,2-propadienel16. 

There are several procedures for synthesis of terminal alkenyl stannanes that involve addition to aldehydes. A well-established three-step sequence culminates in a radical addition to a terminal alkyne.150... 

Crich and Rumthao reported a new synthesis of carbazomycin B using a benzeneselenol-catalyzed, stannane-mediated addition of an aryl radical to the functionalized iodocarbamate 835, followed by cyclization and dehydrogenative aromatization (622). The iodocarbamate 835 required for the key radical reaction was obtained from the nitrophenol 784 (609) (see Scheme 5.85). lodination of 784, followed by acetylation, afforded 3,4-dimethyl-6-iodo-2-methoxy-5-nitrophenyl acetate 834. Reduction of 834 with iron and ferric chloride in acetic acid, followed by reaction with methyl chloroformate, led to the iodocarbamate 835. Reaction of 835 and diphenyl diselenide in refluxing benzene with tributyltin hydride and azobisisobutyronitrile (AIBN) gave the adduct 836 in 40% yield, along with 8% of the recovered substrate and 12% of the deiodinated carbamate 837. Treatment of 836 with phenylselenenyl bromide in dichloromethane afforded the phenylselenenyltetrahydrocarbazole 838. Oxidative... 

C-Glycoside synthesis may be achieved in twro ways. Intermolecular radical addition reactions are observed with (i) polarized, electron-deficient alkenes, (ii) alkenes that provide a high level of stabilization to the initial radical adduct and (in) substrates that undergo a facile fragmentation (e.g. allyl stannanes). Additions to less reactive substrates, though not favored for intermolecular processes, are observed if the two components are tethered in an intramolecular array. 

In summary, intramolecular radical cyclization reactions onto aromatic rings can provide quick access of otherwise not so easily assembled Ca0. -C.ryi bonds, although the yields are generally low and the process suffers from a lack of regioselectivity. Noticeably, Crich found that a catalytic amount of benzeneselenol, which can also be generated in situ reduction of diphenylselenide with stannane [57]. This method should find synthetic utility in intramolecular radical addition to arenes. 

Castle and Widdowson were first to disclose alkyl-alkyl Kumada coupling reactions catalyzed by Pd(dppf)Cl2 [195]. This report was later questioned and corrected by Scott [196]. Matsubara and colleagues established formal Stille-type coupling reactions of perfluoroalkyl halides with allyl, alkynyl, or vinyl stannanes catalyzed by 10 mol% of Pd(PPh3)4, which have to be considered, however, better as radical addition/elimination reactions rather than as coupling reactions (see Sect. 3.1) [184],... 

Free radical additions of phenylthio or stannyl radicals to 2-alkenyl 2-siloxycyclo-propanes afford similar products although a completely different mechanism is operative 84). This direct generation of protected y-oxoesters 144 and 145 is of interest since the silyl enol ether function might be usable for regioselective C-C-bond formation and the allyl stannane moiety in 145 could be activated for subsequent transformations. Yet further examples have to demonstrate utility and scope of this mode of ring opening. 

The reductive cyclization protocol was then applied to a suitably A-protected radical precursor to allow further access to the alkaloid calothrixin B. Satisfactorily, 2-indolylacyl radicals derived from A-(methoxymethyl) selenoester 57 underwent cyclization under TTMSS-AIBN conditions with an even higher efficiency than their A-methyl counterparts. The reaction nevertheless followed a different course as, after the radical addition and quinoline rearomatization, pentacyclic phenol 58, a fully aromatic tautomeric form of ketone P, was isolated in 90% yield. The same phenol 58 was isolated although in lower yields (50-70%) using either stannane-AIBN or AIBN-irradiation protocols. 

Stannyl radical addition-cyclization of oxime ether 183 [31] was also examined (Scheme 44). In this case vinyl stannane 185 was obtained in 77% yield. 

Scheme 44 Stannyl radical addition-cyclization to form a vinyl stannane by Naito [31]...

The reaction of the alkyne with Bu3SnH is a radical addition initially giving the cis (Z-) vinyl stannane but this equilibrates to the trans ( -) if a (slight) excess of BusSnH is used so this is how the reaction must be done. 

SET photochemistry is involved in the reaction between the enones (371) and the a-stannyl ethers (372) in methanol. The products are the 3-sub-stituted cycloalkanes (373) which arise from addition of aryloxymethyl radicals to the enones. Irradiation (X > 400 nm) of the stannanes (374) in the presence of the ketones and aldehydes (375) affords two products identified as (376) and (377). The former of these is dominant and the reaction arises by an electron transfer from the stannane to the ketone. The resultant stannane radical-cation undergoes fission to yield an alkoxy allyl radical and the tin cation. The alkoxyalkyl radical adds to the carbonyl radical-anion with a preference for... 

Several other types of polymer-bound stannanes have since been reported, including a polymer-bound distannane used in radical-mediated cyclizations and radical addition to acetylenes [87], and stannanes with longer tethers to the polymer [88, 89] applied in the Barton-McCombie deoxygenation of alcohols [90, 91] and... 

Phenylmenthyl esters are also suitable chiral groups for inducing stereoselectivity in radical addition reactions, as shown in the allylation of phenylmenthyloxycarbonyl-substituted xanthates. The photoinitiated reaction of the radical precursor with tributyl(2-propenyl)stannane at — 78 =C affords only one diastereomer4. The absolute configuration of (— )-8-phenylmenthyl 2-methyl-2-phenyl-4-pentenoate (5) is not known. 

Scheme 17. Use of the EtjB-stannane-air system for intramolecular radical addition to aldehyde...

With this information in hand, it seemed reasonable to attempt to use force field methods to model the transition states of more complex, chiral systems. To that end, transition state.s for the delivery of hydrogen atom from stannanes 69 71 derived from cholic acid to the 2.2,.3-trimethy 1-3-pentyl radical 72 (which was chosen as the prototypical prochiral alkyl radical) were modeled in a similar manner to that published for intramolecular free-radical addition reactions (Beckwith-Schicsscr model) and that for intramolecular homolytic substitution at selenium [32]. The array of reacting centers in each transition state 73 75 was fixed at the geometry of the transition state determined by ah initio (MP2/DZP) molecular orbital calculations for the attack of methyl radical at trimethyltin hydride (viz. rsn-n = 1 Si A rc-H = i -69 A 6 sn-H-C = 180°) [33]. The remainder of each structure 73-75 was optimized using molecular mechanics (MM2) in the usual way. In all, three transition state conformations were considered for each mode of attack (re or ) in structures 73-75 (Scheme 14). In general, the force field method described overestimates experimentally determined enantioseleclivities (Scheme 15), and the development of a flexible model is now being considered [33]. 

Hydrostannylation and hydroxystannylation. The radical addition of an organotin hydride to a multiple bond is promoted by ultrasonic irradiation. In the presence of air, the product is a /3-hydroxy stannane. 

A useful intermolecular radical reaction that avoids tributyltin hydride and excess alkene makes use of the ready p-ehmination of fin or sulfur radicals. Addition of the carbon-centred radical to the y-position of an unhindered allyl stannane gives an intermediate radical that eliminates a tin radical (4.38). The product is therefore the result of overall allyl addition and the released fin radical reacts... 

Hydrostannylations. Hydrostannanes add to alkynes in uncatalyzed reactions at 60 °C. Phenylacetylene, for instance gives a mixture of ( )- and (Z)-vinylstannanes, wherein the tin atom has added to the terminal carbons. In the presence of Wilkinson s catalyst, however, the hydrostannylation proceeds at 0 °C to give mostly the regioisomeric vinylstannanes (eq 23). Terminal stannanes in the latter process seem to result from competing free radical additions. This may not be a complication with some other catalysts the complexes PdCl2(PPh3)2 and Mo( j -allyl) (CO)2(NCMe)2 also mediate hydrostannylations of alkynes, and they are reported to be 100% cis selective. Hydrostannanes and thiols react in a similar way to silanes and alcohols (eq 24). ... 

Often the cyclized radical opens in the other direction to give rearranged products. This possibility has been used to explain the results of the pyrolysis of cyclopropyl acetates, of the lead tetraacetate oxidation of a-acetoxy alcohols,and of the stannane reduction of a-acetoxy chlorides. In all these cases free radical addition followed by -scission has been proposed, e.g., as in the 1,2-rearrangement of )ff-acyloxyl radicals 205 to 207 (Scheme 90) discovered independently by Teissier and by Tanner,and whose mechanism, while not completely elucidated, is presumed to proceed through 206, which must be considered as a transition state rather than an intermediate. 

Radical addition-fragmentation processes have been exploited in synthetic organic chemistry since the early 1970s. Allyl transfer reartions with allyl stannanes and... 

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