Tin pinacolate

Radical homologation. This tin pinacolate is known to generate trimethyltin radicals at 60° and appears to be superior to tributyltin hydride as a source of stannyl radicals for addition of alkyl halides to O-benzylformaldoxime (equation I).1 Iodides, bromides, and selenides can be used as radical precursors. The same... 

Intermolecular coupling Many papers on hydrodimerization of aromatic carbonyl compounds have appeared indicating the importance of this reaction. The rac/meso ratio for the pinacolization of acetophenone in aqueous ethanol ranges between 0.9 and 1.4 in acidic medium and between 2.5 and 3.2 in basic medium. The diastereoselectivity is independent of the cathode material mercury, tin, or copper. Electrolysis conditions such as current density, potential, or current-controlled electrolysis also do not influence the diastereoselectivity. The same holds for propiophenone. For benzaldehyde, the rac/meso ratio is 1.1 to 1.2 in acidic as well as in basic media [283]. In the presence... 

Addition of chromium(ll ) [114] or tin(li) [115] ions to the electrolyte strongly favours the formation of head-to-head pinacols from the reduction of enones. It is believed that these ions direct tlie dimerization step by coordination through the oxygen centre... 

A new semipinacol rearrangement mediated by Sn(IV) was proposed by Bates and to explain the formation of 579 from 578 (equation 256). As stated by the authors, the mechanism of formation of 579 most likely involves an intermediate hydroxylamine 580 (equation 257). Nucleophilic addition of the hydroxylamine to the ketonic carbonyl leads to 581, which may undergo a tin-mediated pinacol-type rearrangement with preferred migration of the phenyl substituent to produce amide 582. 

Pinacols (61) derived from a variety of aromatic aldehydes have been employed in enantio- and diastereo-selective allylations of aliphatic aldehydes.174 Their allyl-boronate derivatives react under Lewis acid conditions (SnCU) with a variety of aldehyde types, in good yield and ee. Even better results are obtained by addition of (61) as a I irons led acid (auto)catalyst, via coordination/activation of the tin catalyst. 

Asymmetric phosphines, polymer-supported, 12, 701 Asymmetric pinacol coupling reactions, characteristics, 11, 48 Asymmetric reactions, with allylic tins, 9, 355 Asymmetric substitution reactions, with zinc compounds, 9,99 Atmosphere studies... 

Indium mediated Barbier-type cross coupling between carbonyl compounds and allyl halides proceed efficiently under solvent-free conditions. No apparent competing pinacol-coupling or homo-coupling of the allyl halide was observed. The reactions were found to be mediated also by zinc, tin, bismut and copper [45]. 

HOMOALLYLIC ALCOHOLS B-Allyldiisopino-campheylborane. Allyl phenyl selenide. Allyltri-/i-butyltin. Chromium(II) chloride. Crotyltrimethylsilane. Diethylftributyl-stannyl)aluminum. (2R,4R)-Pentanediol. Pinacol chloromethaneboronate. Tin-Aluminum. Titanium(lV) chloride. 

The effective electroreductive pinacolization has been exemplified in the terpene series, i.e. aldehydes such as retinal and ketones such as a- or j8-ionone give the corresponding pinacols in good yields when electrolyzed in a DMF-sodium per-chlorate-Hg system or with a Pt cathode in the presence of tin [8, 9]. 

The enantioselective total synthesis of the polysubstituted tetrahydrofuran (-)-citreoviral, the unnatural enantiomer, was synthesized by L.E. Overman et al. The Prins-pinacol rearrangement of an allylic 1,2-diol with an unsymmetrical ketone proceeded with high stereoselectivity. The jb/s(trimethylsilyl)-1,2-diol was condensed with the dimethyl acetal of the unsymmetrical ketone in the presence of catalytic amounts of TMSOTf, which yielded a nearly 1 1 mixture of the corresponding acetal and rearrangement product. The acetal was converted to the desired tetrahydrofuran product upon exposure to tin tetrachloride. 

Tin. Bu3SnH has recently been demonstrated to effect the intramolecular pinacol coupling of 1,5- and 1,6-dialdehydes and keto aldehydes (Eqs. 3.24 and 3.25) [41]. 1,5-Dicarbonyl compounds undergo cyclization with consistently excellent (> 20 1) cis selectivity, whereas 1,6-dicarbonyl compounds provide variable stereoselection. Mechanistic studies furnish evidence for an interesting new pathway for pinacol coupling, involving homolytic substitution as a key step (Fig. 3-2). 

A different approach involves the intramolecular pinacol coupling of 1,5- or 1,6-dialdehydes or -ketoaldehydes with a tin hydride. The ring is closed by an Sh2 reaction of the oxyalkyl radical at the tin centre.68... 

The Prins-pinacol sequence has been extended to the preparation of oxa bicyclic systems in which hetcrocycle formation is accompanied by one carbon expansion of an existing carbo-cyclic framework39,40. Treatment of dioxolanes 36 with tin(IV) chloride affords, in each case a single, r/v-fused cyclohepta[b]tetrahydrofuran 37. As in the previously noted examples, stereochemical induction derives solely from the homoallylic stereocenter thus rearrangement of the civ-fused dioxolane 36 (R1 = R2 = R3 = H) affords the same product as that obtained from rearrangement of the traws-fused isomer 3840. 

Previously, tin-ketyl radicals have been added to alkenes only in an intramolecular fashion. [9] In recent publications, however, pinacols and amino alcohols have been prepared by cyclisation of dicarbonyl compounds [10] or keto-oximes [11] with tributyltin hydride. Cyclisation of 1,5-ketoaldehydes 1 and 1,5-dialdehydes with tributyltin hydride yields cw-diols 2 with excellent stereoselectivities, whereas the keto-oxime 4 with four benzyloxy-substituents affords a 58 42 cis trans) mixture. The tran -product was transformed in two more steps to the potent glycosidase inhibitor 1-deoxynojirimycin (6). [lib] The reversibility of both the addition of the tributyltin-radical to the carbonyl group and the intramolecular radical C-C bond formation is believed to be responsible for the high selectivity in the formation of 2. In the cyclisation of 1,5-pentanedial the unhydrolyzed coupling product 3 could be isolated, therefore providing evidence for a new mechanistic variant of the pinacol reaction, in which only 1.2 equivalent of the reducing agent are necessary. 

Photochemical reduction in the presence of excess tin hydride can lead to either the alcohol or the pinacol 20d). 

Cydic akenyl ethers of general structure IV (X = O), like dihydrofuran and dihydropyran 50 (n = 1,2) are deprotonated by means of either f-BuLi in a mixture of pentane and THF or with -BuLi and TMEDA in hexane or pentane at 0 °C [41]. These compounds reacted with a great variety of electrophiles, their reaction with carbonyl compounds, followed by add treatment, giving spirocyclic ketones after suffering pinacol rearrangement [46]. Enantiomerically emidied spirocyclic compounds were obtained from the intermediate 51 (prepared from tin-lithium exchange) [47], or a-lithiated glucals 52 and 53 (prepared by direct deprotonation with t-BuLi) [48]. 

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