Alkenes from 1,2-diols

The Barton-McCombie radical deoxygenation [221-223] (vide infra Section 32) has also been applied for transforming v/c-diols into olefins. Commercially available diphenylsilane and the easily formed dixanthates can be conveniently used for this purpose, as in the following example in the sugar series [224]. 

To a solution of the dixanthate (0.185 g, 0.4 mmol) in dry toluene (3 ml), diphenylsilane (147 ml, 0.8 mmol) was added under argon. Then the solution was brought to the boil and treated with portions (300ml) of a solution of AIBN in toluene (262.4 mg AIBN dissolved in 6.0ml of dry toluene) at 30min intervals. The reaction was monitored by TLC. When the reaction was complete (5 h), the solvent was evaporated under vacuum and the olefin isolated by column chromatography on silica gel (eluent CH2C12), giving (1) (93.5 mg, 94%). 

Cyclic sulfates, efficiently prepared from vtc-diols in a two-step one-pot procedure [225], were converted into alkenes by treatment with sodium naphthalenide in THF [226]. 

---

Alkenes from diols. Reaction of v/c-diols with two secondary hydroxyls or one primary and one secondary alcohol with chlorodiphenylphosphine (2 equiv.), imidazole (4 equiv.), and iodine (2 equiv.) results in alkenes. The reaction presumably involves a vic-iododiphenylphosphinate, which can be isolated in some cases and converted to an alkene with zinc in acetic acid. 

Ca.ta.lysts, A small amount of quinoline promotes the formation of rigid foams (qv) from diols and unsaturated dicarboxyhc acids (100). Acrolein and methacrolein 1,4-addition polymerisation is catalysed by lithium complexes of quinoline (101). Organic bases, including quinoline, promote the dehydrogenation of unbranched alkanes to unbranched alkenes using platinum on sodium mordenite (102). The peracetic acid epoxidation of a wide range of alkenes is catalysed by 8-hydroxyquinoline (103). Hydroformylation catalysts have been improved using 2-quinolone [59-31-4] (104) (see Catalysis). 

The hydroxylation of alkenes to diols with potassium osmate-oxidant mixtures has been described, with either hydrogen peroxide or sodium chlorate as the oxidant. The sodium chlorate method is not applicable where the diol is to be distilled from the mixture, because of the danger of explosive oxidation of the product diol by the chlorate. 

Alkenes from vie -diols.2 The 2-dimethylamino-l,3-dioxolanes 1, obtained by reaction of a 1,2-diol with N,N-dimethylformamide dimethyl acetal in quantitative yield, when treated with ethyldiisopropylamine (4 equiv.) and triflic anhy-... 

A synthesis of alkenes from 1,2-diols depends upon the initial formation of the 1,3,2-dioxaphospholans (128), followed by rupture of the heterocyclic ring and removal of phosphorus with Li-NH3 or Na-Ci 0H 8. The reaction is reasonably stereo-... 

The oxidation of alkenes to diols via l,2-bis(boronate) esters was reported for a silver(l) NHC complex 17 [ 109]. 1-Methyl-3-(+)-methyhnenthoxide imi-dazoUum chloride was used as the precursor together with an excess of Ag20 (Scheme 21). The resulting complex was significantly more active than when prepared in situ from Ag20 and the imidazolium salt. 

Alkenes from vie-diols (cf. 9, 507-508). Hexapyranosidc rc-diols arc converted into alkenes by reaction with this combination of reagents. The method is applicable to both cis- and trons-diols, but highest yields of alkenes arc usually obtained from diequatorial tra/is-diols. The reaction is believed to involve formation of a nrc-diiodide derivative (inversion), which then undergoes reductive elimination with imidazole. 

Alkenes from 1,2-diols. Corey and Hopkins2 have reported two improvements in the preparation of alkenes from 1,2-diols via the thionocarbonate (1, 1233-1234 3, 315-316). The thionocarbonate is prepared with thiophosgene and 4-dimethylamino-pyridine, a method generally successful except for ditertiary diols. The thionocarbonate is converted into an alkene when treated with I at 25 40" for 2-24 hours. 

Alkenes from 1,2-diols (7, 385-386). An efficient synthesis of (S)-(—)-y-methoxymethyl-a, /J-butenolide (3) from (+)-5-0-methyl-D-ribonolactone (1) involves conversion to the cyclic orthoformate 2 followed by pyrolysis to give 3 in 66.5% overall yield.1 In this case, the Corey-Winter reaction and the Hanessian route (8,192) were... 

Cyclic orthoesters derived from gem-diols offer a further route to alkenes. As part of a three-step conversion, they may be ring opened with hydrobromic acid to give O-acyl bromodeoxy compounds that undergo reductive elimination with copper-zinc. In this way, unsaturated nucleosides have been made by way of mixed 2y3,-bromo-2y3,-deoxy-3,/2, carboxyl-ates.174 A more direct route to alkenes from cyclic orthoesters involves heating in acetic anhydride together with zirconium oxide.175... 

Alkenes and Potassium Osmate. Sodium chlorate should not be used in the hydroxylation of alkenes to diols in the presence of potassium osmate if the diol is to be distilled from the reaction mixture explosive oxidation may occur.3... 

Peracids themselves produce epoxides and diols from alkenes but are not powerful enough to oxidize these further by cleaving the carbon-carbon bond. However, in the presence of transition metals they will cleave alkenes and diols to give, usually, carboxylic acids. For example, peracids in combination with ruthenium compounds are well known in this capacity (Figure 3.31).146,147 Warwel and co-workers have reported the cleavage of alkenes using peracetic acid and the ruthenium catalyst, Ru(acac)3.148... 

Alkenes from vie-dlols. Hexopyranoside diols are converted into alkenes by reaction with 1 and triphenylphosphine in refluxing toluene. Addition of imidazole improves the yield. The most satisfactory molar proportion of diol, triphenylphosphine, 1, and imidazole is 1 4 1.6 2. Yields are high with t)ic,trans-diols, either diequatorial or diaxial. 

This process occurs for X = O only in two special cases, which have been developed into synthetic methods for alkenes and allylic alcohols. Reaction of BusSnH with 1,2-bis-MDC derivatives gave al-kenes from diols in a process which was independent of the geometry of the starting diol. Examples are the conversions of (84) to (85) and of (86) to (87). 

Diols are easily accessible from alkenes by the reaction with osmium tetroxide, and constitute another protection of the double bond since a variety of methods for the reformation of the alkenes from them is available. Among the important methods which proceed stereospecifically, the desulfurization of 2-thioxo-l,3-dioxolanes (e.g. 102, Corey-Winter reaction),the acid-catalyzed decomposition of... 

The cyclic phosphate and phosphoramidate derivatives of v/c-diols undergo a two-step reductive elimination on treatment with Li/NHs, Na-naphthalene, TiCl4-Mg(Hg) or TiClt-K. In acyclic systems the reaction is only moderately stereoselective. The reaction has particular value in the synthesis of tetra-substituted alkenes from highly hindered v/c-diols. Scheme 20 illustrates its use in the synthesis of [10,10]-betweenanene (49). ... 

Wharton fragmentation Base-induced formation of medium-sized cyclic alkenes from 1,3-diol monosulfonates. 480... 

Alkenes from vic-diols.1 Bis(dithiocarbonates) can be converted into alkenes by reaction with tributyltin hydride/AIBN (8,499). Diphenylsilanc and an initiator are equally effective and avoid use of toxic tin compounds. This radical reaction gives particularly high yields in reactions of nucleoside derivatives. 

Epoxidation and Dihydroxylation of Alkenes There are several ways to convert alkenes to diols. Some of these methods proceed by syn addition, but others lead to anti addition. An important example of syn addition is osmium tetroxide-catalyzed dihydroxylation. This reaction is best carried out using a catalytic amount of OSO4, under conditions where it is reoxidized by a stoichiometric oxidant. Currently, the most common oxidants are f-butyl hydroperoxide, potassium ferricyanide, or an amine oxide. The two oxygens are added from the same side of the double bond. The key step in the reaction mechanism is a [3 + 2] cycloaddition that ensures the syn addition. 

Alkenes from 1,2-diols. The catalytic elimination using Cp ReOj in the presence of PhjP in hot chlorobenzene is stereoselective. This method is potentially competitive with the standard Corey-Winter procedure. 

Alkenes from ic,ds-diols. This conversion can be effected in a two-step, one-pot process. The diol is converted into the l-dimethylamino(methylene) acetal by reaction with DMF dimethyl acetal. The product is converted into a trimethylalkylammonium salt, which is converted into an alkene when heated. Examples ... 

Alkenes from ic-diols. Cyclic and acyclic y c-diols can be converted into olefins by conversion to bisdithiocarbonates (dixanthates) followed by reaction with tri-n-butyltin hydride (equation I). Five examples of the reaction in the carbohydrate field were cited. 

The McMurry reaction (see Section 2.9) can allow the formation of alkenes from dicarbonyl compounds. This reaction generates an intermediate 1,2-diol (pinacol), which is converted on the surface of the titanium to the alkene. The two carbon-oxygen bonds do not break simultaneously and the reaction is not stereospecific. Thus, both anti and syn acycUc 1,2-diols give mixtures of Z- and -alkenes. With cyclic 1,2-diols, the two oxygen atoms must be able to bond to a common titanium surface. Thus, the cis-diol 45 eUnunates to the alkene 46, whereas the trans-dio 47 is inert under these reaction conditions (2.43). ... 

A small number of enantiomerically pure Lewis acid catalysts have been investigated in an effort to develop a catalytic asymmetric process. Initial work in this area was carried out by Narasaka and coworkers using the titanium complex derived from diol (8.216) in the cycloaddition of electron-deficient oxazolidinones such as (8.217) with ketene dithioacetal (8.218), alkenyl sulfides and alkynyl sulfides. Cyclic alkenes can be used in this reaction and up to 73% ee has been obtained in the [2- -2] cycloaddition ofthioacetylene (8.220) and derivatives with2-methoxycarbonyl-2-cyclopenten-l-one (8.221) usingthe copper catalyst generated with bis-pyridine (8.222). Furthermore, up to 99% ee has been obtained in the [2-1-2] cycloaddition of norbornene with alkynyl esters using rhodium/Hs-BINAP catalysts. This reaction is not restricted to the use of transition metal-based Lewis... 

---