Lithium reductive dimerization

Treatment of UCI4 with the lithium complex obtained from dicyclohexylcar-bodiimide followed by crystallization from pyridine afforded a dinuclear uranium(rV) oxalamidinate complex in the form of dark green crystals in 94% yield (Scheme 191). The same compound could also be obtained by first reducing UCI4 to LiUCli (or UQs+LiCl) followed by reductive dimerization of di(cyclo-hexyl)carbodiimide as shown in Scheme 191. The molecular structure of this first oxalamidinato complex of an actinide element is depicted in Figure 31. ° ... 

Reduction in aptotic solvents may be accompanied by side reactions due to the alkaline conditions developed around the cathode. Preparative work is thus limited to substrates undergoing these unwanted side reactions relatively slowly. Reductive dimerizations in aptotic solvents show a high degree of stereoselectivity in favour of carbon-carbon bond formation to yield the ( )-isomer. A templating action is brought about either by co-ordination to a lithium or sodium ion of two reacting... 

Full details of Barton s selenofenchone (212 X = Se) and fenchylidenefeachane (212 X = 2-fenchylidene) synthesis (Vol. 6, p. 40) have been published" and Wynberg has discussed the antipodal interaction effect in the reductive dimerization of (+)- and ( )-camphor to bornylidenebornanes (Vol. 7, p. 41)," sensitized photo-oxidation of which has also been reported." The CuCl2-promoted dimerization of camphor-lithium enolate yields the expected mixture of dia-... 

Piperidinomethylisatin on lithium aluminum hydride gave the indoline 67 while sodium borohydride reduction of the same isatin derivative gave a reductive dimerization to 68 and a reduction-rearrangement to 69.255... 

This disilyloctadiene is obtained as a mixture of (Z,Z)- and (Z,E)-isomers by reductive dimerization of a mixture of butadiene and ClSi(CH3)3 with lithium metal (55% yield). The main by-product is l,4-bis(trimethylsilyl)-2-butene. 

Wittig-type olefinations can also be performed using selenoaldehydes. Phosphorus ylides initially attack the carbon atom of the selenocarbonyl functionality.405 Aromatic selenoketones undergo reductive dimerization with organo-lithium reagents probably via an electron transfer mechanism.406 Also the addition of organolithium reagents takes... 

The structure of the dilithiobutatriene (48) (67), prepared by lithium reduction of 1,4-di-t-butyldiacetylene, is a dimer with nearly parallel, C4 cumulene units. The four lithium atoms serve as bridges between the units. 

By protecting the carbonyl group of a phenacyl bromide through reaction with semicarbazide hydrochloride, by reducing the resultant semicarbazone at mercury in DMF containing lithium perchlorate to obtain a dimer, and by hydrolyzing the electro-lytically formed dimer, one can synthesize a variety of 2,5-diarylfurans in excellent yield [234], In addition, Barba and coworkers [235] demonstrated that reductive dimerization of the semicarbazone derivative of phenacyl bromide... 

Summary Vinylsilanes are known to react with lithium metal either to 1,2-dilithioethanes by reduction or to 1,4-dilithiobutanes by reductive dimerization. The reaction of substituted vinylsilanes with lithium metal is employed in the approach to vicinal and geminal dilithiated vinylsilanes by two consecutive additions of lithium metal and subsequent eliminations of lithium hydride. A mechanistic investigation in the reactivity of a- and (3-substituted vinylsilanes towards lithium metal discloses several new reaction pathways, whereby the choice of solvent plays an important role in apolar solvents like toluene vinyllithium compounds are obtained. Compound 14, R = Ph, which is not stable under the reaction conditions, finally affords the 1,4-dilithium compound 27. Compound 18, R = SiMes, on the other hand either adds to the starting vinylsilane (forming the monolithium compound 39) or shows an unusual dimerization to 47, which is studied in detail. 

Vinylsilanes as mono- and geminal disilyl-substituted C=C-double bonds like 1 [2] or 3 [3] afford, when brought to reaction with lithium metal in THE, the products of reductive dimerization, i. e., the 1,4-dilithiobutanes 2 and 4. This type of reaction is known as Schlenk dimerization [4]. Symmetrically tetrasilyl-substituted C=C-double bonds as in 5 on the other hand add lithium metal with formation of 1,2-dilithioethanes (Scheme 1) as stable intermediates in these reactions radical anions, like 7, can be observed, which are then reduced once again, here to the dianion 6 [5]. These two types of reaction are analogous to the reductions of the corresponding styrene derivatives... 

It has to be mentioned that 2,2-diphenylvinyllithium, which corresponds to structure 18, does not afford l,l-dilithio-2,2-diphenylethene upon treatment with lithium [18], Furthermore, two additional results are of interest when discussing the introduction of substituents into the / position of the vinylsilane (Scheme 4) Seyferth et al. reported that (Z)-propenyltrimethylsilane, (Z)-13, R = Me, was isomerized quantitatively to the corresponding ( )-isomer through the intermediate radical anion 20 when brought into contact with a catalytic amount of lithium metal in THF [19], the alkyl substituent deactivates the double bond, no further reduction was observed. Eisch and Gupta, on the other hand, showed that ( )-styryltrimethylsilane, (A )-13, R = Ph, afforded the expected products of reduction and of reductive dimerization, 21 and 22 meso compound), respectively [20],... 

The azulene anion 752- was reinvestigated by Edlund 92) by the lithium reduction of the neutral azulene (75) in THF-d8. This research followed an earlier study in which a dimeric dianion 192 was mistaken for the dianion 152 93). This anion is a 4nic-... 

REDUCTIVE DIMERIZATION OF TjERT-BUTYLPHENYLACETYLENE. Under nitrogen, a freshly cut suspension of lithium pieces (138 mg, 20 milligram-atoms (mg-at) of wire cut in 2.0-mm segments) in 3.16 g (20 mmol) of tert-butylphenyl-acetylene and 20 mL of ethyl ether was stirred vigorously as 1.2 mL of THF was added dropwise at 25 °C. During the following 3 h, the solution became red, and the amount of lithium diminished. The lithium was removed, and the red solution was treated with water (about 60 mg of Li was recovered). The mixture... 

REDUCTIVE DIMERIZATION AND ISOMERIZATION OF METHYLPHENYL-ACETYLENE. In a manner similar to the foregoing method, 5.8 g (50 mmol) of the methylphenylacetylene and 35 mL of ethyl ether were vigorously stirred with 350 mg (50 mg-at) of lithium pieces at 20 °C for 4 h. The excess lithium was removed, and the red solution was hydrolyzed. Removal of the volatiles in vacuo and crystallization of the residue from absolute ethanol gave 2.2 g (38%) of colorless leaflets of (E,E)-2,3-dimethyl-l,4-diphenyl-l,3-butadiene (mp 132-133 °C) (16). When the reaction was carried out on the same scale but in 30 mL of THF at -10 °C for 24 h, hydrolytic work-up (the addition of water followed by extraction with ether) gave an organic layer containing (E, E)-2,3-dimethyl-1,4-diphenyl-1,3-butadiene and isomers of methylphenylacetylene. 

REDUCTIVE DIMERIZATION OF CYCLOPROPYLTRIPHENYLSILANE. A solution of 1.10 g (3.6 mmol) of the cyclopropyltriphenylsilane in 30 mL of THF and 30 mg (4.6 mg-at) of lithium pieces, after stirring at -78 °C for 20 h and hydrolytic workup, produced an almost quantitative yield of 4,4 -bis(cyclo-propyl(diphenyl)silyl)-l,l, 4,4 -tetrahydrobiphenyl, which after column chromatography and recrystallization from ethyl ether melted at 145-151 °C (22). 

Alkenes can be obtained from aldehydes or ketones on reductive dimerization by treatment with a reagent prepared from titanium(III) chloride and zinc-copper couple (or L1A1H4), or with a species of active titanium metal formed by reduction of titanium(III) chloride with potassium or lithium metal. This McMurry coupling reaction is of wide application, but in intermolecular reactions generally affords a mixture of the E- and Z-alkenes (2.99). 

Lithium (s. a. under Cul) Reductive dimerization of ethylene derivatives with and without substitution... 

Lithium tetrahydridoaluminateftitanium trichloride Reductive dimerization of alcohols... 

Use of strongly anionic nucleophiles such as alkylmagnesium halides and lithium alkyls leads to reductive dimerization or displacement of the olefin. It is quite possible however that alkylation of olefin-iron cations (IX) would be feasible with alkyl derivatives of zinc, cadmium, or tin reagents. Alkylation of dienyliron cations with dialkylzinc and dialkylcadmium reagents has recently been demonstrated (Section III,A,4). Alkylation of olefin-iron cations can be achieved under mild conditions with cr-bonded allyliron compounds [Eqs. (9) and (10), (Rosan et ai, 1973)]. 

A titanium(Il) species formed from titanium trichloride and lithium aluminum hydride is a useful reagent for the reductive coupling of carbonyl compounds to olefins (McMurry, 1974 McMurry and Fleming, 1974). Both aliphatic and aromatic ketones can be converted to tetrasubstituted olefins in excellent yields. Reductive dimerization of retinal (CCLXXFV) affords j6-carotene (CCLXXV) in 85% yield. The course of the reaction can be accounted for by assuming pinacol formation followed by loss of titanium dioxide. 

The pinacol (94) was also obtained by reductive dimerization of retinaldehyde (2) with zinc amalgam (Reedy, 1968) or in the presence of chromium(III) salts (Sopher and Utley, 1979). With a mixture of lithium aluminum hydride and titanium chloride in anhydrous tetrahydrofuran, retinaldehyde (2) underwent reductive coupling to give p-carotene (Akiyama et al., 1979 Mukaiyama, 1977 McMurry et al., 1978 Ishida and Mukaiyama, 1977). 

A synthetic sequence has been discovered in which aldehydes which are not branched at the a-position undergo formal reductive dimerization to give symmetrical olefins (e.g. Scheme 14). At the key step, a-stannylalkyl halides, which are stable and easily handled, smoothly couple on treatment with n-butyl-lithium under very mild conditions (-78 to 0 °C). Furthermore, the difference in reactivity between a-stannylalkyl iodides and chlorides enabled a moderate yield of a cross-coupled olefin from two different aldehydes to be obtained, a-Branched aldehydes exhibit a different pattern of reactivity, furnishing terminal olefins instead of coupled products (Scheme 15), probably for steric reasons. 

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