Hydrodehalogenation of 1,1-dibromocyclopropanes by Grignard reagents promoted by titanium compounds † (original) (raw)
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Conversion of some dihalogenocyclopropanes into unsaturated ketones
Journal of the Chemical Society C: Organic, 1977
ABSTRACT Dibromocyclopropanes, prepared by addition of dibromocarbene to cyclic olefins, give unsaturated ketones, with ring expansion, when treated with silver sulphate in concentrated sulphuric acid at room temperature. Mild hydrogenation of a dibromocyclopropane gave equivalent amounts of the monobromocyclopropane and of a ring-opened bromine-free compound.
2003
This review surveys both data obtained by the authors and published data on the partial or full hydrodehalogenation of di-and polyhalocyclopropanes (chlorides and bromides) with Grignard reagents catalyzed by titanium or zirconium compounds. The factors affecting the efficiency and selectivity of the hydrodebromination of bromocyclopropanes are considered: the nature of Grignard reagents (including isotopically labeled reagents), their transformations and effects in catalyzed and uncatalyzed reactions, the participation of solvents, catalytic and stoichiometric amounts of the catalyst, etc. A scheme is proposed in which the key steps of the mechanism of hydrodebromination of bromocyclopropanes includes three blocks of reactions: (a) the generation of a catalytically active Ti(II) species; (b) the hydrodehalogenation of bromocyclopropanes involving electron transfer from a low-valent catalyst species, formation of the cyclopropyl radical, and stabilization of this radical as a result of hydrogen atom transfer from the solvent molecule; and (c) transformations of previously formed radical species, such as dimerization and disproportionation (for example, of radical species generated from Grignard reagents or ether molecules) or the linking of alkyl radicals to radical species produced from solvent molecules.
Use of Cyclopropanes and Their Derivatives in Organic Synthesis
Under the influence of a variety of chemic- reagents (e.g., electrophiles, nucleophiles, radicals) or external physical forces (e.g., heat, light), cyclopropane deriva- tives undergo a variety of ring-opening reactions. In contrast to normal paraffins, the chemistry of the cy- clopropane C-C single bond resembles that of a car- bon-carbon double bond. Relief of ring strain provides a potent thermodynamic driving force for these pro- cesses. Since numerous methodologies have been de- veloped for the construction of three-membered car- bocycles, the chemistry of cyclopropanes has emerged as a versatile tool in organic synthesis. In this section, the theoretical basis for the “unusual”reactivity and properties of cyclopropane is reviewed.
Bicyclopropylidene: cycloadditions onto a unique olefin
The Journal of Organic Chemistry, 1988
of BH4-, strongly suggests the participation of 8. In-cage transfer of hydrogen at this stage can explain the lack of a primary isotope effect. Acrylonitrile quenches the borohydride reactions to a very small extent. It is likely that aryl radicals are formed when 7 fragments. In the case of 8, the caged radical pair may abstract a hydrogen from within the cage or diffuse into the solvent in the form of a radical. It is likely that acrylonitrile, which appears to be an inefficient quencher of the aryl radical generated from 1, has a lower rate of quenching than the rate of hydrogen transfer from the borohydride in 8 within the radical pair cage. Experimental Section General Procedures. Reagent grade acetonitrile (Baker Chemical Co.) was freshly distilled from phosphorus pentoxide, and its purity was greater than 99% by GLC analysis. Sodium borohydride and borodeuteride (Aldrich) were 99% and 98 atom %, respectively. General Procedure for Photolysis. Irradiations of 1 were carried out in a Rayonet merry-go-round reactor (The Southern New England Co.) equipped with eight 2537-A lamps. A steady stream of air was passed into the reactor to maintain a constant temperature of 40 "C. The photolysis samples (1 mL) were placed in quartz tubes (Ace Glass, 170 mm X 15 mm), each screwed to a nylon adapter bushing containg a Pyrex glass sliding stopper valve, and degassed through three or four freeze-pump-thaw cycles. The tubes were sealed under vacuum and irradiated a t 254 nm for 6 min. Quantum yields of products were determined by using the potassium ferrioxalate'* actinometer. Product Analysis. The photolysis mixtures were analyzed by GLC on a Varian 3300 capillary gas chromatograph equipped with an FID with a 30 m X 0.25 mm DB-225 capillary column (J & W Scientific Inc.) and a Varian 4290 integrator. The column was held a t 60 "C for 5 min and raised to 180 "C at a rate of 5 "C/min with an injection port temperature of 200 "C and detector temperature of 250 "C. Helium was used as carrier gas at 30 mL/min. The photoproducts were identified by comparing their retention times with those of commercially obtained authentic samples. The mass spectral analyses were carried out with a Finnigan 4023 mass spectrometer equipped with a Finnigan 9610 gas chromatograph. Dodecane was used as an internal standard in the determination of yields of products.
Reduction of some halocyclopropanes with sodium naphthalenide
The Journal of Organic Chemistry, 1971
100-101' (4.5 mm) [lit.Io bp 133' (16 mm)]; ir (liquid) 1700 (C=O) and 1120 cm-' (CHsOC); nmr (CCI,) S 3.40 ( 6 , 6, CHsO), 5.03 (s, 1, CH), 7.18-7.53 (m, 3, Ar H), and 8.03-8.20 ppm (m, 2, Ar H). To a stirred suspension of 28.5 g (0.75 mol) of lithium aluminum hydride in 600 ml of tetrahydrofuran, a solution of 77 g (0.43 mol) of 3a in 75 ml of tetrahydrofuran was added, dropwise, over a period of 1 hr. After refluxing for 12 hr the reaction mixture was cooled to O', treated with water, and worked up in the usual way to yield, after distillation of the crude product through a 75-cm spinning band column, 62 g (80%) of mandelaldehyde dimethyl acetal (4a) as a colorless liquid: bp 80-82" (0.5 mm); ir (liquid) 3550 (OH), 2990 (CHa), and 1130 cm-' (CHaOC); nmr (CDCls) 6 3.08 (s, 3, CHsO), 3.28 (9, 4,
Preparation and reactions of some 2,2-difunctional 1,1-dibromocyclopropanes
Tetrahedron, 2007
The synthesis of 2,2-dibromocyclopropane-1,1-dicarboxylic acids is described. Reaction of substituted 1,1-dibromo-2-acyloxymethylcyclopropanes with methyl lithium at low temperature leads to a bromine-lithium exchange and then either formal protonation to give the corresponding monobromocyclopropanes or intramolecular cyclisation to give a substituted 3-oxabicyclo[3.1.0]hexane. Oxidative ring opening of these compounds leads stereoselectively to 1,1,2,2-tetrasubstituted cyclopropanes with four functionalities on the ring.
An improved synthesis of cyclopropanes from homoallenic alcohols
Tetrahedron Letters, 1992
of the readily available g-allene tosylates with LDA or nBuLi provides an expeditious, although stereorandom, synthesis of functionalized cyclopropanes which can be easily prepared in high enantiomeric purity. Homoallenic groups are well-known to participate in solvolysis reactions in a manner analogous to homoallylic groups. For example, g-allenic tosylates or halides (1) are readily cyclized under solvolytic conditions to afford cyclopropylketones (2) or methylenecyclobutanols (3) as the major product, mainly depending upon the RI substituent of the starting allenes (Scheme I).* By taking advantage of the relative acidity of the allenyl proton, it occurred to us that treatment of these g-allene tosylates with a base should furnish alkynylcyclopropanes (4) with clean inversion of configuration at the sp3 carbon center.3 This ring formation would lend itself to the synthesis of enantiomerically pure cyclopropanes of defined absolute stereochemistry from the readily available p-allenic alcohols. 4~5 Herein we report the successful implementation of such approach to an expeditious, although stereorandom, synthesis of functionalized cyclopropanes which can be easily prepared in high enantiomeric purity. Scheme I R, =H +Ts *r-i 1 I RI = alkyl base (LDA or nBuLi) Rl COCH, A R2 2 R3 RI HO a R2 t f% 3a As outlined in Scheme II, we have employed two general synthetic methods for the preparation of the requisite starting J3-allenic alcohols (5a-i).6<7 In Method A the starting materials (5a-e) were readily available (75-86%) by the one-carbon homologation of propargylic alcohols (7a-e) by the procedure of Crabbe .s The latter alcohols were prepared either by addition 4703
Transformations of halocyclopropanes-VI
Tetrahedron, 1981
The reactions of 7,7dichlorobicyclo-[Cl.O]heptane, 1 with organic bases were carried out in benzene and THF. The reaction course of 1 in nonpolar media is different from that in DMSO; the rcarrangements of cyclopropene 3 taking place in the former. 8 is produced by the shift of double bond into the C, ring. The cyclopropene-carbine isomerization and reactions of carhenes 14 and IS with alkoxide anions are the probable route leading to the other products. Reactions of 7,7-dichlorobicyclo [4.l.O]heptane, 1 with a variety of organic anions and potassium t-butoxide in DMSO have been reported.' Generally they lead to the carbon 1 substituted derivatives of 7chlorobicyclo [4. I.0 Jheptane 2. Apart from frequently appearing products of further rearrangements of 2, a significant amount of aromatic hydrocarbons C8-C9, mostly aethyltoluene and ethylbenzene, were found in the post-reaction mixture.