3.4- Dichlorocyclobutene

Typical examples occur with the trans and cis 3,4 dichlorocyclobutenes which open stereo specifically to trans, trans and cis, trans-1,4-dichlorobutadiene respectively. 

A number of examples involving nitrile oxide cycloadditions to cyclic cis-disubstituted olefinic dipolarophiles was presented in the first edition of this treatise, notably to cyclobutene, cyclopentene, and to 2,5-dihydrofuran derivatives (15). The more recent examples discussed here also show, that the selectivity of the cycloaddition to 1,2-cis-disubstituted cyclobutenes depends on the type of substituent group present (Table 6.8 Scheme 6.41). The differences found can be explained in terms of the nonplanarity (i. e., pyramidalization) of the double bond in the transition state (15) and steric effects. In the cycloaddition to cis-3,4-diacetyl-(197) and cis-3,4-dichlorocyclobutene (198), the syn-pyramidalization of the carbon atoms of the double bond and the more facile anti deformability of the olefinic hydrogens have been invoked to rationalize the anti selectivity observed. 

Studies of perfluoro(3,4-dichlorocyclobutene),2 perfluoro(l,2-dimethylcyclobutene),4 per-fluoro(3,4-dimethylcyclobutene) (l).5,6 perfluoro(3-methylcyclobutene) (3),6,7 and... 

Several transition-metal complexes of cyclobutadiene have been prepared, and this is all the more remarkable because of the instability of the parent hydrocarbon. Reactions that logically should lead to cyclobutadiene give dimeric products instead. Thus, 3,4-dichlorocyclobutene has been de-chlorinated with lithium amalgam in ether, and the hydrocarbon product is a dimer of cyclobutadiene, 5. However, 3,4-dichlorocyclobutene reacts with diiron nonacarbonyl, Fe2(CO)9, to give a stable iron tricarbonyl complex of cyclobutadiene, 6, whose structure has been established by x-ray analysis. The 7r-electron system of cyclobutadiene is considerably stabilized by complex formation with iron, which again attains the electronic configuration of krypton. 

A still purer product is obtained if the dried petroleum ether solution is evaporated to dryness with a water aspirator and the residual crude product distilled through a Vigreux column After a small fore-run, pure tetramethyl-3,4-dichlorocyclobutene distils at 59-60° at 12 mm. The product melts at 58° and is more stable at room temperature than the recrystallized but undistilled material... 

Nicolaou and co-workers reported the use of c/.s-3,4-dichlorocyclobutene (59) as a useful building block to prepare complex polyether systems via ROM-RCM <01AG(E)4441> (Scheme 43). The GMC failed to give product in this transformation. 

Cyclobutadieneiron tricarbonyl may also be produced through reaction of 3,4-dichlorocyclobutene with Na2Fe(CO)4,5 and by irradiation of a-pyrone followed by treatment with Fe2(CO)9 . The method outlined here is the most convenient procedure especially when considerable quantities (10 g. or more) of cyclobutadieneiron tricarbonyl are required. The analogous reaction of derivatives of 3,4-dihalocyclobutenes with Fe2(CO)9 affords the corresponding cyclobutadieneiron tricarbonyl complexes. Cyclobutadieneiron tricarbonyl can be oxidized to generate cyclobutadiene in situ.7... 

This method of preparation, which is due to Nenitzescu, Avram, Marica, Dinulescu, Farcasiu, Elian, and Mateescu,2 is the only practical method available at this time for the preparation of 3,4-dichlorocyclobutene. 

Reaction of Ni(CO)4, with l,2,3,4-tetramcthyl-3,4-dichlorocyclobutene gives the dimeric [ / -(CH3)4C4]NiCl2 2 in 70% yield by prolonged heating ... 

Examination of the cyclobutadiene system indicates that it possesses four 7c-electrons and is thus an unstable 4 system. Cyclobutadiene itself only exists at very low temperatures, though some of its derivatives are stable to some extent at room temperature. Cyclobutadiene is a rectangular diene. Loss of two electrons through the departure of two chloride ions from the 3,4-dichlorocyclobutene derivative creates a 2jc-electron aromatic system, the square, stable cyclobutenyl dication (Scheme 1.2). 

The area of cyclobutadiene-transition metal chemistry has expanded rapidly since these initial findings, largely through the work of Maitlis 163), Nakamura 183), Freedman 104), and others, but details will not be presented here. Several recent important discoveries by Pettit and co-workers 22, 79,102, 24I), however, relate to the formation and chemistry of cyclobutadiene-iron tricarbonyl (XVII). This product is formed from the reaction of cis-3,4-dichlorocyclobutene and diiron nonacarbonyl and can be isolated in the form of yellow crystals of excellent stability. Cyclobutadiene can be liberated by treating the complex with oxidizing agents such as ferric or ceric ion. The free ligand has been trapped and demonstrated to possess a finite lifetime. It has also been shown to... 

In 1964 we prepared cyclobutadiene-iron tricarbonyl (III), a complex possessing an unsubstituted cyclobutadiene ligand (5). The reaction employed in this preparation involves the interaction of c2s-3,4-dichlorocyclobutene with Fe2(CO)9. It has subsequently been found that... 

The mechanism of the formation of these cyclobutadiene complexes remains obscure it is interesting to note that either the cis or trans isomer of 3,4-dichlorocyclobutene reacts with Fe2(CO)9 to give Complex III and that Complex IV can be produced from either the cis or trans isomer of diiodobenzocyclobutene (7). 

Cyclobutanols substituted by a leaving group at the 2-position are transformed into cyclopropanecarbaldehydes or cyclopropyl ketones upon treatment with base . The reaction of a 3,4-dichlorocyclobutene derivative with aqueous base to give a 2-cyclopropen-l-yl ketone probably proceeds via a 4-chlorocyclobut-2-en-l-ol (equation 104) 67 jjj oxidation of 1-methylcyclobutene with metallic salts in aqueous medium to... 

This addition-elimination reaction can be applied to the preparation of squaric acid derivatives. Thus, the treatment of 3,4-dichlorocyclobutene-l,2-dione 27 with two different zinc-copper reagents furnishes polyfunctional squaric acid derivatives like 28, provided the first zinc-copper reagent bears a secondary or tertiary alkyl group (Scheme 9-26) [56]. 

The particularly reactive cyano(vinyl)carbene 3 was generated by photolysis of 3,3-dimethyl-3//-pyrazole-4,5-dicarbonitrile it is able to cyclopropanate benzene to give 4, and, to a minor extent, also 3,4-dichlorocyclobutene to give 5. ... 

Reaction of l,2,3-tri-ter/-butyl-3,4-dichlorocyclobutene with aqueous base (i.e. sodium hydrogen carbonate) gives l,2-di-/cr/-butyl-3-(2,2-dimethylpropanoyl)cyclopropene (17) in nearly quantitative yield, most probably also proceeding via a 4-chlorocyclobut-2-en-l-ol. ... 

The hydrolysis of the 3,4-dichlorocyclobutene 1 resulted in ring contraction to give cyclo-propene 4. The reaction probably proceeds via the cation 2 and the cyclobutenol 3 which ring contracts with loss of hydrogen chloride. 

The reduction of 3,4-dichlorocyclobutene (222) in the presence of metal carbonyls has been utilized to prepare the parent complex [223, MLn = Cr(CO)4, Mo(CO)3, W(CO)3, Fe(CO)3, Ru(CO)3 orCo2(CO)6] (equation 32) .Morerecently, reaction ofNi(CO)4 with 3,4-dihalocyclobutenes (X = Br or I) or with 222 in the presence of AICI3 produced the corresponding (cyclobutadiene)nickel dihalides . Methodology for the preparation of 1,2- or 1,3-disubstituted (cyclobutadiene)Fe(CO)3 complexes from 1,2- or 1,3-disubsli-tuted-3,4-dibromocyclobutenes has been presented - In turn, the substituted dibromo-cyclobutenes are prepared from squaric esters. The reaction of cz5-3,4-carbonyldioxycy-clobutene and substituted variants with l c2(CO)9 orNa2Fe(CO)4 also produces (cyclobu-tadiene)Fe(CO)3 complexes . Photolysis of a-pyrone generates 3-oxo-2-oxabicyclo [2.2.0]hex-5-ene (224) which undergoes photolysis with a variety of metal carbonyls to afford the parent cyclobutadiene complex 223 [MLn = CpV(CO)2, Fe(CO)3, CoCp. or RhCp] (equation 33) 2 0. 

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