Four-membered ring compounds

The first important conclusion from all the theoretical studies is that the calculations can reproduce fairly well the experimental structures, provided that polarized basis sets are used for geometry optimizations. The large effect that the addition of polarization 

TABLE 12. Experimentally determined nonbonded Si. .. Si distances (in A) and angles (in degrees) in substituted planar 1,3-disubstituted four-membered rings (R2SiX)2 (31) and calculated Si-Si distances in the parent monosubstituted three-membered rings (H2Si)2X (27)a. 

FIGURE 8. Projected electron-density maps and schematic diagrams of the highest six MOs of cyclodisiloxane 33 at 3-21G( Contour levels for the density maps are from 0.001 to 0.101 by 0.01 e au-2. Reprinted with permission from J. Am. Chem. Soc., 107, 1186 (1985). Copyright (1985) American Chemical Society136. 

The most recent paper that analyzes the bonding in 31 reached yet another conclusion126. Thus, Grev and Schaefer concluded that the Dewar-Chatt-Duncanson 

Other four-membered rings containing heteroatoms have been studied. These are mostly polycyclic carbon skeletons which have one or more silicon atoms. For example, Boatz and Gordon122 studied the silabicyclo[1.1.0]butanes 35 and the silacyclobutenes 36, containing 1-4 silicon atoms. The calculated geometries of the bicyclobutanes with two heteroatoms (37a-c) exhibit a number of interesting features.  

---

Examples of four-membered ring formation are rare. The cyclization of the cyclic allylic acetate 42 afforded a 2 1 mixture of the four-membered ring compound 43 and the six-membered ring compound 44[45]. 

Cyclic and Polymeric Substances Containing Arsenic—Arsenic Bonds. A number of organoarsenic compounds containing rings of four, five, or six arsenic atoms have been prepared (cycHc polyarsines). The first such four-membered ring compound to be adequately characterized, tetrakis (trifluoromethyl) tetrarsetane [7547-15-17, was obtained by the interaction of a diiodoarsine and mercury (107,108) ... 

Tlie present chapter reviews the chemistry of three- and four-membered ring compounds containing an S—S bond in their ring. Dithiiranes 1,2-dithietanes and 1,2-dithietes are the compounds of this type. Although 1,3-dithietanes are four-membered heterocycles which are prepared much more easily and are seemingly more familiar, they have no S-S bond in the ring and hence are not included in this chapter. 

By a photochemically induced elimination of CO, a chromium carbene complex with a free coordination site is generated. That species can coordinate to an alkyne, to give the alkyne-chromium carbonyl complex 4. The next step is likely to be a cycloaddition reaction leading to a four-membered ring compound 5. A subsequent electrocyclic ring opening and the insertion of CO leads to the vinylketene complex 6 ... 

As typical examples of crystal-to-crystal thermal reactions, the cyclization of allene derivatives to four-membered ring compounds and the transformation of a racemic complex into a conglomerate complex are described. 

While the decomposition of silacyclobutanes as a source of silenes has continued to be studied in the last two decades, the interest has largely focused on mechanisms and kinetic parameters. However, a few reports are listed in Table I of the presumed formation of silenes having previously unpublished substitution patterns, prepared either thermally or photo-chemically from four-membered ring compounds containing silicon. Two cases of particular interest involve the apparent formation of bis-silenes. Very low-pressure pyrolysis of l,4-bis(l-methyl-l-silacyclobutyl)ben-zene94 apparently formed the bis-silene 1, as shown in Eq. (2), which formed a high-molecular-weight polymer under conditions of chemical vapor deposition. 

Disilenes are very reactive toward a variety of unsaturated bonds, such as C=C, C=C, C=N, C=N, C=0, C=S, N=N, and N=0, giving interesting four-membered ring compounds that are otherwise difficult to synthesize.3b Reactions using stable tetraaryl- or dialkyldiaryldisilenes and marginally stable tetra-t-butyldisilene (21), which is generated by photolysis of the corresponding cyclotrisilane, are summarized in Schemes 10 and 11, respectively. Some of them merit further comments. 

Summary The formation, reactivity, and cycloaddition behavior of neopentylsilenes towards suitable reaction partners is described. Especially l,l-dichloro-2-neopentylsilene. Cl2Si=CHCH2Bu (2) - easily obtained from vinyltrichlorosilane and LiBu - is a useful building block for the synthesis of SiC four membered ring compounds. These can be converted into the isomeric Diels-Alder and retro ene products upon thermolysis reactions. The mode of the silenes cycloaddition reactions ([4+2] vs [2+2] addition) can be directed by either the substitution pattern at the Si=C moiety, the choice of reaction partners or the conditions. Furthermore the products resulting from cycloaddition reactions open up a wide variety of following reactions, which possibly will lead to new organosilicon materials or pharmaceutical compounds. 

All results so far obtained prove the high synthetic potential of differently substituted neopentylsilenes. Especially silicon dichloro substituted silenes are useful for the preparation of a wide variety of new silacyclobutanes and -butenes. These SiC four membered ring compounds can be utilized as pre-... 

Dichloroneopentylsilene is formed in situ by reaction of trichlorovinylsilane with LirBu [1], The [2+2] cycloaddition to imines yields Si-dichloro functionalized 2-silaazetidines in a preparative scale [2], When aldimines are used as trapping agents for the silene, the resulting SiN-four membered ring compounds are isolated as syn/anti-isomers (syn/anti 2/1). 

Addition of elemental chalcogens to the double bonds between silicon and phosphorus gives three-membered ring compounds (Scheme 9).59 A 1,2-di-bromodigermene reacts with S8 to give the four-membered ring compound,60 in... 

If the cycloaddition and cycloreversion steps occurred under the same conditions, an equilibrium would establish and a mixture of reactant and product olefins be obtained, which is a severe limitation to its synthetic use. In many cases, however, the two steps can very well be separated, with the cycloreversion under totally different conditions often showing pronounced regioselectivity, e.g. for thermodynamic reasons (product vs. reactant stability), and this type of olefin metathesis has been successfully applied to organic synthesis. In fact, this aspect of the synthetic application of four-membered ring compounds has recently aroused considerable attention, as it leads the way to their transformation into other useful intermediates. For example aza[18]annulene (371) could be synthesized utilizing a sequence of [2 + 2] cycloaddition and cycloreversion. (369), one of the dimers obtained from cyclooctatetraene upon heating to 100 °C, was transformed by carbethoxycarbene addition to two tetracyclic carboxylates, which subsequently lead to the isomeric azides (368) and (370). Upon direct photolysis of these, (371) was obtained in 25 and 28% yield, respectively 127). Aza[14]annulene could be synthesized in a similar fashion I28). 

Of the other possible structures (95) of the condensation product, the Schiff s base (CXXX) is not resolvable, nor, because of the tautomeric nature of the guanidine system, is the four-membered ring compound (CXXXI). A compound of the improbable structure (CXXXII) should yield a copper complex, which the condensation products failed to do. 

In a later study of these reactions, differently substituted thioureas were compared in relation to their effectiveness in producing four-membered rings. Compounds 177 and 178 produced four-membered intermediates that collapsed to the more stable chlorformamidines. This phenomenon is most likely the result of the greater basicity of the N-alkyl than that of the N-aryl group. 

Michael addition has been employed as a key step in the formation of four-membered ring compounds.6 As outlined, indirect generation of carbanions from 3,3-dimethyl-4-tosyloxybut-... 

In addition to the application of malonic esters, four-membered ring compounds can also be realized by using methyl methylsulfanylmethyl sulfoxide (14) as the active methylene component.25 27 As demonstrated in the following scheme, when the potassium salt of methyl methylsulfanylmethyl sulfoxide is allowed to react with 1,3-dibromopropane, cyclobutanone dimethyl dithioacetal 5-oxide (16) is isolated in 97% yield.8,10 Of particular interest is that the... 

There is much current interest in four-membered ring compounds containing heteroatoms that are isoelectronic with a cyclobuta-l, 3-dienediide. The first thermally robust parent homocyclic dianion [ CfSiMe,), " was only reported in 2000 the first neutral heterocyclic analogue [P(C6H2Bu 3-2,4,6) p-C( Cl) ]2, containing a Group 14 element (C) dates from 1995 Extensions to the 4" and 5" quantum group elements Ge and Sn respectively, were published in 2004. 

When simple alkenes were employed as reaction partners for silenes of the type (Me3Si)2Si=C(OSiMe3)R1, silacyclobutanes were obtained, provided that no allylic hydrogen is present in the alkene. In the reaction with alkenes with allylic hydrogens the ene reaction becomes predominant (see Table 11). Thus, while the reaction with styrene exclusively gives the four-membered ring compound 454, with 1-methylstyrene the ene products 455 were obtained (equation 144). Similarly, from the reaction of 150 with 1-octene only the ene product 456 was isolated (equation 145). 

---