Synthesis and characterization of a planarized, trimethylenemethane-type bis(semiquinone) biradical
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Keywords:
Trimethylenemethane
Semiquinone
Geminal
Characterization
Carbon fibers
Single bond
Trimethylenemethane
Semiquinone
Geminal
Characterization
Carbon fibers
Single bond
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Internal fusion: The stable fused tricyclic tetraalkyl disilenes cis-1 and trans-1 were synthesized as yellow and red-purple crystals, respectively (see picture). X-ray analysis has shown that the geometry around the SiSi bond in trans-1 is unusually distorted, but is rather normal in cis-1. Disilene trans-1 isomerizes stereospecifically to a tetracyclic isomer by unprecedented addition of a SiSi single bond to a SiSi double bond.
Single bond
Tricyclic
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Geminal
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Single bond
Triple bond
Carbon fibers
Hamiltonian (control theory)
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In an attempt to examine the effects of different numbers and positions of cis double bonds in thesn-2-acyl chain of phosphatidylethanolamine (PE) on the bilayer's melting behavior, 21 molecular species of PE were first semisynthesized, and their Tm and ΔHvalues were subsequently determined by high resolution differential scanning calorimetry. In the plot of Tm versus the number of the cis double bond, some characteristic profiles were observed for the various series of PEs. For instance, if the cis double bond was first introduced into the sn-2-acyl chain of C(20):C(20)PE at the Δ5-position, the Tm was observed to reduce drastically. Subsequent stepwise additions of up to fivecis double bonds at the methylene-interrupted positions toward the methyl end resulted in a progressive yet smaller decrease inTm. If, on the other hand, the cisdouble bonds were introduced sequentially at the Δ11-, Δ11,14-, and Δ11,14,17-positions along thesn-2-acyl chain of C(20):C(20)PE, theTm profile in the Tm versus the number of the cis double bond showed a down-and-up trend. Most interestingly, for positional isomers of C(20):C(20:3Δ5,8,11)PE, C(20):C(20:3Δ8,11,14)PE, and C(20):C(20:3Δ11,14,17)PE, an inverted bell-shapedTm profile was detected in the plot ofTm against the position of the ω-carbon for these isomers. Similar Tm profiles were also observed for C(18):C(20)PE, C(20):C(18)PE, and their unsaturated derivatives. This work thus demonstrated that both the positions and the numbers ofcis double bonds in the sn-2 acyl chain could exert noticeable influence on the gel-to-liquid crystalline phase transition behavior of the lipid bilayer. Finally, a molecular model was presented, with which the behavior of the gel-to-liquid crystalline phase transition observed for lipid bilayers composed of varioussn-1-saturated/sn-2-unsaturated lipids can be rationalized.
Phosphatidylethanolamine
Chain (unit)
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The first compound featuring an As[double bond, length as m-dash]Ge double bond, arsagermene Mes*As[double bond, length as m-dash]Ge(SiMetBu2)2, was synthesized, isolated and fully characterized. Crystallographic studies revealed that arsagermene has a planar Ge[double bond, length as m-dash]As bond of 2.2731(8) Å, which is notably shorter than the standard Ge-As single bond. The double bond character of arsagermene was further supported by computational data.
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Quadruple bond
Triple bond
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The rhodium allenylidenes trans-[RhCl[[double bond]C[double bond]C[double bond]C(Ph)R](PiPr(3))(2)] [R = Ph (1), p-Tol (2)] react with NaC(5)H(5) to give the half-sandwich type complexes [(eta(5)-C(5)H(5))Rh[[double bond]C[double bond]C[double bond]C(Ph)R](PiPr(3))] (3, 4). The reaction of 1 with the Grignard reagent CH(2)[double bond]CHMgBr affords the eta(3)-pentatrienyl compound [Rh(eta(3)-CH(2)CHC[double bond]C[double bond]CPh(2))(PiPr(3))(2)] (6), which in the presence of CO rearranges to the eta(1)-pentatrienyl derivative trans-[Rh[eta(1)-C(CH[double bond]CH(2))[double bond]C[double bond]CPh(2)](CO)(PiPr(3))(2)] (7). Treatment of 7 with acetic acid generates the vinylallene CH(2)[double bond]CH[bond]CH[double bond]=C=CPh(2) (8). Compounds 1 and 2 react with HCl to give the five-coordinate allenylrhodium(III) complexes [RhCl(2)[CH[double bond]C[double bond]C(Ph)R](PiPr(3))(2)] (10, 11). An unusual [C(3) + C(2) + P] coupling process takes place upon treatment of 1 with terminal alkynes HC[triple bond]CR', leading to the formation of the eta(3)-allylic compounds [RhCl[eta(3)-anti-CH(PiPr(3))C(R')C[double bond]C[double bond]CPh(2)](PiPr(3))] [R' = Ph (12), p-Tol (13), SiMe(3) (14)]. From 12 and RMgBr the corresponding phenyl and vinyl rhodium(I) derivatives 15 and 16 have been obtained. The previously unknown unsaturated ylide iPr(3)PCHC(Ph)[double bond]C[double bond]C[double bond]CPh(2) (17) was generated from 12 and CO. A [C(3) + P] coupling process occurs on treatment of the rhodium allenylidenes 1, 2, and trans-[RhCl[[double bond]C[double bond]C[double bond]C(p-Anis)(2)](PiPr(3))(2)] (20) with either Cl(2) or PhICl(2), affording the ylide-rhodium(III) complexes [RhCl(3)[C(PiPr(3))C[double bond]C(R)R'](PiPr(3))] (21-23). The butatrienerhodium(I) compounds trans-[RhCl[eta(2)-H(2)C[double bond]C[double bond]C[double bond]C(R)R'](PiPr(3))(2)] (28-31) were prepared from 1, 20, and trans-[RhCl[[double bond]C[double bond]C[double bond]C(Ph)R](PiPr(3))(2)] [R = CF(3) (26), tBu (27)] and diazomethane; with the exception of 30 (R = CF(3), R' = Ph), they thermally rearrange to the isomers trans-[RhCl[eta(2)-H(2)C[double bond]C[double bond]C[double bond]C(R)R'](PiPr(3))(2)] (32, 33, and syn/anti-34). The new 1,1-disubstituted butatriene H(2)C[double bond]C[double bond]C[double bond]C(tBu)Ph (35) was generated either from 31 or 34 and CO. The iodo derivatives trans-[RhI(eta(2)-H(2)C[double bond]C[double bond]C[double bond]CR(2))(PiPr(3))(2)] [R = Ph (38), p-Anis (39)] were obtained by an unusual route from 1 or 20 and CH(3)I in the presence of KI. While the hydrogenation of 1 and 26 leads to the allenerhodium(I) complexes trans-[RhCl[eta(2)-H(2)C[double bond]C[double bond]C(Ph)R](PiPr(3))(2)] (40, 41), the thermolysis of 1 and 20 produces the rhodium(I) hexapentaenes trans-[RhCl(eta(2)-R(2)C[double bond]C[double bond]C[double bond]C[double bond]C[double bond]CR(2))(PiPr(3))(2)] (44, 45) via C-C coupling. The molecular structures of 3, 7, 12, 21, and 28 have been determined by X-ray crystallography.
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Triple bond
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Abstract The hot spots of molecules that I have identified as double bonds (two shared pairs of electrons lying between the same two carbon atoms) and their triple bond cousins are often desirable entities. They are desirable either in their own right or because they can be used in the course of the construction of an elaborate molecule. For instance, a double bond can make the molecule stiffer and resistant to twisting. In Reaction 28 you will see that one particular natural product, quinine, must have a double bond in a particular position for it to be able to function—Nature is very particular about the shape of a molecule that she uses—and the drug’s synthesizers had to find a way to introduce it. How, though, can a double bond be introduced into a molecule that begins life with only single bonds? One approach is ‘elimination’, the expulsion of groups of atoms on neighbouring carbon atoms, leaving those two atoms free to form a second or even third bond to each other. One approach is to pull an H atom (as a proton) or some other group of atoms off one C atom, and then hope that the ensuing convulsions of the electron cloud will result in its accumulation to form a double bond between that C atom and its neighbour. There are two common approaches, one involving an acid and the other a base. Let’s watch what happens when sulfuric acid, 1, is added to 2. The acid, a proton donor, generates H3O+ ions in the usual way by transferring a proton to a neighbouring water molecule and leaving behind an HSO4– ion, and we see one of these ions sidle up to the target molecule. A proton hops across onto the O atom from the H3O+ ion, so forming a positively charged –OH2+ group. There is an immediate convulsion of the electron cloud, and that group escapes as an H2O molecule, leaving behind a positively charged ion with the positive charge mostly localised on the C atom. This ion is unstable but survives briefly.
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Abstract Hydroboration of the allenes (I) with the boranes (II) gives the alkenylboranes (III) with an exocyclic double bond.
Geminal
Hydroboration
Boranes
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Cyclopropane
Methylenecyclopropane
Single bond
Bent bond
Triple bond
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