Alkynyl group, alkyl stabilization

The bonding situation in the stannanorbornyl cation 62 is reminiscent of the intriguing zwitterions of the types 63 and 64 in which triorganotin cations are intramolecularly stabilized by side-on coordination with an alkynyl group. The zwitterions 63 with a [3-l-1] coordination for the tin atom are characterized by a significantly deshielded Sn NMR resonance and sizeable j(SnC) couplings to the alkynyl carbon atoms (for = alkyl and R = Et 5" Sn = 160-215 J(SnC ) = 41 8Hz, J(SnC ) = 60-80 Coordination with the amino... 

Although the activity of methoxylated monobactams could be improved by appropriate side-chain modifications, difficulty of synthesis and poor chemical stability focused attention on the nonmethoxylated analogues. Both high intrinsic activity and excellent p-lactamase stability are exhibited by monobactams that combine C-3 aminothiazole oxime side chains and 4-alkyl, 4-alkenyl, and 4-alkynyl groups (19). 

With the successful chemistry of the cymantrenes and the (cyclobuta-diene)tricarbonyl iron, the quest for tetraethynylated cyclobutadienes based on CpCo-stabilized complexes arose. Why would they be interesting Whereas all derivatives of 63 and 68 exhibit reasonable stability when their alkynyl substituents are protected by either an alkyl or a trimethylsilyl group, the desilylated parents are isolated only with difficulty and are much more sensitive. 

At equilibrium, therefore, the Li becomes attached preferentially to the organic group best able to stabilize the negative charge. Hence, extensive exchanges and useful preparations are available from reaction of alkyl Li with aryl halides, cyclopropyl halides, alkenyl halides, alkynyl halides and a-heterosubstituted alkyl halides (including per- and polyhaloalkanes), but not with simple alkyl halides e.g., interaction of EtLi and Mel provides a mixture of EtLi and MeLi. 

Alkenyl (vinyl), aryl, and alkynyl carbocations are particularly unstable with respect to alkyl carbocations. Let s compare the isopropyl cation with the iso-propenyl cation. In the latter, the central C has two cr bonds, one it bond, and one empty orbital, so it is sp-hybridized (linear). Both ions are stabilized by the C(sp3)-H cr bonds of the CH3 group on the right. In the isopropyl cation there is an additional interaction with C(sp3)-H cr bonds on the left, whereas in the isopropenyl cation there is an additional interaction with C(sp2)-H cr bonds on the left. Because C(sp2) orbitals are lower in energy than C(sp3) orbitals, the... 

When the alkynyl moiety is substituted with an alkyl group or a proton only, the corresponding benzoquinones were detected as products of thermolysis. But when R = Ph, OR, or MesSi, a mixture of both products was formed. Furthermore, the use of 4-alkynylcyclobutanones with electron-withdrawing substituents, like C02Et or CH=CHOMe, resulted in the formation of cyclopentediones as a sole product. These results can be explained by the mechanism presented in the scheme above. Intermediate 10 is favored over intermediate 8 when substituent R better stabilizes the adjacent radical site. A ferrocenyl group at the alkyne also provides a stabilization that favors the formation of cyclopentene-l,3-diones. ... 

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