Vinylidene from alkenes

The fluoropolymer family consists of polymers produced from alkenes in which one or more hydrogens have been replaced by fluorine. The most important members of this family are polytetrafluoroethylene (PTFE) (XLVII), polychlorotrifluoroethylene (PCTFE) (XLVIII), poly(vinyl fluoride) (PVF) (XLIX), poly(vinylidene fluoride) (PVDF) (L) copolymers of... 

The synthesis of 2-chloro-2,3,3-trifluorocyclobutyl acetate illustrates a general method of preparing cyclobutanes by heating chlorotrifluoroethylene, tetrafluoroethylene, and other highly fluorinated ethylenes with alkenes. The reaction has recently been reviewed.11 Chlorotrifluoroethylene has been shown to form cyclobutanes in this way with acrylonitrile,6 vinylidene chloride,3 phenylacetylene,7 and methyl propiolate.3 A far greater number of cyclobutanes have been prepared from tetrafluoroethylene and alkenes 4,11 when tetrafluoroethylene is used, care must be exercised because of the danger of explosion. The fluorinated cyclobutanes can be converted to a variety of cyclobutanes, cyclobutenes, and butadienes. 

The nucleophilic attack of f-butyllithium on lithium vinylidene carbenoids has also been used for synthetic purposes in as far as the reaction permits to generate sterically hindered alkenes. Thus, treatment of the dibromoalkene 78 generated from adamantanone with an excess of f-butyllithium results in the formation of the alkene 79 that contains three bulky substituents at the double bond (equation 43) . In an analogous way, a f-butyl residue is introduced into chloroenamine 80 (equation 44) . 

In the absence of H2 and other transfer agents, polymer molecular weight is limited by various P-hydride transfers—from normal (1,2-) and reverse (2,1-) propagating centers, before and after rearrangement [Lehmus et al., 2000 Resconi et al., 2000 Rossi et al., 1995, 1996 Zhou et al., 2001] (Sec. 8-4i-2). Vinylidene, vinylene, and trisubstituted double-bond end groups are formed in 1-alkene polymerizations, vinyl and vinylene in ethylene polymerization. [Vinyl groups are also produced in some 1-alkene polymerizations, not by P-hydride transfer, but by P-alkyl transfer (Sec. 8-4i-2).]... 

Much of the chemistry of vinylidene complexes has been developed with catalytic applications in mind, as detailed later in this volume. Early examples had low activity for alkene metathesis, although complexes containing imidazolylidene ligands showed improved efficiencies [35]. However, in many cases, reactions of the vinylidene ligand have resulted in transformation to other carbon-based ligands which have not been released from the metal fragment. 

This stoichiometric reaction constitutes a new contribution to vinylidene chemistry and a novel method to generate alkenylcarbene ligand from simple propargyl alkyl ethers rather than via activation of cyclopropenes [4] or by stoichiometric activation of butadiene [6[. When linked to a suitable metal-ligand moiety this carbene constitutes an alkene metathesis initiator. 

It is interesting to note that the yield of 39 can be increased to 95 % in the presence of a Ni(0) catalyst (see Section I.3.I.2.).43 In unsymmetrically substituted buta-l,2,3-trienes, head-to-head dimerization takes place as well, as seen in the examples below.44 The intermediate cyclopropy-lidenecumulenes 41 are formed from vinylidene insertions into alkenes and reactions occur at the terminal unsaturated site of the cyclopropylidene group. Structures 42 were confirmed by X-ray crystallographic analysis and revised the original assignment of the head-to-tail structures for these derivatives.45... 

The cyclo addition of the alkene to the ruthenium vinylidene species leads to a ruthenacyclobutane which rearranges into an allylic ruthenium species resulting from / -elimination or deprotonation assisted by pyridine and produces the diene after reductive elimination (Scheme 16). This mechanism is supported by the stoichiometric C-C bond formation between a terminal alkyne and an olefin, leading to rf-butatrienyl and q2-butadienyl complexes via a ruthenacyclobutane resulting from [2+2] cycloaddition [62]. 

The major products from 325 and 1-alkenes are substituted vinylidenes 355 (R=CH=CHR, R = H, Me, Ph, C02Me), resulting from insertion... 

Alkenes and alkynes can substitute for carbonyl ligands on Os3(CO)i2. Carbon monoxide slowly dissociates from Os3(CO)ii(j7 -PhC=CPh) and a cluster with a bridging alkyne ligand forms, Os3(CO)io(/n-PhC=CPh). The addition of ethene to the cluster gives a bridging vinylidene complex, Os3(CO)9(/u-H)2(/u-C=CH2). The oxidative addition of terminal alkynes to Os3(CO)i2 gives Os3(CO)9(/u.-H)(/u,3-CCR). Nucleophiles add to the bridging alkyne unit. 

The formation of reactive intermediates provides possible opportunities for new reaction design. An attractive highly reactive intermediate, carbenes, which demonstrate numerous useful synthetic pathways, most notably by addition to alkenes and alkynes and also insertion into X-H bonds, where X is both carbon and heteroatoms, suffers from problems associated with their accessibility. Undoubtedly, the most useful class of precursor is the diazo compounds, whose safety problems restrict their use. For the specific case of vinylidenes, an attractive possibility is a terminal alkyne which is isomeric with a vinylidene. Although the thermolysis appears to effect this transformation (Equation 1.1, path a), the extraordinarily high temperatures required make the prospect of a transition metal-catalyzed version (Equation 1.1, path b) attractive. The early studies of Werner [6] using Rh and Bruce and co-workers [7] using Ru proved the facility with which such species would form however, the studies focused on the formation and isolation of the vinylidene-metal complexes and their stoichiometric reactions. 

The difluoroiodosilanes RpSiFtI (Rf = CFa or CaFs) required in the above work were synthesized via insertion of silicon difluoride into the C—bonds of the corresponding perfluoroiodoalkanes (c/. Vol. 1, p. 89). The alkenes CF.iCH SiFa plus CF CH SiF, SiF, and CHjtCF SiF, plus CHjiCfSiF have been identified as volatile products of attack on trifluoroethylene and vinylidene fluoride, respectively, by silicon difluoride, and the cyclic compounds (16) and (17) from reactions involving 3,3,3-trifluoropropyne. ... 

The addition of a terminal alkyne to a Ru precursor with the objective of creating a metathesis catalyst is a known strategy. It has already been used by Grubbs to generate an efficient metathesis catalyst from [RuCl2(p-cymene)]2, a NHC and ieri-butylacetylene [68]. Indeed, when die reaction was performed with the catalytic system B, under the same conditions but with an acetylene atmosphere instead of an inert gas atmosphere, an active alkene metathesis catalyst was generated, and no cycloisomerization was observed. The metathesis products 75-78 were thus formed in 68-82% yield (Eq. 17) [66,67]. To understand this change of catalytic activity, the fast formation of a Ru vinylidene, precursor of Ru carbene species, is proposed [69]. 

If the carbenium ion undergoes termination or chain transfer, the polymerization system deviates from living behavior. In Scheme 2.10, P-proton elimination is a common type of chain transfer that generates another carbenium ion while producing vinylidene (1,1-disubstituted) alkenes as... 

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