Synthesis of alkenyl halides

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β-Oxido phosphonium ylides, generated in situ from aldehydes and Wittig reagents react readily with electrophilic halogen sources to form predominantly or exclusively E-bromo- or iodosubstituted alkenes. The stereochemical outcome on halogenation is remarkably sensitive to alkylidene size [ethylidene(triphenyl)phosphorane is highly Z-selective].
D. M. Hodgson, T. Arif, J. Am. Chem. Soc., 2008, 130, 16500-16501.

(E)-β-Aryl vinyl halides are synthesized in good yield with excellent stereoselectivity and functional group tolerance from benzyl bromides. The convenient conditions involve a nucleophilic substitution with anions of CH₂I₂, ICH₂Cl, or CH₂Br₂ and a subsequent stereoselective base-induced elimination. A facile purification allows rapid access to a wide range of functionalized vinyl halides.
J. A. Bull, J. J. Mousseau, A. B. Charette, Org. Lett., 2008, 10, 5485-5488.

Previously unknown halo-substituted molybdenum alkylidene species are exceptionally reactive and are able to participate in high-yielding olefin metathesis reactions that afford acyclic 1,2-disubstituted Z-alkenyl halides. Transformations are promoted by small amounts of a catalyst that is generated in situ and used with unpurified, commercially available and easy-to-handle liquid 1,2-dihaloethene reagents.
M. J. Koh, T. T. Ngyuen, H. Zhang, R. R. Schrock, A. H. Hoveyda, Nature, 2016, 531, 459-465.

An efficient protocol for a highly stereoselective one-pot synthesis of (E)-β-aryl vinyl iodides and (E)-β-aryl vinyl bromides from styrenes is based on a ruthenium-catalyzed silylative coupling followed by a N-halosuccinimide-mediated halodesilylation reaction.
P. Pawluć, G. Hreczycho, J. Szudkowska, M. Kubicki, B. Marciniec, Org. Lett., 2009, 11, 3390-3393.

Trapping of a diazo species in an intermolecular fashion by two independent ion species in tandem at the carbene center installs an electrophile and a nucleophile on the same carbon. This metal-free concept enables regioselective syntheses of various vinyl halides, vinyl sulfones, and alkyne derivatives.
D. P. Ojha, K. R. Prabhu, Org. Lett., 2015, 17, 18-21.

An optimized Julia olefination between readily available α-halomethyl sulfones and a variety of aldehydes afforded alkenyl halides in good to excellent yields with high E/Z stereoselectivities.
M.-E. Lebrun, P. Le Marquand, C. Berthelette, J. Org. Chem., 2006, 71, 2009-2013.

A stereoselectiv hydrodefluorination reaction of trifluoromethylated alkenes with LiAlH₄ provides terminal monofluoroalkenes in very good yields and diastereoselectivities. Mechanistic studies suggested a hydroalumination reaction followed by a stereoselective fluoride elimination.
P. Poutrel, X. Pannecoucke, P. Jubault, T. Poisson, Org. Lett., 2020, 22, 4858-4863.

An efficient synthesis of α-iodo/bromo-α,β-unsaturated aldehydes/ketones directly from propargylic alcohols is catalyzed collaboratively by Ph₃PAuNTf₂ and MoO₂(acac)₂, and Ph₃PO as an additive helps suppress undesired enone/enal formation. Notable features of this method include low catalyst loadings, mild reaction conditions, and mostly good diastereoselectivity.
L. Ye, L. Zhang, Org. Lett., 2009, 11, 3646-3649.

A copper-catalyzed formal S_N2' defluorinative borylation of 3-substituted 3,3-difluoropropenes provides 3-fluoroallylboronic esters in high yields with excellent Z:E ratios. The primary 3-fluoroallylboronic esters undergo several synthetic sequences involving sigmatropic rearrangements, S_E2' substitutions, and S_N2' substitutions to give tertiary allylic fluorides.
T. W. Butcher, J. L. Yang, J. F. Hartwig, Org. Lett., 2020, 22, 6805-6809.

A copper-amine catalyst system enables a general and facile synthetic method for β-fluoroenones from silyl enol ethers or ketones via tandem process of difluoroalkylation-hydrolysis-dehydrofluorination. This reaction offers high yields, excellent Z/E ratios, a low-cost catalyst, and a broad substrate scope.
Y. Li, J. Liu, S. Zhao, Y. Du, M. Guo, W. Zhao, X. Tang, G. Wang, Org. Lett., 2018, 20, 917-920.

Stoichiometric bromotrichloromethane in acetonitrile can replace solvent quantities of carbon tetrachloride in the synthesis of gem-dichloroalkenes from aldehydes in the presence of triphenylphosphine. Similarly, bromotrichloromethane in dichloromethane can be used for the Appel reaction of benzyl alcohols to form benzyl chlorides at r.t., which is commonly carried out in refluxing carbon tetrachloride.
S. G. Newman, C. S. Bryan, D. Perez, M. Lautens, Synthesis, 2011, 342-346.

The Ramberg-Bäcklund reaction of alkyl triflones provides structurally diverse, fully substituted gem-difluoroalkenes that are difficult to prepare by treatment with specific Grignard reagents. The Grignard reagent serves both as a base to remove the α-proton and as a Lewis acid to assist C-F bond activation.
Y. Maekawa, M. Nambo, D. Yokogawa, C. M. Crudden, J. Am. Chem. Soc., 2020, 142, 15667-15672.

A transition-metal-free cross-coupling of two carbene fragments proceeds through the direct nucleophilic addition of diazo compounds to difluorocarbene derived from Ruppert-Prakash reagent (TMSCF₃) or TMSCF₂Br followed by elimination of N₂. This transition-metal-free protocol efficiently affords various disubstituted 1,1-difluoroalkenes, including difluoroacrylates, diaryldifluoroolefins, as well as arylalkyldifluoroolefins.
M. Hu, C. Ni, L. Li, Y. Han, J. Hu, J. Am. Chem. Soc., 2015, 137, 14496-14501.

A Cu-catalyzed gem-difluoroolefination of diazo compounds with TMSCF₃ affords the desired 1,1-difluoroalkene products via trifluoromethylation followed by C-F bond cleavage.
M. Hu, Z. He, B. Gao, L. Li, C. Ni, J. Hu, J. Am. Chem. Soc., 2013, 135, 17302-17305.

A fast and practical cross-coupling between difluorocarbene and carbene-derived intermediates generated from diazocompounds provides hindered gem-difluoroolefins. Whereas the cross-coupling between difluorocarbene and aryl diazoacetates proceeded smoothly in the presence of a copper source, the coupling with diaryl diazomethanes occurred well under metal-free conditions.
J. Zheng, J.-H. Lin, L.-Y. Yu, Y. Wei, X. Zheng, J.-C. Xiao, Org. Lett., 2015, 17, 6126-6129.

Difluoromethyl 2-pyridyl sulfone acts as a novel and efficient gem-difluoroolefination reagent for both aldehydes and ketones. A fluorinated sulfinate intermediate in the reaction is relatively stable and can be observed by ¹⁹F NMR and trapped with CH₃I.
Y. Zhao, W. Huang, L. Zhu, J. Hu, Org. Lett., 2010, 12, 1444-1447.

A gem-difluoroolefination of trifluoroacetaldehyde N-triftosylhydrazone with organoboronic acids - including alkylboronic acids - is operationally simple, free of transition metals, and suitable for a broad range of substrates. Moreover, the utility of the products was demonstrated by further conversion of the gem-difluorovinyl group.
Y. Ma, B. R. P. Reddy, X. Bi, Org. Lett., 2019, 21, 9860-9863.

A copper-catalyzed enantioselective defluoroalkylation of linear 1-(trifluoromethyl)alkenes with arylboronate-activated alkyl Grignard reagents provides various gem-difluoroalkenes. Tetraorganoborate complexes generated in situ were the key reactive species for this transformation.
M. Wang, X. Pu, Y. Zhao, P. Wang, Z. Li, C. Zhu, Z. Shi, J. Am. Chem. Soc., 2018, 140, 9061-9065.

Zn mediates a decarboxylative/defluorinative alkylation of α-trifluoromethyl alkenes with N-hydroxyphthalimide esters as radical precursors. Several α-trifluoromethyl alkenes were readily coupled to a wide range of primary, secondary, and tertiary radicals, affording the desired gem-difluoroethylenes in good yields.
H.-W. Du, Y. Chen, J. Sun, Q.-S. Gao, H. Wang, M.-D. Zhou, Org. Lett., 2020, 22, 9342-9345.

An organic photoredox-catalyzed gem-difluoroallylation of α-trifluoromethyl alkenes with alkyl iodides provides gem-difluoroalkene derivatives via C-F bond cleavage. This transition-metal-free transformation utilizes a readily available organic dye (4CzIPN) as the sole photocatalyst and N,N,N',N'-tetramethylethylenediamine as the radical activator.
S. Yan, W. Yu, J. Zhang, H. Fan, Z. Lu, Z. Zhang, T. Wang, J. Org. Chem., 2022, 87, 1574-1584.

An efficient photoredox catalytic radical addition/defluoroalkylation coupling reaction between derivatives of primary amines and trifluoromethyl-substituted alkenes provides a series of gem-difluoroalkenes under visible light irradiation. This reaction offers a broad substrate scope and good functional group tolerance.
B. Wang, C.-T. Wang, X.-S. Li, X.-Y. Liu, Y.-M. Liang, Org. Lett., 2022, 24, 6566-6570.

Hydroboration of α,β-unsaturated carbonyl substrates followed by 1,2-elimination provides gem-difluoroalkenes under mild conditions. gem-Difluoroalkenes can be converted into β-CF₂H- and β-CFH₂-α,β-unsaturated carbonyls.
S. An, J. Zhang, G. Jiang, Synlett, 2021, 32, 91-94.

The generation of a tribromomethyl radical from tetrabromomethane (CBr₄) with the assistance of a persulfate salt (K₂S₂O₈) enables a single step conversion of terminal alkynes to gem-dibromoenones in the presence of water. The reaction features readily available chemicals, a broad substrate scope, a green solvent, and mild reaction conditions.
X. Zeng, Y. Xu, J. Liu, Y. Deng, Org. Lett., 2021, 23, 9058-9062.

An efficient, practical photocatalytic deoxygenation/defluorination protocol provides γ,γ-difluoroallylic ketones from commercially available aromatic carboxylic acids and α-trifluoromethyl alkenes in the presence of triphenylphosphine. The protocol offers good functional group tolerance and a broad substrate scope.
Y.-Q. Guo, R. Wang, H. Song, Y. Liu, Q. Wang, Org. Lett., 2020, 22, 709-713.

Bis(2,2,2-trifluoroethyl)bromophosphonoacetate, a Novel HWE Reagent for the Preparation of (E)-α-Bromoacrylates: A General and Stereoselective Method for the Synthesis of Trisubstituted Alkenes
K. Tago, H. Kogen, Org. Lett., 2000, 2, 1975-1978.

Deprotonation of a 64:36 mixture of ethyl bromo(diphenylphosphono)acetate and ethyl dibromo(diphenylphosphono)acetate using NaH as base at 0 °C and subsequent reaction with various aldehydes provided unsaturated α-bromoesters in good yields and high E-selectivities.
T. Olpp, R. Brückner, Synthesis, 2004, 2135-2152.

A highly stereoselective olefination reaction of α-fluoro-β-keto esters enables the synthesis of α-fluoro-α,β-unsaturated esters via nucleophilic addition, intramolecular nucleophilic addition, and elimination. α-Fluoro-α,β-unsaturated esters are important units in many biologically active compounds and useful precursors in a variety of functional-group transformations.
J. Qian, W. Yi, M. Lv, C. Cai, Synlett, 2015, 26, 127-132.

The reaction of aromatic ketones with bis(trichloromethyl) carbonate afforded aryl-(Z)-vinyl chlorides in the presence of scandium triflate, DMF, and benzoyl chloride as catalysts. A plausible addition-elimination mechanism is proposed. This catalytic method enables a green route to aryl-(Z)-vinyl chlorides.
W. Su, C. Jin, Org. Lett., 2007, 9, 993-996.

(E)-β-Arylvinyl bromides were efficiently and stereoselectively prepared in high yields by microwave irradiation of the corresponding anti-3-aryl-2,3-dibromopropanoic acids in AcOH in the presence of AgOAc.
C. Kuang, H. Senboku, M. Tokuda, Tetrahedron, 2005, 61, 637-642.

(Z)-1-Bromo-1-alkenes were stereoselectively efficiently prepared in high yields by microwave irradiation of the corresponding anti-2,3-dibromoalkanoic acids in DMF using Et₃N as base. Microwave-assisted one-pot syntheses of terminal alkynes and enynes from 2,3-dibromoalkanoic acids were also developed.
C. Kuang, Q. Yang, H. Senboku, M. Tokuda, Tetrahedron, 2005, 61, 4043-4052.

Application of (PhO)₃P-halogen-based reagents allows the preparation of vinyl halides in good to excellent yields from enolizable ketones, whereas aldehydes afforded the corresponding gem-dihalides. The halogenation proceeded smoothly under mild conditions.
A. Spaggiari, D. Vaccari, P. Davoli, G. Torre, F. Prati, J. Org. Chem., 2007, 72, 2216-2219.

Allylic gem-dichlorides undergo regio- and enanantioselective copper-catalyzed allylic alkylation with Grignard reagents to afford chiral Z-vinyl chlorides. Subsequent Suzuki cross coupling reactions afford optically active Z-alkenes and 1,3-cis,trans dienes.
M. Giannerini, M. Fañanás-Mastral, B. L. Feringa, J. Am. Chem. Soc., 2012, 134, 4108-4111.

An efficient copper-catalyzed reductive defluorination of β-trifluoromethylated enones proceeds smoothly with a wide range of substrates to provide gem-difluoroolefin ketones in the presence of Grignard reagents as reducing agents. The simultaneous homocoupling process of the Grignard reagents is effective for both Csp²-Csp² and Csp³-Csp³ bond formations to afford coupling products with high yields.
X. Wu, F. Xie, I. D. Gridnev, W. Zhang, Org. Lett., 2018, 20, 1638-1642.