Alkyl Carbagermatranes Enable Practical Palladium-Catalyzed sp2-sp3 Cross-Coupling

Pd-catalyzed cross-coupling reactions have achieved tremendous accomplishments in the past decades. However, C(sp³)-hybridized nucleophiles generally remain as challenging coupling partners due to their sluggish transmetalation compared to the C(sp²)-hybridized counterparts. While a single-electron-transfer-based strategy using C(sp³)-hybridized nucleophiles had made significant progress recently, fewer breakthroughs have been made concerning the traditional two-electron mechanism involving C(sp³)-hybridized nucleophiles. In this report, we present a series of unique alkyl carbagermatranes that were proven to be highly reactive in cross-coupling reactions with our newly developed electron-deficient phosphine ligands. Generally, secondary alkyl carbagermatranes show slightly lower, yet comparable activity to its Sn analogue. Meanwhile, primary alkyl carbagermatranes exhibit high activity, and they were also proved stable enough to be compatible with various reactions. Chiral secondary benzyl carbagermatrane gave the coupling product under base/additive-free conditions with its configuration fully inversed, suggesting that transmetalation was carried out in an "S_E2(open) Inv" pathway, which is consistent with Hiyama's previous observation. Notably, the cross-coupling of primary alkyl carbagermatranes could be performed under base/additive-free conditions with excellent functional group tolerance and therefore may have potentially important applications such as stapled peptide synthesis.

Pd-catalyzed one-pot sequential unsymmetrical cross-coupling reactions of aryl/heteroaryl 1,2-dihalides

The present methodology allows the conversion of easily available aryl/heteroaryl 1,2-dihalides into synthetically useful unsymmetrically substituted arenes/heteroarenes in good to excellent yields under mild reaction conditions.

Palladium-catalyzed Hiyama cross-coupling of aryltrifluorosilanes with aryl and heteroaryl chlorides

An efficient, palladium-catalyzed Hiyama cross-coupling reaction of aryltrifluorosilanes with aryl chlorides has been developed. A wide variety of functionalized biaryl derivatives were isolated in good to excellent yields. The scope of this reaction has also been extended to heteroaryl chlorides, affording the corresponding heterobiaryl compounds in high yields.

Dual role of silanol groups in cyclopropanation and Hiyama-Denmark cross-coupling reactions

Di-tert-butoxy(alkenyl)silanols serve as substrates in the Simmons-Smith cyclopropanation reaction furnishing the corresponding di-tert-butoxy(cyclopropyl)silanols, which may be included in a Hiyama-Denmark cross-coupling reaction. The silanol group bears two distinct roles as it provides a directing group during the cyclopropanation and mediates the transmetalation event in the cross-coupling. The nature of the ligands on the silicon atom had a profound effect on reactivity in the cross-coupling, whereby substituting the alkoxide groups for fluorides allowed for efficient cross-coupling.

Silicon-based cross-coupling reactions in the total synthesis of natural products

Unlike other variants of transition-metal-catalyzed cross-coupling reactions, those based on organosilicon donors have not been used extensively in natural product synthesis. However, recent advances such as: 1) the development of mild reaction conditions, 2) the expansion of substrate scope, 3) the development of methods to stereoselectively and efficiently introduce the silicon-containing moiety, 4) the development of a large number of sequential processes, and 5) the advent of bifunctional bis(silyl) linchpin reagents, signify the coming of age of silicon-based cross-coupling reactions. The following case studies illustrate how silicon-based cross-coupling reactions play a strategic role in constructing carbon-carbon bonds in selected target molecules.

Iron-catalyzed cross-coupling reactions

Simple iron salts such as FeCl(n), Fe(acac)(n) (n = 2,3) or the salen complex 4 turned out to be highly efficient, cheap, toxicologically benign, and environmentally friendly precatalysts for a host of cross-coupling reactions of alkyl or aryl Grignard reagents, zincates, or organomanganese species with aryl and heteroaryl chlorides, triflates, and even tosylates. An "inorganic Grignard reagent" of the formal composition [Fe(MgX)(2)] prepared in situ likely constitutes the propagating species responsible for the catalytic turnover, which occurs in many cases at an unprecedented rate even at or below room temperature. Because of the exceptionally mild reaction conditions, a series of functional groups such as esters, ethers, nitriles, sulfonates, sulfonamides, thioethers, acetals, alkynes, and -CF(3) groups are compatible. The method also allows for consecutive cross-coupling processes in one pot, as exemplified by the efficient preparation of compound 12, and has been applied to the first synthesis of the cytotoxic marine natural product montipyridine 8. In contrast to the clean reaction of (hetero)aryl chlorides, the corresponding bromides and iodides are prone to a reduction of their C-X bonds in the presence of the iron catalyst.

Organolanthanide-catalyzed cyclization/boration of 1,5- and 1,6-Dienes

[figure: see text] 1,5- and 1,6-Dienes undergo a cyclization/boration reaction in the presence of a catalytic amount of Cp*2Sm.THF. The resulting organoboranes can be oxidized to the corresponding primary cyclic alcohols using standard conditions.