Boron compounds, conjugate addition

When the metallic additive to the intermediate 374 was zinc dihalide (or another Lewis acid, such as aluminum trichloride, iron trichloride or boron trifluoride), a conjugate addition to electrophilic olefins affords 381 . In the case of the lithium-zinc transmetallation, a palladium-catalyzed Negishi cross-coupling reaction with aryl bromides or iodides allowed the preparation of arylated componnds 384 ° in 26-77% yield. In addition, a Sn2 allylation of the mentioned zinc intermediates with reagents of type R CH=CHCH(R )X (X = chlorine, bromine) gave the corresponding compounds 385 in 52-68% yield. ... 

A diastereoselective Rh(I)-catalysed conjugate addition reaction of aryl- and alkenyl-boronic acids to unprotected 2-phenyl-4-hydroxycyclopentenone (207) has been investigated. The free OH group on the substrate was found to be responsible for the (g) stereochemistry, which is cis for arylboronic derivatives (208). In the case of the alkenylboronic compounds, the stereochemistry can be tuned to either a cis (with a base as additive) or trans addition (209) (with CsF as additive), without the need for protecting groups.249... 

Boron reagents add to conjugated carbonyl compounds. Alkynyl borate esters (p. 815) give conjugate addition in the presence of boron trifluoride ethe-... 

There are currently three general methods for the production of boron-stabilized carbanions, each of which has analogies in caibonyl chemistry. The cleavage of a 1,1-diboiylalkane by base readily yields the desired anion in a fashion similar to the base cleavage of a -dicaibonyl compound. Deprotonation a to a boron atom can be accomplished directly in special circumstances, as can the addition of an organo-metallic compound to a vinylborane, which is similar to conjugate addition to an a,p-unsaturated carbonyl compound. Each of these methods is treated in detail in Ae next three sections. 

This commoner type of reaction involves the attack of carbon or heteroatom nucleophiles onto carbonyl compounds, by direct or conjugate addition, and onto imines. Because we have just dealt with boranes we shall start with the reaction of allylic boron compounds with such electrophiles. You might strictly not call this nucleophilic attack. 

Boron or tin (II) Z-enolates are generated by reaction with the corresponding triflates with a carbonyl compound in the presence of tertiary amines like r-P NEt or. M-ethylpiperidine (except when using dicyclopentylboron triflate [407]). E-Enolates are prepared by using dicyclohexyl- or other cyclic chloroboranes in the presence of Et3N or Me NEt [407, 685, 686, 1246, 1247, 1248], Because enolization does not take place under such conditions with esters or aliphatic tertiary amides, thiophenyl esters RGH COSPh have been used as ester/amide substitutes. Furthermore, Z-boron enolates of ketones can be prepared by conjugate addition of acid derivatives of dialkylboranes to a-enones [687],... 

The a,(3-unsaturated ester (8), aldehyde (9), and nitro (10) compounds participate as electrophiles in a number of useful conjugate addition reactions. Copper-catalyzed addition of Grignard reagents provides access to aryl butanoic acid derivatives substituted with an oxetane (equation 1 in scheme 13.7). The ester, aldehyde, and nitro electrophiles also undergo mild Rh-catalyzed additions of aryl and vinyl boronic acids (equation 2 through equation 4 in scheme 13.7). Interestingly, the unsaturated aldehyde participates readily in an amine conjugate addition to afford oxetane substituted 3-amino-acetaldehyde derivatives. 

The reaction of aromatic and aliphatic primary selenoamides with a,/3-unsaturated ketones in the presence of boron trifluoride affords a variety of 477-1,3-selenazines in high yields (Scheme 41, and Tables 4 and 5) <1999J(P1)453>. The reaction proceeds by conjugate addition of the selenium to the /3-position of the unsaturated system followed by cyclization. In cases where diastereomeric compounds are formed, the m-isomer predominates. 

The empty 2/j,-orbital on boron interacting with the ji-electrons of the butadienylidene system in borole (5) is proved by the pronounced bathochromic shift in the visible spectrum compared with that of its pyridine adduct. This Ti-electron delocalization in boroles is destabilizing and produces a 471-electron antiaromatic system. The antiaromaticity is further evidenced by the rapidity of reactions that remove the 2p ,-orbital on boron from conjugation. These reactions include Lewis complexation, transition metal complexation, or borole ring opening reactions such as Diels-Alder addition, oxidation, and protodeboration. This removal of the antiaromaticity from borole results in transforming the compound to a typical, low reacting, sterically hindered arylborane. 

Mixing trialkylboranes with a, 3-unsaturated carbonyl compounds in the presence of water gives conjugate addition of one of the alkyl groups from the boron. The carbonyl compound can be an aldehyde (R = H) or a ketone (R = alkyl). 

Miyaura and eo-workers found that conjugate addition of arylboronic acids to a -unsaturated carbonyl compounds can be carried out by using cationic Pd(II) complexes such as [Pd(dppe)(PhCN)2]X2 as catalysts. Neutral Pd complexes such as PdCl2(dppe) are not active. As anions, X = C104, OTf , Bp4 , PFe", and SbFe are effective. Coordination of DPPE and PhCN is important. The reaction proceeds at room temperature in THF or dioxane, and requires the presence of water. The addition of boronic acid 48 to cyclohexenone took place to give 49 in 95% yield [18]. Also 3-phenylcyclohexanone (50) is prepared by the conjugate addition of PhSi(OEt)3 to cyclohexenone [19]. 

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