Alkylboronates

Solution polymerization of VDE in fluorinated and fluorochlorinated hydrocarbons such as CEC-113 and initiated with organic peroxides (99), especially bis(perfluoropropionyl) peroxide (100), has been claimed. Radiation-induced polymerization of VDE has also been investigated (101,102). Alkylboron compounds activated by oxygen initiate VDE polymerization in water or organic solvents (103,104). Microwave-stimulated, low pressure plasma polymerization of VDE gives polymer film that is <10 pm thick (105). Highly regular PVDE polymer with minimized defect stmcture was synthesized and claimed (106). Perdeuterated PVDE has also been prepared and described (107). 

Cumene hydroperoxide [95], benzoyl peroxide, or tert-h iiy peroxide [96]. can be used as accelerators with alkylboron initiators. The chain transfer constant for MMA to tributylborane has been estimated to be 0.647, which is comparable to tripropylamine [97]. 

The reaction of [ Co"(DMG)2py 2] with B( -C4H,)3 in benzene followed by extraction with CH2Cl2/aq. NaOH produces [ -BuCo(DMG)2py] the reaction is less successful with [pyCo "(DMG)2Cl] 161). This may be a convenient method using the more stable arylboron derivatives, but is probably experimentally more difficult than the Grignard route if a lower alkyl is to be introduced, because the lower alkylboron compounds inflame spontaneously in air, though they do not react with water, while the aryls are stable in air. 

Through the use of arenediazonium salts, the straightforward transformation of amines into cross-coupling products can be realized. Whenever the diazonium salts do not tolerate bases and strong nucleophiles (e.g., phosphines), base- and phosphine-free protocols have to be used. Heterocyclic carbene ligands serve well in cross-coupling of Aryl- and vinylboronic acids, or alkylboronates with arenediazonium salts.369,370 Several convenient phosphine-free protocols have been developed for the same purpose.371-373... 

The rhodium-catalyzed borylation of methyl C-H bonds is compatible with several moieties containing oxygen, nitrogen, and fluorine.154 For example, the reaction of pinacol acetal of 2-hexanone with bis(pinacolato)diborane (B2pin2) in the presence of Cp Rh(774-C6Me6) catalyst gives the alkylboronate ester in 74% yield (Equation (111)). The rhodium-catalyzed C-H activation and borylation occur at the least hindered and least electron-rich methyl group. 

In addition, with respect to the Suzuki coupling, various alkylboron reagents have also been successfully coupled with electrophile partners. 

As in the Negishi reaction, various alkylboron reagents have also been successfully coupled with electrophile partners. Suzuki et al. coupled 1-bromo-l-phenylthioethene with 9-[2-(3-cyclohexenyl)ethyl]-9-BBN (27), prepared by a simple addition of 9-borabicyclo[3.3.1]nonane (9-BBN) to 4-vinyl-1-hexene (26), to furnish 4-(3-cyclohexenyl)-2-phenylthio-1-butene (28) in good yield [36],... 

Diastereoselective homologation of chiral alkylboronates (cf. 12, 80-81). Investigations of this reaction have been carried out mainly on boronic esters (1)... 

The catalytic hydroboration of vinylarenes has also been well studied and, depending on the rhodium or iridium catalytic system used, the product distribution can be tuned. [lrCl2(T -C5Me5)]2 catalyzed the hydroboration of 4-vinylanisole in the presence of HBcat with the exclusive formation of the terminal hydroboration product, in contrast to the analogue rhodium complexes which mainly afford the branched alkylboronate ester (Scheme 7.13) [14]. 

A mild one-pot procedure based on a platinum-catalyzed diborylation of 1,3-butadienes (see Eq. 30) gives doubly allylic boronate 144, which adds to an aldehyde to form a quaternary carbon center in the intermediate 145 (Eq. 105). The use of a tartrate auxiliary in this process leads to good levels of enantiose-lectivity in the final diol product, which is obtained after oxidation of the primary alkylboronate intermediate. Although examples of aliphatic, aromatic, and unsaturated aldehydes have been described, enantioselectivities vary widely (33 to 74% ee), and are good only for aliphatic aldehydes. An intramolecular variant of this interesting tandem reaction is also known. ... 

While the introduction of aryl groups has been well documented, the use of alkylboronic acids to decorate the pyridazine core is hitherto not well explored. Wermuth and co-workers nicely showed that hydroboration of alkenes with 9-BBN followed by Suzuki coupling with 3-iodopyridazines 180 and 182 yielded the corresponding 3-alkylpyr-idazines 181 and 183 in good yield (Schemes 43 and 44) <2002SL1123>. 

Yet another use for bis(oxazoline) ligands is in the synthesis of (a-chloroalkyl) boronates. " " As shown in Figure 9.71, the alkylboronate 240 was converted to the a-chloro derivative 241 in 86% ee through the use of ytterbium(lll) triflate-complexed phe-box 6. 

A)-Pinanediol ethylboronate with (l,l-dichloroelhyl)lithium yields the (S)-a-chloro boronic ester (89 11 d.r.), which is converted by phenylmagnesium bromide to the (S)-tert-alkylboronate, the same isomer obtained from (S )-pinanediol phenylboronate. The enantiomeric excess of the derived (R)-2-phenyl-2-butanol is 70%. 

Homologation of boronic esters. The reagent 1 can be formed in situ by reaction of CH2C12 with lithium 2,2, 6,6 -tetramethylpiperidide (LiTMP) or even LDA. The product obtained on reaction of 1 with alkylboronic esters, an a-chlo-roboronic ester, is reduced by potassium triisopropoxyborohydride (KIPBH) to the homologated boronic ester.1... 

---