Carbonyl compounds reaction with trialkylboranes

Reaction with trialkylboranes. The reaction of trialkylborancs with acrolein (2, 15) and with methyl vinyl ketone (2. 284-185) yields, respectively, aldehydes and methyl ketones. However, the reaction is not directly applicable to all a,/3-unsaturated carbonyl compounds. Thus /ram-crotonaldehyde (I) and mesityl... 

The reaction of a chiral alkene with borane in the proper stoichiometry may afford alkyl boranes R BH2 or dialkyl boranes R BH, where R is a chiral ligand. Attempts to achieve highly selective reductions of ketones using such reagents have met with little success, however. Trialkyl boranes R3B were first reported to reduce aldehydes and ketones (under forcing conditions) in 1966 by Mikhailov [50]. Mechanistic studies (summarized in ref. [46]) showed that there are two limiting mechanisms for the reduction of a carbonyl compound by a trialkylborane, as shown in Scheme 7.4 a pericyclic process reminiscent of the Meerwein-Pondorf-Verley reaction (Scheme 7.4a), and a two step process that involves dehydro-... 

Organoboranes can also be used to construct carbon-carbon bonds by several other types of reactions that involve migration of a boron substituent to carbon. One such reaction involves a-halo carbonyl compounds.20 For example, ethyl bromoac-etate reacts with trialkylboranes in the presence of base to give alkylated acetic acid derivatives in excellent yield. The reaction is most efficiently carried out with a 9-BBN derivative. These reactions can also be effected with (3-alkenyl derivatives of 9-BBN to give (3,y-unsaturated esters.21... 

Extension of this reaction to electrophiles other than aldehydes was unsuccessful [22, 23], However, propargylic boronates were found to react with allylic halides and various carbonyl compounds [23], The boronates were prepared by lithiation of a methyl-substituted alkyne with t-butyllithium followed by treatment with a trialkylborane. The propargylic boronate preferentially reacts with the electrophile at the y-position to yield propargylic products (Eq. 9.20). The methodology has also been applied to alanates with comparable results. 

Since the catalytic effect of Cu ions with Fenton s reagent has been recognized, continuing interest has been focused on the role of the Cu species in oxidizing systems. The electrochemical alkyltransfer reaction of trialkylboranes to carbonyl compounds has been performed in an undivided cell by using the sacrificial Cu anode in a DMF-BU4NI system, giving the alkylated products in 62 77% yields [580]. 

Figure 3.27 shows reaction equations and the energy relationships of the hydroboration of enantiomerically pure a-pinene with 9-BBN. The reagent approaches only the side of the C=C double bond that lies opposite the isopropylidene bridge. The addition is thus completely diastereoselective. Moreover, the trialkylborane obtained is a pure enantiomer, since the starting material is a pure enantiomer. It is used as Alpine-Borane for the enantioselec-tive reduction of carbonyl compounds (Section 10.4). 

Addition to ot, -acetylenic esters and ketones (3, 108 6, 163-164). The addition of organocopper reagents to conjugated acetylenic carbonyl compounds is usually not stereospeciflc, although cis-addition predominates. Japanese chemists have found that the stereoselectivity in the reaction with alkylcopper reagents is markedly enhanced by use of the complex RCu BRj. Thus the reaction of dimethyl acetylenedicarboxylate with n-butylcopper complexed with tri-n-butylboron (or triethylboron) results in exclusive formation of the cis-adduct. In the absence of a trialkylborane the cis- and adducts are formed in the-ratio 85 15. 

The products (8) that result from the reaction of trialkylboranes with a,(3-unsaturated carbonyl compounds followed by hydrolysis (Scheme 2), provided the first strong evidence to permit assignment of the alkenyloxyborane structure to the precursors (7). 

Chiral boranes can be used as reducing agents for carbonyl compounds. 30 Brown prepared a variety of chiral boranes in connection with extensive studies of hydroboration reactions (sec. 5.4.B). Reaction of diborane and a-pinene, for example, gave (-)-( / ,25,3/ ,5/ )-diisopinocampheylborane (206), which reduced carbonyl derivatives to alcohols with high asymmetric induction.33 Trialkylboranes are also capable of... 

It is known that organoboranes readily undergo autoxidation in the presence of oxygen and this reaction has been used to prepare alcohols and alkyl hydroperoxides as well as to induce free radical reactions. Organoboranes can also be used to alkylate a,P-unsaturated carbonyl compounds through a free radical 1,4-addition reaction in the presence of oxygen. However, organoboranes do not normally react with saturated carbonyl compounds except for the reaction of formaldehyde with trialkylboranes in the presence of air. ... 

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