Organoborates

The organoborate intermediates can also be generated from alkenylboronic esters and alkyllithium or Grignard reagents, or from ttialkylboranes and alkenyllithium compounds. Conjugated symmetrical and unsymmetrical diynes (289—291), stereochemically pure 1,3-dienes (292,293), and 1,3-enynes (294) including functionali2ed systems can be prepared (289,295). 

OC-All lation of Carbonyl Compounds and Derivatives. The organoborate iatermediates generated by the reaction of alkylboranes with carbanions derived from a-halocarbonyl compounds and a-halonitriles rearrange to give a-alkylated products. 

Michailov, B. M., T. A. Schegoleva, E. M. Schaskova, and V. D. Sche-ludjakov Organobor-Verbindungen, 102. Mitt., Mono (alky lthio)-borane. Isv. Akad. Nauk, SSSR 1962, 1143. 

Multinuclear ( H, 13C, nB) NMR spectroscopy is undoubtly the most useful and powerful method for the investigation of the organoboration of 1-alkynyltin, -germanium, and -silicon compounds allowing not only the structure of the final products to be determined but also the product ratio to be ascertained (Tables 6-8). 

Table 7 Selected NMR data for 3-borahomoadamantanes and bis-3-borahomoadamantanes prepared via 1,1 -organoboration of 1 -alkynylsilicon, -germanium, and -tin compounds with 1 -boraadamantane... 

It should be noted that 1,1-organoboration of Me3SiC=CMe with Et3B requires harsher conditions 100 °C and 24 h. Bis(trimethylsilyl)ethyne does not react with Et3B, but readily (20 °C) inserts into the 1-boraadamantane core giving rise to an equilibrium mixture containing c. 80% of 92 (Scheme 35). Treatment of this mixture with bis(trialkylstannyl)ethynes leads to the formation of distannane derivatives 93 (R = Me, Et) which seem to be more stable than 92 <2001JOM(620)51>. 

The two acetylenic functions in the silicon or tin derivative make possible a competition between 1 1 and 2 1 reactions (Scheme 39, pathway b), and the product ratio depends significantly on the reaction conditions. For silicon derivatives an excess of 1-boraadamantane in the reaction mixture (acetylenic component added to 1-boraadamantane) leads to the octacyclic compounds 88 (up to 60%), while when the much more reactive 1-alkynyltin analogues are used, a second intramolecular 1,1-organoboration often takes place <2001CEJ775>. The stmcture of 88c (M = Sn, R = Me, R1 = SiMe3) was confirmed by X-ray analysis <2001CEJ775>. 

As mentioned above the 1,1-organoboration reaction is reversible, and exchange is slow on the NMR timescale. This statement is in agreement with the chemical behavior of equilibrated mixtures of products. Thus, treatment of silicon borahomoadamantane derivative 92 with bis(trimethylstannyl)ethyne leads to the tin-containing compound 93 and liberation of bis(trimethylsilyl)ethyne (Scheme 44). With pyridine, the equilibrium is moved toward 1-boraadamantane completely due to the complexation <2001JOM(620)51>. 

According to the literature, the bulkier the substituents on the methylideno group, the more reversible is the 1,1-organoboration reaction <2001JOM(620)51>. In contrast to 92 and 93, (/. )-4-( 1-trimethyl si lyl)ethylidcno-3-boraho-moadamantane 89 does not react with pyridine (Scheme 45). 

Organoboration of diacetylenic derivatives proceeds stepwise and in a number of cases is also accompanied by 1,2-anionotropic rearrangement giving rise to polycyclic compounds (see Section 12.13.4.2) <2001CEJ775, 2002CEJ1537>. 

As was mentioned above (see Section 12.13.4.2), the 1,1-organoboration of alkynylsilanes, -stannanes, and -germans with trialkylboranes and boracyclanes occurs under very mild (Sn), mild (Ge), or harsh (Si, > 100 °C) reaction conditions <1995CCR125>. Similar reactions with triallylboranes proceed in general under milder conditions in comparison with trialkylboranes and furthermore a competition between 1,1- and 1,2-allylboration often takes place... 

Despite considerable efforts, the formulation in equation (42) remains incomplete owing to the high reactivity of organocuprates as well as their oligomeric nature. Accordingly, we select organoborates as stable electron donors to study alkyl additions to various pyridinium acceptors (by thermal and photoinduced electron transfer) via charge-transfer salts as follows. 

Alkylation of pyridinium acceptors with organoborates as electron donors... 

Spirostannanes can also be prepared by organoboration of tetrakis(trimethylsilyl-ethynyl)tin with acyclic trialkylboranes BR3 to give the stannaspiro[4,4]nona-l,3,6,8-tetraenes117 74 (R = ethyl, neopentyl). A stannacyclopentadiene was prepared by this type of organoboration of alkynylstannanes123. Treating bis(trimethylsilylethynyl)dimethyltin with a trialkylboron leads to the substituted stannacyclopentadiene 75. 

By organoboration of bis(alkynyl)plumbanes 85, the compound 86, stabilized by a jt-bonding interaction, is prepared180. (Scheme 22a). The cation 86 is unstable and, at room temperature, decomposes rapidly to rearrange into a l,4-plumbabora-2,5-cyclohexa-diene, 87. 

A variety of boron-substituted stannoles such as those shown in formulas 10 and 12 have been prepared by Wrackmeyer s 1,1-organoboration of dialkynes.234... 

Wrackmeyer s organoboration of dialkynylstannanes to give borylstannacyclopentadienes is referred to above. By this type of reaction, sfiiro-stannacycloalkanes and cycloalkenes have been prepared (Scheme 12). 

Alkynes XC=CR cannot be organoborated, when X represents an alkyl group. In the case of X = H 74) or X = Me3Sn (75), a particular type of alkyloboration, coupled to the migration of X, is possible [Eq. (36)]. Such a reaction was not observed with iminoboranes. 

The amination of organoboranes with H2NOZ-type reagents involves initial nucleophilic attack of NH2OZ on the organoborane to yield an organoborate complex followed... 

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